Name: complementary use of microscopic techniques and fluorescence reading in studying cryptococcus-amoeba interactions
Uploaded: 2019-06-22T13:00:00
Description: Watch this Scientific Journal Video about Complementary Use of Microscopic Techniques and Fluorescence Reading in Studying Cryptococcus-Amoeba Interactions at JoVE.com

The work was supported by a grant from the National Research Foundation of South Africa (grant number: UID 87903) and the University of the Free State. We are also grateful to services and assistance offered by Pieter van Wyk and Hanlie Grobler during our microscopy studies.

Cryptococcus neoformans and some Acanthamoeba castellanii strains are regarded as biosafety level-2 (BSL-2) pathogens; thus, researchers must take proper precautions when working with these organisms. For example, laboratory personnel should have specific training and personal protective equipment (PPE) such as lab coats, gloves, and eye protection. A biological safety cabinet (level-2) should be used for procedures that can cause infection14.
1. Cultiva...

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In the paper, different techniques were successfully employed to reveal the possible outcome that may arise when amoeba interact with cryptococcal cells. Also, we were interested to show the effects of 3-hydroxy fatty acids on the outcome of Cryptococcus-amoeba interactions.
The first technique used was confocal microscopy, which rendered still images. The major drawback of this technique here was that it only gave us information that is limited to a particular timepoint. Any conclusi...

Microbes have evolved over time to occupy and thrive in different ecological niches such as the open physical boundaries of the soil and water, among others1. In these niches, microbes often engage in the direct competition for limited resources; importantly, for nutrients that they use for supporting their growth or space, which they need to accommodate the expanding population2,3. In certain instances, some holozoic organisms like amoeba may even predate on cryptococcal cells as a way of extracting nutrients from their biomass4,5. In turn, this allows such organisms to establish territorial dominance via controlling the population numbers of its prey. Because of this predatory pressure, some prey may be selected to produce microbial factors, such as the cryptococcal capsule6, to reconcile the negative effects of the pressure. However, as an unintended consequence of this pressure, some microbes acquire factors that allow them to cross the species barrier and seek out new niches to colonize7, like the confined spaces of the human body that are rich in nutrients and have ideal conditions. The latter may explain how a terrestrial microbe like Cryptococcus (C.) neoformans can transform to become pathogenic. 
To this end, it is important to study the initial contact that cryptococcal cells may have with amoeba and how this may select them to become pathogenic. More specifically, this may give clues on how cryptococcal cells behave when acted upon by macrophages during infection. It is for this reason that amoeba was chosen as a model for macrophages here, as it is relatively cheap and easy to maintain a culture of amoeba in a laboratory8. Of interest was to also examine how cryptococcal secondary metabolites viz. 3-hydroxy fatty acids9,10 influence the interaction between amoebae and cryptococcal cells.
A simple way of perceiving the interaction between amoeba and its prey with the naked eye is to create a lawn using its prey on the surface of an agar plate and spot amoeba. The visualization of plaques or clear zones on the agar plate depicts areas where amoeba may have fed on its prey. However, at this macro level, only the outcome of the process is noted, and the process of phagocytosis is mechanized cannot be observed. Therefore, to appreciate the process on a cell-to-cell basis, there are several microscopic methods that can be used11,12. For example, an inverted microscope with an incubation chamber can be used to video record a time-lapse of events between a phagocytic cell and its target13. Unfortunately, due to the cost of a microscope with a time-lapse functionality, it is not always possible for laboratories to purchase such a microscope, especially in resource poor-settings. 
To circumvent the above limitation, this study presents a sequential exploratory design that evaluates the interaction of C. neoformans viz C. neoformans UOFS Y-1378 and C. neoformans LMPE 046 with Acanthamoeba castellani. First, a qualitative method is used that precedes a quantitative method. Still images are captured using an inverted fluorescence microscope, as well as a transmission electron microscope to depict amoeba-Cryptococcus interactions. This was followed by quantifying fluorescence using a plate reader to estimate the efficiency of amoeba to internalize cryptococcal cells. When reconciling findings from these methods during the data-interpretation stage, this may equally reveal as much critical information as perusing a phagocytosis time-lapse video.

