Antibody Neutralization-Resistant Primary Isolates of Human Immunodeficiency Virus Type 1

Although typical primary isolates of human immunodeficiency virus type 1 (HIV-1) are relatively neutralization resistant, three human monoclonal antibodies and a small number of HIV-1⁺ human sera that neutralize the majority of isolates have been described. The monoclonal antibodies (2G12, 2F5, and b12) represent specificities that a putative vaccine should aim to elicit, since in vitro neutralization has been correlated with protection against primary viruses in animal models. Furthermore, a neutralization escape mutant to one of the antibodies (b12) selected in vitro remains sensitive to neutralization by the other two (2G12 and 2F5) (H. Mo, L. Stamatatos, J. E. Ip, C. F. Barbas, P. W. H. I. Parren, D. R. Burton, J. P. Moore, and D. D. Ho, J. Virol. 71:6869–6874, 1997), supporting the notion that eliciting a combination of such specificities would be particularly advantageous. Here, however, we describe a small subset of viruses, mostly pediatric, which show a high level of neutralization resistance to all three human monoclonal antibodies and to two broadly neutralizing sera. Such viruses threaten antibody-based antiviral strategies, and the basis for their resistance should be explored.     

Chemokine Coreceptor Usage by Diverse Primary Isolates of Human Immunodeficiency Virus Type 1

We tested chemokine receptor subset usage by diverse, well-characterized primary viruses isolated from peripheral blood by monitoring viral replication with CCR1, CCR2b, CCR3, CCR5, and CXCR4 U87MG.CD4 transformed cell lines and STRL33/BONZO/TYMSTR and GPR15/BOB HOS.CD4 transformed cell lines. Primary viruses were isolated from 79 men with confirmed human immunodeficiency virus type 1 (HIV-1) infection from the Chicago component of the Multicenter AIDS Cohort Study at interval time points. Thirty-five additional well-characterized primary viruses representing HIV-1 group M subtypes A, B, C, D, and E and group O and three primary simian immunodeficiency virus (SIV) isolates were also used for these studies. The restricted use of the CCR5 chemokine receptor for viral entry was associated with infection by a virus having a non-syncytium-inducing phenotype and correlated with a reduced rate of disease progression and a prolonged disease-free interval. Conversely, broadening chemokine receptor usage from CCR5 to both CCR5 and CXCR4 was associated with infection by a virus having a syncytium-inducing phenotype and correlated with a faster rate of CD4 T-cell decline and progression of disease. We also observed a greater tendency for infection with a virus having a syncytium-inducing phenotype in men heterozygous for the defective CCR5 Δ32 allele (25%) than in those men homozygous for the wild-type CCR5 allele (6%) (P = 0.03). The propensity for infection with a virus having a syncytium-inducing phenotype provides a partial explanation for the rapid disease progression among some men heterozygous for the defective CCR5 Δ32 allele. Furthermore, we did not identify any primary viruses that used CCR3 as an entry cofactor, despite this CC chemokine receptor being expressed on the cell surface at a level commensurate with or higher than that observed for primary peripheral blood mononuclear cells. Whereas isolates of primary viruses of SIV also used STRL33/BONZO/TYMSTR and GPR15/BOB, no primary isolates of HIV-1 used these particular chemokine receptor-like orphan molecules as entry cofactors, suggesting a limited contribution of these other chemokine receptors to viral evolution. Thus, despite the number of chemokine receptors implicated in viral entry, CCR5 and CXCR4 are likely to be the physiologically relevant chemokine receptors used as entry cofactors in vivo by diverse strains of primary viruses isolated from blood.     

Molecular Basis for Cell Tropism of CXCR4-Dependent Human Immunodeficiency Virus Type 1 Isolates

