Syncytium-inducing HIV-1 Research Articles

Relevance of early detection of HIV type 1 SI/CXCR4-using viruses in vertically infected children.

The aim of the study was to investigate the prevalence and persistence of syncytium-inducing (SI) strains in HIV-1-infected children along time of infection and to evaluate the influence of antiretroviral therapy and host factors on viral tropism. This is a retrospective analysis carried out in 267 HIV-1 vertically infected children from an Argentinean cohort. The viral phenotype was screened in MT-2 cells and coreceptor usage confirmed by the GHOST cell assay. Also, CD4(+) T cell count, viral load, antiretroviral therapy, and human CCR5-Δ32 and CCR2-64I genotypes were analyzed. A high frequency of HIV-1 SI/CXCR4-using variants (22%) was found among children within the first trimester of life, reaching 46% after 10 years of infection. At acute infection, zidovudine prophylaxis did not significantly affect the proportions of SI HIV-1 strains, while their presence was favored by the CCR5(+)/Δ32 genotype. Interestingly, the majority of the early SI strains did not persist over time, probably due to a higher susceptibility to antiretroviral (ARV) treatment or immunologic pressure. At the chronic stage, SI variants emerged even in the presence of HAART reaching 36% at 120 months of infection. Also the HIV-1 SI phenotype was associated with lower CD4(+) T cell counts all along the course of infection. These findings highlight the need to evaluate the presence of SI/CXCR4 variants early at primary infection. This will make it possible to optimize the use of CCR5 inhibitors in children who are apparently carriers of the R5 virus preventing early therapeutic failure due to the reemergence of SI strains from reservoirs.

Limited efficiency of endogenous interleukin-7 levels in T cell reconstitution during HIV-1 infection: will exogenous interleukin-7 therapy work?

Limited efficiency of endogenous interleukin-7 levels in T cell reconstitution during HIV-1 infection: will exogenous interleukin-7 therapy work?

Nuclear factor 90 mediates activation of the cellular antiviral expression cascade.

Viral infection triggers a cascade of interferon response genes, but the mechanisms that prime such innate antiviral defenses are poorly understood. Among candidate cellular mediators of the antiviral response are the double-stranded RNA (dsRNA)-binding proteins. Here we show that a C-terminal variant of the ubiquitous dsRNA-binding protein, nuclear factor 90 (NF90ctv), can activate the interferon response genes in the absence of viral infection. NF90ctv-expressing cells were infected with the syncytium-inducing HIV-1 strain NL4-3 and were shown to inhibit viral replication. To gain insight into this mechanism of protection, we analyzed the expression profiles of NF90ctv-positive cells as compared with parental cells transduced with the empty vector. Of the 5600 genes represented on the expression arrays, 90 displayed significant (4-fold or more) changes in mRNA levels in NF90-expressing cells. About 50% are known interferon alpha/beta-stimulated genes. The microarray expression data were confirmed by quantitative reverse transcriptase-polymerase chain reaction analysis of six representative interferon-inducible genes. Electrophoretic mobility shift assays showed that the biological response is mediated by the activation of transcription factors in NF90ctv-expressing cells. Functional significance of the activated transcription complex was evaluated by transfection assays with luciferase reporter constructs driven by the interferon-inducible promoter from the 2'-5'-oligoadenylate synthetase (p69) gene. Resistance to HIV-1, caused by the expression of NF90ctv in the cell culture system, appears to be mediated in part by the induction of interferon response genes. This leads to a hypothesis as to the mechanism of action of NF90 in mediating endogenous antiviral responses.

Stromal-cell-derived factor 1 prevents the emergence of the syncytium-inducing phenotype of HIV-1 in vivo.

In a correlative study, the mean plasma level of the chemokine stromal-cell-derived factor 1 (SDF-1) was lower in subjects with syncytium-inducing (SI) than in subjects with non-syncytium-inducing (NSI) HIV isolates, regardless of the CD4 cell count or when compared with HIV-negative individuals. Individuals with high SDF-1 had an 81% probability of having an NSI virus phenotype compared with individuals with lower SDF-1. Increased expression of SDF-1 may help explain why the more pathogenic SI HIV-1 variants do not appear in some individuals.

Differential coreceptor expression allows for independent evolution of non-syncytium-inducing and syncytium-inducing HIV-1

Differential coreceptor expression allows for independent evolution of non-syncytium-inducing and syncytium-inducing HIV-1

Differential coreceptor expression allows for independent evolution of non-syncytium-inducing and syncytium-inducing HIV-1.