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			<td height="21" style="height:21px;width:312px;">1,4-Diazabicyclo-[2.2.2]-octane</td>
			<td style="width:176px;">Sigma-Aldrich</td>
			<td style="width:123px;">D27802</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">1.5-mL plastic tube&#160;</td>
			<td>Thermo Fisher Scientific</td>
			<td style="width:123px;">69715</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">15-mL Centrifuge tube&#160;</td>
			<td>Thermo Fisher Scientific</td>
			<td style="width:123px;">7252018</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">50-mL Centrifuge tube&#160;</td>
			<td>Thermo Fisher Scientific</td>
			<td style="width:123px;">1132017</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">8-Well chamber slide</td>
			<td style="width:176px;">Thermo Fisher Scientific</td>
			<td style="width:123px;">1109650</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Acetone</td>
			<td style="width:176px;">Merck</td>
			<td style="width:123px;">SAAR1022040LC</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="23">
			<td height="23" style="height:23px;width:312px;">Amoeba strain</td>
			<td style="width:176px;">ATCC&#210;</td>
			<td style="width:123px;">30234TM</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="23">
			<td height="23" style="height:23px;width:312px;">ATCC medium 712</td>
			<td style="width:176px;">ATCC&#210;</td>
			<td style="width:123px;">712TM</td>
			<td style="width:172px;">Amoeba medium</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Black 96-well microtiter plate</td>
			<td>Thermo Fisher Scientific</td>
			<td style="width:123px;">152089</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Centrifuge</td>
			<td style="width:176px;">Hermle</td>
			<td style="width:123px;">-</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Chloroform</td>
			<td style="width:176px;">Sigma-Aldrich</td>
			<td style="width:123px;">C2432</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Confocal microscope</td>
			<td style="width:176px;">Nikon</td>
			<td>Nikon TE 2000</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td colspan="4" height="21" style="height:21px;width:783px;">Epoxy resin:</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">[1] NSA</td>
			<td style="width:176px;">[1] ALS</td>
			<td style="width:123px;">[1] R1054</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">[2] DER 736</td>
			<td style="width:176px;">[2] ALS</td>
			<td style="width:123px;">[2] R1073</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">[3] ERL Y221 resin</td>
			<td style="width:176px;">[3] ALS</td>
			<td style="width:123px;">[3] R1047R</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">[4] S1 (2-dimethylaminoethanol)</td>
			<td style="width:176px;">[4] ALS&#160;</td>
			<td style="width:123px;">[4] R1067</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Fluorescein isothiocyanate</td>
			<td style="width:176px;">Sigma-Aldrich</td>
			<td style="width:123px;">F4274</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Formic Acid</td>
			<td style="width:176px;">Sigma-Aldrich</td>
			<td style="width:123px;">489441</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Fluoroskan Ascent FL</td>
			<td>Thermo Fisher Scientific</td>
			<td style="width:123px;">374-91038C</td>
			<td style="width:172px;">Microplate reader</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Glucose</td>
			<td style="width:176px;">Sigma-Aldrich</td>
			<td style="width:123px;">G8270</td>
			<td>-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Glutaraldehyde</td>
			<td style="width:176px;">ALS</td>
			<td style="width:123px;">R1009</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Hemocytometer</td>
			<td style="width:176px;">Boeco</td>
			<td style="width:123px;">-</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Lead citrate</td>
			<td style="width:176px;">ALS</td>
			<td style="width:123px;">R1209</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Liquid Chromatography Mass Spectrometer</td>
			<td>Thermo Fisher Scientific</td>
			<td style="width:123px;"></td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Methanol</td>
			<td style="width:176px;">Sigma-Aldrich</td>
			<td style="width:123px;">R 34,860</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Orbital shaker</td>
			<td style="width:176px;">Lasec&#160;</td>
			<td style="width:123px;">-</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Osmium tetroxide</td>
			<td style="width:176px;">ALS</td>
			<td style="width:123px;">R1015</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:312px;">pHrodo Green Zymosan A BioParticles</td>
			<td style="width:176px;">Life Technologies</td>
			<td style="width:123px;">P35365</td>
			<td style="width:172px;">This is the pH-sensitive dye</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Physiological buffer solution</td>
			<td style="width:176px;">Sigma-Aldrich</td>
			<td style="width:123px;">P4417-50TAB</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Rotary shaker</td>
			<td style="width:176px;">Labcon</td>
			<td style="width:123px;">-</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td colspan="4" height="21" style="height:21px;width:783px;">Sodium phosphate buffer:</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:312px;">[1] di-sodium hydrogen orthophosphate dihydrate&#160;</td>
			<td style="width:176px;">[1] Merck</td>
			<td style="width:123px;">[1] 106580</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:312px;">[2] sodium di-hydrogen orthophosphate dihydrate</td>
			<td style="width:176px;">[2] Merck&#160;</td>
			<td style="width:123px;">[2] 106345</td>
			<td style="width:172px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Transmission electron microscope</td>
			<td style="width:176px;">Philips</td>
			<td>Philips EM 100&#160;</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Trypan blue&#160;</td>
			<td style="width:176px;">Sigma-Aldrich</td>
			<td style="width:123px;">T8154</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Ultramicrotome</td>
			<td style="width:176px;">Leica</td>
			<td style="width:123px;">EM UC7</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Uranyl acetate</td>
			<td style="width:176px;">ALS</td>
			<td style="width:123px;">R1260A</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Vacuum dessicator</td>
			<td style="width:176px;">Lasec&#160;</td>
			<td style="width:123px;">-</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">Vial</td>
			<td style="width:176px;">Sigma-Aldrich</td>
			<td style="width:123px;">29651-U</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">YNB</td>
			<td style="width:176px;">Lasec&#160;</td>
			<td style="width:123px;">239210</td>
			<td style="width:172px;">-</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:312px;">YPD agar</td>
			<td style="width:176px;">Sigma-Aldrich</td>
			<td style="width:123px;">Y-1500</td>
			<td>-</td>
		</tr>
	</tbody>
</table>

