INTRODUCTION
Nonhuman primate (NHP) models of HIV/AIDS are important tools for studying aspects of HIV-1 transmission, pathogenesis, and immunity. The most commonly used NHPs in these studies are Asian macaques, particularly rhesus macaques (Rhm;
Macaca mulatta) and pig-tailed macaques (Ptm;
Macaca nemestrina) (
2). HIV-1 is thought to be able to utilize the macaque CD4 and CXCR4/CCR5 receptors for viral entry (
13,
28); however, HIV-1 replication is restricted by other factors, most notably, TRIM5α in Rhm and the APOBEC3 family of cytidine deaminases in both species (
15,
36,
58). Because of these species-specific restrictions, the direct study of HIV-1 infection and pathogenesis in macaques is not possible, and instead research has focused on the development of SIV/HIV chimeric viruses (SHIVs). SHIVs have traditionally been constructed by inserting the region of the HIV-1 genome encompassing the
tat,
rev,
vpu, and
env genes into the context of the pathogenic SIV
mac239 backbone (
55). More recently, directed approaches that rely on engineering HIV-1 to encode SIV
gag and
vif sequences to specifically counter macaque TRIM5α and APOBEC3 have led to the creation of minimal SHIVs that primarily encode HIV-1 proteins (
16,
24). The
env gene is a particularly important component of SHIVs, because the envelope (Env) determines viral tropism and is the target of neutralizing antibodies, which are considered an important part of the protective HIV-1 immune response (reviewed in reference
37).
If SHIVs are to be effective as predictors of human disease and vaccine efficacy, they should closely mimic the transmitted strains in human infection. The Envs from most circulating strains of HIV-1 require CCR5 as a coreceptor for entry, whereas many of the current SHIVs make use of envelopes from CXCR4-tropic or dual-tropic clones, such as NL4-3, HXB2, HIV
SF33, and HIV
89.6 (
31,
35,
51,
55). Moreover, the Envs encoded by these SHIVs are highly sensitive to neutralization compared to circulating HIV-1 variants (
4,
5,
38,
60). Among subtype B CCR5-tropic SHIVs (
14,
41,
45), SHIV
SF162 passaged isolates are the most commonly used; however, the SF162 Env encoded by these SHIVs is also extremely sensitive to neutralization (
54,
63). Thus, current SHIVs do not provide a realistic benchmark for neutralizing antibody protection from circulating strains of HIV-1, and many also do not model the dominant CCR5-mediated mode of transmission.
The worldwide epidemic is comprised of very diverse HIV-1 genotypes, termed clades or subtypes. In sub-Saharan Africa, which carries the highest burden of new HIV-1 infections and HIV-1-related deaths, subtypes C and A predominate (
17). SHIVs that are infectious to macaques have been generated using subtype C
env sequences (
9,
56,
57), as well as
env sequences from the circulating recombinant CRF_AE, which is the most common HIV-1 subtype in Southeast Asia (
20,
26). Despite the relative prominence of subtype A strains in the most afflicted regions of the world, attempts to make subtype A-based SHIVs (SHIV-As) have thus far been unsuccessful, as the SHIV-As tested to date failed to replicate in macaque cells (
19).
In order to gain further insight into barriers to SHIV-A replication in macaque cells, we created HIVA
Q23/SIV
vif, a minimal SHIV encoding the
vif gene from SIV
mac239 in the context of the Q23-17 provirus, which is a CCR5-tropic subtype A HIV-1 molecular clone obtained soon after seroconversion (
48). This minimal SHIV approach takes advantage of the fact that the APOBEC3-mediated restriction to HIV-1 replication in Ptm cells can be countered by SIV Vif (
15) and the fact that, in contrast to rhmTRIM5α, the ptmTRIM5 isoforms and TRIMCyp do not antagonize HIV-1 infection (
6,
7,
33,
62). In this study, the replicative properties and adaptation of HIVA
Q23/SIV
vif to Ptm cells were explored. Two adaptive mutations were identified that, when introduced into different subtype A Envs, permit much more efficient usage of ptmCD4, resulting in a dramatic increase in the infectivity of Ptm cells. These findings identify the inefficient use of ptmCD4 as a previously uncharacterized barrier to subtype A HIV-1 replication in Ptm cells and provide approaches to increase SHIV-A infection in macaque cells.
