These studies were funded by US Public Health Services grants P01AI057005, R01AI047734, R01AI111806 and the Functional Profiling and Computational Biology Core of the University of Washington&#8217;s Center for AIDS Research (P30 AI027757).

<ol>
	<li>Domingo, E., Holland, J. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=RNA+virus+mutations+and+fitness+for+survival.">RNA virus mutations and fitness for survival.</a> Annu Rev Microbiol. 51, 151-178 (1997).</li><li>Domingo, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mechanisms+of+viral+emergence.">Mechanisms of viral emergence.</a> Veterinary research. 41 (6), 38 (2010).</li><li>Martinez-Picado, J., Martinez, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=HIV-1+reverse+transcriptase+inhibitor+resistance+mutations+and+fitness:+a+view+from+the+clinic+and+ex+vivo.">HIV-1 reverse transcriptase inhibitor resistance mutations and fitness: a view from the clinic and ex vivo.</a> Virus Res. 134 (1-2), 104-123 (2008).</li><li>Troyer, R. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Variable+fitness+impact+of+HIV-1+escape+mutations+to+cytotoxic+T+lymphocyte+(CTL)+response.">Variable fitness impact of HIV-1 escape mutations to cytotoxic T lymphocyte (CTL) response.</a> PLoS Pathog. 5 (4), e1000365 (2009).</li><li>Rolland, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Amino-acid+co-variation+in+HIV-1+Gag+subtype+C:+HLA-mediated+selection+pressure+and+compensatory+dynamics.">Amino-acid co-variation in HIV-1 Gag subtype C: HLA-mediated selection pressure and compensatory dynamics.</a> PLoS One. 5 (9), e12463 (2010).</li><li>Abraha, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=CCR5-+and+CXCR4-tropic+subtype+C+human+immunodeficiency+virus+type+1+isolates+have+a+lower+level+of+pathogenic+fitness+than+other+dominant+group+M+subtypes:+implications+for+the+epidemic.">CCR5- and CXCR4-tropic subtype C human immunodeficiency virus type 1 isolates have a lower level of pathogenic fitness than other dominant group M subtypes: implications for the epidemic.</a> J Virol. 83 (11), 5592-5605 (2009).</li><li>Quinones-Mateu, M. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+dual+infection/competition+assay+shows+a+correlation+between+ex+vivo+human+immunodeficiency+virus+type+1+fitness+and+disease+progression.">A dual infection/competition assay shows a correlation between ex vivo human immunodeficiency virus type 1 fitness and disease progression.</a> J Virol. 74 (19), 9222-9233 (2000).</li><li>Ball, S. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparing+the+ex+vivo+fitness+of+CCR5-tropic+human+immunodeficiency+virus+type+1+isolates+of+subtypes.">Comparing the ex vivo fitness of CCR5-tropic human immunodeficiency virus type 1 isolates of subtypes.</a> B and C. J Virol. 77 (2), 1021-1038 (2003).</li><li>Lanxon-Cookson, E. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Factors+affecting+relative+fitness+measurements+in+pairwise+competition+assays+of+human+immunodeficiency+viruses.">Factors affecting relative fitness measurements in pairwise competition assays of human immunodeficiency viruses.</a> J Virol Methods. 194 (1-2), 7-13 (2013).</li><li>Garcia-Lerma, J. G., MacInnes, H., Bennett, D., Weinstock, H., Heneine, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transmitted+human+immunodeficiency+virus+type+1+carrying+the+D67N+or+K219Q/E+mutation+evolves+rapidly+to+zidovudine+resistance+in+vitro+and+shows+a+high+replicative+fitness+in+the+presence+of+zidovudine.">Transmitted human immunodeficiency virus type 1 carrying the D67N or K219Q/E mutation evolves rapidly to zidovudine resistance in vitro and shows a high replicative fitness in the presence of zidovudine.</a> J Virol. 78 (14), 7545-7552 (2004).</li><li>Sharma, P. L., Crumpacker, C. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Attenuated+replication+of+human+immunodeficiency+virus+type+1+with+a+didanosine-selected+reverse+transcriptase+mutation.">Attenuated replication of human immunodeficiency virus type 1 with a didanosine-selected reverse transcriptase mutation.</a> J Virol. 71 (11), 8846-8851 (1997).</li><li>Martinez-Picado, J., Savara, A. V., Sutton, L., D'Aquila, R. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Replicative+fitness+of+protease+inhibitor-resistant+mutants+of+human+immunodeficiency+virus+type+1.">Replicative fitness of protease inhibitor-resistant mutants of human immunodeficiency virus type 1.</a> J Virol. 73 (5), 3744-3752 (1999).</li><li>Wang, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+HIV-1+reverse+transcriptase+mutants+G190S+and+G190A,+which+confer+resistance+to+non-nucleoside+reverse+transcriptase+inhibitors,+demonstrate+reductions+in+RNase+H+activity+and+DNA+synthesis+from+tRNA(Lys,+3)+that+correlate+with+reductions+in+replication+efficiency.">The HIV-1 reverse transcriptase mutants G190S and G190A, which confer resistance to non-nucleoside reverse transcriptase inhibitors, demonstrate reductions in RNase H activity and DNA synthesis from tRNA(Lys, 3) that correlate with reductions in replication efficiency.</a> Virology. 348 (2), 462-474 (2006).</li><li>Song, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Impact+of+immune+escape+mutations+on+HIV-1+fitness+in+the+context+of+the+cognate+transmitted/founder+genome.">Impact of immune escape mutations on HIV-1 fitness in the context of the cognate transmitted/founder genome.</a> Retrovirology. 9 (89), (2012).</li><li>Ali, A., Yang, O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+novel+small+reporter+gene+and+HIV-1+fitness+assay.">A novel small reporter gene and HIV-1 fitness assay.</a> Journal of Virological Methods. 133 (1), 41-47 (2006).</li><li>Maarseveen, N. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+novel+real-time+PCR+assay+to+determine+relative+replication+capacity+for+HIV-1+protease+variants+and/or+reverse+transcriptase+variants.">A novel real-time PCR assay to determine relative replication capacity for HIV-1 protease variants and/or reverse transcriptase variants.</a> J Virol Methods. 133 (2), 185-194 (2006).</li><li>Liu, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Evolution+of+human+immunodeficiency+virus+type+1+cytotoxic+T-lymphocyte+epitopes:+fitness-balanced+escape.">Evolution of human immunodeficiency virus type 1 cytotoxic T-lymphocyte epitopes: fitness-balanced escape.</a> J Virol. 81 (22), 12179-12188 (2007).</li><li>Anastassopoulou, C. G., Ketas, T. J., Klasse, P. J., Moore, J. