WFDC1 expression identifies memory CD4 T- lymphocytes rendered vulnerable to cell-cell HIV-1 transfer by promoting intercellular adhesive junctions
© Alvarez et al; licensee BioMed Central Ltd. 2011
Received: 18 October 2010
Accepted: 5 May 2011
Published: 5 May 2011
Elucidating mechanisms that promote HIV-1 transfer between CD4+ T-lymphocytes and their subsequent loss is of importance to HIV-1 pathogenesis. We recently reported that whey acidic protein, ps20, promotes cell-free HIV-1 spread through ICAM-1 modulation. Since ICAM-1 is pivotal in cell conjugation and intercellular HIV-1 transfer, this study examines ps20 effects on HIV-1 spread between T lymphocytes.
We demonstrate intrinsic ps20 variability in primary CD4+ T-lymphocyte clonal populations and a significant positive correlation between endogenous ps20 levels and virus transfer involving fusion resulting in a spreading infection that could be reversed by the addition of reverse transcriptase inhibitors. Blocking anti-ps20 antibody or siRNA mediated ps20 knockdown, significantly reduced virus transfer. Conversely, virus transfer was promoted by ectopic ps20 expression or by exogenous addition of recombinant ps20. A higher frequency of virological synapse formation was evident in cocultures of HIV-1 infected donor T-cells with ps20high v ps20low/intermediate targets. Blocking ps20 inhibited T-lymphocyte conjugate formation and ICAM-1 expression, and was as potent as ICAM-1 in inhibiting HIV-1 transfer.
Therefore ps20 is a novel marker of CD4+ T-cells rendered vulnerable to HIV-1 infection by regulating the fundamental biologic process of intercellular conjugate formation and consequently of potential importance in HIV-1 pathogenesis.
Understanding the mechanisms by which retroviruses spread from one cell to another is of central importance to disease pathogenesis as this process enables viruses to effectively escape immune responses. Three modes of cell contact have been described which are capable of transmitting retroviruses. One mode is through the formation of filopodial bridges, which are protrusions that originate from uninfected target cells that become tethered to infected donor cells through the surface expression of viral ENV proteins . After tethering, both MLV and HIV-1 were shown to travel along the outside of these bridge structures onto the surface of target cells . A similar mode of retroviral transfer involves thin elongated structures called nanotubes, which form when two T cells come into contact and begin to move apart, independent of virus protein expression and described in HIV-1 transmission . Lastly, a highly prevalent mode of virus transfer, occurs through the close apposition of infected and uninfected cells which form cellular conjugates [3, 4] leading to the formation of virological synapses (VS). A VS forms when CD4 and HIV-1 Env and Gag polarize to conjugate interfaces in a microtubule- and actin- dependent manner, allowing for the rapid and direct transfer of virus from infected to uninfected cells [3–10]. A recent study demonstrated conjugate formation preceding and leading to Gag redistribution/polarization with VS formation detected in 80% of conjugates formed . Similarly, the formation of multiple conjugates precedes the formation of multiple VS termed "polysynapses"  and is postulated as an efficient mode of virus dissemination in vivo, enabling a single infected cell to infect multiple target cells, as observed in the cervix and lymph nodes of SIV+ Macaques .