complementary use of microscopic techniques and fluorescence reading in studying cryptococcus-amoeba interactions

To simulate Cryptococcus infection, amoeba, which is the natural predator of cryptococcal cells in the environment, can be used as a model for macrophages. This predatory organism, similar to macrophages, employs phagocytosis to kill internalized cells. With the aid of a confocal laser-scanning microscope, images depicting interactive moments between cryptococcal cells and amoeba are captured. The resolution power of the electron microscope also helps to reveal the ultrastructural detail of cryptococcal cells when trapped inside the amoeba food vacuole. Since phagocytosis is a continuous process, quantitative data is then integrated in the analysis to explain what happens at the timepoint when an image is captured. To be specific, relative fluorescence units are read in order to quantify the efficiency of amoeba in internalizing cryptococcal cells. For this purpose, cryptococcal cells are stained with a dye that makes them fluoresce once trapped inside the acidic environment of the food vacuole. When used together, information gathered through such techniques can provide critical information to help draw conclusions on the behavior and fate of cells when internalized by amoeba and, possibly, by other phagocytic cells.

Microbes are microscopic organisms that cannot be perceived with the naked eye. However, their impact may result in observable clinically evident illnesses, such as skin infections. When studying certain aspects of microbes, ranging from their morphology, byproducts, and interactions, being able to provide pictorial and video evidence is of the utmost importance.
We first sought to visualize the interaction between cryptococcal ...

Watch this Scientific Journal Video about Complementary Use of Microscopic Techniques and Fluorescence Reading in Studying Cryptococcus-Amoeba Interactions at JoVE.com

Complementary Use of Microscopic Techniques and Fluorescence Reading in Studying Cryptococcus-Amoeba Interactions

This paper details a protocol for preparing a co-culture of cryptococcal cells and amoebae that is studied using still, fluorescent images and high-resolution transmission electron microscope images. Illustrated here is how quantitative data can complement such qualitative information.

Complementary Use of Microscopic Techniques and Fluorescence Reading in Studying Cryptococcus-Amoeba Interactions

To simulate Cryptococcus infection, amoeba, which is the natural predator of cryptococcal cells in the environment, can be used as a model for ...