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Resistance+to+CCR5+inhibitors+caused+by+sequence+changes+in+the+fusion+peptide+of+HIV-1+gp41.">Resistance to CCR5 inhibitors caused by sequence changes in the fusion peptide of HIV-1 gp41.</a> Proc Natl Acad Sci U S A. 106 (13), 5318-5323 (2009).</li><li>Abraha, A., Troyer, R. M., Quinones-Mateu, M. E., Arts, E. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Methods+to+determine+HIV-1+ex+vivo+fitness.">Methods to determine HIV-1 ex vivo fitness.</a> Methods Mol Biol. 304, 355-368 (2005).</li><li>Liu, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+sensitive+real-time+PCR+based+assay+to+estimate+the+impact+of+amino+acid+substitutions+on+the+competitive+replication+fitness+of+human+immunodeficiency+virus+type+1+in+cell+culture.">A sensitive real-time PCR based assay to estimate the impact of amino acid substitutions on the competitive replication fitness of human immunodeficiency virus type 1 in cell culture.</a> J Virol Methods. 189 (1), 157-166 (2013).</li><li>Brumme, Z. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reduced+replication+capacity+of+NL4-3+recombinant+viruses+encoding+reverse+transcriptase-integrase+sequences+from+HIV-1+elite+controllers.">Reduced replication capacity of NL4-3 recombinant viruses encoding reverse transcriptase-integrase sequences from HIV-1 elite controllers.</a> J Acquir Immune Defic Syndr. 56 (2), 100-108 (2011).</li><li>Nomura, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Significant+reductions+in+Gag-protease-mediated+HIV-1+replication+capacity+during+the+course+of+the+epidemic+in.">Significant reductions in Gag-protease-mediated HIV-1 replication capacity during the course of the epidemic in.</a> Japan. J Virol. 87 (3), 1465-1476 (2013).</li><li>Rolland, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=HIV-1+conserved-element+vaccines:+relationship+between+sequence+conservation+and+replicative+capacity.">HIV-1 conserved-element vaccines: relationship between sequence conservation and replicative capacity.</a> J Virol. 87 (10), 5461-5467 (2013).</li><li>Martinez-Picado, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fitness+cost+of+escape+mutations+in+p24+Gag+in+association+with+control+of+human+immunodeficiency+virus+type+1.">Fitness cost of escape mutations in p24 Gag in association with control of human immunodeficiency virus type 1.</a> J Virol. 80 (7), 3617-3623 (2006).</li><li>Brockman, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Escape+and+Compensation+from+Early+HLA-B57-Mediated+Cytotoxic+T-Lymphocyte+Pressure+on+Human+Immunodeficiency+Virus+Type+1+Gag+Alter+Capsid+Interactions+with+Cyclophilin+A.">Escape and Compensation from Early HLA-B57-Mediated Cytotoxic T-Lymphocyte Pressure on Human Immunodeficiency Virus Type 1 Gag Alter Capsid Interactions with Cyclophilin A.</a> J Virol. 81 (22), 12608-12618 (2007).</li><li>Miura, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=HLA-associated+alterations+in+replication+capacity+of+chimeric+NL4-3+viruses+carrying+gag-protease+from+elite+controllers+of+human+immunodeficiency+virus+type+1.">HLA-associated alterations in replication capacity of chimeric NL4-3 viruses carrying gag-protease from elite controllers of human immunodeficiency virus type 1.</a> J Virol. 83 (1), 140-149 (2009).</li><li>Lee, P. Y., Costumbrado, J., Hsu, C. Y., Kim, Y. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Agarose+gel+electrophoresis+for+the+separation+of+DNA+fragments.">Agarose gel electrophoresis for the separation of DNA fragments.</a> Journal of visualized experiments : JoVE. 62 (Apr 20), (2012).</li><li>McClure, J., van't Wout, A. B., Tran, T., Mittler, J. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Granulocyte-monocyte+colony-stimulating+factor+upregulates+HIV-1+replication+in+monocyte-derived+macrophages+cultured+at+low+density.">Granulocyte-monocyte colony-stimulating factor upregulates HIV-1 replication in monocyte-derived macrophages cultured at low density.</a> J Acquir Immune Defic Syndr. 44 (3), 254-261 (2007).</li><li>Reed, L. J., Muench, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+simple+method+of+estimatingfifty+percent+endpoints.">A simple method of estimatingfifty percent endpoints.</a> Am J Hyg. 27 (3), 493-497 (1938).</li><li>Nickle, D. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Consensus+and+ancestral+state+HIV+vaccines.">Consensus and ancestral state HIV vaccines.</a> Science. 299 (5612), 1515-1518 (2003).</li><li>Rolland, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reconstruction+and+function+of+ancestral+center-of-tree+human+immunodeficiency+virus+type+1+proteins.">Reconstruction and function of ancestral center-of-tree human immunodeficiency virus type 1 proteins.</a> J Virol. 81 (16), 8507-8514 (2007).</li><li>Bryksin, A. V., Matsumura, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Overlap+extension+PCR+cloning:+a+simple+and+reliable+way+to+create+recombinant+plasmids.">Overlap extension PCR cloning: a simple and reliable way to create recombinant plasmids.</a> Biotechniques. 48 (6), 463-465 (2010).</li><li>Spira, A. I., Ho, D. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+different+donor+cells+on+human+immunodeficiency+virus+type+1+replication+and+selection+in+vitro.">Effect of different donor cells on human immunodeficiency virus type 1 replication and selection in vitro.</a> J. Virol. 69 (1), 422-429 (1995).</li><li>Manocheewa, S., Swain, J. V., Lanxon-Cookson, E., Rolland, M., Mullins, J. I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fitness+costs+of+mutations+at+the+HIV-1+capsid+hexamerization+interface.">Fitness costs of mutations at the HIV-1 capsid hexamerization interface.</a> PLoS One. 8 (6), e66065 (2013).</li><li>Zakeri, H., Amparo, G., Chen, S. M., Spurgeon, S., Kwok, P. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Peak+height+pattern+in+dichloro-rhodamine+and+energy+transfer+dye+terminator+sequencing.">Peak height pattern in dichloro-rhodamine and energy transfer dye terminator sequencing.</a> Biotechniques. 25 (3), 406-410 (1998).</li><li>Lalonde, M. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sensitive+oligonucleotide+ligation+assay+for+low-level+detection+of+nevirapine+resistance+mutations+in+human+immunodeficiency+virus+type+1+quasispecies.">Sensitive oligonucleotide ligation assay for low-level detection of nevirapine resistance mutations in human immunodeficiency virus type 1 quasispecies.</a> J Clin Microbiol. 45 (8), 2604-2615 (2007).</li></ol>