Laboratory isolates of human immunodeficiency virus type 1 (HIV-1) that utilize CXCR4 as a coreceptor infect primary human macrophages inefficiently even though these express a low but detectable level of cell surface CXCR4. In contrast, infection of primary macrophages by primary CXCR4-tropic HIV-1 isolates is readily detectable. Here, we provide evidence suggesting that this difference in cell tropism results from a higher requirement for cell surface CXCR4 for infection by laboratory HIV-1 isolates. Transfected COS7 cells that express a high level of CD4 but a low level of CXCR4 were infected significantly more efficiently by two primary CXCR4-tropic HIV-1 isolates compared to the prototypic laboratory HIV-1 isolate IIIB. More importantly, overexpression of either wild-type or signaling-defective CXCR4 on primary macrophages dramatically enhanced the efficiency of infection by the laboratory HIV-1 isolate yet only modestly enhanced infection by either primary CXCR4-tropic virus. Overexpression of CD4 had, in contrast, only a limited effect on macrophage infection by the laboratory HIV-1, although infection by the primary isolates was markedly enhanced. We therefore conclude that the laboratory CXCR4-tropic HIV-1 isolate exhibits a significantly higher CXCR4 requirement for efficient infection than do the primary CXCR4-tropic isolates and that this difference can explain the poor ability of the laboratory HIV-1 isolate to replicate in primary macrophages. More generally, we propose that the cell tropisms displayed by different strains of HIV-1 in culture can largely be explained on the basis of differential requirements for cell surface CD4 and/or coreceptor expression levels.     

A Microassay for Determination of the Cytopathogenicity of Human Immunodeficiency Virus Type-1 Isolates

Correlations between the in vitro biological properties of HIV strains isolated from patients and the prognosis of their disease have been reported. We developed a technique to study the phenotype of HIV strains isolated from patients. We used the P4 cell line, derived from HeLa cells, which has been transfected with receptor CD4 gene. HIV laboratory strain (HIV_LAI) and peripheral blood leukocytes (PBLs) from donors infected with HIV_LAI induce syncytium in P4 cell cultures in vitro. The presence of reporter gene (LacZ gene) under the control of the HIV-1 long terminal repeat (LTR) in these cells allows colorimetric visualization of syncytia in the cytoplasm using a β-galactosidase (βgal) assay in the presence of X-gal. We cocultivated 1 × 10⁶ patient PBLs with 2 × 10⁶ normal PHA-activated normal PBLs for 4 days in the presence of IL-2 in 24-well plates. Half of the medium was replaced twice a week and PHA-activated normal PBLs were added every 7 days. HIV-1 was isolated from cocultured PBLs of 18 patients with advanced-stage HIV infection as assessed by the production of HIV p24 detected with a commercially available HIV-1 p24 ELISA. Supernatant and 10⁵ cells were collected twice a week from cocultured PBLs and were added to P4 cells in 96-well microtiter plates. The cultures were observed every day for 3 days and then the βgal assay was performed. We did not observe any effect with cells and supernatant from 8 patients, harvested from cultures incubated for as long as 28 days. The phenotype of these isolates was called NC (noncytopathic). In cells from 2 patients, we obtained blue multinucleated giant cells; the phenotype of these strains was called SI (syncytium inducing). In cultures from 8 other patients, we obtained the death of P4 cells without syncytium formation, and the phenotype of these strains was called CI (cell-killing inducing). In every case, the cytopathic effect of HIV-1 isolates could be detected with cocultured PBLs collected as early as day 4 of culture. Cocultured PBLs from 13 healthy controls did not alter the P4 cells. We displayed the replication of CI strains of HIV-1, but not the one of NC strains in P4 cell line. Our micromethod allowed the detection of cytopathic effects of HIV isolates. Further investigations should define the clinical applications of this method.     

Chemokine Receptor Utilization by Human Immunodeficiency Virus Type 1 Isolates That Replicate in Microglia

The role of human immunodeficiency virus (HIV) strain variability remains a key unanswered question in HIV dementia, a condition affecting around 20% of infected individuals. Several groups have shown that viruses within the central nervous system (CNS) of infected patients constitute an independently evolving subset of HIV strains. A potential explanation for the replication and sequestration of viruses within the CNS is the preferential use of certain chemokine receptors present in microglia. To determine the role of specific chemokine coreceptors in infection of adult microglial cells, we obtained a small panel of HIV type 1 brain isolates, as well as other HIV strains that replicate well in cultured microglial cells. These viruses and molecular clones of their envelopes were used in infections, in cell-to-cell fusion assays, and in the construction of pseudotypes. The results demonstrate the predominant use of CCR5, at least among the major coreceptors, with minor use of CCR3 and CXCR4 by some of the isolates or their envelope clones.     