The overall purpose of this procedure is to use microscopy as a tool to study the phagocytosis of the pathogenic fungal species Cryptococcus neoformans by its environmental predator amoeba. This video details a protocol for preparing a co-culture of cryptococcal cells and amoebae which I will study using confocal laser-scanning microscopy and transmission electron microscopy. Information obtained from using these techniques could shed insights into the behavior of cryptococcal cells when internalized during infection.Streak out the test fungal strains on YPD agar plates. Incubate the plate for 48 hours at 30 degrees Celsius. Scrape off a loopful of cells from the 48-hour-old plate and inoculate them into a 250-milliliter conical flask containing 100 milliliters of YNB broth that is supplemented with 4%glucose.Incubate the flask at 30 degrees Celsius for 24 hours on a rotary shaker. After a 24-hour incubation period, count the fungal cells using a hemocytometer and adjust the cell number to one million cells per milliliter with physiological buffer solution. Cultivate amoeba cells in peptone yeast extract glucose broth for a week at 30 degrees Celsius.Count the amoeba cells using a hemocytometer and adjust the cell number to 10 million cells per milliliter with fresh sterile ATCC medium 712. Following this, perform a viability assay using a trypan blue stain. It is advisable to work with cells that show at least 80%viability.Dispense a 200-microliter suspension of standardized amoeba cells into chambers of an adherent slide. Allow two hours to pass for cells to adhere to the surface at 30 degrees Celsius. While amoeba cells are settling down, stain the standardized cryptococcal cells with fluorescein isothiocyanate.Gently agitate the fungal cells for two hours at room temperature and in the dark. Dispense 200-microliter suspension of stained cryptococcal cells after washing to appropriate chambers that already contain amoeba cells. Incubate the prepared co-culture at 30 degrees Celsius for an additional two-hour period.At the end of the coincubation period, wash the chambers twice with PBS to remove any unbound amoeba cells including non-internalized cryptococcal cells. Aspirate the glutaraldehyde that was used to fix the cells, and wash the chamber twice with PBS. Dismantle the chamber slide.View the co-cultured cells using a confocal laser-scanning microscope. To complement the above assay, stain the cryptococcal cells with pHrodo dye which selectively allows for the reading of cells trapped inside the food vacuole. Dispense these stained cells into the wells of a black adherent microtiter plate that already contains seeded amoeba cells.Measure fluorescence on a microplate reader that can convert logarithmic signals to relative fluorescence units. Add five milliliters of standardized cryptococcal cells to the same centrifuge tube that already contains five milliliters of standardized amoeba cells. Allow the tube to stand for two hours at 30 degrees Celsius.Aspirate the supernatant after centrifugation. Fix the pellet which represents the co-cultured cells with one molar of sodium phosphate buffered 3%glutaraldehyde at pH 7 for three hours. Wash the co-cultured cells once with sodium phosphate buffer.Fix again for 1 1/2 hours in similarly buffered 1%osmium tetroxide followed by washing. Dehydrate the TEM material also known as the co-cultured cells in a graded acetone series of 30%50%70%95%and 100%for 15 minutes each. Here, repeat the final dehydration step in two changes.Prepare the epoxy resin by weighing off epoxy of normal consistency. Embed material into the epoxy resin. Polymerize the TEM material for eight hours at 70 degrees Celsius.Cut 16-nanometer sections with glass knives mounted on the ultratome. Stain the sections with uranyl acetate for 10 minutes in the dark followed by lead citrate for 10 minutes, also in the dark. View the sections with a transmission electron microscope.For illustrative purposes, in the video, two isolates are examined, one that produces 3-hydroxy fatty acid and the other that does not. 3-hydroxy fatty acids are secondary metabolites that do not play role in the growth of cryptococcal cells. Here are snapshots that show interactive moments between Cryptococcus and amoebae.To the point, a cryptococcal cell before and after internalization can be observed. When studying phagocytosis, it is ideal to make use of live imaging. However, it is not always possible for researchers to have access to such a microscope in order to follow the process of phagocytosis continuously.A possible solution to compensate for this limitation may be to complement images with quantitative data through the reading of relative fluorescence units. The bar graph is an example of data that I used to complement the images. Importantly, both the qualitative data in the form of images and the quantitative data can be taken at different time points over the course of time.By piecing all the information, one can start to construct a bigger image of what transpires during the process of phagocytosis. From the quantitative data, it is clear to see that the cells with 3-hydroxy fatty acids are less susceptible to amoeba action when compared to cells that do not produce 3-hydroxy fatty acid as fewer cells are internalized. The TEM is particularly a powerful tool as it can give a bird's-eye view into the lumen of the food vacuole because of its high resolution power, and this is the kind of detail that is often missed when examining live imaging as well as still images.In this specific TEM micrograph, protuberances on the surface of cryptococcal cells can clearly be seen. Previously, it was theorized that these cell surface structures are channels by which 3-hydroxy fatty acids are released into the surrounding environment. To the point, these channels may help to deliver 3-hydroxy fatty acids into the lumen of the food vacuole of amoebae to alter the internal conditions, hence the promotion of cell survival.pHrodo or like FITC may be a much better stain to use as it only fluoresce at a low pH, like inside the food vacuole. To be specific, pHrodo only allows the researcher to obtain data concerning internalized cells. Towards this end, it is important that cells are suspended in phosphate buffer solution before staining and that amoeba media is of a neutral pH.Transmission electron microscope operators are to be well trained to section properly as this is often the point where the results could be distorted and micrograph damaged by electron bombardment. It is envisaged that researchers will be able to take enough information from the presented protocol and optimize them in their own studies. Moreover, researchers may also develop antibodies against targeted metabolite and apply them during their immunofluorescent microscopy study including immunogold labeling during TEM examination.

