We would like to thank Patrick C. Wilson for monoclonal antibody CR9114. We also thank Jennifer Debeauchamp and Richard Webby for the original A/Anhui/1/13 plasmids. Partial support for this work was provided by the National Institute for Allergy and Infectious Diseases-funded Program Project Grant AI097092-01A1 and CEIRS (Centers of Excellence for Influenza Research and Surveillance, HHSN26620070010C). FK was supported by an Erwin Schr&ouml;dinger fellowship (J 3232) from the Austrian Science Fund (FWF).

1. Generation of Recombinant Baculovirus
<ol>
	<li>Cloning into baculovirus transfer-plasmids
	<ol>
		<li>Clone H7 HA and N9 NA ectodomain (or any other HA or NA gene) into modified pFastBac transfer plasmids (see introduction). Vectors for HA containing a C-terminal trimerization domain and a hexahistidine tag, as well as for NA containing an N-terminal tetramerization domain and hexahistidine tag, have been described1,10,11,22 (Figure 1) and can be requested from the authors. 
		Explanatory note: HA ectodomains expressed without a trimerization domain will be folded incorrectly, leading to loss of conformational neutralizing epitopes,&#160;especially in the stalk domain. Similarly, the NA enzymatic function relies on proper tetramerization.</li>
		<li>Generation of bacmids 
		Transform baculovirus transfer-plasmids containing HA and NA genes into DH10Bac bacteria according using a baculovirus expression system per manufacturer&#39;s instructions. Isolate and purify bacmids using a Midiprep kit according to manufacturer&#39;s instructions (Figure 2).</li>
	</ol>
	</li>
	<li>Rescue of recombinant baculovirus and verification of protein expression 
	Explanatory note: The following steps have to be performed aseptically in a laminar flow hood. Perform this procedure independently for HA and NA (or any other gene of interest). Insect cells are usually grown at 28 &#176;C without CO2. Sf9 cells for this protocol are grown in full TNM-FH insect cell media supplemented with 10% FBS, 1% Pen-Strep* and 0.1% Pluronic F68 solution, if not indicated otherwise, and passaged 1:4 twice a week. High Five cells are grown in SFX serum free media and passaged 1:15 twice a week. 
	* Addition of antibiotics should be considered optional throughout the protocol and adjusted according to the practices used in each laboratory.
	<ol>
		<li>Plate Sf9 insect cells in 6-well plates at a density of 2 x 105&#160;cells/cm2 and let sit for 20 min in a 28 &#176;C incubator (without CO2).</li>
		<li>Mix 2 &#956;g (conc. 0.5-2 &#956;g/ul) of recombinant bacmid with 100 &#956;l of TNM-FH medium (serum free, 1% Pen-Strep). Mix 6 &#956;l of transfection agent with 100 &#956;l of TNM-FH medium (serum free, Pen-Strep). Combine the two volumes, mix gently and let sit for 20 min at room temperature (transfection mixture). Include one mock-transfection as control (Figure 3).&#160;</li>
		<li>Remove supernatant from plated Sf9 cells (cells will now stick to the plate) and replace with 2 ml of serum free TNM-FH medium containing Pen-Strep (1%). Add the complete volume of the transfection mixture (from step&#160;1.2.2) and rock the 6-well plate carefully for 5 sec. Incubate in a 28 &#176;C incubator without CO2 for 6 hr (Figure 3).</li>
		<li>Replace media with fresh full TNM-FH media (containing 10% FBS, 1% Pen-Strep and 0.1% Pluronic F68). Incubate in a 28 &#176;C incubator without CO2 for 6 days. Check cells occasionally under a microscope. Infected cells usually appear bigger, rounder, have enlarged nuclei, and start to detach (Figure 4).&#160;</li>
		<li>Harvest of cells and confirmation of protein expression
		<ol>
			<li>Harvest cells 6 days post transfection by centrifugation at 2,000 x g for 5 min at room temperature. The supernatant contains the recombinant baculovirus - this will be used to generate working stocks right away, or can be stored at 4 &#176;C for years.</li>
			<li>To check protein expression collect pellets and resuspend in 500 &#956;l of PBS. Mix 50 &#956;l of the suspension with 50 &#956;l of SDS-PAGE loading buffer (containing a reducing agent). Run SDS-PAGE and perform a Western blot analysis with an anti-hexahistidine antibody to confirm gene expression. Include cells from the mock transfection as negative controls (Figure 3).</li>
		</ol>
		</li>
		<li>Preparation of working stocks
		<ol>
			<li>Plate 5 x 105 Sf9 cells/cm2 in a T175 cm2 flask and let sit for 20 min allow them to attach to the dish. Replace complete TNM-FH media with TNM-FH media containing 3% FBS (and 1% Pen-Strep, 0.1% Pluronic F68).</li>
			<li>Infect cells with 100 &#956;l of the rescue supernatant. Incubate in a 28 &#176;C incubator without CO2 for 6 days and check occasionally if cells get infected (as described in step 1.2.5.).</li>
			<li>Spin at 2,000 x g at room temperature for 5 min and collect supernatant. This is now your P2 stock. This stock can be stored indefinitely at 4 &#176;C. Repeat the infection procedure using 100 &#956;l of the P2 stock to infect cells. The resulting stock is called P3 or working stock, which can be stored at 4 &#176;C&#160;or used directly for protein expression. The P3 stock usually contains 107-109 plaque forming units/ml.</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
2. Protein Expression and Purification