Neutralization Profiles of Primary Human Immunodeficiency Virus Type 1 Isolates in the Context of Coreceptor Usage

Most strains of human immunodeficiency virus type 1 (HIV-1) which have only been carried in vitro in peripheral blood mononuclear cells (primary isolates) can be neutralized by antibodies, but their sensitivity to neutralization varies considerably. To study the parameters that contribute to the differential neutralization sensitivity of primary HIV-1 isolates, we developed a neutralization assay with a panel of genetically engineered cell lines (GHOST cells) that express CD4, one of eight chemokine receptors which function as HIV-1 coreceptors, and a Tat-dependent green fluorescent protein reporter cassette which permits the evaluation and quantitation of HIV-1 infection by flow cytometry. All 21 primary isolates from several clades could grow in the various GHOST cell lines, and their use of one or more coreceptors could easily be defined by flow cytometric analysis. Ten of these primary isolates, three that were CXCR4 (X4)-tropic, three that were CCR5 (R5)-tropic, and four that were dual- or polytropic were chosen for study of their sensitivity to neutralization by human monoclonal and polyclonal antibodies. Viruses from the X4-tropic category of viruses were first tested since they have generally been considered to be particularly neutralization sensitive. It was found that the X4-tropic virus group contained both neutralization-sensitive and neutralization-resistant viruses. Similar results were obtained with R5-tropic viruses and with dual- or polytropic viruses. Within each category of viruses, neutralization sensitivity and resistance could be observed. Therefore, sensitivity to neutralization appears to be the consequence of factors that influence the antibody-virus interaction and its sequelae rather than coreceptor usage. Neutralization of various viruses by the V3-specific monoclonal antibody, 447-52D, was shown to be dependent not only on the presence of the relevant epitope but also on its presentation. An epitope within the envelope of a particular virus is not sufficient to render a virus sensitive to neutralization by an antibody that recognizes that epitope. Moreover, conformation-dependent factors may overcome the need for absolute fidelity in the match between an antibody and its core epitope, permitting sufficient affinity between the viral envelope protein and the antibody to neutralize the virus. The studies indicate that the neutralization sensitivity of HIV-1 primary isolates is a consequence of the complex interaction between virus, antibody, and target cell.     

Grossly Defective nef Gene Sequences in a Human Immunodeficiency Virus Type 1-Seropositive Long-Term Nonprogressor

We have been investigating a long-term nonprogressor who was found to be human immunodeficiency virus type 1 (HIV-1) seropositive in 1985 and has survived with stable CD4⁺ T-cell counts (>1,000 CD4 cells/μl) without any AIDS-related illness. We have previously reported that repeated attempts to measure HIV-1 RNA in the peripheral mononuclear cells obtained from this subject have invariably failed. In the present study, we have analyzed the molecular nature of the HIV-1 quasispecies infecting this patient by PCR amplification of two proviral regions, the 5′ long terminal repeat (5′LTR)/gag leader and the nef gene, directly from fresh uncultured peripheral mononuclear cells, followed by length polymorphism analysis (with 1994, 1995, and 1996 samples) and sequencing (with a 1996 sample). Only proviral forms with nef deletions were revealed by length polymorphism analysis in samples from all three time points. Sequence analysis of the nef gene from the 1996 sample confirmed the presence of similar proviral quasispecies characterized by the presence of several deletions located in the nef-alone and the nef/U3 overlapping regions. Length polymorphism analysis of the 5′LTR/gag leader region suggested the existence of two major quasispecies populations, one characterized by the presence of forms carrying deletions in the U3 region and the other showing a completely intact, full-length 5′LTR. Evidence of the role of nef gene defects in long-term survival of HIV-1-infected patients has been provided so far in two independent investigations involving patients infected with HIV through blood transfusion. Here we show the existence of a similar condition in a subject who acquired HIV-1 seropositivity through the sexual route.     

Differences in Variation of Human Immunodeficiency Virus Type 1 Sequences from Henan and Shanghai Regions of China

Illegally paid blood donation was a risk factor for HIV acquisition exclusively in Henan and Hubei Provinces of China,and not in Shanghai.Nucleotide sequences in the gag and env genes of HIV-1 were compared between isolates from Henan and Shanghai regions of China to test whether an expected higher degree of a common source of infections from this unique blood donation transmission risk would be evident as decreased variation among Henan isolates in an exploratory cross-sectional analysis.Among 38 isolates studied,23 of 23(100%)from Henan and 8 of 15(54%)from Shanghai were subtype B.In addition,fewer sequence differences were found in gp41 of subtype B isolates from Henan than from Shanghai isolates.Further studies with additional controls are therefore warranted to confirm the role of the degree of a common source of infections in differences in HIV variation across populations.     