The authors declare that they have no competing financial interests. The opinions expressed herein are those of the authors and should not be construed as official or representing the views of the US Department of Defense or the Department of the Army.&#160;

The protocols presented consisted of two main parts: construction of recombinant HIV-1 molecular clones and growth competition assays. In order to distinguish two viruses in a dually infected cell culture, it is important that the competing molecular clones contain sequence tags, which can be detected by a RT-qPCR primer-probe assay or by Sanger sequencing. This protocol makes use of the VifA and VifB tags, which occupy the same region of HIV-1 NL4-3 vif and encode the same amino acid sequence but differ by six synonymous mutations. These mutations were shown not to affect viral replication fitness20. The PCR-based cloning method used in this protocol provides more flexibility in selecting cloning sites, compared to restriction site based cloning. However, the efficiency of PCR cloning decreases as the insert size increases. The current limit of the insert size is ~5 kb34. For PCR-based cloning and site directed mutagenesis, the use of a high-fidelity DNA polymerase is crucial to reduce the likelihood of extra mutations. Use of the minimal number of PCR cycles needed to yield adequate amounts of products is also recommended. In our experience, we did not detect any extra mutations after the PCR-based cloning and site-directed mutagenesis steps described here. Nevertheless the PCR products should be sequenced to check for any undesired mutations. Ideally, the entire HIV coding region should be resequenced.
At least three sampling time points should be examined within the exponential growth phase9. The viral growth kinetics must first be established using daily sampling to determine the appropriate culture period and sampling time points for the growth competition. One factor that affects viral growth kinetics is the multiplicity of infection (MOI). This protocol uses a total MOI of 0.005 for both monoinfection and growth competition, as it was shown to yield more robust results than lower MOIs9. Nonetheless, lower MOIs can be used to obtain a longer exponential growth phase if necessary, but at the expense of result consistency. This protocol suggests an initial infection ratio of 50:50, assuming that the fitness differences of the viruses are generally unknown beforehand. However, the use of unequal input ratios are appropriate when there is preliminary data suggesting significant differences in viral replication kinetics. In these cases, an infection ratio of 70:30 is recommended to allow for the detection of a large fitness difference where the less fit virus is placed in excess9.
The protocol for determining the TCID50, viral growth kinetics, and for performing the growth competition assays were optimized using HIV-1 subtype B, NL4-3 COTB-p24, and PBMCs from a single donor whose PBMC have demonstrated consistent susceptibility to HIV-1 infection in vitro. The culture period and sampling time points presented in this protocol are likely to be suitable for studying HIV-1 group M viruses in human PBMCs. Using PBMCs from a single source is highly recommended to obtain consistent results as the viral replication can vary in different donor cells35. Although less desirable, PBMCs pooled from multiple donors can be used as a substitution provided that the same pool is used across all experiments. Another alternative is the use of cell lines. The protocol presented here was used successfully with the T-cell line CEMx17423,36. However, it is important that the numbers of cells seeded are re-optimized to achieve consistent cell growth. Viral growth kinetics must also be re-established, as it is likely to vary in different cell lines, to determine the appropriate sampling time points in the growth competition steps.
Two different methods to determine the viral ratio for calculating fitness are included in the protocol. The first uses RT-qPCR to measure viral cDNA copy number at each sampling time point. Viral replication fitness was then calculated from the viral ratio, based on cDNA copy number. Alternatively, the viral ratio can be determined based on the ratio of chromatogram peak height at the VifAB tag sites. The two methods yielded comparable results (Figure 4). The chromatogram peak height method can be applied to other HIV-1 strains without an engineered sequence tag. For RT-qPCR, the use of primers or probes to distinguish viral variants must first be carefully evaluated (see20). Nevertheless, RT-qPCR provides better sensitivity for samples with a small amount of viral RNA, such as those from the first time point within the exponential growth phase. Direct measurement of viral cDNA also allows detection of technical problems that may arise from the RNA extraction and cDNA synthesis. Using molecular clones with sequence tags provides a cost effective solution to the RT-qPCR methods, as only two pairs of primers and probes are needed to study multiple viruses. This strategy also avoids the problem of peak-height variation in sequence chromatograms due to neighboring bases37, as sequencing is done at the same site across all viruses in the study. The PCR products produced for RT-qPCR and Sanger sequencing can also be use with other methods to determine viral ratio such as bulk sequencing of individual clones12,13, HTA4,7,17,19, or oligonucleotide ligation assay (OLA)38.

Viral replication fitness is defined as the capacity of a virus to produce infectious progeny in a given environment1 and is an important contributing factor in determining the prevalence of a virus variant at the population level over time2. As in vivo fitness studies are not feasible with pathogenic human viruses, such as HIV-1, various in vitro and ex vivo replication fitness assays have been developed to study the effects on fitness arising from drug resistance and immune escape mutations, epistasis and the evolution of viral populations3-6. Among different fitness assays, growth competition assays are recognized to yield more sensitive and valid measures of fitness differences, as two or more viral variants compete for the same cell population under precisely the same environmental conditions, as occurs in vivo1,7,8. Before starting growth competition experiments, several variables need to be determined, including the use of different multiplicities of infection (MOI), viral input ratio, and timing of sampling for analysis. We have studied the effects of these parameters on viral growth kinetics and on the outcome of competition experiments, and have identified key factors necessary for robust measurements of HIV-1 fitness in cell culture9.
In addition to assay variables, there are a variety of methods for quantitating viral variants in growth competition experiments. Bulk10,11 or clonal sequencing12,13 has been used to determine the ratio of the competing viruses based on the nucleotide frequencies at the site(s) of interest. Relative fitness is derived from changes in this ratio over time. This method is convenient as DNA sequencing services are widely available. The parallel allele-specific sequencing (PASS) method enables sequencing at multiple sites and the detection of recombinants14, but it also requires specifically developed reagents and detection systems. Essentially, these methods were developed to study viral strains with a small number of nucleotide differences in a region of interest. Other methods use a small reporter gene15 or synonymous mutations4,16-18 as tags to distinguish the competing viruses by sequencing, heteroduplex tracking assays (HTA)4,7,17,19 or reverse transcription-quantitative real-time polymerase chain reaction (RT-qPCR)15,16,18,20, all of which can be made applicable to study competing strains regardless of sequence similarity. An additional step is required to introduce tags into viral genomes and the RT-qPCR assay also requires specific reagents and instrumentation. We have found that bulk Sanger sequencing yields comparable results9.
Following growth competition, viral replication fitness is presented as relative fitness, or a fitness ratio between two viral variants. The relative fitness of a virus can be defined as the final proportion of a viral variant normalized by its initial proportion in the inoculum or as the net growth rate difference between the two competing viruses. We found that the latter method, using longitudinal data points only within the exponential growth phase, produced the most robust results9,20.
In vitro fitness assays are used primarily to study biological clones6-8 and infectious molecular clones of HIV-1. The latter, being amenable to genetic manipulation, are often employed to study the effect on fitness from particular mutations or specific sequences of interest3-5,21,22. The following protocols describe a workflow from the point of constructing new full-length infectious HIV-1 molecular clones using HIV-1 vectors containing a sequence tag, introducing mutations of interest, making viral stocks and establishing viral growth kinetics, to performing the growth competition assay and calculating relative fitness (Figure 1).
Using our optimized procedures, we created three recombinant HIV-1 mutants and determined their replication fitness. The recombinant molecular clone was first constructed by replacing the HIV-1 gag-p24 gene region of pNL4-3, a plasmid containing a full length infectious genome of HIV-1 lab strain NL4-3, with a synthetic COTB (Center-Of-Tree, subtype B) gag-p24 sequence23 to create the prototype strain. Single amino changes (T186M, T242N, and I256V) were then introduced to create three mutant clones. Each mutant was competed against the prototype virus to observe the fitness impact of each mutation in the given genetic background. The three mutants demonstrated varying levels of replication fsitness from slight to significantly lower than the prototype virus. The T242N mutation was previously reported to have a moderate fitness cost24-26, similar to the result shown in this study. The fitness cost of the other two mutations had not been reported previously.