Research

JoVE Journal

Immunology and Infection

Use of an Optical Trap for Study of Host-Pathogen Interactions for Dynamic Live Cell Imaging

Dynamic live cell imaging allows direct visualization of real-time interactions between cells of the immune system1, 2; however, the lack of spatial and ...

Use of the EpiAirway Model for Characterizing Long-term Host-pathogen Interactions

Nontypeable Haemophilus influenzae  (NTHi) are human-adapted Gram-negative bacteria that can cause recurrent and chronic infections of the respiratory ...

Assessing Hepatic Metabolic Changes During Progressive Colonization of Germ-free Mouse by 1H NMR Spectroscopy

Assessing Hepatic Metabolic Changes During Progressive Colonization of Germ-free Mouse by 1H NMR Spectroscopy

It is well known that gut bacteria contribute significantly to the host homeostasis, providing a range of benefits such as immune protection and vitamin ...

Studying Interactions of Staphylococcus aureus with Neutrophils by Flow Cytometry and Time Lapse Microscopy

Studying Interactions of Staphylococcus aureus with Neutrophils by Flow Cytometry and Time Lapse Microscopy

We present methods to study the effect of phenol soluble modulins (PSMs)  and other toxins produced and secreted by Staphylococcus  aureus on neutrophils. ...

Use of Shigella flexneri to Study Autophagy-Cytoskeleton Interactions

Use of Shigella flexneri to Study Autophagy-Cytoskeleton Interactions

Shigella flexneri is an intracellular pathogen that can escape from phagosomes to reach the cytosol, and polymerize the host actin cytoskeleton to promote ...

A Novel In vitro Model for Studying the Interactions Between Human Whole Blood and Endothelium

A Novel In vitro Model for Studying the Interactions Between Human Whole Blood and Endothelium

The majority of all known diseases are accompanied by disorders of the cardiovascular system. Studies into the complexity of the interacting pathways ...

Evaluation of the Efficacy And Toxicity of RNAs Targeting HIV-1 Production for Use in Gene or Drug Therapy

Small RNA therapies targeting post-integration steps in the HIV-1 replication cycle are among the top candidates for gene therapy and have the potential ...

Visual and Microscopic Evaluation of Streptomyces Developmental Mutants

Visual and Microscopic Evaluation of Streptomyces Developmental Mutants

Streptomycetes are filamentous soil bacteria belonging to the phylum Actinobacteria that are found throughout the world and produce a wide array of ...

Co-immunoprecipitation Assay for Studying Functional Interactions Between Receptors and Enzymes

Receptor-associated enzymes are the major mediators of cellular activation. These enzymes are regulated, at least in part, by physical interactions with ...

Analysis of SAMHD1 Restriction by Flow Cytometry in Human Myeloid U937 Cells

Sterile &#945;-motif/histidine-aspartate domain-containing protein 1 (SAMHD1) inhibits replication of HIV-1 in quiescent myeloid cells. U937 cells are ...