MATERIALS AND METHODS
Construction of HIVAQ23/SIVvif.
HIVA
Q23/SIV
vif, a full-length replication-competent clone expressing
vif from SIV
mac239, was created from Q23Δ
vif (
46). Q23Δ
vif was derived from the Q23-17 full-length molecular clone (
48) and was engineered with unique SalI and MluI restriction sites at the 5′ and 3′ ends of
vif, causing a frameshift in the endogenous
vif gene and allowing for the insertion and expression of different
vif variants (
53). To make HIVA
Q23/SIV
vif, the entire SIV
mac239 vif open reading frame was amplified from SIV
mac239Δ
env (a gift from David Evans) by using forward primer 5′-GAAG
GTCGACATGGAGGAGGAAAAGA-3′ and reverse primer 5′-AGTG
ACGCGTTCATGCCAGTATTCCCAA-3′ (restriction sites are underlined). The PCR product was then digested with the SalI and MluI restriction enzymes, ligated into Q23Δ
vif, and verified by sequencing.
Env clones and mutagenesis.
In addition to Q23ENV.17 (
50) (referred to here as Q23-17), the following plasmids expressing subtype A Envs were used in the study: QF495.23 M.ENV.A3 (
4), referred to here as QF495.A3; BG505.W6M.ENV.B1 and MG505.W0M.ENV.H3 (
64), referred to here as BG505.B1 and MG505.H3, respectively; and Q259.D2.26 and Q259.D2.17 (
34). Additionally, the SF162P3 clone, constructed by inserting the predominant V1-V5 sequence from the SHIV
SF162P3 isolate into the HIV-1
SF162 Env clone (
21) (a gift from Cecilia Cheng-Mayer) and the SIV Mne CL8 Env clone (
47) were used.
Mutations were introduced to the subtype A Env clones by site-directed mutagenesis using primers designed using the QuikChange site-directed mutagenesis kit (Stratagene; primer sequences are available upon request) to amplify 25 ng of plasmid with Pfu Turbo (Invitrogen) under the following reaction conditions: 95°C for 5 min, followed by 18 cycles of 95°C for 30 s, 55°C for 1 min, and 68°C for 16 min. The Env mutants were sequenced through the entirety of the env open reading frame to verify that no undesired nucleotide changes had occurred.
Construction of other full-length molecular clones.
Chimeric full-length molecular clones were constructed by digesting the subtype B pNL-DT5R (
24) (a gift from Malcolm Martin) and HIVA
Q23/SIV
vif with EcoRI (restriction site located in
vpr) and XhoI (restriction site located in
nef) and ligating the heterologous fragments. The resulting chimeras were named Q/N
vpr-nef and N/Q
vpr-nef to indicate the origin of the
vpr-to-
nef portion in each clone (
Fig. 1a).
The HIVA
Q23/SIV
vif provirus was engineered to express different
env genes of interest by using a previously described method (
49). In short, HIVA
Q23/SIV
vif was digested with SmaI (restriction site located in
vpr) and XhoI to excise an ∼3-kb sequence encompassing the Q23-17
env gene. Heterologous
env genes were then introduced into HIVA
Q23/SIV
vif by digesting the
env clones of interest with SmaI and XhoI and ligating the fragment into HIVA
Q23/SIV
vif.
Virus production and titration.
HEK 293T cells (referred to throughout as 293Ts) were used to produce all virus preparations and were maintained in Dulbecco's modified Eagle's medium (DMEM; Invitrogen) supplemented with 10% heat-inactivated fetal calf serum (FCS) and 2 mM l-glutamine.
Full-length replication-competent viruses were produced by transfecting 293T cells using polyethyleneimine (PEI; Polysciences). Briefly, 2 × 106 293Ts were plated 24 h prior to transfection in a T75 tissue culture flask. The next day 6 μg of DNA was incubated with 60 μg of PEI in 600 μl of serum-free DMEM for 10 min before adding the mixture to cells. Viral supernatants were collected 72 h after transfection, filtered through a 0.22-μm filter (Millipore Corporation), and stored at −80°C until use.