<table border="0" cellpadding="0" cellspacing="0" width="1172">
	<tbody>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Name of Material/ Equipment</td>
			<td style="width:484px;">Company</td>
			<td style="width:137px;">Catalog Number</td>
			<td style="width:196px;">Comments/Description</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Construction of recombinant clones</td>
			<td style="width:484px;"></td>
			<td style="width:137px;"></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Chimeric primer/Mutagenic/Sequencing primers</td>
			<td style="width:484px;">IDT</td>
			<td style="width:137px;">N/A</td>
			<td>Custom DNA oligos</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">pNL4-3VifA and pNL4-3VifB plasmid</td>
			<td style="width:484px;">N/A</td>
			<td style="width:137px;">N/A</td>
			<td>Please contact Dr. James I Mullins (jmullins@uw.edu)</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">High-Fidelity DNA polymerase</td>
			<td style="width:484px;">Thermo Scientific</td>
			<td>F-549S</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">High-fidelity buffer</td>
			<td style="width:484px;">Thermo Scientific</td>
			<td>F-549S</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">dNTP</td>
			<td style="width:484px;">Bioline</td>
			<td style="width:137px;">Bio-39026</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Thermal cycler</td>
			<td style="width:484px;">Life Technologies</td>
			<td style="width:137px;">N/A</td>
			<td>Applied Biosystems&#174; GeneAmp&#174; PCR System 9700</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">DNA loading dye</td>
			<td style="width:484px;">Thermo Scientific</td>
			<td style="width:137px;">R0631</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">1kb Plus DNA ladder</td>
			<td style="width:484px;">Invitrogen</td>
			<td style="width:137px;">10787-018</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">QIAquick PCR Purification Kit</td>
			<td style="width:484px;">Qiagen</td>
			<td style="width:137px;">28104</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">QIAquick gel extraction kit</td>
			<td style="width:484px;">Qiagen</td>
			<td style="width:137px;">28704</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">DpnI enzyme</td>
			<td style="width:484px;">New England Biolabs</td>
			<td>R0176S</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">TOP10 chemically competent Escherichia coli</td>
			<td style="width:484px;">Invitrogen</td>
			<td>C4040-10</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Luria Broth base powder</td>
			<td style="width:484px;">Invitrogen</td>
			<td style="width:137px;">12795-084</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Carbenicillin</td>
			<td style="width:484px;">Research Products International</td>
			<td style="width:137px;">C46000-25.0</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">QIAprep Spin Miniprep kit&#160;</td>
			<td style="width:484px;">Qiagen</td>
			<td style="width:137px;">27104</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">EndoFree Plasmid Maxi Kit</td>
			<td style="width:484px;">Qiagen</td>
			<td style="width:137px;">12362</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Transfection</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">X-tremeGENE 9 DNA transfection reagent</td>
			<td style="width:484px;">Roche</td>
			<td>6365787001</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">HEK 293T-17</td>
			<td style="width:484px;">ATCC</td>
			<td style="width:137px;">CRL-11268</td>
			<td>http://www.atcc.org/</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">0.22um filter top tube</td>
			<td style="width:484px;">VWR International</td>
			<td>89220-716&#160;</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Cell culture</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Dulbecco&#39;s Modified Eagle Medium (DMEM)</td>
			<td style="width:484px;">Life Technologies</td>
			<td>10566016</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Iscove&#8242;s Modified Dulbecco&#8242;s Medium (IMDM)</td>
			<td style="width:484px;">Life Technologies</td>
			<td>31980-030</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">RPMI-1640 media</td>
			<td style="width:484px;">Life Technologies</td>
			<td>61870-036</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Phytohemagglutinin (PHA)</td>
			<td style="width:484px;">Thermo Scientific</td>
			<td>R30852801</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Human Interleukin-2</td>
			<td style="width:484px;">Roche</td>
			<td>11147528001</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Fetal bovine serum</td>
			<td style="width:484px;">JR Scientific</td>
			<td style="width:137px;">43640</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Penicillin and Streptomycin</td>
			<td style="width:484px;">Corning Cellgro</td>
			<td>30-001-CI</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">6 well plate</td>
			<td style="width:484px;">VWR Scientific</td>
			<td style="width:137px;">73520-906</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">48 well plate</td>
			<td style="width:484px;">VWR Scientific</td>
			<td style="width:137px;">62407-338</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">96 well flat-bottomed plate</td>
			<td style="width:484px;">ISC Bioexpress</td>
			<td style="width:137px;">T-3015-4</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">96 well round-bottomed plate</td>
			<td style="width:484px;">BD Falcon</td>
			<td style="width:137px;">353077</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">1.5 ml Microcentrifuge Tube</td>
			<td style="width:484px;">Mt. Baker Bio</td>
			<td style="width:137px;">MBD-1500</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">1.5 mL tubes w/ O-ring</td>
			<td style="width:484px;">VWR Scientific</td>
			<td style="width:137px;">89004-290</td>
			<td></td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:355px;">50 mL conical tube</td>
			<td style="width:484px;">ISC Bioexpress</td>
			<td>C-3317-6</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">ELISA</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Triton-X 100</td>
			<td style="width:484px;">Sigma-Aldrich</td>
			<td>X100</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Phosphate buffer saline (PBS)</td>
			<td style="width:484px;">Invitrogen</td>
			<td style="width:137px;">14190-250</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Fetal bovine serum (FBS)</td>
			<td style="width:484px;">Sigma-Aldrich</td>
			<td>F0392</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">glycerol</td>
			<td style="width:484px;">Sigma-Aldrich</td>
			<td style="width:137px;">G5516</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Tween 20</td>
			<td style="width:484px;">Sigma-Aldrich</td>
			<td>P1379</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Bovine Serum Albumin (BSA)</td>
			<td style="width:484px;">Sigma-Aldrich</td>
			<td>A2153</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Rabbit anti-HIV-1 SF2 p24 antiserum</td>
			<td style="width:484px;">NIH AIDS Reagent Program</td>
			<td style="width:137px;">4250</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Goat anti-rabbit HRP</td>
			<td style="width:484px;">KPL</td>
			<td style="width:137px;">474-1516</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">TMB Microwell Peroxidase Substrate</td>
			<td style="width:484px;">KPL</td>
			<td style="width:137px;">52-00-01</td>
			<td></td>
		</tr>
		<tr height="26">
			<td height="26" style="height:26px;">H2SO4</td>
			<td>Fisher Scientific</td>
			<td>A300-500</td>
			<td>Causes severe burns by all exposure routes. Use personal protective equipment. Use only under a chemical fume hood. Wash off immediately with plenty of water for at least 15 minutes.</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">HIV p24 standard</td>
			<td style="width:484px;">AIDS and Cancer Virus program (NCI-Frederick)</td>
			<td style="width:137px;">N/A</td>
			<td>For order details please contact Julian Bess, Jr. (bessjw@mail.nih.gov); http://ncifrederick.cancer.gov/Programs/Science/Acvp/bio/Bess.aspx</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Microplate reader</td>
			<td style="width:484px;">Molecular Devices</td>
			<td style="width:137px;">N/A</td>
			<td>VMax Kinetic ELISA Microplate Reader</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">RNA isolation cDNA synthesis</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">QIAxtractor</td>
			<td style="width:484px;">Qiagen</td>
			<td style="width:137px;">N/A</td>
			<td>http://www.qiagen.com/products/catalog/automated-solutions/sample-prep/qiaxtractor</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">QIAamp Viral RNA Mini Kit</td>
			<td style="width:484px;">Qiagen</td>
			<td style="width:137px;">52906</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">dNTP</td>
			<td style="width:484px;">Bioline</td>
			<td style="width:137px;">Bio-39026</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">SuperscriptIII</td>
			<td style="width:484px;">Invitrogen</td>
			<td>18080-085</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">First-strand buffer&#160;</td>
			<td style="width:484px;">Invitrogen</td>
			<td>18080-085</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Dithiothreitol (DTT)</td>
			<td style="width:484px;">Invitrogen</td>
			<td>18080-085</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Rnase inhibitor</td>
			<td style="width:484px;">Roche</td>
			<td>3335402001</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">RnaseH</td>
			<td style="width:484px;">Invitrogen</td>
			<td>18021-071</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">qPCR</td>
			<td style="width:484px;"></td>
			<td style="width:137px;"></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Real-time qPCR machine</td>
			<td style="width:484px;">Life Technologies</td>
			<td style="width:137px;">N/A</td>
			<td>Applied Biosystems 7300 Real-Time PCR&#160;</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">TaqMan&#174; Gene Expression Master Mix</td>
			<td style="width:484px;">Life Technologies</td>
			<td>4369016</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Forward, reverse primer</td>
			<td style="width:484px;">IDT</td>
			<td style="width:137px;">N/A</td>
			<td>Custom order</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Probe</td>
			<td style="width:484px;">Life Technologies</td>
			<td style="width:137px;">N/A</td>
			<td>Custom order</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">optical 96 well reaction plate</td>
			<td style="width:484px;">Life Technologies</td>
			<td style="width:137px;">I19N3Q216</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">PCR+sequencing</td>
			<td style="width:484px;"></td>
			<td style="width:137px;"></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Taq DNA polymerase (Biolase)</td>
			<td style="width:484px;">Bioline</td>
			<td style="width:137px;">Bio-21043</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">NH4 buffer&#160;</td>
			<td style="width:484px;">Bioline</td>
			<td style="width:137px;">Bio-21043</td>
			<td></td>
		</tr>
		<tr height="26">
			<td height="26" style="height:26px;">MgCl2</td>
			<td style="width:484px;">Bioline</td>
			<td style="width:137px;">Bio-21043</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">Sequencing primer</td>
			<td style="width:484px;">IDT</td>
			<td style="width:137px;">N/A</td>
			<td>Custom order</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:355px;">QIAxcel</td>
			<td style="width:484px;">Qiagen</td>
			<td style="width:137px;"></td>
			<td>http://www.qiagen.com/products/catalog/automated-solutions/detection-and-analysis/qiaxcel-advanced-system</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">DNA sequencing service provider</td>
			<td>http://www.htseq.org/</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;"></td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;"></td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">GRC web tool</td>
			<td>http://indra.mullins.microbiol.washington.edu/grc/</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">ChromatQuan web tool</td>
			<td>http://indra.mullins.microbiol.washington.edu/cgi-bin/chromatquant.cgi</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