Explanatory note: Sf9 cells support the growth of baculovirus very well. However, their capacity to secrete large amounts of recombinant protein is limited. Therefore we use another insect cell line, High Five cells, for expression of secreted recombinant proteins. These cells do not support baculovirus replication to very high titers13 but have high secretory capacity.
<ol>
	<li>Growing up High Five cells 
	Grow High Five cells in SFX insect cell culture medium in T175 cm2 flasks. Passage every other day 1:3. For a 200 ml expression culture you will need to grow 5 confluent flasks.</li>
	<li>Harvesting and infecting High Five cells
	<ol>
		<li>Detach High Five cells from 5 T175 cm2 flasks by tapping the flasks with your hand. Collect the cell suspension and transfer into 50 ml centrifugation tubes.</li>
		<li>Spin at 1,200 x g for 7 min at room temperature. Remove supernatant by decanting and unite the pellets by resuspending them in 15 ml of P3 stock.</li>
		<li>Let sit for 15 min in the flow hood (Figure 5).</li>
		<li>Transfer 200 ml of Hyclone SFX culture medium into a clean, sterile 1,000 ml shaker flask (without buffles, sealed with aluminium foil before autoclaving, should not have been used for bacterial cultures before).</li>
		<li>Transfer the P3/cell suspension into the shaker flask. Seal with sterile aluminium foil and transfer flask into a shaker. Shake at 70 rpm at 28 &#176;C for 72-96 hr.</li>
	</ol>
	</li>
	<li>Harvesting recombinant protein from High Five cell expression supernatants
	<ol>
		<li>72-96 hr post infection transfer culture supernatants into 500 ml centrifugation buckets. Spin at 5,500 x g for 20 min at 4 &#176;C. In the meantime rinse the shaker flasks 3x with double distilled water (ddH2O).</li>
		<li>Transfer 3 ml Ni-resin slurry into a 50 ml tube with 45 ml of PBS (one needed per 200 ml culture). Shake well and spin at 3,000 x g at room temperature for 10 min. Decant the PBS and keep the pellet. Mix the insect cell supernatant with the Ni-resin pellet and transfer into the already rinsed shaker flasks. Incubate at 4 &#176;C for 2-4 hr, while shaking (Figure 6).&#160;</li>
		<li>Mount 10 ml polypropylene column on a clamp on a support stand. Remove both caps and place a beaker below the column to collect flow through (waste). Take the shaker flask out of the shaker and start to transfer the supernatant/slurry mixture onto the column; it will retain the Ni-resin and only the supernatant will flow through. Pass the entire volume over the column (Figure 7).</li>
		<li>Wash the resin 4x with 15 ml of washing buffer (50 mM Na2HCO3, 300 mM NaCl, 20 mM imidazole, pH 8). Let all the washing buffer drain from the column and close it with the cap (lower side).</li>
		<li>Add 2 ml of elution buffer (50 mM Na2HCO3, 300 mM NaCl, 300 mM imidazole, pH 8) and let sit for 5 min. Open column and collect the eluate. Repeat this 3 more times (with a total of 8 ml of elution buffer). Eluate that contains high concentrations of protein typically shows foam when shaken (Figure 7). Keep eluate on wet ice whenever possible.</li>
	</ol>
	</li>
	<li>Buffer exchange and concentration
	<ol>
		<li>Prespin 15 ml ultrafiltration centrifugation units (30 kDa cut-off for HA/NA but may vary depending on the purified protein size) with PBS (pH 7.4-7.6) at 3,000 x g at 4 &#176;C for 20 min. Discard flow through and load eluate onto the spin column. Fill up with 5 ml of sterile, ice cold PBS and spin for 60 min at 3,000 x g at 4 &#176;C.</li>
		<li>Discharge flow through again, refill with 15 ml of ice cold PBS and spin for 60 min at 3,000 x g at 4 &#176;C. Repeat this procedure one more time.</li>
		<li>Take the spin column out of the centrifuge and collect the buffer exchanged concentrate (typically 200 &#956;l). This is highly concentrated recombinant protein. Rinse the spin column with 400 &#956;l of sterile ice cold PBS and add this to the concentrate. Keep the concentrate on ice at all times.</li>
	</ol>
	</li>
</ol>
3. Characterization and Quality Control
<ol>
	<li>Measuring protein concentration 
	Take an aliquot of the concentrate and measure it by using your protein quantification method of choice. Set protein concentration to 1 mg/ml by adding PBS and freeze small aliquots to -80 &#176;C. Freeze thaw cycles harm the protein structure and can lead to aggregation.</li>
	<li>SDS-PAGE and Coomassie staining 
	Run 500 ng of your protein on a SDS-PAGE and perform a Coomassie staining. For fast and convenient staining use Coomassie reagent in combination with a microwave protocol. HA proteins typically appear at 64 kDa whereas NA proteins should run at a size of around 56 kDa (Figures 8A and&#160;B). Using the described protocol you should not see any degradation products. If degradation occurs it is usually a sign of yeast/fungal contamination during expression. In this case repeat the procedure and make sure to keep everything sterile.</li>
	<li>Western blot/ELISA 
	In order to verify protein identity we recommend do to a Western blot or ELISA assay with a specific antibody. Ideally, ELISA assays are performed with an antibody that binds to conformational epitopes (e.g. anti-HA stalk antibodies1). Using such an antibody the correct folding of the protein can be confirmed (Figure 8C).</li>
	<li>Measuring NA activity 
	Correct folding of the viral NA can also be determined by measuring NA activity. We recommend to perform this test using the commercially available NA*STAR assay (just follow the user guidelines).</li>
</ol>