We demonstrated previously that CD45RA(+) CD4(+) T cells are infected primarily by syncytium-inducing (SI) HIV-1 variants, whereas CD45RO(+) CD4(+) T cells harbor both non-SI (NSI) and SI HIV-1 variants. Here, we studied evolution of tropism for CD45RA(+) and CD45RO(+) CD4(+) cells, coreceptor usage, and molecular phylogeny of coexisting NSI and SI HIV-1 clones that were isolated from four patients in the period spanning SI conversion. NSI variants were CCR5-restricted and could be isolated throughout infection from CD45RO(+) CD4(+) cells. SI variants seemed to evolve in CD45RO(+) CD4(+) cells, but, in time, SI HIV-1 infection of CD45RA(+) CD4(+) cells equaled infection of CD45RO(+) CD4(+) cells. In parallel with this shift, SI HIV-1 variants first used both coreceptors CCR5 and CXCR4, but eventually lost the ability to use CCR5. Phylogenetically, NSI and SI HIV-1 populations diverged over time. We observed a differential expression of HIV-1 coreceptors within CD45RA(+) and CD45RO(+) cells, which allowed us to isolate virus from purified CCR5(+) CXCR4(-) and CCR5(-) CXCR4(+) CD4(+) cells. The CCR5(+) subset was exclusively infected by CCR5-dependent HIV-1 clones, whereas SI clones were preferentially isolated from the CXCR4(+) subset. The differential expression of HIV-1 coreceptors provides distinct cellular niches for NSI and SI HIV-1, contributing to their coexistence and independent evolutionary pathways.

Human immunodeficiency virus type 1 pathogenesis in SCID-hu mice correlates with syncytium-inducing phenotype and viral replication.

Human immunodeficiency virus type 1 (HIV-1) patient isolates and molecular clones were used to analyze the determinants responsible for human CD4(+) thymocyte depletion in SCID-hu mice. Non-syncytium-inducing, R5 or R3R5 HIV-1 isolates from asymptomatic infected people showed little or no human CD4(+) thymocyte depletion in SCID-hu mice, while syncytium-inducing (SI), R5X4 or R3R5X4 HIV-1 isolates from the same individuals, isolated just prior to the onset of AIDS, rapidly and efficiently eliminated CD4-bearing human thymocytes. We have mapped the ability of one SI HIV-1 isolate to eliminate CD4(+) human cells in SCID-hu mice to a region of the env gene including the three most amino-terminal variable regions (V1 to V3). We find that for all of the HIV-1 isolates that we studied, a nonlinear relationship exists between viral replication and the depletion of CD4(+) cells. This relationship can best be described mathematically with a Hill-type plot indicating that a threshold level of viral replication, at which cytopathic effects begin to be seen, exists for HIV-1 infection of thymus/liver grafts in SCID-hu mice. This threshold level is 1 copy of viral DNA for every 11 cells (95% confidence interval = 1 copy of HIV-1 per 67 cells to 1 copy per 4 cells). Furthermore, while SI viruses more frequently achieve this level of replication, replication above this threshold level correlates best with cytopathic effects in this model system. We used GHOST cells to map the coreceptor specificity and relative entry efficiency of these early- and late-stage patient isolates of HIV-1. Our studies show that coreceptor specificity and entry efficiency are critical determinants of HIV-1 pathogenesis in vivo.

Suppression of human immunodeficiency virus type 1 replication by a soluble factor produced by CD8+ lymphocytes from HIV-2-infected baboons

Human immunodeficiency virus type 2 (HIV-2)-infected baboons (Papio cynocephalus) provide a valuable animal model for the study of acquired immunodefidency syndrome (AIDS) pathogenesis since many features of disease progression resemble HIV-1-infection of humans. In some HIV-2-infected baboons that are clinically healthy, a CD8+ cell antiviral response, that is partly mediated by a soluble factor, controls viral replication in vitro. In the present study, we demonstrate that CD8+ cells derived from HIV-2-infected baboon peripheral blood, lymph nodes, adenoids and tonsils had antiviral activity in co-cultures of CD8+ and CD4+ cells that inversely correlates with viral load. A soluble factor was found to be active against the chemokine-resistant, syncytium-inducing HIV-1 SF2 and HIV-1 SF33 isolates and was relatively heat stable at 100°C for 10 min. Moreover, inhibition of the transcription from the long terminal repeat of HIV-1 was observed in 1G5 cells after activation with phorbol 12-myristate 13-acetate. Therefore, the soluble suppressing activity of CD8+ cells in HIV-2-infected baboons may be analogous to the CD8+ cell antiviral factor described in human HIV-infected asymptomatic people.