pairwise growth competition assay for determining the replication fitness of human immunodeficiency viruses

In vitro fitness assays are essential tools for determining viral replication fitness for viruses such as HIV-1. Various measurements have been used to extrapolate viral replication fitness, ranging from the number of viral particles per infectious unit, growth rate in cell culture, and relative fitness derived from multiple-cycle growth competition assays. Growth competition assays provide a particularly sensitive measurement of fitness since the viruses are competing for cellular targets under identical growth conditions. There are several experimental factors to consider when conducting growth competition assays, including the multiplicity of infection (MOI), sampling times, and viral detection and fitness calculation methods. Each factor can affect the end result and hence must be considered carefully during the experimental design. The protocol presented here includes steps from constructing a new recombinant HIV-1 clone to performing growth competition assays and analyzing the experimental results. This protocol utilizes experimental parameter values previously shown to yield consistent and robust results. Alternatives are discussed, as some parameters need to be adjusted according to the cell type and viruses being studied. The protocol contains two alternative viral detection methods to provide flexibility as the availability of instruments, reagents and expertise varies between laboratories.

To study the fitness impact of single amino acid changes in HIV-1 Gag-p24, we used oligonucleotide directed mutagenesis to introduce mutations into a pNL4-3 plasmid containing the HIV-1 COTB-p24 gene23,32,33. Viral stocks were generated by transfection of 293T cells and harvested after 48 hr. We estimated the 50% tissue culture infectious dose (TCID50) of each viral stock by the Reed-Muench method31. The TCID50 of the prototype and mutant viruses ranged from 104 to 105 IU/ml (Figure 3A).
The growth kinetics of the recombinant viruses were established in PBMCs from a single donor at an MOI of 0.005. RT-qPCR was used to measure viral cDNA copy number daily for six days. All mutant and prototype viruses grew exponentially between day 2 and day 4, following which viral growth slowed, as indicated by a decreased slope of the viral RNA copy number increase (Figure 3B). The decrease in cDNA copy number between day 0 (corresponding to the inoculum) and day 2 is due to the absorption of virus to cells and the removal of unbound virions by the day 1 wash (step 5.1.6). In the exponential growth phase, all three mutants had a slower growth rate (g) than the prototype virus (Figure 3C).
All three mutants were competed against the prototype virus in growth competition assays at a total MOI of 0.005. Viral growth kinetics in dually infected cultures were similar to that of monoinfection; the viral exponential growth phase was between days 2 and 4, and viral growth reached a plateau around day 5 (Figure 4A).
Viral growth rate differences were derived from the change in the viral ratio over time. The viral ratio was calculated based on cDNA copy number of the reference and the mutant viruses using RT-qPCR, and by comparing peak heights in sequence chromatograms at nucleotide sites distinguishing the two viruses (Figure 4B). The growth rate differences determined using the peak-height and viral RNA copy number methods yielded similar results (Figure 4C). All three mutants had lower replication fitness than the prototype viruses with the mutant I256V having the lowest fitness (Figure 4C).
<img alt="Figure 1" src="/files/ftp_upload/52610/52610fig1.jpg" /> 
Figure 1: Flow diagram of the protocols presented in this paper. The Virus Preparation protocols concern construction of HIV-1 recombinant clones and generation of viral stocks. The Fitness Assays protocols are for establishing viral growth kinetics and determining relative viral fitness. Dashed lines represent alternative flows for the protocols. For example, a mutation can be introduced directly into HIV-1 NL4-3 molecular clone without generating a recombinant clone.
<img alt="Figure 2" src="/files/ftp_upload/52610/52610fig2.jpg" /> 
Figure 2: Construction of HIV-1 NL4-3 COTB Gag-p24 recombinant molecular clones using overlap extension PCR.&#160;(A) Design the chimeric primers. The 5&#8217; halves of the primers contain the NL4-3 vector sequence and the 3&#8217; halves contain the ends of the insert sequence. (B) Chimeric primers are used to amplify the insert fragment, COTB Gag-p24. (C) The insert fragments are used as primers for a second PCR to generate a new recombinant plasmid.
<img alt="Figure 3" src="/files/ftp_upload/52610/52610fig3.jpg" /> 
Figure 3: Viral growth characteristics in PBMCs.&#160;(A) Log10 TCID50 of viral stocks. (B) Viral growth kinetics in monoinfections begun at an MOI = 0.005. (C) Viral growth rates over six days, including the exponential growth phase (days 2-4) derived from panel (B). The values shown represent the average of three replicates from one experiment. The error bars represent 95% confidence intervals.
<img alt="Figure 4" src="/files/ftp_upload/52610/52610fig4.jpg" /> 
Figure 4: Viral growth and relative fitness determinations. (A) Viral growth in dually infected PBMC cultures. (B) Ratios of T242N and prototype viruses determined using RT-qPCR or the sequencing peak-height method. (C) Net viral growth rate differences (d) between the mutant and the prototype viruses in dually infected cultures, as shown in (A). d values were calculated from the viral ratio data shown in (B). The values shown represent the average of three replicates from one experiment. The error bars represent 95% confidence intervals.

Watch this Scientific Journal Video about Pairwise Growth Competition Assay for Determining the Replication Fitness of Human Immunodeficiency Viruses at JoVE.com

Pairwise Growth Competition Assay for Determining the Replication Fitness of Human Immunodeficiency Viruses

Growth competition between nearly isogenic viruses provides a sensitive measurement for determining relative replication fitness. The protocols described here include the construction of recombinant HIV-1 clones, virus propagation and growth competition and analysis methods optimized to yield sensitive and consistent results.

In vitro fitness assays are essential tools for determining viral replication fitness for viruses such as HIV-1. Various measurements have been used to ...

pairwise-growth-competition-assay-for-determining-replication-fitness

Research

JoVE Journal

Immunology and Infection

11.0K Views.  University of Washington. Growth competition between nearly isogenic viruses provides a sensitive measurement for determining relative replication fitness. The protocols described here include the construction of recombinant HIV-1 clones, virus propagation and growth competition and analysis methods optimized to yield sensitive and consistent results.