The following
env-deficient HIV-1 proviruses were used to generate pseudoviruses: Q23Δ
env and pLai3Δ
envLuc2 (a gift from Michael Emerman), both of which have been described previously (
34,
65), and Q23Δ
env-GFP, which was constructed by subcloning enhanced green fluorescent protein (eGFP) from pEGFPN1 (Clontech) into Q23Δ
env. For this purpose, BamHI and NotI sites were introduced at nucleotides 31 and 63 in the
nef open reading frame of Q23Δ
env, and the eGFP sequence was introduced using these same restriction sites.
Pseudoviruses were produced by cotransfecting 293T cells with one of the plasmids encoding env-deficient proviruses described above and plasmids encoding env clones at a 2:1 mass ratio. To do this, 2.5 × 105 293T cells were plated in each well of a six-well dish 24 h prior to transfection. For each well, 1 μg of total DNA was mixed with 10 μg of PEI in 100 μl of serum-free DMEM. In some cases, 3 μl of Fugene 6 (Roche) was used in place of PEI per the manufacturer's instructions. Pseudoviruses lacking Env [Env(−)] were used to determine the background levels of some assays and were generated by cotransfecting the empty pCI-neo (Promega) mammalian expression vector in place of an env-expressing plasmid. Viral supernatants were harvested 48 to 72 h posttransfection and cleared of cellular debris by centrifuging at 1,300 rpm for 5 min. In some cases, cleared supernatants were concentrated ∼20- to 50-fold by using Amicon Ultra 10K filters (Millipore Corporation). All viral preparations were frozen at −80°C until use.
Viral titers were determined by infecting TZM-bl reporter cells (NIH AIDS Research and Reference Reagent Program) with thawed cell-free virus in the presence of 10 μg/ml of DEAE-dextran. Forty-eight hours later, the cells were fixed and stained for beta-galactosidase activation, and blue foci were counted to obtain infectious titers. Infectious titers were reported as the number of infectious particles (IP)/ml (
5).
Virus replication assays.
Full-length proviral clones were assessed for the production of infectious virus in human peripheral blood mononuclear cells (PBMCs), primary Ptm PBMCs, and immortalized Ptm lymphocytes. PBMCs from HIV-negative donors were isolated by the Ficoll gradient method, activated for 72 h with 10 U of phytohemagglutinin M/ml (Roche), and maintained in RPMI 1640 medium (Invitrogen) with 10% heat-inactivated FCS, 2 mM
l-glutamine, 100 U of penicillin/ml, 100 μg of streptomycin/ml, and 10 U/ml of interleukin-2 (Roche) for 48 h prior to infection and thereafter. Ptm PBMCs were isolated using a 95% Ficoll gradient, activated for 40 h in with 4 U of phytohemagglutinin M/ml, and maintained in RPMI 1640 medium with 25 mM HEPES, 20% heat-inactivated FCS, 2 mM
l-glutamine, 100 U of penicillin/ml, 100 μg of streptomycin/ml, and 100 U/ml of recombinant human interleukin-2 (Roche). The immortalized Ptm lymphocytes used in this study have been described previously (
40) (a gift from Nina Munoz and Hans-Peter Kiem). Ptm lymphocytes were maintained in Iscove's modified Dulbecco's medium (Invitrogen) with 10% heat-inactivated FCS, 2 mM
l-glutamine, 100 U of penicillin/ml, 100 μg of streptomycin/ml, and 100 U of interleukin-2/ml (Chiron Corporation).