Low T-cell responses to CD3 plus CD28 monoclonal antibodies are predictive of development of AIDS.

Decreased T-cell reactivity in vitro is strongly associated with progression to AIDS and low CD4+ T-cell numbers. Low T-cell responses in vitro induced by CD3 monoclonal antibody (mAb) are predictive for progression to AIDS independent of low CD4+ T-cell counts and high HIV-1 RNA levels. We developed a whole-blood lymphocyte culture system in which T cells were stimulated by a combination of CD3 and CD28 mAb. Combined stimulation of CD28, a costimulatory molecule, and CD3 considerably enhances T-cell responses in vitro and reduces variation coefficients, which may increase the prognostic power of T-cell responses. A prospective study of HIV-1-infected homosexual men followed for 35 months. The predictive value of low T-cell responses to CD3 plus CD28 mAb relative to low CD4+ T-cell counts, high HIV-1 RNA levels and the presence of syncytium-inducing (SI) HIV-1 variants was evaluated longitudinally in 202 HIV-1-infected homosexual men followed for 35 months. In multivariate analysis, decreased T-cell responses at baseline were predictive of development of AIDS, independent of low CD4+ T-cell numbers and high HIV-1 RNA levels. In a time-dependent model, HIV-1 RNA levels lost their predictive value, whereas low T-cell responses, low CD4+ T-cell numbers and the presence of SI HIV-1 variants independently predicted AIDS. These data demonstrate that combined use of virological and immunological markers may be useful in monitoring disease progression and response to antiretroviral therapy.

The role of a stromal cell-derived factor-1 chemokine gene variant in the clinical course of HIV-1 infection

A G-to-A transition in the 3' untranslated region (UTR) of stromal cell-derived factor (SDF)-1 gene (SDF1-3'A) has recently been described, which in the homozygous state was associated with delayed disease progression. To analyse the effect of the SDF-1 polymorphism on AIDS-free survival and survival after AIDS diagnosis, also in relation to viral phenotype. Retrospective longitudinal study among 344 homosexual HIV-1-infected men. A more rapid progression to AIDS (Centers for Disease Control and Prevention 1993 definition) was observed in SDF1-3'A/3'A subjects than in wild-type (SDF1-wt/wt) subjects (relative hazard, 1.75; P = 0.07). Using death as an endpoint, accelerated progression was no longer observed (relative hazard, 0.93; P = 0.84), suggesting a late protective effect of the SDF1-3'A/3'A genotype. Indeed, survival after AIDS diagnosis was significantly delayed in SDF1-3'A/3'A subjects (relative hazard, 0.40; P = 0.02). No effect of the SDF1-3'A/wt genotype on disease progression was observed. Interestingly, a higher frequency of Kaposi's sarcoma was observed as the AIDS-defining event among SDF1-3'A/3'A (40.0%) and SDF1-3'A/wt (30.6%) subjects than in SDF1-wt/wt subjects (17.0%). At the end of the study the total frequency of syncytium-inducing (SI) HIV-1 variants was lower in SDF1-3'A/3'A subjects (22.2%) than in SDF1-3'A/wt (32.5%) and SDF1-wt/wt subjects (40.5%), although not significantly. SDF-1 genotype did not influence the rate of evolution to SI HIV-1. Progression to AIDS after the emergence of SI HIV-1 was accelerated in SDF1-3'A/3'A subjects compared with the SDF1-wt/wt genotypic group (relative hazard, 4.04; P = 0.06). In our study group, homozygosity for a G-to-A transition in the 3' UTR of SDF-1 is associated with an accelerated progression to AIDS but a subsequent prolonged survival after AIDS diagnosis.

Efficient inhibition of both syncytium-inducing and non-syncytium-inducing wild-type HIV-1 by lamivudine in vivo.