Coreceptor Usage of Human Immunodeficiency Virus Type 2 Primary Isolates and Biological Clones Is Broad and Does Not Correlate with Their Syncytium-Inducing Capacities

Entry of human immunodeficiency virus type 1 (HIV-1) into target cells is mediated by binding of the surface envelope glycoprotein to the CD4 molecule. Interaction of the resulting CD4-glycoprotein complex with α- or β-chemokine receptors, depending on the biological phenotype of the virus, then initiates the fusion process. Here, we show that primary HIV-2 isolates and biological clones, in contrast to those of HIV-1, may use a broad range of coreceptors, including CCR-1, CCR-3, CCR-5, and CXCR-4. The syncytium-inducing capacity of these viruses did not correlate with the ability to infect via CXCR-4 or any other coreceptor. One cell-free passage of the intermediate isolates in mitogen-stimulated, CD8⁺ cell-depleted peripheral blood mononuclear cells resulted in the outgrowth of variants with CCR-5 only, whereas the coreceptor usage of late and early isolates did not change. Since HIV-2 is less pathogenic in vivo than HIV-1, these data suggest that HIV pathogenicity in vivo is not directly related to the spectrum of coreceptors used in in vitro systems.     

HIV-1的表型及其感染的细胞嗜性

HIV-1的表型分为合胞体诱导型(syncytium-inducing,SI)和非合胞体诱导型(non-syncytium-inducing,NSI)。依据所用辅助受体和感染靶细胞的不同,HIV-1又被分为R5、X4和R5X4型。R5和X4型病毒分别利用CCR5和CXCR4作为辅助受体,而R5X4型病毒可利用这两种辅助受体。在病毒的复制力、细胞嗜性以及合胞体诱导能力上,SI型与X4型病毒一致,NSI型与R5型病毒一致。在HIV-1感染过程中,疾病的发展伴随着病毒从NSI型向SI型、及R5型向X4型的转变。HIV-1的表型影响和决定着HIV-1的感染、传播及AIDS的疾病进程。HIV-1的表型和细胞嗜性主要由病毒gp120的V3区(特别是第11和25位的氨基酸)决定。V3区的氨基酸序列信息,将为预测HIV-1的表型,以及病毒感染后的疾病进程提供生物信息学的依据。     

Analysis of Minimal Human Immunodeficiency Virus Type 1 gag Coding Sequences Capable of Virus-Like Particle Assembly and Release

We have constructed a series of human immunodeficiency virus (HIV) gag mutants by progressive truncation of the gag coding sequence from the C terminus and have combined these mutants with an assembly-competent matrix domain deletion mutation (ΔMA). By using several methods, the particle-producing capabilities of each mutant were examined. Our analysis indicated that truncated Gag precursors lacking most of C-terminal gag gene products assembled and were released from 293T cells. Additionally, a mutant with a combined deletion of the MA (ΔMA) and p6 domains even produced particles at levels comparable to that of the wild-type (wt) virus. However, most mutants derived from combination of the ΔMA and the C-terminal truncation mutations did not release particles as well as the wt. Our smallest HIV gag gene product capable of virus-like particle formation was a 28-kDa protein which consists of a few MA amino acids and the CA-p2 domain. Sucrose density gradient fractionation analysis indicated that most mutants exhibited a wt retrovirus particle density. Exceptions to this rule were mutants with an intact MA domain but deleted downstream of the p2 domains. These C-terminal truncation mutants possessed particle densities of 1.13 to 1.15 g/ml, lower than that of the wt. The N-terminal portions of the CA domain, which have been shown to be dispensable for core assembly, became critical when most of the MA domain was deleted, suggesting a requirement for an intact CA domain to assemble and release particles.     