Viral stocks were mixed with either 3 × 10
6 stimulated donor human PBMCs, 2 × 10
6 Ptm PBMCS, or 1 × 10
6 immortalized Ptm lymphocytes at a multiplicity of infection (MOI) of 0.02 or 0.2 in a final volume of 250 μl. After spinoculation (
43) at room temperature for 2 to 3 h at 1,200 ×
g, cells were washed three times in 1.5 ml of the appropriate medium and resuspended into duplicate 600-μl cultures in a 48-well dish. Cultures were maintained for 15 days, with approximately two-thirds of the medium being replaced every 3 days. The p24
gag levels were determined by measuring cleared culture supernatants with a p24
gag antigen enzyme-linked immunosorbent assay kit (ZeptoMetrix, Buffalo, NY).
Luciferase assays.
Infections of 8 × 104 immortalized Ptm lymphocytes were performed in triplicate by spinoculation with luciferase reporter viruses for 2 h at an MOI of 0.2 in a final volume of 100 μl. Infections were allowed to proceed for 72 h before lysis with Brite-Glo reagent (Invitrogen) according to the manufacturer's guidelines. Luciferase activity was immediately read on a Fluoroskan Ascent FL luminometer (Thermo LabSystems) with a 1,000-ms integration period. Infections with Env(−) pseudoviruses were used to determine background levels for the assay.
Long-term culturing of HIVAQ23/SIVvif and sequencing of outgrowth variants.
Cultures of 1 × 106 immortalized Ptm lymphocytes were infected with HIVAQ23/SIVvif at an MOI of 0.2 in an initial volume of 1 ml in 12-well dishes. Cells were maintained at a concentration of 0.8 × 106 to 3 × 106 cells/ml without discarding cells and with at least two-thirds of the medium being replenished every 4 to 5 days. Supernatant was assessed for the presence of virus every 4 to 5 days by infecting TZM-bl cells.
Using the Qiagen blood DNA kit, total DNA was extracted from cells in cultures in which viral outgrowth was identified. The number of integrated proviral copies was determined using a previously described real-time PCR assay (
3). To sequence viral variants, 1,000 copies of the integrated provirus were amplified with four nested PCRs, resulting in overlapping amplicons spanning the entirety of the HIV-1 genome. The primers and conditions used for this are available upon request. The nested PCRs were performed in triplicate, and the PCR products were detected as single prominent bands by gel electrophoresis. All reaction mixtures were treated with ExoSap (Amersham Biosciences), and the amplicons were sequenced directly without gel purification.
Infection of Ptm lymphocytes in the presence of TAK779.
TAK779-treated immortalized Ptm lymphocytes were preincubated with 1 μM TAK779 (NIH AIDS Reference and Reagent Program) for 2 h at 37°C and maintained thereafter in 1 μM TAK779. Triplicate infections of 8 × 104 TAK779-treated or untreated cells were then performed by spinoculation for 2 h with pseudotyped luciferase reporter viruses at an MOI of 0.2 in a final volume of 100 μl. Infected cultures were maintained for 72 h before lysis and subsequent measurement of luciferase activity. The percent inhibition by TAK779 was determined by comparing relative luciferase levels in TAK779-treated cells and untreated cells.
Neutralization assays.
Neutralization assays were performed using the TZM-bl neutralization assay as described previously (
5). Approximately 500 IP of Q23Δ
env-derived pseudoviruses was incubated with six serial 3-fold dilutions of IgG1b12 (
8) (referred to here as b12) or soluble CD4 (sCD4) in a 96-well plate. One hour later, 1 × 10
4 TZM-bl cells were added to each well with DEAE-dextran to a final concentration of 10 μg/ml. The relative levels of infection were determined by assessing β-galactosidase activity in triplicate wells after 48 h. Median inhibitory concentrations (IC
50s) were defined as the concentration of b12 or sCD4 that resulted in 50% inhibition of β-galactosidase, and these values were calculated using the linear fit model.
Construction and transient transfection of CD4 and CCR5 expression plasmids.