To determine the effect of lamivudine (3TC) on syncytium-inducing (SI) and non-SI (NSI) HIV-1 populations in vivo. Responses in virus load and 3TC resistance in 40 3TC-treated subjects were analysed in relation to the presence or absence of SI HIV-1 variants. Peripheral blood samples were collected at regular intervals from 40 HIV-1-infected subjects during 3TC treatment. Virus isolates obtained at the start of treatment were typed for SI-capacity by coculture with MT-2 cells. Changes in levels of viral RNA in plasma were determined by quantitative reverse transcriptase PCR. The relative amount of wild-type and mutant virus at codon 184, associated with HIV resistance to 3TC, was determined using a primer-guided nucleotide incorporation assay after amplification of part of the reverse transcriptase gene. In five subjects the frequency of productively infected CD4+ cells with SI or NSI variants was determined in relation to codon 184 genotype. Twenty-six subjects harboured only NSI variants at baseline, whereas 14 subjects also harboured SI variants. Although baseline plasma viral RNA load and CD4 cell counts were different between the two groups, no differences in the response to 3TC therapy were observed for these parameters. In-depth analysis of five subjects showed that the kinetics of virus load changes and emergence of 3TC resistance mutations were similar in plasma and cells, and comparable for the SI and NSI populations present in one individual. These data show that, in contrast to didanosine and zidovudine, the pressure exerted by 3TC is similar for SI and NSI M184 populations.

Association between CCR5 genotype and the clinical course of HIV-1 infection.

Heterozygosity for a 32-nucleotide deletion in the C-C chemokine receptor 5 gene (CCR5 delta 32) is associated with delayed disease progression in persons infected with HIV-1. To compare the predictive value of CCR5 genotype with that of established markers in the clinical course of HIV-1 infection. Retrospective longitudinal study and nested case-control study. The latter included only long-term survivors, who were individually matched with progressors. Amsterdam, the Netherlands. 364 homosexual men with HIV-1 infection. Polymerase chain reaction was used for CCR5 genotyping. Univariate and multivariate Cox proportional hazard analyses were done for disease progression with CCR5 genotype, CD4+ T-lymphocyte counts, T-lymphocyte function, HIV-1 biological phenotype (syncytium-inducing or non-syncytium-inducing HIV-1), and viral RNA load in serum as covariates. In the case-control study, 48% of long-term survivors were heterozygous for CCR5 delta 32 compared with 9% of progressors (odds ratio, 6.9 [95% CI, 1.9 to 24.8]). In the total study sample, CCR5 delta 32 heterozygotes had significantly delayed disease progression (P < 0.001; relative hazard, 0.4 [CI, 0.3 to 0.6]), a 1.5-fold slower decrease in CD4+ T-lymphocyte count (P = 0.01), and a 2.6-fold lower viral RNA load (P = 0.01) at approximately 2.3 years after seroconversion compared with CCR5 wild-type homozygotes. At the end of the study, both groups showed the same prevalence of syncytium-inducing HIV-1, but CCR5 delta 32 heterozygotes had a delayed conversion rate. The protective effect of CCR5 delta 32 heterozygosity was stronger in the presence of only non-syncytium-inducing HIV-1. The CCR5 genotype predicted disease progression independent of viral RNA load, CD4+ T-lymphocyte counts, T-lymphocyte function, and HIV-1 biological phenotype. The addition of CCR5 genotype to currently available laboratory markers may allow better estimation of the clinical course of HIV-1 infection.

Selective inhibition of syncytium-inducing and nonsyncytium-inducing HIV-1 variants in individuals receiving didanosine or zidovudine, respectively.

By studying changes in the clonal composition of HIV-1 populations during the first weeks of zidovudine (ZDV) treatment before the development of ZDV resistance-conferring mutations, we demonstrated previously a selective inhibition of nonsyncytium-inducing (NSI) HIV-1, even when present as coexisting population in individuals also harboring syncytium-inducing (SI) HIV-1. In this study, we observed the opposite in individuals receiving didanosine (ddI) treatment. In these individuals (n = 7) a median -0.98 log change (range -1.55-0.08) in infectious cellular SI load was observed, whereas the coexisting NSI load was only minimally affected (median -0.15 log, range -1.27-0.50; P = 0.03). The virus phenotype-dependent treatment responses were independent of the clonal composition of HIV-1 populations at baseline. Individuals treated with a combination of ZDV and ddI revealed an equal decline of both NSI and SI infectious cellular load (n = 4; NSI: median -1.55 log, range -2.19 to -1.45; SI: median -1.47 log, range -1.81 to -0.86; P = 0.56). To test the hypothesis that the previously reported optimal activation of ZDV and ddI in activated and resting T cells, respectively, in combination with the differential T cell tropism of NSI and SI HIV-1 is the basis for the observed virus phenotype specific efficacy of nucleoside analogs, we studied the effect of treatment with a protease inhibitor that does not require intracellular activation. Individuals receiving ritonavir (n = 4) indeed showed equal declines in NSI and SI infectious cellular load (NSI: median -2.37 log, range -2.59 to -2.16; SI: median -2.82 log, range -3.14 to -2.50; P = 0.25). Our data suggest HIV-1 phenotype as an additional parameter in the design of optimal treatment regimens.