Several host factors beyond the HIV-1 receptor/coreceptor complex can regulate the process of cell-cell HIV-1 transfer depending on whether the conjugates formed are between CD4+ T cells or between CD4+ T cells and dendritic cells. These include adhesion molecules, lipid raft components, signalling molecules and the tetraspanins [6, 13–22]. More recently, our laboratory identified a novel HIV-1 enhancing pathway, namely the whey acidic protein, ps20, in memory CD4+ T lymphocytes that promotes cell-free HIV-1 replication through the modulation of ICAM-1 surface expression . Blocking endogenous ps20 suppressed HIV-1 replication, while the exogenous addition of recombinant ps20 promoted infection. Furthermore, blocking anti-ps20 Ab suppressed ICAM-1 surface expression . Cell adhesion antigens like ICAM-1 and integrins (e.g. like LFA-1 and α4β7 [17, 18, 24–27]), can be exploited by viruses like HIV-1 to promote spreading infection. Specifically, budding cell-free HIV-1 particles that incorporate ICAM-1 bind target cells better through cognate LFA-1 binding [24–27]. Additionally, ICAM-1 can promote cell-to-cell HIV spread by stabilising virus fusion to target cells and VS formation [17, 26, 27] and anti-ICAM-1 blocking antibody can reduce VS formation by ~30% . Together, these observations prompted us to test the hypothesis that ps20 can promote cell-cell HIV transfer by modulating ICAM-1 expression.
WFDC1/ps20 is a member of the extended whey acidic protein (WAP) family, identified by a highly conserved 4-disulphide core domain, which includes a number of small, secreted proteins found within mucosal secretions [28, 29]. Of the 18 human members, only three, namely secretory lymphocyte protease inhibitor (SLPI), Elafin and more recently ps20, have ascribed functions. All three proteins appear multifunctional; SLPI and Elafin possess anti-microbial activity, including anti-HIV-1 activity, as well anti-protease and anti-inflammatory activity [28–30]. Consequently, these proteins are implicated in innate immunity by providing broad anti-microbial cover and by negating the damaging effects of host and pathogen proteases and limiting immune activation [28–30]. To date, ps20 has not been ascribed with anti-microbial activity or anti-protease activity, and in contrast to SLPI and Elafin , ps20 promotes HIV-1 infection . A previous study highlighted the ability of ps20 to promote wound healing, cell migration and angiogenesis . All these processes require the modulation of adhesion molecules [32, 33], and therefore ps20 function is postulated to involve cell-extracellular matrix or cell-cell interactions [31, 34]. In this paper, we provide data in support of this contention by demonstrating that HIV-1 exploits ps20-mediated regulation of the quality and quantity of T lymphocyte-T lymphocyte (T-T) conjugate formation and ICAM-1 expression in the process of cell-cell virus transfer and ps20 to be a novel marker of CD4+ T cells that are highly vulnerable to HIV-1 infection.
Jurkat CD4+ T cells stably transduced to express ps20, are rendered more susceptible to T-T HIV-1 transfer
Screening steady state ps20 mRNA in ten primary clones from multiple donors confirmed profound heterogeneity in ps20 levels spanning 5 logs (Additional file 1 figure S1A) and confirmed ps20 expression, in the transduced J-ps20high cells, falls within the range seen in primary clones. As ps20 expression in this panel segregated naturally into three distinct clusters, we arbitrarily assigned populations to be ps20high (RCN above 0.1), ps20Intermediate (ps20inter)(RCN 0.001-0.1) and ps20low (RCN below 0.001). Ps20 mRNA expression in J-ps20high cells was 3-logs higher than J-ps20inter cells; accordingly, J-ps20high cultures were clearly ps20 protein positive (Additional file 1 figure S1B). A 23-fold higher level of infection in J-ps20high vs. J-ps20inter cells was noted in a spreading infection assay (Additional file 1 figure S1C). Blocking anti-ps20 Ab reduced single-cycle infection by 2.8-fold in the J-ps20high population (Additional file 1 figure S1D). These data extend previous observation that human ps20 promotes cell-free HIV-1 infection .
We next tested the ps20-dependency of an R5 HIV-1 strain (YU2) and additionally used a PCR-based assay to verify infection levels. Following co-culture with YU2 infected donor cells at a 1:0.2 T:D ratio, J-ps20high targets had a 3-fold higher level of Gag transfer, after 4 hours compared to J-ps20inter target cells (Figure 1C). In parallel, the co-cultured populations were FACS sorted for dye-positive single target cells and HIV-1 DNA measured in the sorted population. This sorting procedure ensured that infection levels were determined in single target cells, excluding possible target-target or donor-target conjugates , thereby providing an accurate estimation of infection in the infected target cells. qPCR on these samples showed a 6-fold higher level of HIV-1 LTR in the J-ps20high vs. J-ps20inter target cells (Figure 1D).