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The authors have nothing to disclose.

Although expression of influenza virus HA and NA proteins in insect cells using the baculovirus expression system is generally straight forward, there are several important points to consider. It is important, as pointed out in the introduction, that the ectodomains of HA and NA are expressed in fusion with trimerization or tetramerization domains, respectively. These domains ensure correct folding of the molecules usually assisted by the transmembrane domain, which is not present if only the ectodomain is expressed. For...

As a first response to the recent emergence of the novel H7N9 influenza virus strain in China, we acquired plasmids carrying the genomic sequences for the hemagglutinin (HA) and neuraminidase (NA) genes of the first two isolates. Using these plasmids we were able to quickly construct baculoviral expression vectors for production of recombinant HA and NA in insect cells. This expression system is well established in our laboratory and has proven pivotal to many of our ongoing research projects1-8. Insect cells are able to correctly fold and post-translationally modify complex proteins. These modifications include N-linked glycosylation, which is important for many viral glycoproteins. As such, it is not surprising that the baculovirus system is quite established for expressing influenza virus surface antigens. In fact, many of the HA and NA crystal structures have been solved using insect cell expressed proteins9-11. Another advantage of the system lies in its reliable high protein yields of up to 30 mg of HA/L cell culture (based on our experience with more than 50 different HA proteins) - a consequence of virus-infection driven expression from the strong polyhedrin promoter.
Removal of the transmembrane and endodomain of the HA and NA proteins allows for expression of secretable, soluble versions of the proteins and thus greatly facilitates the purification process. In addition, these ectodomains are hexahistidine tagged and can therefore be purified using affinity chromatography using Ni2+-resin. The transmembrane domains of HA and NA proteins contribute to homotrimer and homotetramer formation, respectively. In order to preserve these oligomerizations, we add a C-terminal trimerization domain to the HA-, and a N-terminal tetramerization domain to the NA constructs (Figure 1). We have shown conclusively that such a trimerization domain stabilizes functional, conserved conformational epitopes on the stalk-domain of HA1. The correct tetramerization of the NA might also contribute to correct folding and NA function10. Sequences and baculo-transfer vectors harboring trimerization or tetramerization domains can be requested from the authors or from other laboratories in the field ,9,10,12.
Another important issue for expression of secreted proteins in insect cells is the choice of the right cell line. While Spodoptera frugiperda derived Sf9 cells support baculovirus replication very well and are generally used for virus rescue and propagation, they have limited capacity to secrete large amounts of protein. Trichoplusia ni derived BTI-TN-5B1-4 cells (commonly known as High Five) have a higher secretion capacity and are the cell line of choice for expression in this protocol13,14. Furthermore, it is helpful to reduce or even remove fetal bovine serum (FBS) from expression cultures. We therefore use media with reduced (3%) FBS content for growing the virus working stocks, and we perform the protein expression in serum free media.
Recombinant HA and NA proteins are used for many immunological techniques. Perhaps the most common one is in ELISA assays for measuring serum conversion upon vaccination in humans or animals4,6,7,15. HAs and NAs are also used in ELISPOT assays as both stimulatory molecules and detection reagents for antibody secreting cells16. Their use as molecular baits allows to specifically sort for plasmablasts or other types of B-cells that can then be used to isolate (therapeutic) monoclonal antibodies (mAbs). Recombinant HA and NA molecules are further used in the characterization of these mAbs8. Other examples include study the pH stability or receptor specificity of HAs expressed by different virus isolates, or quantitatively assessing specific NA enzymatic activity or resistance to inhibitors. Furthermore, recombinant, purified HA can be used to standardize HA content of inactivated influenza virus vaccines.
Importantly, rHA (and to a certain degree NA) vaccine candidates are currently being tested in animal models and human clinical trials with one vaccine candidate being licensed by the FDA in 20132,17-19. The advantage of these novel vaccines is that, due to the recombinant nature of these proteins, the labor-intensive process of generating of high growth reassortant viruses could be avoided. Perhaps even more relevant is the fact that their HA yield is high and reproducible from strain to strain. Also, since these vaccines are produced in an egg-free system, they do not contain protein contaminants that are problematic for individuals with egg allergies.
Here we chose to express the HA and NA proteins from two isolates of the novel Chinese H7N9 influenza virus, A/Anhui/1/13 and A/Shanghai/1/1320,21. These are timely examples, but the described protocol can be used for expression of any influenza A and B HA or NA proteins and can be adapted to express any other secreted viral or cellular proteins. Trimeric or tetrameric transmembrane proteins can be cloned into the expression cassettes used for HA and NA, respectively. However, most soluble secreted cellular proteins, like interferons for example, do not need an additional domain for stabilization and can be expressed solely with a C-terminal hexahistidine tag.