Video: Pairwise Growth Competition Assay for Determining the Replication Fitness of Human Immunodeficiency Viruses

Determining the Reactivity and Titre of Serum using a Haemagglutination Assay

Haemagglutination is a specific form of agglutination and is used when antibodies bind to red blood cells, which act as a particulate antigen. Red blood cells are particularly useful targets as they are readily available and agglutination is observable using the naked eye. This technique is commonly used to determine the titre of an antibody (Ab), for blood grouping and viral quantification. In this video, the steps involved in preparing and performing a haemagglutination assay is demonstrated using antibodies specific to blood group A-antigens added to red blood cells (Revercells). The antiserum is serially diluted in a 96 well U-bottom microtitre tray, to which is added a suspension of Revercells. The samples are mixed and then incubated at 37&deg;C for 60 minutes. After this time, the samples can then be easily scored for  ve, +ve and intermediate (-/+) haemagglutination reactions. This approach allows for the reactivity and titre of a serum sample to be assessed using a rapid and simple technique. The video will cover the theory behind the assay, how the results are read and interpreted, how the titre is determined, how the assay can be modified and any issues associated with the use of this technique.

Haemagglutination is a form of agglutination where antibodies bind to red blood cells. Red blood cells are both readily available and the results are readily observable using the naked eye. This video demonstrates the steps involved in a haemagglutination assay, interpreting the results and determining the titre.

Haemagglutination is a specific form of agglutination and is used when antibodies bind to red blood cells, which act as a particulate antigen. Red blood ...

Cost-effective Method for Microbial Source Tracking Using Specific Human and Animal Viruses

Microbial contamination of the environment represents a significant health risk. Classical bacterial fecal indicators have shown to have significant limitations, viruses are more resistant to many inactivation processes and standard fecal indicators do not inform on the source of contamination. The development of cost-effective methods for the concentration of viruses from water and molecular assays facilitates the applicability of viruses as indicators of fecal contamination and as microbial source tracking (MST) tools. Adenoviruses and polyomaviruses are DNA viruses infecting specific vertebrate species including humans and are persistently excreted in feces and/or urine in all geographical areas studied. In previous studies, we suggested the quantification of human adenoviruses (HAdV) and JC polyomaviruses (JCPyV) by quantitative PCR (qPCR) as an index of human fecal contamination. Recently, we have developed qPCR assays for the specific quantification of porcine adenoviruses (PAdV) and bovine polyomaviruses (BPyV) as animal fecal markers of contamination with sensitivities of 1-10 genome copies per test tube. In this study, we present the procedure to be followed to identify the source of contamination in water samples using these tools. As example of representative results, analysis of viruses in ground water presenting high levels of nitrates is shown.
Detection of viruses in low or moderately polluted waters requires the concentration of the viruses from at least several liters of water into a much smaller volume, a procedure that usually includes two concentration steps in series. This somewhat cumbersome procedure and the variability observed in viral recoveries significantly hamper the simultaneous processing of a large number of water samples.
In order to eliminate the bottleneck caused by the two-step procedures we have applied a one-step protocol developed in previous studies and applicable to a diversity of water matrices. The procedure includes: acidification of ten-liter water samples, flocculation by skimmed milk, gravity sedimentation of the flocculated materials, collection of the precipitate and centrifugation, resuspension of the precipitate in 10 ml phosphate buffer. The viral concentrate is used for the extraction of viral nucleic acids and the specific adenoviruses and polyomaviruses of interest are quantified by qPCR. High number of samples may be simultaneously analyzed using this low-cost concentration method.
The procedure has been applied to the analysis of bathing waters, seawater and river water and in this study, we present results analyzing groundwater samples. This high-throughput quantitative method is reliable, straightforward, and cost-effective.

The study describes a cost-effective method for the identification of the source of fecal/urine contamination or contamination by nitrates in water using qPCR for the specific quantification of human/porcine/bovine DNA viruses, adenoviruses and polyomaviruses, proposed as MST tools.

Microbial contamination of the environment represents a significant health risk. Classical bacterial fecal indicators have shown to have significant ...

Determining Optimal Cytotoxic Activity of Human Her2neu Specific CD8 T cells by Comparing the Cr51 Release Assay to the xCELLigence System

Cytotoxic CD8 T cells constitute a subgroup of T cells that are capable of inducing the death of infected or malignant host cells1. These cells express a specialized receptor, called the T cell receptor (TCR), which can recognize a specific antigenic peptide bound to HLA class I molecules2. Engagement of infected cells or tumor cells through their HLA class I molecule results in production of lytic molecules such as granzymes and perforin resulting in target cell death. While it is useful to determine frequencies of antigen-specific CD8 T cells using assays such as the ELIspot or flow cytometry, it is also helpful to ascertain the strength of CD8 T cell responses using cytotoxicity assays3. The most recognizable assay for assessing cytotoxic function is the Chromium Release Assay (CRA), which is considered a standard assay 4. The CRA has several limitations, including exposure of cells to gamma radiation, lack of reproducibility, and a requirement for large numbers of cells. Over the past decade, there has been interest in adopting new strategies to overcome these limitations. Newer approaches include those that measure caspase 
release 4, BLT esterase activity 5 and surface expression of CD107 6. The impedance-based assay, using the Roche xCelligence system, was examined in the present paper for its potential as an alternative to the CRA. Impedance or opposition to an electric current occurs when adherent tumor cells bind to electrode plates. Tumor cells detach following killing and electrical impedance is reduced which can be measured by the xCelligence system. The ability to adapt the impedance-based approach to assess cell-mediated killing rests on the observation that T cells do not adhere tightly to most surfaces and do not appear to have much impact on impedance thus diminishing any concern of direct interference of the T cells with the measurement. Results show that the impedance-based assay can detect changes in the levels of antigen-specific cytotoxic CD8 T cells with increased sensitivity relative to the standard CRA. Based on these results, impedance-based approaches may be good alternatives to CRAs or other approaches that aim to measure cytotoxic CD8 T cell functionality.

The chromium release assay, a common assay for detecting cytotoxic T cell activity, has several limitations. Using antigen-specific CD8 T cells and the human breast cancer tumor line, SKBR3, in the present article, an impedance-based approach was examined for the capability of detecting cell killing.

Cytotoxic CD8 T cells constitute a subgroup of T  cells that are capable of inducing the death of infected or malignant host  cells1. These cells express ...