Gp 120s derived from four syncytium-inducing HIV-1 strains induce different patterns of CD4 association with lymphocyte surface molecules.

This work extends our previous finding that lymphocyte treatment with gp120IIIB specifically induces CD4 association with several surface molecules to other molecules and to three other gp120s from different HIV-1 strains. The ability to induce this association was displayed by the four gp120s employed, i.e. gp120IIIB, gp120SF2, gp120MN and gp120(451), and the association patterns were different, as shown by both co-capping and immunoprecipitation. Co-capping showed that all four gp120s significantly potentiated CD4 association with CD3, CD45RA, CD45RB, CD38, CD26, CD59 and class I MHC molecules. By contrast, CD4 association with CD95 was induced only by gp120(451) and gp120MN; that with CD11a only by gp120SF2 and gp120MN; and that with CD27 and CD45RO only by gp120MN and gp120(451) respectively. All gp120s induced significant CD4 association with CD49d, but gp120SF2 displayed a significantly weaker effect than gp120IIIB. Induction of association was not mediated by inside-out signaling via the CD4-associated tyrosine kinase p58lck, since it was not inhibited by the tyrosine kinase inhibitors herbymicin and genistein, nor by CD45 bridging between CD4 and the associating molecule, since similar patterns of association were detected IN cells expressing different CD45 isoform patterns. Moreover, it was not mediated by chemokine receptors interacting with the gp120 V3 loop, since RANTES did not alter the gp120-induced CD4 association pattern. By contrast, the observation that gp120s from four HIV-1 strains induce different CD4 association patterns suggests that gp120 directly interacts with the associating molecules, possibly via their hypervariable regions.

Human Immunodeficiency Virus (HIV)-Resistant CD4+ UT-7 Megakaryocytic Human Cell Line Becomes Highly HIV-1 and HIV-2 Susceptible Upon CXCR4 Transfection: Induction of Cell Differentiation by HIV-1 Infection

AbstractRecent findings have shown that the expression of the seven trans-membrane G-protein–coupled CXCR4 (the receptor for the stromal cell-derived factor [SDF]-1 chemokine) is necessary for the entry of T-lymphotropic human immunodeficiency virus (HIV) strains, acting as a coreceptor of the CD4 molecule. In the human system, the role of CXCR4 in HIV infection has been determined through env-mediated cell fusion assays and confirmed by blocking viral entry in CD4+/CXCR4+ cells by SDF-1 pretreatment. We observed that the human megakaryoblastic CD4+ UT-7 cell line fails to express CXCR4 RNA and is fully resistant to HIV entry. Transfection of an expression vector containing the CXCR4 c-DNA rendered UT-7 cells readily infectable by different T-lymphotropic syncytium-inducing HIV-1 and HIV-2 isolates. Interestingly, HIV-1 infection of CXCR4 expressing UT-7 cells (named UT-7/fus) induces the formation of polynucleated cells through a process highly reminiscent of megakaryocytic differentiation and maturation. On the contrary, no morphologic changes were observed in HIV-2–infected UT-7/fus cells. These findings further strengthen the role of CXCR4 as a molecule necessary for the replication of T-lymphotropic HIV-1 and HIV-2 isolates and provide a useful model to study the functional role of CD4 coreceptors in HIV infection.