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			<td>Name of Material/ Equipment</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments/Description</td>
		</tr>
		<tr>
			<td>Bac-to-Bac Baculovirus expression system</td>
			<td>Invitrogen</td>
			<td>10359-016</td>
			<td>Follow manufacturer&#39;s instructions</td>
		</tr>
		<tr>
			<td>TNM-FH insect medium</td>
			<td>Gemini Bioproducts</td>
			<td>600-311</td>
			<td></td>
		</tr>
		<tr>
			<td>HyClone SFX-Insect</td>
			<td>Thermo Fisher</td>
			<td>SH30278.02</td>
			<td></td>
		</tr>
		<tr>
			<td>Cellfectin II Reagent</td>
			<td>Invitrogen</td>
			<td>P/N58760</td>
			<td></td>
		</tr>
		<tr>
			<td>Pluronic F68</td>
			<td>Sigma</td>
			<td>1000702664</td>
			<td></td>
		</tr>
		<tr>
			<td>SimplyBlue SafeStain</td>
			<td>Invitrogen</td>
			<td>LC6060</td>
			<td></td>
		</tr>
		<tr>
			<td>Ni-NTA Agarose</td>
			<td>Qiagen</td>
			<td>1018240</td>
			<td></td>
		</tr>
		<tr>
			<td>Amicon Ultra Centrigual filters Ultracel 30K</td>
			<td>Millipore</td>
			<td>UFC902024</td>
			<td></td>
		</tr>
		<tr>
			<td>Pen Strep</td>
			<td>Gibco</td>
			<td>15140-122</td>
			<td></td>
		</tr>
		<tr>
			<td>Polypropylene Columns (5 ml)</td>
			<td>Qiagen</td>
			<td>34964</td>
			<td></td>
		</tr>
		<tr>
			<td>Max efficiency DH10Bac bacteria</td>
			<td>Invitrogen</td>
			<td>10361-012</td>
			<td></td>
		</tr>
		<tr>
			<td>PureLink HiPure Plasmid Filter Midiprep Kit</td>
			<td>Invitrogen</td>
			<td>K210015</td>
			<td></td>
		</tr>
		<tr>
			<td>Imidazole</td>
			<td>Sigma-Aldrich</td>
			<td>I2399-100G</td>
			<td>for elution and wash buffers</td>
		</tr>
		<tr>
			<td>Sf9 cells</td>
			<td>ATCC</td>
			<td><a href="http://www.atcc.org/Products/All/CRL-1711.aspx" target="_blank">CRL-1711</a></td>
			<td></td>
		</tr>
		<tr>
			<td>High Five cells</td>
			<td>Invitrogen</td>
			<td>B85502</td>
			<td></td>
		</tr>
	</tbody>
</table>

expression of functional recombinant hemagglutinin and neuraminidase proteins from the novel h7n9 influenza virus using the baculovirus expression system

The baculovirus expression system is a powerful tool for expression of recombinant proteins. Here we use it to produce correctly folded and glycosylated versions of the influenza A virus surface glycoproteins - the hemagglutinin (HA) and the neuraminidase (NA). As an example, we chose the HA and NA proteins expressed by the novel H7N9 virus that recently emerged in China. However the protocol can be easily adapted for HA and NA proteins expressed by any other influenza A and B virus strains. Recombinant HA (rHA) and NA (rNA) proteins are important reagents for immunological assays such as ELISPOT and ELISA, and are also in wide use for vaccine standardization, antibody discovery, isolation and characterization. Furthermore, recombinant NA molecules can be used to screen for small molecule inhibitors and are useful for characterization of the enzymatic function of the NA, as well as its sensitivity to antivirals. Recombinant HA proteins are also being tested as experimental vaccines in animal models, and a vaccine based on recombinant HA was recently licensed by the FDA for use in humans. The method we describe here to produce these molecules is straight forward and can facilitate research in influenza laboratories, since it allows for production of large amounts of proteins fast and at a low cost. Although here we focus on influenza virus surface glycoproteins, this method can also be used to produce other viral and cellular surface proteins.