Determining Soil-transmitted Helminth Infection Status and Physical Fitness of School-aged Children

Soil-transmitted helminth (STH) infections are common. Indeed, more than 1 billion people are affected, mainly in the developing world where poverty prevails and hygiene behavior, water supply, and sanitation are often deficient1,2. Ascaris lumbricoides, Trichuris trichiura, and the two hookworm species, Ancylostoma duodenale and Necator americanus, are the most prevalent STHs3. The estimated global burden due to hookworm disease, ascariasis, and trichuriasis is 22.1, 10.5, and 6.4 million disability-adjusted life years (DALYs), respectively4. Furthermore, an estimated 30-100 million people are infected with Strongyloides stercoralis, the most neglected STH species of global significance which arguably also causes a considerable public health impact5,6. Multiple-species infections (i.e., different STHs harbored in a single individual) are common, and infections have been linked to lowered productivity and thus economic outlook of developing countries1,3.
For the diagnosis of common STHs, the World Health Organization (WHO) recommends the Kato-Katz technique7,8, which is a relatively straightforward method for determining the prevalence and intensity of such infections. It facilitates the detection of parasite eggs that infected subjects pass in their feces.
With regard to the diagnosis of S.stercoralis, there is currently no simple and accurate tool available. The Baermann technique is the most widely employed method for its diagnosis. The principle behind the Baermann technique is that active S.stercoralis larvae migrate out of an illuminated fresh fecal sample as the larvae are phototactic9. It requires less sophisticated laboratory materials and is less time consuming than culture and immunological methods5.
Morbidities associated with STH infections range from acute but common symptoms, such as abdominal pain, diarrhea, and pruritus, to chronic symptoms, such as anemia, under- and malnutrition, and cognitive impairment10. Since the symptoms are generally unspecific and subtle, they often go unnoticed, are considered a normal condition by affected individuals, or are treated as symptoms of other diseases that might be more common in a given setting. Hence, it is conceivable that the true burden of STH infections is underestimated by assessment tools relying on self-declared signs and symptoms as is usually the case in population-based surveys.
In the late 1980s and early 1990s, Stephenson and colleagues highlighted the possibility of STH infections lowering the physical fitness of boys aged 6-12 years11,12. This line of scientific inquiry gained new momentum recently13,14,15. The 20-meter (m) shuttle run test was developed and validated by L&eacute;ger et al.16 and is used worldwide to measure the aerobic fitness of children17. The test is easy to standardize and can be performed wherever a 20-m long and flat running course and an audio source are available, making its use attractive in resource-constrained settings13. To facilitate and standardize attempts at assessing whether STH infections have an effect on the physical fitness of school-aged children, we present methodologies that diagnose STH infections or measure physical fitness that are simple to execute and yet, provide accurate and reproducible outcomes. This will help to generate new evidence regarding the health impact of STH infections.

Chronic infection with soil-transmitted helminths (STHs) causes malabsorption, stunting, and wasting in the growing child. Hence, it is plausible that these infections also reduce the physical fitness of children. Here, we visualize two techniques for the diagnosis of STHs and the 20-meter shuttle run test for assessing children's physical fitness.

Soil-transmitted helminth (STH) infections are common. Indeed, more than 1 billion people are affected, mainly in the developing world where poverty ...

A Visual Assay to Monitor T6SS-mediated Bacterial Competition

Type VI secretion systems (T6SSs) are molecular nanomachines allowing Gram-negative bacteria to transport and inject proteins into a wide variety of target cells1,2. The T6SS is composed of 13 core components and displays structural similarities with the tail-tube of bacteriophages3. The phage uses a tube and a puncturing device to penetrate the cell envelope of target bacteria and inject DNA. It is proposed that the T6SS is an inverted bacteriophage device creating a specific path in the bacterial cell envelope to drive effectors and toxins to the surface. The process could be taken further and the T6SS device could perforate other cells with which the bacterium is in contact, thus injecting the effectors into these targets. The tail tube and puncturing device parts of the T6SS are made with Hcp and VgrG proteins, respectively4,5.
The versatility of the T6SS has been demonstrated through studies using various bacterial pathogens. The Vibrio cholerae T6SS can remodel the cytoskeleton of eukaryotic host cells by injecting an "evolved" VgrG carrying a C-terminal actin cross-linking domain6,7. Another striking example was recently documented using Pseudomonas aeruginosa which is able to target and kill bacteria in a T6SS-dependent manner, therefore promoting the establishment of bacteria in specific microbial niches and competitive environment8,9,10.
In the latter case, three T6SS-secreted proteins, namely Tse1, Tse2 and Tse3 have been identified as the toxins injected in the target bacteria (Figure 1). The donor cell is protected from the deleterious effect of these effectors via an anti-toxin mechanism, mediated by the Tsi1, Tsi2 and Tsi3 immunity proteins8,9,10. This antimicrobial activity can be monitored when T6SS-proficient bacteria are co-cultivated on solid surfaces in competition with other bacterial species or with T6SS-inactive bacteria of the same species8,11,12,13.
The data available emphasized a numerical approach to the bacterial competition assay, including time-consuming CFU counting that depends greatly on antibiotic makers. In the case of antibiotic resistant strains like P. aeruginosa, these methods can be inappropriate. Moreover, with the identification of about 200 different T6SS loci in more than 100 bacterial genomes14, a convenient screening tool is highly desirable. We developed an assay that is easy to use and requires standard laboratory material and reagents. The method offers a rapid and qualitative technique to monitor the T6SS-dependent bactericidal/bacteriostasis activity by using a reporter strain as a prey (in this case Escherichia coli DH5&alpha;) allowing a-complementation of the lacZ gene. Overall, this method is graphic and allows rapid identification of T6SS-related phenotypes on agar plates. This experimental protocol may be adapted to other strains or bacterial species taking into account specific conditions such as growth media, temperature or time of contact.

We describe a qualitative assay to monitor bacterial competition mediated by the Pseudomonas aeruginosa type VI secretion system (T6SS). The assay relies on the survival/killing of Escherichia coli target cells carrying a lacZ-reporter. This technique is adjustable to assess the bactericidal/bacteriostasis activity of T6SS-proficient microorganisms.

Type VI secretion systems (T6SSs) are molecular  nanomachines allowing Gram-negative bacteria to transport and inject proteins  into a wide variety of ...

Establishing a Liquid-covered Culture of Polarized Human Airway Epithelial Calu-3 Cells to Study Host Cell Response to Respiratory Pathogens In vitro

The apical and basolateral surfaces of airway epithelial cells demonstrate directional responses to pathogen exposure in vivo. Thus, ideal in vitro models for examining cellular responses to respiratory pathogens polarize, forming apical and basolateral surfaces. One such model is differentiated normal human bronchial epithelial cells (NHBE). However, this system requires lung tissue samples, expertise isolating and culturing epithelial cells from tissue, and time to generate an air-liquid interface culture.
Calu-3 cells, derived from a human bronchial adenocarcinoma, are an alternative model for examining the response of proximal airway epithelial cells to respiratory insult1, pharmacological compounds2-6, and bacterial7-9 and viral pathogens, including influenza virus, rhinovirus and severe acute respiratory syndrome - associated coronavirus10-14. Recently, we demonstrated that Calu-3 cells are susceptible to respiratory syncytial virus (RSV) infection in a manner consistent with NHBE15,16 . Here, we detail the establishment of a polarized, liquid-covered culture (LCC) of Calu-3 cells, focusing on the technical details of growing and culturing Calu-3 cells, maintaining cells that have been cultured into LCC, and we present the method for performing respiratory virus infection of polarized Calu-3 cells.
To consistently obtain polarized Calu-3 LCC, Calu-3 cells must be carefully subcultured before culturing in Transwell inserts. Calu-3 monolayer cultures should remain below 90% confluence, should be subcultured fewer than 10 times from frozen stock, and should regularly be supplied with fresh medium. Once cultured in Transwells, Calu-3 LCC must be handled with care. Irregular media changes and mechanical or physical disruption of the cell layers or plates negatively impact polarization for several hours or days. Polarization is monitored by evaluating trans-epithelial electrical resistance (TEER) and is verified by evaluating the passive equilibration of sodium fluorescein between the apical and basolateral compartments17,18 . Once TEER plateaus at or above 1,000 &Omega;&times;cm2, Calu-3 LCC are ready to use to examine cellular responses to respiratory pathogens.

Establishing a Liquid-covered Culture of Polarized Human Airway Epithelial Calu-3 Cells to Study Host Cell Response to Respiratory Pathogens In vitro

The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the Centers for Disease Control and Prevention.