HIV-1 transfer into J-ps20high cells is fusion dependent and leads to productive infection
HIV-1 transfer correlates directly with ps20 expression in primary CD4+ T cell clones
Virus transfer into primary clones was next confirmed to be fusion-dependent resulting in spreading infection. Representative Clone 7 (ps20high) was treated with either 5 μM RT-inhibitors or 5 μg/ml T-20 for 1 hour prior to co-culturing with 2044 infected Jwt-ps20inter donor cells at a T:D ratio of 1:0.2 for 48 hours. The presence of RT inhibitors reduced Gag accumulation by 43-fold (Figure 3C). In the presence of the T-20 fusion inhibitor, inhibition was even more pronounced with a >100-fold reduction of Gag expression (Figure 3C). These data confirm that virus transfer into primary ps20inter and ps20high clones is fusion dependent and can lead to productive infection, with more marked suppression noted in ps20high cells due to higher levels of virus transfer and spread in these cells.
Blocking endogenous ps20 inhibits HIV-1 transfer in primary CD4+ T cell clones
ps20high CD4+ T cells form a higher frequency of conjugates, multiple conjugates and virological synapse with HIV-1 infected donor cells
ps20 promotes conjugate and multiple conjugate formation more effectively than ICAM-1
Cell-to-cell HIV transfer is a significant mode of virus spread amongst CD4+ T cells in-vitro [3–10] and also likely to be predominant in vivo, since memory CD4+ T cells are more likely to become infected while trafficking through secondary lymphoid tissue, where lymphocyte velocities decrease allowing for cell-virus and cell-cell interactions to take place [10, 39, 40]. Therefore, identifying host factors that regulate this process is of importance to understanding HIV-1 dissemination in-vivo.
This paper highlights ps20 to be a novel rate-limiting step in T-T HIV-1 transfer. We demonstrate this by utilising a flow-cytometry and a PCR-based HIV transfer assay in a ps20 transduced Jurkat model system, as well as in a panel of primary CD4+ T cell clones. We report a significant positive correlation between endogenous ps20 expression levels and T-T virus transfer. Blocking ps20 activity with siRNA or specific antibody significantly inhibited T-T transfer. Conversely, gain of function studies using ps20-transduced Jurkat CD4+ T cells or the exogenous addition of rps20 confirmed ps20 to promote HIV-1 transfer. We further show inhibition of virus transfer and spread into ps20high cells by the T-20 fusion inhibitor and RT inhibitors respectively, with differences in virus spread between ps20high vs. ps20low/inter populations reaching upto 5.7-fold in Jurkat cells (Figure 2) and 8.7-fold in primary clones (Figure 3), highlighting ps20 to be a potentially potent pathway in promoting T-T virus dissemination.
Divergent data exist with regard to cell-cell transfer mediated by fusion vs. endocytosis contributing to productive HIV infection [6–8, 36, 37, 41]. In studies using unstimulated CD4+ T-cells as target populations, virus transfer through endocytosis [8, 36, 37] was noted. However, other studies show T-T virus transfer to be both co-receptor- and virus fusion- dependent [6, 41]. Whilst the infected donor cell in these divergent studies was Jurkat or Molt 4, a key difference appears to be the state of target cell activation with evidence of fusion dependent entry into activated memory CD4+ T-cells targets . Our observations with activated clonal CD4 T-cells or Jurkat cells are therefore compatible with this data. Taken together, these findings suggest that the activation state of the target cell may account for observed differences in the mode of virus uptake during cell-cell virus transfer. Indeed, these differences may account for other data showing ICAM-1 and LFA-1 as not being critical for HIV transfer to unstimulated ex vivo CD4 T cells , which have been shown to take up virus via endocytosis [8, 36, 37]. Thus it is reasonable to hypothesize that, differences in the molecular determinants and the mechanisms that govern virus transfer are at least partly dependent on the state of activation of the target CD4+ T-cell.