HA molecules from A/Shanghai/1/13 and A/Anhui/1/13 expressed well with yields of about 20 mg/L culture. NA molecules of both strains showed moderate expression levels ranging from 0.2 mg/L culture for A/Shanghai/1/13 to 0.7 mg/L for A/Anhui/1/13. All four protein preparations had very little to no impurities (Figures 8A and&#160;B) with HAs being of higher purity than NAs which can be explained by the expression level. A/Shanghai/1/13 HA was further analyzed using ELISA with the broadly ...

Watch this Scientific Journal Video about Expression of Functional Recombinant Hemagglutinin and Neuraminidase Proteins from the Novel H7N9 Influenza Virus Using the Baculovirus Expression System at J

Expression of Functional Recombinant Hemagglutinin and Neuraminidase Proteins from the Novel H7N9 Influenza Virus Using the Baculovirus Expression System

Here we describe a way to express correctly folded and functional influenza virus surface antigens derived from the novel Chinese H7N9 virus in insect cells. The technique can be adapted to express ectodomains of any viral or cellular surface proteins.

The baculovirus expression system is a powerful tool for expression of  recombinant proteins. Here we use it to produce correctly folded and  glycosylated ...

expression-functional-recombinant-hemagglutinin-neuraminidase

Research

JoVE Journal

Immunology and Infection

23.8K Views.  Icahn School of Medicine at Mount Sinai. Here we describe a way to express correctly folded and functional influenza virus surface antigens derived from the novel Chinese H7N9 virus in insect cells. The technique can be adapted to express ectodomains of any viral or cellular surface proteins.

Video: Expression of Functional Recombinant Hemagglutinin and Neuraminidase Proteins from the Novel H7N9 Influenza Virus Using the Baculovirus Expression System

Generation of Recombinant Influenza Virus from Plasmid DNA

Efforts by a number of influenza research groups have been pivotal in the development and improvement of influenza A virus reverse genetics. Originally established in 1999 1,2 plasmid-based reverse genetic techniques to generate recombinant viruses have revolutionized the influenza research field because specific questions have been answered by genetically engineered, infectious, recombinant influenza viruses. Such studies include virus replication, function of viral proteins, the contribution of specific mutations in viral proteins in viral replication and/or pathogenesis and, also, viral vectors using recombinant influenza viruses expressing foreign proteins 3.

Rescue of influenza A viruses from plasmid DNA is a basic and essential experimental technique that allows influenza researchers to generate recombinant viruses to study multiple aspects in the biology of influenza virus, and to be used as potential vectors or vaccines.

Efforts by a number of influenza research groups have been pivotal in the development and improvement of influenza A virus reverse genetics. Originally ...

Using Bioluminescent Imaging to Investigate Synergism Between Streptococcus pneumoniae and Influenza A Virus in Infant Mice

During the 1918 influenza virus pandemic, which killed approximately 50 million people worldwide, the majority of fatalities were not the result of infection with influenza virus alone. Instead, most individuals are thought to have succumbed to a secondary bacterial infection, predominately caused by the bacterium Streptococcus pneumoniae (the pneumococcus). The synergistic relationship between infections caused by influenza virus and the pneumococcus has subsequently been observed during the 1957 Asian influenza virus pandemic, as well as during seasonal outbreaks of the virus (reviewed in 1, 2). Here, we describe a protocol used to investigate the mechanism(s) that may be involved in increased morbidity as a result of concurrent influenza A virus and S. pneumoniae infection. We have developed an infant murine model to reliably and reproducibly demonstrate the effects of influenza virus infection of mice colonised with S. pneumoniae. Using this protocol, we have provided the first insight into the kinetics of pneumococcal transmission between co-housed, neonatal mice using in vivo imaging 3.

Using Bioluminescent Imaging to Investigate Synergism Between Streptococcus pneumoniae and Influenza A Virus in Infant Mice

A concurrent infection with influenza A virus is one of the factors implicated in the induction of invasive pneumococcal disease during asymptomatic Streptococcus pneumoniae carriage. Here we describe a mixed infection method using infant mice to investigate the synergism between these two respiratory pathogens.

During the 1918 influenza virus pandemic, which killed approximately 50 million people worldwide, the majority of fatalities were not the result of ...