The  apical and basolateral surfaces of airway epithelial cells demonstrate  directional responses to pathogen exposure in  vivo. Thus, ideal in vitro ...

A Restriction Enzyme Based Cloning Method to Assess the In vitro Replication Capacity of HIV-1 Subtype C Gag-MJ4 Chimeric Viruses

The protective effect of many HLA class I alleles on HIV-1 pathogenesis and disease progression is, in part, attributed to their ability to target conserved portions of the HIV-1 genome that escape with difficulty. Sequence changes attributed to cellular immune pressure arise across the genome during infection, and if found within conserved regions of the genome such as Gag, can affect the ability of the virus to replicate in vitro. Transmission of HLA-linked polymorphisms in Gag to HLA-mismatched recipients has been associated with reduced set point viral loads. We hypothesized this may be due to a reduced replication capacity of the virus. Here we present a novel method for assessing the in vitro replication of HIV-1 as influenced by the gag gene isolated from acute time points from subtype C infected Zambians. This method uses restriction enzyme based cloning to insert the gag gene into a common subtype C HIV-1 proviral backbone, MJ4. This makes it more appropriate to the study of subtype C sequences than previous recombination based methods that have assessed the in vitro replication of chronically derived gag-pro sequences. Nevertheless, the protocol could be readily modified for studies of viruses from other subtypes. Moreover, this protocol details a robust and reproducible method for assessing the replication capacity of the Gag-MJ4 chimeric viruses on a CEM-based T cell line. This method was utilized for the study of Gag-MJ4 chimeric viruses derived from 149 subtype C acutely infected Zambians, and has allowed for the identification of residues in Gag that affect replication. More importantly, the implementation of this technique has facilitated a deeper understanding of how viral replication defines parameters of early HIV-1 pathogenesis such as set point viral load and longitudinal CD4+ T cell decline.

A Restriction Enzyme Based Cloning Method to Assess the In vitro Replication Capacity of HIV-1 Subtype C Gag-MJ4 Chimeric Viruses

HIV-1 pathogenesis is defined by both viral characteristics and host genetic factors. Here we describe a robust method that allows for reproducible measurements to assess the impact of the gag gene sequence variation on the in vitro replication capacity of the virus.

The protective effect of many HLA class I alleles on HIV-1 pathogenesis and disease progression is, in part, attributed to their ability to target ...

Intranasal Administration of Recombinant Influenza Vaccines in Chimeric Mouse Models to Study Mucosal Immunity

Vaccines are one of the greatest achievements of mankind, and have saved millions of lives over the last century. Paradoxically, little is known about the physiological mechanisms that mediate immune responses to vaccines perhaps due to the overall success of vaccination, which has reduced interest into the molecular and physiological mechanisms of vaccine immunity. However, several important human pathogens including influenza virus still pose a challenge for vaccination, and may benefit from immune-based strategies. 
Although influenza reverse genetics has been successfully applied to the generation of live-attenuated influenza vaccines (LAIVs), the addition of molecular tools in vaccine preparations such as tracer components to follow up the kinetics of vaccination in vivo, has not been addressed. In addition, the recent generation of mouse models that allow specific depletion of leukocytes during kinetic studies has opened a window of opportunity to understand the basic immune mechanisms underlying vaccine-elicited protection. Here, we describe how the combination of reverse genetics and chimeric mouse models may help to provide new insights into how vaccines work at physiological and molecular levels, using as example a recombinant, cold-adapted, live-attenuated influenza vaccine (LAIV). We utilized laboratory-generated LAIVs harboring cell tracers as well as competitive bone marrow chimeras (BMCs) to determine the early kinetics of vaccine immunity and the main physiological mechanisms responsible for the initiation of vaccine-specific adaptive immunity. In addition, we show how this technique may facilitate gene function studies in single animals during immune responses to vaccines. We propose that this technique can be applied to improve current prophylactic strategies against pathogens for which urgent medical countermeasures are needed, for example influenza, HIV, Plasmodium, and hemorrhagic fever viruses such as Ebola virus.

There is an overall lack of knowledge about how vaccines work. Here we propose the combined use of reverse genetics and bone marrow chimeric mice to gain insight into the early host immune responses to vaccines with a special focus on dendritic cells and T cell immunity.

Vaccines are one of the greatest achievements of mankind, and have saved millions of lives over the last century. Paradoxically, little is known about the ...

Fluorescence-based Neuraminidase Inhibition Assay to Assess the Susceptibility of Influenza Viruses to The Neuraminidase Inhibitor Class of Antivirals

The neuraminidase (NA) inhibitors are the only class of antivirals approved for the treatment and prophylaxis of influenza that are effective against currently circulating strains. In addition to their use in treating seasonal influenza, the NA inhibitors have been stockpiled by a number of countries for use in the event of a pandemic. It is therefore important to monitor the susceptibility of circulating influenza viruses to this class of antivirals. There are different types of assays that can be used to assess the susceptibility of influenza viruses to the NA inhibitors, but the enzyme inhibition assays using either a fluorescent substrate or a chemiluminescent substrate are the most widely used and recommended. This protocol describes the use of a fluorescence-based assay to assess influenza virus susceptibility to NA inhibitors. The assay is based on the NA enzyme cleaving the 2&#8242;-(4-Methylumbelliferyl)-&#945;-D-N-acetylneuraminic acid (MUNANA) substrate to release the fluorescent product 4-methylumbelliferone (4-MU). Therefore, the inhibitory effect of an NA inhibitor on the influenza virus NA is determined based on the concentration of the NA inhibitor that is required to reduce 50% of the NA activity, given as an IC50 value.

We describe the use of a phenotypic fluorescence-based neuraminidase inhibition assay to assess the susceptibility of influenza A and B viruses to the neuraminidase inhibitor class of antivirals.

The neuraminidase (NA) inhibitors are the only class of antivirals approved for the treatment and prophylaxis of influenza that are effective against ...

Ex Vivo Infection of Human Lymphoid Tissue and Female Genital Mucosa with Human Immunodeficiency Virus 1 and Histoculture

Histocultures allow studying intercellular interactions within human tissues, and they can be employed to model host-pathogen interactions under controlled laboratory conditions. Ex vivo infection of human tissues with human immunodeficiency virus (HIV), among other viruses, has been successfully used to investigate early disease pathogenesis, as well as a platform to test the efficacy and toxicity of antiviral drugs. In the present protocol, we explain how to process and infect with HIV-1 tissue explants from human tonsils and cervical mucosae, and maintain them in culture on top of gelatin sponges at the liquid-air interface for about two weeks. This non-polarized culture setting maximizes access to nutrients in culture medium and oxygen, although progressive loss of tissue integrity and functional architectures remains its main limitation. This method allows monitoring HIV-1 replication and pathogenesis using several techniques, including immunoassays, qPCR, and flow cytometry. Of importance, the physiologic variability between tissue donors, as well as between explants from different areas of the same specimen, may significantly affect experimental results. To ensure result reproducibility, it is critical to use an adequate number of explants, technical replicates, and donor-matched control conditions to normalize the results of the experimental treatments when compiling data from multiple experiments (i.e., conducted using tissue from different donors) for statistical analysis.

Ex Vivo Infection of Human Lymphoid Tissue and Female Genital Mucosa with Human Immunodeficiency Virus 1 and Histoculture

Infection of human tissues with human immunodeficiency virus (HIV) ex vivo provides a valuable 3D model of virus pathogenesis. Here, we describe a protocol to process and infect tissue specimens from human tonsils and female genital mucosae with HIV-1 and maintain them in culture at the liquid-air interface.