Human CCR5 and Ptm CCR5 (referred to here as huCCR5 and ptmCCR5, respectively) in the pBABE-puro vector were described previously (
11,
25). To clone human and Ptm CD4 (referred to here as huCD4 and ptmCD4, respectively), total RNA was isolated from human PBMCs and Ptm lymphocytes by using the Qiagen RNeasy minikit. Total cDNA was obtained by reverse transcribing 2 μg of RNA with SuperScript II reverse transcriptase (Invitrogen) using oligo(dT) primers. The CD4 open reading frame was PCR amplified using forward primer 5′-GAT
GGATCCATGAACCGGGGAGTCCC-3′ with reverse primer 5′-GGT
GTCGACTCAAATGGGGCTACATG-3′ for huCD4 and forward primer 5′-GAT
GGATCCATGAACCGGGGAATCCC-3′ with the same reverse primer for ptmCD4. The PCR products were digested with BamHI and SalI (underlined in the primers), cloned into the pBABE-puro plasmid (
39), and verified by sequencing.
CD4 and CCR5 expression plasmids were cotransfected in equal amounts into 293T cells using Fugene 6 at a ratio of 3 μl of transfection reagent to 1 μg of DNA as per the manufacturer's protocol. In some cases, transfections were performed with 6 μg of DNA in T75 tissue culture flasks with 18 μl of Fugene in a total volume of 200 μl of serum-free DMEM.
Analysis of receptor expression levels by flow cytometry.
CD4 and CCR5 expression levels were determined by flow cytometry using allophycocyanin-conjugated mouse anti-human CD4 antibody (catalog number 551980; BD Biosciences) and phycoerythrin-conjugated mouse anti-human CCR5 (catalog number 550632; BD Biosciences). Briefly, 1 × 105 to 2 × 105 cells were washed in phosphate-buffered saline (PBS)–2% FBS and incubated in ∼100 μl of PBS–2% FBS with 2 μl of the anti-CD4 antibody and/or 5 μl of the anti-CCR5 antibody at room temperature for 30 min. The cells were then washed once in 1 ml of PBS–2% FBS and resuspended in 200 μl PBS–2% FBS for analysis by flow cytometry.
DISCUSSION
In this study, the limited ability of subtype A Envs to use ptmCD4 for entry was identified as a major barrier to the replication of SHIV-As in macaque cells. A minimal SHIV-A, HIVAQ23/SIVvif, replicated poorly in Ptm lymphocytes, but variants displaying increased replication in Ptm lymphocytes were selected after long-term culturing. Two independent adaptive amino acid changes in Env, G312V and A204E, conferred more efficient entry into Ptm cells when introduced into the Q23-17 parental Env. These same changes conferred high levels of replication in Ptm lymphocytes when introduced into the parental HIVAQ23/SIVvif proviral clone. Importantly, increased entry was also observed when either of these changes was independently introduced into multiple subtype A Envs, suggesting that these amino acid positions have a general impact on the interactions between subtype A Env and ptmCD4. Env variants encoding G312V and A204E maintained CCR5 tropism but were much more sensitive to neutralization by sCD4 and mediated more efficient entry by ptmCD4. These findings implicate Env/CD4 interactions in the restriction of SHIV-A replication in macaque cells and provide insight into specific amino acid positions in gp120 that can enhance these interactions.
Prior studies with subtype B minimal SHIVs suggested that the inclusion of the
vif gene from SIV
mac239 in HIVA
Q23/SIV
vif would be sufficient for the virus to evade APOBEC3-mediated restriction, thus allowing for replication in Ptm cells (
15). However, HIVA
Q23/SIV
vif replicated poorly in immortalized Ptm lymphocytes, in contrast to NL-DT5R, a CXCR4-tropic minimal SHIV that encodes both the
vif gene and the CypA-binding loop of SIV
mac239 (
24), which achieved high levels of replication. Similar results were observed when these SHIVs were used to infect primary Ptm lymphocytes (data not shown). The SIV CypA binding loop is required to evade Rhm TRIM5α but should not be necessary for replication in Ptm lymphocytes, because the capsid-directed TRIM5 isoforms and TRIM Cyp expressed by Ptms do not restrict HIV-1 (
6,
7,
15,
33,
62). Indeed, the lack of restriction to the HIV-1 capsid in Ptm cells was further verified here by using chimeras between NL-DT5R and HIVA
Q23/SIV
vif, which showed that the HIVA
Q23/SIV
vif capsid supported replication in Ptm lymphocytes. Instead, the chimeras demonstrated that it was the 3′ portion of HIVA
Q23/SIV
vif, including
tat,
rev,
vpu, and
env, that limited the replication of the SHIV-A in Ptm lymphocytes. These findings were consistent with previous studies using traditional SHIVs encoding the 3′ elements of HIV-1, which implicated
env as the main determinant for infection of macaque cells (
19). The identification of the Q23-17 Env as the reason for reduced replication in Ptm cells was definitively shown when mutations encoding the G312V or A204E amino acid changes to gp120, which were identified in viruses selected for increased replication in immortalized Ptm lymphocytes, were introduced into the parental clone, and the resulting viruses replicated in immortalized Ptm lymphocytes to levels that were comparable to NL-DT5R. Viruses bearing the G312V or A204E mutations also showed increased replication compared to the parental virus in primary Ptm PBMCs, but the differences were much more modest than those observed in immortalized CD4 lymphocytes, perhaps reflecting the fact that CD4/CCR5-positive lymphocytes comprise only a fraction of the PBMCs.