Packaging HIV- or FIV-based Lentivector Expression Constructs &amp; Transduction of VSV-G Pseudotyped Viral Particles

As with standard plasmid vectors, it is possible to transfect lentivectors in plasmid form into cells with low-to-medium efficiency to obtain transient expression of effectors. Packaging lentiviral expression constructs into pseudoviral particles, however, enables up to 100% transduction, even with difficult-to-transfect cells, such as primary, stem, and differentiated cells. Moreover, the lentiviral delivery does not produce the specific cellular responses typically associated with chemical transfections, such as cell death resulting from toxicity of the transfection reagent 1, 2. When transduced into target cells, the lentiviral construct integrates into genomic DNA and provides stable expression of the small hairpin RNA (shRNA), cDNA, microRNA or reporter gene 3, 4. Target cells stably expressing the effector molecule can be isolated using a selectable marker contained in the expression vector construct such as puromycin or GFP. After pseudoviral particles infect target cells, they cannot replicate within target cells because the viral structural genes are absent and the long terminal repeats (LTRs) are designed to be self-inactivating upon transduction 5, 6.
There are three main components necessary for efficient lentiviral packaging 1, 5, 6, 7. 
1. The lentiviral expression vector that contains some of the genetic elements required for packaging, stable integration of the viral expression construct into genomic DNA, and expression of the effector or reporter. 
2. The lentiviral packaging plasmids that provide the proteins essential for transcription and packaging of an RNA copy of the expression construct into recombinant pseudoviral particles. This protocol uses the pPACK plasmids (SBI) that encode for gag, pol, and rev from the HIV or FIV genome and Vesicular Stomatitis Virus g protein (VSV-G) for the viral coat protein. 
3. 293TN producer cells (derived from HEK293 cells) that express the SV40 large T antigen, which is required for high-titer lentiviral production and a neomycin resistance gene, useful for reselecting the cells for maintenance.
An overview of the viral production protocol can be seen in Figure 1. Viral production starts by co-transfecting 293TN producer cells with the lentiviral expression vector and the packaging plasmids. Viral particles are secreted into the media. After 48-72 hours the cell culture media is harvested. Cellular debris is removed from the cell culture media, and the viral particles are precipitated by centrifugation with PEG-it for concentration. Produced lentiviral particles are then titered and can be used to transduce target cells. Details of viral titering are not included in this protocol, but can be found at:
<a href="http://www.systembio.com/downloads/global_titer_kit_web_090710.pdf" target="_blank">http://www.systembio.com/downloads/global_titer_kit_web_090710.pdf</a> 8.
This protocol has been optimized using the specific products indicated. Other reagents may be substituted, but the same results cannot be guaranteed.

Lentiviral expression vectors are the most effective vehicles for stably expressing different effector molecules or reporter constructs in dividing and non-dividing mammalian cells and whole organisms. Here we provide a protocol on how to package lentivector expression constructs in pseudoviral particles and to transduce target cells using the pseudoviral particles.

As with standard plasmid vectors, it is possible to transfect lentivectors in plasmid form into cells with low-to-medium efficiency to obtain transient ...

Affinity Purification of Influenza Virus Ribonucleoprotein Complexes from the Chromatin of Infected Cells

Like all negative-strand RNA viruses, the genome of influenza viruses is packaged in the form of viral ribonucleoprotein complexes (vRNP), in which the single-stranded genome is encapsidated by the nucleoprotein (NP), and associated with the trimeric polymerase complex consisting of the PA, PB1, and PB2 subunits. However, in contrast to most RNA viruses, influenza viruses perform viral RNA synthesis in the nuclei of infected cells. Interestingly, viral mRNA synthesis uses cellular pre-mRNAs as primers, and it has been proposed that this process takes place on chromatin1. Interactions between the viral polymerase and the host RNA polymerase II, as well as between NP and host nucleosomes have also been characterized1,2.
Recently, the generation of recombinant influenza viruses encoding a One-Strep-Tag genetically fused to the C-terminus of the PB2 subunit of the viral polymerase (rWSN-PB2-Strep3) has been described. These recombinant viruses allow the purification of PB2-containing complexes, including vRNPs, from infected cells. To obtain purified vRNPs, cell cultures are infected, and vRNPs are affinity purified from lysates derived from these cells. However, the lysis procedures used to date have been based on one-step detergent lysis, which, despite the presence of a general nuclease, often extract chromatin-bound material only inefficiently. 
Our preliminary work suggested that a large portion of nuclear vRNPs were not extracted during traditional cell lysis, and therefore could not be affinity purified. To increase this extraction efficiency, and to separate chromatin-bound from non-chromatin-bound nuclear vRNPs, we adapted a step-wise subcellular extraction protocol to influenza virus-infected cells. Briefly, this procedure first separates the nuclei from the cell and then extracts soluble nuclear proteins (here termed the "nucleoplasmic" fraction). The remaining insoluble nuclear material is then digested with Benzonase, an unspecific DNA/RNA nuclease, followed by two salt extraction steps: first using 150 mM NaCl (termed "ch150"), then 500 mM NaCl ("ch500") (Fig. 1). These salt extraction steps were chosen based on our observation that 500 mM NaCl was sufficient to solubilize over 85% of nuclear vRNPs yet still allow binding of tagged vRNPs to the affinity matrix.
After subcellular fractionation of infected cells, it is possible to affinity purify PB2-tagged vRNPs from each individual fraction and analyze their protein and RNA components using Western Blot and primer extension, respectively. Recently, we utilized this method to discover that vRNP export complexes form during late points after infection on the chromatin fraction extracted with 500 mM NaCl (ch500)3.