An important step in HIV transfer is the conjugation between an infected and an uninfected cell, leading to VS formation, through which virus can be directly transferred [3–10]. A time-lapse microscopy study highlighted that up to 80% of T-T conjugates, at some point after conjugation/contact, form a VS . Consequently, we examined the role of ps20 in the quality and quantity of T-T conjugate formation. Evidence is provided in support of ps20 promoting intercellular conjugation and VS formation. As the overall proportion of conjugates containing VS was similar between ps20high and ps20low populations, the capacity of ps20 to promote VS formation may be attributable to the protein enhancing T-cell conjugation. In addition, we observed a higher frequency of multiple conjugates and polysynapses in the presence of ps20. Therefore, the ability of ps20 to promote multiple conjugates and polysynapses may be of critical importance to virus dissemination in lymphoid and mucosal tissue by allowing for fewer transient interactions between cells [see ]. This notion is further supported by the observation that the conjugate interface between infected donor cells and ps20high targets was significantly larger compared to ps20low targets. These characteristics were attributable to ps20, since knocking down ps20 expression significantly reduced the number of single conjugates, multiple conjugates, the size of the conjugate interface and resulting virus transfer.
Molecular determinants of cell-cell conjugate and VS formation include the actin and microtubule cytoskeletal networks, cell signalling, tetraspanin and lipid raft recruitment [6, 13–22, 42]. In addition to the HIV-1 receptor complex, several adhesion molecules can polarize to, and stabilize these supramolecular synapses [13, 15, 17, 20, 21]. In particular, Jolly et al. showed that anti-ICAM-1 inhibited VS by 30%, while anti-LFA-1 inhibited VS and conjugate formation from 15-90% depending on the blocking Ab used . Previously, we demonstrated that ps20 enhanced HIV-1 infection through ICAM-1 modulation . Here we confirm and extend these observations. Knocking down ps20 by 60% specifically suppressed ICAM-1 mRNA by 40% and ICAM-1 surface staining by 45% and both ICAM-1 and ps20 individually contributed to conjugate formation and virus transfer. Strikingly, knocking down ps20 inhibited multiple conjugate formation by 90% compared to 50% inhibition by siRNA against ICAM-1. Furthermore, ICAM-1 did not impact the size of the conjugate interface, whilst ps20 did so, albeit marginally (Figure 6E). These data suggest that ps20-induced ICAM-1 modulation though important, may not fully account for the ps20 effect. As ps20 is implicated in regulating extracellular matrix (ECM) components [31, 34] and given recent data that another retrovirus, HTLV-1, can be captured, and then transmitted through ECM glycoproteins' , the identification of other adhesion and ECM targets regulated by ps20 and their role in cell-cell HIV-1 transfer could enhance understanding of mechanisms that drive HIV-1 dissemination in-vivo.
The molecular mechanisms by which ps20 regulates ICAM-1 expression and clustering through putative binding partners and signalling functions are part of on-going work in our laboratory. Other work highlights a fundamental role for ps20 in cell migration and angiogenesis . Both these processes are recognised to modulate adhesion molecules [32, 33]. As cell-cell adhesion plays a significant role in successful virus infections in general , it could be argued that the potency of ps20 to promote HIV-1 infection is linked with it's fundamental role in regulating cell adhesion. The novel observation that CD4+ T cells can be segregated into stable subsets on the basis of ps20 expression coupled with the observation that ps20high CD4+ T cells are more susceptible to infection than ps20low cells, strongly suggests that ps20+ CD4+ T cells may be preferentially targeted and lost in-vivo. Our contention is that local concentrations of ps20, in tissue such as the gut, may drive HIV-1 infection and CD4+ T cell loss by increasing adhesion antigen expression on CD4+ T cells through autocrine and paracrine effects, thereby highlighting a novel role for the ancient whey acidic protein, WFDC1/ps20, in HIV-1 pathogenesis.