The increases in entry into Ptm lymphocytes by the G312V and A204E variants in luciferase reporter assays were not necessarily directly correlated with increases in virus spread, as exemplified by the G312V change in the context of Q23-17 and BG505.B1. The G312V changes conferred similar increases in entry in the single-cycle reporter assay as the A204E mutants, yet in the viral replication assays, peak replication levels were much lower for a full-length clone encoding the Q23-17 G312V Env compared to one encoding the BG505.B1 G312V Env. The same immortalized Ptm lymphocytes were used for both the single-cycle and replication assays, ruling out the possibility of differences in cell targets. This suggests that differences in infectivity may be due to differences in how the viruses were generated. Depending on the Env being expressed, pseudoviruses have been shown to express artificially large amounts of Env, both in the processed and unprocessed forms (
18), and this can result in higher levels of infectivity compared to replication-competent virus (
49).
The G312 residue, located at the tip of the V3 loop, and the A204 residue, located adjacent to the β3-strand of the bridging sheet, are in regions of gp120 that are known to mediate interaction with the coreceptor (
22,
52). However, the increased ability of the G312V and A204E variants to infect Ptm cells was not due to a change in coreceptor use or an increased ability to mediate infection by ptmCCR5. Surprisingly, subtype A Envs were deficient in their ability to mediate infection of cells expressing ptmCD4, and the G312V and A204E amino acid changes rescued this deficiency. Flow cytometric analyses demonstrated that the differences in infectivity could not be explained by differences in cell surface CD4 expression. These results argue that the barrier to HIVA
Q23/SIV
vif replication in Ptm lymphocytes was due to inefficient use of ptmCD4 and that the G312V and A204E changes arose to circumvent this barrier.
A notable property of the G312V and A204E variants was their increased sensitivity to sCD4, with both mutations conferring increased sensitivity to sCD4 in all Envs tested, in most cases by more than 100-fold. This finding suggests that sensitivity to sCD4 may predict how well a particular envelope uses ptmCD4 for entry. If this is the case, then most circulating HIV-1 Env variants, which tend to be relatively insensitive to sCD4 (
5,
32), may not promote efficient entry via ptmCD4. However, these findings also raise the interesting possibility that sensitivity to sCD4 may provide a means to identify representative transmitted Env variants that would be the best candidates for a successful SHIV. In support of this hypothesis, it is significant that many of the existing CCR5-tropic SHIVs incorporate HIV-1 Envs that are relatively sensitive to soluble CD4, including the SF162 lineage (
10,
27), ADA (
60), BaL (
12), and the subtype C isolate HIV2873i (
56).