Common Themes of Antibody Maturation to Simian Immunodeficiency Virus, Simian-Human Immunodeficiency Virus, and Human Immunodeficiency Virus Type 1 Infections

Characterization of virus-specific immune responses to human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) is important to understanding the early virus-host interactions that may determine the course of virus infection and disease. Using a comprehensive panel of serological assays, we have previously demonstrated a complex and lengthy maturation of virus-specific antibody responses elicited by attenuated strains of SIV that was closely associated with the development of protective immunity. In the present study, we expand these analyses to address several questions regarding the nature of the virus-specific antibody responses to pathogenic SIV, SIV/HIV-1 (SHIV), and HIV-1 infections. The results demonstrate for the first time a common theme of antibody maturation to SIV, SHIV, and HIV-1 infections that is characterized by ongoing changes in antibody titer, conformational dependence, and antibody avidity during the first 6 to 10 months following virus infection. We demonstrate that this gradual evolution of virus-specific antibody responses is independent of the levels of virus replication and the pathogenicity of the infection viral strain. While the serological assays used in these studies were useful in discriminating between protective and nonprotective antibody responses during evaluation of vaccine efficacy with attenuated SIV, these same assays do not distinguish the clinical outcome of infection in pathogenic SIV, SHIV, or HIV-1 infections. These results likely reflect differences in the immune mechanisms involved in mediating protection from virus challenge compared to those that control an established viral infection, and they suggest that additional characteristics of both humoral and cellular responses evolve during this early immune maturation.     

Human Immunodeficiency Virus Type 1 Populations in Blood and Semen

Transmission of human immunodeficiency virus type 1 (HIV-1) usually results in outgrowth of viruses with macrophage-tropic phenotype and consensus non-syncytium-inducing (NSI) V3 loop sequences, despite the presence of virus with broader host range and the syncytium-inducing (SI) phenotype in the blood of many donors. We examined proviruses in contemporaneous peripheral blood mononuclear cells (PBMC) and nonspermatozoal semen mononuclear cells (NSMC) of five HIV-1-infected individuals to determine if this preferential outgrowth could be due to compartmentalization and thus preferential transmission of viruses of the NSI phenotype from the male genital tract. Phylogenetic reconstructions of ~700-bp sequences covering the second constant region through the fifth variable region (C2 to V5) of the viral envelope gene revealed distinct variant populations in the blood versus the semen in two patients with AIDS and in one asymptomatic individual (patient 613), whereas similar variant populations were found in both compartments in two other asymptomatic individuals. Variants with amino acids in the V3 loop that predict the SI phenotype were found in both AIDS patients and in patient 613; however, the distribution of these variants between the two compartments was not consistent. SI variants were found only in the PBMC of one AIDS patient but only in the NSMC of the other, while they were found in both compartments in patient 613. It is therefore unlikely that restriction of SI variants from the male genital tract accounts for the observed NSI transmission bias. Furthermore, no evidence for a semen-specific signature amino acid sequence was detected.     

Efficiency of Cell-Free and Cell-Associated Virus in Mucosal Transmission of Human Immunodeficiency Virus Type 1 and Simian Immunodeficiency Virus

Effective strategies are needed to block mucosal transmission of human immunodeficiency virus type 1 (HIV-1). Here, we address a crucial question in HIV-1 pathogenesis: whether infected donor mononuclear cells or cell-free virus plays the more important role in initiating mucosal infection by HIV-1. This distinction is critical, as effective strategies for blocking cell-free and cell-associated virus transmission may be different. We describe a novel ex vivo model system that utilizes sealed human colonic mucosa explants and demonstrate in both the ex vivo model and in vivo using the rectal challenge model in rhesus monkeys that HIV-1-infected lymphocytes can transmit infection across the mucosa more efficiently than cell-free virus. These findings may have significant implications for our understanding of the pathogenesis of mucosal transmission of HIV-1 and for the development of strategies to prevent HIV-1 transmission.     

Biological Signature Characteristics of Primary Isolates from Human Immunodeficiency Virus Type 1 Group O in Ex Vivo Human Tonsil Histocultures