This study highlights three novel aspects of T-T HIV transfer. First, using three approaches to probe T-T HIV transfer, namely, flow cytometry, PCR and confocal microscopy, this study highlights ps20 to be a novel host factor that promotes cell-cell conjugation and virological synapse formation in gain and loss of function assay systems. Second, one mechanism by which ps20 promotes intercellular HIV transfer is by regulating surface ICAM-1 expression levels. Importantly, ps20 promoted multiple cell conjugation more efficiently than ICAM-1 and was identified to promote poly-synapse formation. Host factors that promote poly-synapse formation may be particularly potent in promoting virus dissemination in vivo  and thereby impact HIV-1 pathogenesis. Thirdly, the observation that primary CD4 T-lymphocyte clones segregate naturally into distinct subsets based on endogenous ps20 expression and that ps20 levels correlate with intercellular HIV transfer, identifies ps20 to a novel marker of CD4 T cells that are vulnerable to HIV infection. Together, these observations highlight that ps20 is a novel host factor that could promote virus dissemination by promoting T-T cell conjugation.
CD4+ T cells
Jurkat CCR5+ T cells (from National Centre for Biological Standards & Controls, NIBSC, UK) were maintained in RPMI 1640 (GIBCO Invitrogen, UK) + 10% Fetal Calf Serum (FCS) (Helena Biosciences UK), 20 ug/ml Gentamycin (Sigma-Aldrich, UK). A panel of random blood CD4+ T-cell clones generated by standard limiting dilution cloning [23, 45] was screened for ps20 mRNA, and cells were identified to be ps20high, ps20inter, ps20low (see Additional file figure S1A). Additionally, two CD4+ T cell clones isolated from Endoscopy sections taken from non-lesional or lesional portion of the colon of a patient with Crohns disease (kind gift Dr. Deena Gibbons and Prof. Adrian Hayday King's College London) were identified to be ps20low and ps20high respectively. Clones were maintained in RPMI 1640, 10% FCS + 10% Human AB+ Serum (First Link, UK), 20 ug/ml Gentamycin. A typical feeding cycle of CD4+ T cell clones included activation with irradiated allogeneic PBMC (1:1 ratio) + 2 μg/ml PHA (Biostat Diagnostic Systems, Germany) + 20 IU/ml IL2 (Proleukin, Chiron, UK) every 10-14 days. Cells were split and fed every 3-4 days with fresh 30 IU/ml IL2.
Stable ps20high Jurkat CD4+ T cells
The WFDC1 gene was digested out from the pHA/WFDC1 expression plasmid (kind gift D. Rowley, Baylor University, USA) with EcoRI and cloned into an MMLV based bi-cistronic retroviral vector, pCxCR encoding red fluorescent protein (RFP) under the control of a cytomegalovirus (CMV) promoter (kind gift Greg Towers, University College London). Retroviral particles encoding WFDC1/RFP (pCps20CR), or RFP alone (pCxCR, empty vector control) were made by transient transfection of 293T cells with pCxCR, or pCG9CR, along with the packaging construct pCpg (MMLV Gag/Pol) and an envelope construct encoding VSV-G (pMD.G) (kind gift D. Trono, Geneva, Switzerland). Retroviral particles were harvested 48 hrs after transfection, clarified and titrated onto 2 × 105 CCR5+ CD4+ Jurkat cells, three times over 3 days. Cells were sorted for RFP, expanded and ps20 expression confirmed by qRT-PCR. The ps20 transduced population is referred to as J-ps20high; the empty vector control as J- ps20inter.
The primary HIV-1 X4 strain 2044 was propagated in PHA activated PBMC . The HIV-1 full-length molecular clones NL4-3 and YU2 (kind gift, M Malim, King's College London) were produced by the transient transfection of 293T cells, using Fugene 6 reagent (Roche, Switzerland). Viral stocks were standardised based on Gag p24 concentrations determined by p24 ELISA (NIH Reagents).