One model that may explain how changes to G312 and A204 elicit their effect is by increasing exposure of the CD4-binding site, which could then allow for better usage of ptmCD4. G312 is found in the V3 loop, which has been shown to be a determinant for increased sensitivity to sCD4 (
23,
42,
59), implying that changes to the V3 loop may participate in quaternary interactions in Env that could lead to a more exposed CD4-binding site. A204 is adjacent to the highly conserved C205 residue, whose replacement has been shown to increase the susceptibility of the HIV-1 Env to sCD4, presumably by abrogating a highly conserved disulfide bond, resulting in a more open Env conformation (
61). It is possible that changes to A204, particularly the introduction of a negative charge, as in the A204E and A204D variants, may also serve to modulate this disulfide bond or may open up the Env structure via other steric interactions. The increased exposure of the CD4-binding site in the G312V and A204E variants is further supported by the sensitivity that some of the variants display to the b12 monoclonal antibody, whose epitope overlaps the CD4-binding site (
8,
66). The high degree of conservation of the G312 and A204 residues may indicate that they play some role in maintaining the structural integrity of the envelope trimer. This may explain why the G312V/A204E double mutant and, in some contexts, the single G312V or A204E changes (for example, in the case of the Q259.D2.17 Env) result in an envelope that does not support efficient entry or viral replication.
Little is known regarding how differences in macaque and human CD4 impact the ability of HIV-1 Envs to mediate infection of macaque cells, although one study concluded that ptmCD4 is not a barrier to HIV-1 infection (
13). The lab-adapted LAI IIIb strain of HIV-1 was used in that previous study, and the results are consistent with observations that CXCR4-tropic subtype B SHIVs are infectious in macaque cells. The differences between that study and the data presented here most likely reflect differences in the biology of lab-adapted strains compared to CCR5-tropic variants cloned directly from infected individuals, and this serves as a reminder of the difficulty of extrapolating findings with lab-adapted HIV variants to more relevant, circulating strains of HIV-1.
The primary determinants of the HIV Env-CD4 interaction have been mapped to the D1 and D2 domains of human CD4, and human and ptmCD4 differ at 17 amino acid positions in these domains. Structural studies indicate that F43 and R58 from CD4 form contacts with gp120 (
29). There is an amino acid change at R58 (K in Ptm) that could alter Ptm CD4-Env interactions, but it is a conservative amino acid difference. There are other, nonconservative amino acid differences at S23 (N in Ptm) and N52 (S in Ptm). Although these residues have not been recognized as contact residues in structural analyses, they have been identified in mutagenesis studies as being critical for gp120 binding (
1). Thus, disruption of CD4-gp120 binding, either by modulation of direct amino acid interactions or indirectly, through effects on binding affinity, may explain the differences in HIV-mediated entry between human and Ptm CD4 observed here.
Prior studies showed that SHIV-As are not infectious in Rhm cells (
19), and our findings may provide an explanation for these results. The published sequence of Rhm CD4 is identical to the Ptm CD4 at each of the four residues noted above (data not shown). Further studies are needed to determine whether Rhm CD4 presents a similar barrier to infection as ptmCD4 and whether amino acid changes at positions A204 and G312, which are highly conserved across all subtypes, can overcome this barrier. Such studies may also help define the precise changes that alter HIV Env-macaque CD4 interactions.
The usefulness of SHIV models, particularly as tools to examine the biology of HIV-1 transmission and strategies to prevent infection, depends on how well they mimic HIV-1 transmission and early infection in humans. There are numerous barriers to HIV-1 infection in macaques, and the SHIV proviruses tested to date have required further adaptation in the animal to increase replication. The improved understanding of host restriction factors has permitted more targeted approaches to developing infectious SHIVs, although the initial chimeric viruses did not replicate to high levels in infected animals (
15,
16,
24). Here, we have identified differences in CD4 as a barrier to HIV-1 infection of pig-tailed macaque cells and showed that a single amino acid change in Env is sufficient to surmount this limitation, at least for subtype A Envs. These data indicate it may be possible to develop SHIVs that are derived from more relevant HIV-1 variants with only minor modifications. However, in using these findings to develop more relevant SHIV models, it will be important to consider how changes that permit efficient CD4 interaction impact other key biological properties of the envelope, such as sensitivity to neutralization. Identifying inefficient use of CD4 as a barrier to HIV-1 infection of macaque cells provides a critical first step in the process of designing SHIVs based on biologically relevant HIV-1 variants.