Human immunodeficiency virus type 1 (HIV-1) group M viruses have achieved a global distribution, while HIV-1 group O viruses are endemic only in particular regions of Africa. Here, we evaluated biological characteristics of group O and group M viruses in ex vivo models of HIV-1 infection. The replicative capacity and ability to induce CD4 T-cell depletion of eight group O and seven group M primary isolates were monitored in cultures of human peripheral blood mononuclear cells and tonsil explants. Comparative and longitudinal infection studies revealed HIV-1 group-specific activity patterns: CCR5-using (R5) viruses from group M varied considerably in their replicative capacity but showed similar levels of cytopathicity. In contrast, R5 isolates from group O were relatively uniform in their replicative fitness but displayed a high and unprecedented variability in their potential to deplete CD4 T cells. Two R5 group O isolates were identified that cause massive depletion of CD4 T cells, to an extent comparable to CXCR4-using viruses and not documented for any R5 isolate from group M. Intergroup comparisons found a five- to eightfold lower replicative fitness of isolates from group O than for isolates from group M yet a similar overall intrinsic pathogenicity in tonsil cultures. This study establishes biological ex vivo characteristics of HIV-1 group O primary isolates. The current findings challenge the belief that a grossly reduced replicative fitness or inherently impaired cytopathicity of viruses from this group underlies their low global prevalence.Independent cross-species transmission events from simian immunodeficiency virus-infected apes have led to four distinct phylogenetic lineages of human immunodeficiency virus (HIV) in humans (45). The main (M) group of HIV type 1 (HIV-1) is responsible for the HIV pandemic, while HIV-1 group O (outlier) and HIV-2 are endemic only in west and central Africa, and HIV-1 group N (non-M/non-O) infection has been documented only in a small number of Cameroonians (56). These cross-species transmissions are believed to have occurred in western Africa around the same time, but only HIV-1 group M founded the pandemic (33, 37).The global distribution of HIV-1 group O is remarkably restricted. The relative seroprevalence of group O is reported to be highest in the Republic of Cameroon, Equatorial Guinea, and Gabon (7, 42, 57), implicating this area as the possible starting point of this HIV-1 lineage''s epidemic. Rare group O infections have been documented in industrialized countries, the majority comprising patients of Cameroonian descent (8, 25, 30, 40, 46). Notably, the prevalence of group O among HIV-1-positive blood samples in Cameroon showed a marked decline from the period 1986 to 1988 (20.6% of all HIV-1 infections) to the period 1997 to 1998 (1.4%) (7) with evidence of a low, but stabilized, prevalence in the subsequent period up to 2004 (10, 55). Primary isolates from group O and group M display pronounced genetic differences (24, 54), yet the reasons for the decreasing prevalence of HIV-1 group O relative to group M in west Africa and the almost exclusive contribution of group M to the AIDS pandemic are unclear. Many factors could, in principle, have contributed to this variable spread through the human population, including host genetic effects, transmission bottlenecks, behavioral and environmental restrictions, founder effects, and other factors (33, 53).Clinical observations do not suggest major differences in disease progression in patients infected with HIV-1 groups O and M (23, 24, 35, 39). This notion is based on limited data on the immune status and virological parameters for group O-infected individuals. Few experimental in vitro studies have compared the replicative fitness of HIV types or groups (1, 2, 50, 52, 54). In head-to-head replication competition experiments of pairs of primary isolates from group M and group O in peripheral blood mononuclear cell (PBMC) cultures, Arien et al. reported a greater than 100-fold reduced replicative fitness of group O viruses (2). They suggested that grossly reduced “ex vivo pathogenic fitness” and impaired transmission from dendritic cells to cocultured T cells (“ex vivo transmission fitness”) are intrinsic properties of group O viruses that may contribute to their low prevalence and limited geographical spread (2, 3).Here, we evaluated characteristics of a panel of primary isolates from HIV-1 group O compared to a panel from group M in three primary cell models of HIV infection. In addition to replication studies in single-donor PBMCs used in a previous fitness study (2), we employed multidonor pools of PBMCs and an ex vivo human tonsil lymphoid aggregate culture (HLAC) model. HIV readily replicates to high titers in tonsil cultures that maintain the cell composition and cytokine milieu of a lymphoid target organ in vivo (17). Previously, studies in this model have shed light on key pathogenic properties of HIV, including cell tropism and cytopathic effects in relation to coreceptor usage, productive infection of resting CD4 T cells, early host responses to infection, and viral coinfections (5, 6, 14, 18-20, 27, 38, 43, 48-50). A unique characteristic of this ex vivo model is that it allows parallel assessment of an isolate''s replicative fitness and cytopathicity, the latter determined by its ability to deplete CD4 T cells. The current investigation may enhance our understanding of parameters critical for HIV-1 spread in the human population and could thus potentially also provide clues to prevention and therapy.