Influenza viruses replicate their RNA genome in association with host-cell chromatin. Here, we present a method to purify intact viral ribonucleoprotein complexes from the chromatin of infected cells. Purified viral complexes can be analyzed by both Western blot and primer extension of protein and RNA content, respectively.

Like all negative-strand RNA viruses, the genome of influenza viruses is packaged in the form of viral ribonucleoprotein complexes (vRNP), in which the ...

Differentiating Functional Roles of Gene Expression from Immune and Non-immune Cells in Mouse Colitis by Bone Marrow Transplantation

To understand the role of a gene in the development of colitis, we compared the responses of wild-type mice and gene-of-interest deficient knockout mice to colitis. If the gene-of-interest is expressed in both bone marrow derived cells and non-bone marrow derived cells of the host; however, it is possible to differentiate the role of a gene of interest in bone marrow derived cells and non- bone marrow derived cells by bone marrow transplantation technique. To change the bone marrow derived cell genotype of mice, the original bone marrow of recipient mice were destroyed by irradiation and then replaced by new donor bone marrow of different genotype. When wild-type mice donor bone marrow was transplanted to knockout mice, we could generate knockout mice with wild-type gene expression in bone marrow derived cells. Alternatively, when knockout mice donor bone marrow was transplanted to wild-type recipient mice, wild-type mice without gene-of-interest expressing from bone marrow derived cells were produced. However, bone marrow transplantation may not be 100% complete. Therefore, we utilized cluster of differentiation (CD) molecules (CD45.1 and CD45.2) as markers of donor and recipient cells to track the proportion of donor bone marrow derived cells in recipient mice and success of bone marrow transplantation. Wild-type mice with CD45.1 genotype and knockout mice with CD45.2 genotype were used. After irradiation of recipient mice, the donor bone marrow cells of different genotypes were infused into the recipient mice. When the new bone marrow regenerated to take over its immunity, the mice were challenged by chemical agent (dextran sodium sulfate, DSS 5%) to induce colitis. Here we also showed the method to induce colitis in mice and evaluate the role of the gene of interest expressed from bone-marrow derived cells. If the gene-of-interest from the bone derived cells plays an important role in the development of the disease (such as colitis), the phenotype of the recipient mice with bone marrow transplantation can be significantly altered. At the end of colitis experiments, the bone marrow derived cells in blood and bone marrow were labeled with antibodies against CD45.1 and CD45.2 and their quantitative ratio of existence could be used to evaluate the success of bone marrow transplantation by flow cytometry. Successful bone marrow transplantation should show a vast majority of donor genotype (in term of CD molecule marker) over recipient genotype in both the bone marrow and blood of recipient mice.

Bone marrow transplantation provides a way to change the genotype of the bone marrow derived cells. If the gene of interest is expressed in both bone marrow derived cells and non-bone marrow derived cells, bone marrow transplantation can change the bone marrow derived cells to a different genotype without changing the non-bone marrow derived cell genotype.

To understand the role of a gene in the development of colitis, we compared the responses of wild-type mice and gene-of-interest deficient knockout mice ...

Rescue of Recombinant Newcastle Disease Virus from cDNA

Newcastle disease virus (NDV), the prototype member of the Avulavirus genus of the family Paramyxoviridae1, is a non-segmented, negative-sense, single-stranded, enveloped RNA virus (Figure 1) with potential applications as a vector for vaccination and treatment of human diseases. In-depth exploration of these applications has only become possible after the establishment of reverse genetics techniques to rescue recombinant viruses from plasmids encoding their complete genomes as cDNA2-5. Viral cDNA can be conveniently modified in vitro by using standard cloning procedures to alter the genotype of the virus and/or to include new transcriptional units. Rescue of such genetically modified viruses provides a valuable tool to understand factors affecting multiple stages of infection, as well as allows for the development and improvement of vectors for the expression and delivery of antigens for vaccination and therapy. Here we describe a protocol for the rescue of recombinant NDVs.

Newcastle disease virus (NDV) has been extensively studied in the last few years in order to develop new vectors for vaccination and therapy, among others. These studies have been possible due to techniques to rescue recombinant virus from cDNA, such as those we describe here.

Newcastle disease virus (NDV), the prototype member of  the Avulavirus genus of the family Paramyxoviridae1, is a non-segmented,  negative-sense, ...