Anti-CD54 RPE (clone 15.2; Serotec, U.K.); anti-ps20 (IG7; kind gift D. Rowley); anti-ICAM-1 Ab (LB-2) (BD Pharmingen, CA); FITC- or PE- conjugated Gag-p24 antibody (clone KC57; Beckman Coulter, UK) were used. In addition: anti-CD4 (L120), anti-HIV ENV and rabbit anti-HIV Gag p24 and p17 were obtained from NIBSC, UK. Isotype-matched mAbs were from BD Pharmingen, CA.
Affinity purified polyclonal rabbit anti-ps20 antibody (202-254) specific for residues 206-220 of the ps20 amino acid sequence was generated through Eurogentec Ltd, Belgium. Nunc 96-well plates were coated over-night at 4°C with 1 ug/mL 202-254 diluted in PBS. Plates were blocked with PBS, 1% BSA for 2H at room temperature (RT), washed x3 with PBS-0.2% Tween-20 and test samples added for 2 hours at RT. Plates were washed 6 times to remove unbound material. Detection was with 2 ug/ml IG7 anti-ps20 conjugated to horseradish peroxidase in PBS-1% BSA-0.2% Tween-20 for 2 hours, RT followed by further 6 washes. 150 μL of substrate OPD (Sigma) was added for 30 min. at room temperature and stopped with 50 μL of 4 M sulphuric acid. Optical densities were determined at 492 nm in a Bio-Rad ELISA plate reader. Ps20 concentrations were determined in relation to a standard curve using recombinant ps20 of known concentration generated in drosophila2.
Spreading HIV-1 infection
Cells were challenged with virus stocks standardised on Gag p24 concentration, and unbound virus was removed by washing after 24 hours. Productive infection was monitored by intracellular staining for HIV-1 Gag p24 using a Fix and Perm kit (AD Serotec, U.K.). Cells were fixed for 10 minutes at RT in fixation buffer, washed once with cold PBS, then resuspended in permeabilization buffer and a 1/10 final dilution of KC57 RD1 or FITC added for 25 minutes at RT. Samples were washed twice with PBS and resuspended in PBS 2%FCS, 2% paraformaldehyde and analysed on a FACSCalibur instrument and data analysed using Flow Jo software.
Cell to cell HIV-1 transfer assay
A modified version of an assay described by Sourisseau et al [10, 15] was used. Briefly, WT Jurkat CD4+ T cells (Donor) were infected with HIV-1 strains till cultures were 10-60% Gag p24+. Targets cells comprising ps20high, ps20inter or ps20low CD4+ T cell population were first labelled with the Cell Trace FarRed DDAO-SE vital dye (Invitrogen UK). 1 × 106/ml target cells were incubated in PBS with 10 μM of the DDAO-SE dye for 7 minutes at 37°C, washed twice in PBS 5% FCS and co-cultured with infected donor cells at varying ratios in a final volume of 500 μl in a 24-well plate. Infection inhibitors included were 5 μg/ml T-20 (Roche, Hertfordshire, UK), or 5 μM each of AZT/Lamirudine (AIDS Repository Reagent Program, MD, USA). Gag transfer was measured 4 hours post co-culture by enumerating dye-labelled targets cells that stained positive for HIV-1 Gag p24 by flow cytometry.
Accell siRNA smart pools targeting ps20 (E-013097-00), ICAM-1 (E-003502-00-0010) or a non-specific targeting control (D-001910-10) were purchased from Dharmacon, CO, USA. Cell populations were washed 3 times in PBS prior to resuspension at a density of 3 × 105/ml in Accell siRNA passive uptake media containing, 3% FCS and 1 μM of specified Accell siRNA pools. For primary clones, the medium was supplemented with 30 IU/ml IL-2. 3 days later cells were washed and re-incubated at 3 × 104 cells/well in a 96-well plate in a fresh aliquot of complete passive uptake media containing 1 μM of specified Accell siRNA pool. 3 days later, cells were washed, and target gene knockdown efficiency was assessed by qRT-PCR before use in functional assays.