Broader Tropism and Higher Cytopathicity for CD4+T Cells of a Syncytium-Inducing Compared to a Non-Syncytium-Inducing HIV-1 Isolate as a Mechanism for Accelerated CD4+T Cell Declinein Vivo

The emergence of syncytium-inducing (SI) HIV-1 isolates in infected individuals precedes an accelerated CD4+T cell decline and is associated with high virus load and rapid disease progression. The exact mechanism by which SI HIV-1 variants may cause this enhanced clinical progression is unknown. Here we demonstrate that an SI HIV-1 isolate had a broader tropism for CD4+T cell clones (TCC) compared to a macrophage-tropic non-syncytium-inducing (NSI) HIV-1 isolate. Whereas the NSI isolate replicated poorly in 6 of 12 TCC and completely failed to replicate in 3 of 12 TCC, the SI isolate replicated efficiently in all 12 TCC tested. Restriction for replication occurred early in the viral replication cycle, before provirus formation. Infection of TCC with the SI but not with the NSI HIV-1 isolate resulted in massive death of T cells, independent of the extent of virus replication and proportion of infected cells. The high cytopathicity and broader tropism of the SI isolate for primary CD4+T cells may be directly related to the increased rate of CD4 cell decline and rapid disease progression in carriers of SI variants.

Acquisition of syncytium-inducing HIV-1 strains during therapy with zidovudine alone or combined with alpha interferon or didanosine.

The effect of antiretroviral treatment on HIV-1 phenotype was studied in a group of 83 nucleoside-naive patients. These initially nonsyncytium-inducing HIV-1 positive patients were followed prospectively for their HIV-1 phenotype. Syncytium-inducing variants were detected by cocultivation of peripheral blood mononuclear cells with the MT-2 T-cell line. Overall, 16 of 83 (19%) patients underwent a shift to syncytium-inducing phenotype: 11 of 67 during zidovudine treatment, 3 of 10 during zidovudine plus alpha interferon treatment, and 2 of 6 under initial zidovudine plus didanosine therapy. The results of this study demonstrate that neither zidovudine therapy alone nor combined with interferon or didanosine prevents the acquisition of syncytium-inducing strains.

Transient High Titers of HIV-1 in Plasma and Progression of Disease

Periodic quantitative HIV-1 plasma cultures were performed on 28 seropositive individuals who had CD4 cells < or = 300/mm3 and who were enrolled in three clinical trials testing the efficacy of didanosine versus zidovudine monotherapy. Most plasma cultures were negative or of low titer (1-100 tissue culture infective dose/ml of plasma), but there were 14 instances of high-titered plasma viremia (> or = 1,000 tissue culture infective dose/ml of plasma) seen in 11 individuals. These peaks in plasma culture titers were significantly associated either with rapidly decreasing CD4 cell numbers or with CD4 cells already < 50/mm3. In addition, patients who experienced these episodes of high-titered plasma viremia were more apt to have clinical complaints of fever, rash, flu-like illness, and/or opportunistic infection and also the syncytium-inducing HIV-1 phenotype and progression of disease.

Analysis of long-term viral expression in CEM cells persistently infected with non syncytium-inducing HIV-1 strains.

CEM cells were infected with three HIV-1 non syncytium-inducing (NSI) strains obtained from AIDS patients or seropositive individuals. The surviving cells were followed for several months in the persistently infected cultures designated 65870/CEM, 65871/CEM and 3929/CEM, and analyzed for virus expression using light and electron microscopy, immunofluorescence, reverse transcriptase assay, polymerase chain reaction amplification (PCR), nucleic acid hybridization and flow cytometry. The virus isolates induced relatively few syncytia and other cytopathic effects in the corresponding cell lines and the number of cells positive for virus expression never rose above 44%. Distinct peaks of antigen-positive cells were obtained, coincident with high levels of reverse transcriptase activity. The cultures were strongly resistant to superinfection by laboratory strain Lai, with the exception of 65870/CEM which expressed HIV antigens in up to 15% of the cells for a few days. However, cell lysis was minimal in all cases. After long-term cultivation of the three cultures, no antigen-positive cells were detected and no trace of virus expression could be observed. The remaining cells consisted entirely of CD4-negative cells. PCR analyses indicated that cells harboring a provirus were progressively eliminated from the cultures, leaving only virus-free cells. In this system, cells carrying a latent provirus survive for a limited period of time before virus activation induces cell lysis. These results suggest that at least three types of cells exist in the CEM cell line: CD4-positive cells which are rapidly killed by the virus, a second type harboring a latent viral genome after the infection and which grow normally until activation of the resident genome by external or internal signal(s), and a third type which represents rare CD4-negative cells present in the initial CEM population and which are selected for by the NSI isolates. This is the first study documenting specific interactions between NSI strains of HIV-1 and distinct subpopulations of CEM cells grown as a single cell culture.