Co-immunoprecipitation of the Mouse Mx1 Protein with the Influenza A Virus Nucleoprotein

Studying the interaction between proteins is key in understanding their function(s). A very powerful method that is frequently used to study interactions of proteins with other macromolecules in a complex sample is called co-immunoprecipitation. The described co-immunoprecipitation protocol allows to demonstrate and further investigate the interaction between the antiviral myxovirus resistance protein 1 (Mx1) and one of its viral targets, the influenza A virus nucleoprotein (NP). The protocol starts with transfected mammalian cells, but it is also possible to use influenza A virus infected cells as starting material. After cell lysis, the viral NP protein is pulled-down with a specific antibody and the resulting immune-complexes are precipitated with protein G beads. The successful pull-down of NP and the co-immunoprecipitation of the antiviral Mx1 protein are subsequently revealed by western blotting. A prerequisite for successful co-immunoprecipitation of Mx1 with NP is the presence of N-ethylmaleimide (NEM) in the cell lysis buffer. NEM alkylates free thiol groups. Presumably this reaction stabilizes the weak and/or transient NP&#8211;Mx1 interaction by preserving a specific conformation of Mx1, its viral target or an unknown third component. An important limitation of co-immunoprecipitation experiments is the inadvertent pull-down of contaminating proteins, caused by nonspecific binding of proteins to the protein G beads or antibodies. Therefore, it is very important to include control settings to exclude false positive results. The described co-immunoprecipitation protocol can be used to study the interaction of Mx proteins from different vertebrate species with viral proteins, any pair of proteins, or of a protein with other macromolecules. The beneficial role of NEM to stabilize weak and/or transient interactions needs to be tested for each interaction pair individually.

This co-immunoprecipitation protocol allows to study the interaction between the influenza A virus nucleoprotein and the antiviral Mx1 protein in human cells. The protocol emphasizes the importance of N-ethylmaleimide for successful co-immunoprecipitation of Mx1 and influenza A virus nucleoprotein.

Studying the interaction between proteins is key in understanding their function(s). A very powerful method that is frequently used to study interactions ...

Nasal Wipes for Influenza A Virus Detection and Isolation from Swine

Surveillance for influenza A viruses in swine is critical to human and animal health because influenza A virus rapidly evolves in swine populations and new strains are continually emerging. Swine are able to be infected by diverse lineages of influenza A virus making them important hosts for the emergence and maintenance of novel influenza A virus strains. Sampling pigs in diverse settings such as commercial swine farms, agricultural fairs, and live animal markets is important to provide a comprehensive view of currently circulating IAV strains. The current gold-standard ante-mortem sampling technique (i.e. collection of nasal swabs) is labor intensive because it requires physical restraint of the pigs. Nasal wipes involve rubbing a piece of fabric across the snout of the pig with minimal to no restraint of the animal. The nasal wipe procedure is simple to perform and does not require personnel with professional veterinary or animal handling training. While slightly less sensitive than nasal swabs, virus detection and isolation rates are adequate to make nasal wipes a viable alternative for sampling individual pigs when low stress sampling methods are required. The proceeding protocol outlines the steps needed to collect a viable nasal wipe from an individual pig.

The authors present a protocol to collect swine nasal wipes to detect and isolate influenza A viruses.

Surveillance for influenza A viruses in swine is critical to human and animal health because influenza A virus rapidly evolves in swine populations and ...

A Miniaturized Glycan Microarray Assay for Assessing Avidity and Specificity of Influenza A Virus Hemagglutinins

Influenza A virus (IAV) hemagglutinins recognize sialic acids on the cell surface as functional receptors to gain entry into cells. Wild waterfowl are the natural reservoir for IAV, but IAV can cross the species barrier to poultry, swine, horses and humans. Avian viruses recognize sialic acid attached to a penultimate galactose by a &#945;2-3 linkage (avian-type receptors) whereas human viruses preferentially recognize sialic acid with a &#945;2-6 linkage (human-type receptors). To monitor if avian viruses are adapting to human type receptors, several methods can be used. Glycan microarrays with diverse libraries of synthetic sialosides are increasingly used to evaluate receptor specificity. However, this technique is not used for measuring avidities. Measurement of avidity is typically achieved by evaluating the binding of serially diluted hemagglutinin or virus to glycans adsorbed to conventional polypropylene 96-well plates. In this assay, glycans with &#945;2-3 or &#945;2-6 sialic acids are coupled to biotin and adsorbed to streptavidin plates, or are coupled to polyacrylamide (PAA) which directly adsorb to the plastic. We have significantly miniaturized this assay by directly printing PAA-linked sialosides and their non PAA-linked counterparts on micro-well glass slides. This set-up, with 48 arrays on a single slide, enables simultaneous assays of 6 glycan binding proteins at 8 dilutions, interrogating 6 different glycans, including two non-sialylated controls. This is equivalent to 18x 96-well plates in the traditional plate assay. The glycan array format decreases consumption of compounds and biologicals and thus greatly enhances efficiency.

Using a printed glycan microarray strategy, a conventional 96-well plate assay was miniaturized for analysis of influenza A virus hemagglutinin avidity and specificity for sialic acid containing receptors.

Influenza A virus (IAV) hemagglutinins recognize sialic acids on the cell surface as functional receptors to gain entry into cells. Wild waterfowl are 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 ...