Total RNA was extracted using an RNaeasy kit (Qiagen, UK), then converted to cDNA (Ambion, Inc., TX, USA). ps20 was measured using a custom designed Taqman™ assay as described previously . ICAM-1 was measured using a TaqMan® primer and probe set (Hs99999152_m1). ps20/ICAM-1 expression was measured relative to HPRT 1(Hs01003268_g1) or β-actin (Hs99999903_m1) or GAPDH (Hs99999905_m1) (TaqMan®; Applied Biosystems, UK), according to manufacturer's instructions, on an ABI Prism 7900 HT (Applied Biosystems, CA, USA). Data was analyzed using SDS 2.3 software (Applied biosystems, CA, USA). Relative copy number (RCN) was calculated by determining the delta ct values, which was used in the following equation (RCN) = 2^ (-ΔCt).
HIV-1 LTR measurement
DNA was extracted using a DNeasy kit (Qiagen, UK). Primer sequences for HIV-1 LTR were; L2:CTGTGGATCTACCACACACAAGGCTAC (forward) and L3:GCTGCTTATATGTAGCATCTGAGGGC (reverse)  and measured using TaqMan® probe assay relative to β-actin (Applied Biosystems, CA, USA). Delta Ct was first calculated, then normalized by subtracting the delta ct values generated from 8E5 cells, which contain one integrated copy of HIV-1. Relative copy number = 2^ (-ΔΔCt).
Far Red DDAO-SE dye-labelled target cells were cultured with HIV-1 infected donor cells along with 5 ug/ml anti-CD4 and anti-HIV ENV Abs on poly-L-lysine coated glass cover slips (Sigma, UK). Cells were washed twice with PBS, fixed for 10 minutes in 4% paraformaldehyde, 1% BSA in PBS at RT, washed twice with PBS 1% BSA and permeabilized using 0.2% triton X-100 for 10 minutes at RT. The cells were then blocked and quenched in 50 mM NH4CL, 2% mouse serum, 2% BSA, 0.05% sodium azide. Cells were then stained with an anti-Gag Ab, KC57-PE (Beckman Coulter), and an anti-CD4, L120-FITC (BD, San Jose, CA) at 1/20 total volume dilutions in block/quench buffer for 60 minutes at RT. After incubation, cells were washed 3× with PBS and mounted on Superfrost Plus glass slides (VWR International, UK), using ultramount aqueous permanent mounting medium (Dako, Denmark). Slides were allowed to dry overnight at 4°C in the dark. The next day the cells were visualized by scanning laser confocal microscopy on a Leica DM IR2 (LEICA Microsystems, Germany) and analyzed using LEICA TCS SP2 confocal software (LEICA, Microsystems, Germany). Images were acquired using a 63× oil-immersion objective and processed using Adobe Photoshop CS (Adobe Systems, San Jose, CA).
Statistical analysis of data was performed using Graph Pad PRISM Software, San Diego, CA. All p-values reported are two-tailed. P values of less than or equal to 0.05 were deemed significant.
Conflict of interests
The authors declare that they have no competing interests.
This work was funded by a PhD studentship funded by the Medical Research Council, UK & by grants to AV from the Guy's & St Thomas' Charity and the International Consortium for Novel Anti-Virals. The authors thank Professor M Malim for discussions and critical review of the manuscript; Dr Nathan Shearer for expert advice on confocal microscopy and critical review of manuscript; Dr T Tree for CD4 T cell clones from blood and Dr Deena Gibbons and Professor AH Hayday for CD4+ T cell clones isolated from gut tissue. The authors thank Professor Q Sattentau and Dr C Jolly for helpful discussions, and review of the manuscript.
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