Expression of the CD8alpha beta-heterodimer on CD8(+) T lymphocytes in peripheral blood lymphocytes of human immunodeficiency virus- and human immunodeficiency virus+ individuals

CD8(+) T lymphocytes play a pivotal role in controlling human immunodeficiency virus (HIV)-1 replication in vivo. We have performed four-color flow cytometric analysis of CD8(+) peripheral blood lymphocytes (PBL) from 21 HIV-1 seronegative and 103 seropositive individuals to explore the phenotypic heterogeneity of CD8beta-chain expression on CD8(+) T lymphocytes and to clarify how its expression on CD8(+) T lymphocytes may relate to acquired immunodeficiency syndrome (AIDS) clinical progression. We showed that the single monoclonal antibody (MoAb) 2ST8-5H7, directed against the CD8alpha beta-heterodimer, identifies CD8(+) T lymphocytes as effectively as the conventional combination of anti-CD3 and anti-CD8alpha antibodies. However, we detected a significantly lower mean fluorescence (MF) of anti-CD8alpha beta staining on PBL from HIV-1 seropositive donors as compared with seronegative donors. In fact, CD8(+) T lymphocytes from HIV-1-infected individuals with the lowest CD4 counts showed the lowest levels of CD8alpha beta MF. To explore further this change in CD8alpha beta expression, we assessed the expression of 14 different cell surface molecules on CD8alpha beta+ T lymphocytes of PBL from 11 HIV-1 seronegative and 22 HIV-1 seropositive individuals. The MF of anti-CD8alpha beta staining was significantly reduced on CD8(+) T lymphocyte subsets that showed immunophenotypic evidence of activation. The subset of lymphocytes expressing low levels of CD8alpha beta expressed higher levels of activation, adhesion, and cytotoxic-associated molecules and was predominantly CD45RO+ and CD28(-). Finally, we monitored the expression of the CD8alpha beta-heterodimer on PBL of eight HIV-1-infected individuals over a 16-week period after the initiation of highly active antiretroviral therapy (HAART), including zidovudine (ZDV), lamivudine (3TC), and indinavir (IDV), and found a significant increase in the expression of the CD8alpha beta-heterodimer. These results suggest that antibodies recognizing the CD8alpha beta-heterodimer are useful tools to specifically identify CD8(+) T lymphocytes. Moreover, the quantitative monitoring of CD8alpha beta expression allows the detection of discrete CD8(+) T lymphocyte subsets and may be useful for assessing the immune status of individuals infected with HIV-1.     

CD11b+CD28-CD4+ human T cells: activation requirements and association with HLA-DR alleles

Engagement of CD28 induces a major costimulatory pathway required by many CD4+ T cells in addition to activation via the TCR. In the absence of signals provided by CD28, ligation of the TCR alone can induce anergy or apoptosis in CD28+ cells. However, we report here characterization of a distinct subset of CD4+ T cells that are CD28-. Three autoreactive CD4+ human T cell clones that could be activated to produce IL-2 and proliferate by anti-CD3 alone were found to lack expression of CD28. CD28- clones that were activated with anti-CD3 alone were not anergic to restimulation via CD3. The presence of CD28-CD4+ T cells was verified in peripheral blood, and their frequency ranged from 0% to >22% of CD4+ T cells in different individuals. The percentage of CD28-CD4+ T cells in the peripheral blood of 57 individuals was significantly correlated with specific class II MHC alleles. Persons with HLA-DRB1*0401 and DR1 alleles had significantly higher numbers of CD28- T cells, while individuals with HLA-DR2(15) had significantly fewer CD28-CD4+ T cells than the mean. Like the CD28- clones, CD28-CD4+ T cells isolated from peripheral blood proliferated upon CD3 cross-linking in the absence of costimulation. The finding that CD28-CD4+ T cells resist induction of anergy following engagement of the TCR in the absence of conventional costimulation demonstrates one mechanism by which autoreactive T cells can escape processes that censor self-reactivity. The MHC associations observed suggest a relationship with autoimmunity and loss of self-tolerance.     

Evidence for selective in vivo expansion of synovial tissue-infiltrating CD4+ CD45RO+ T lymphocytes on the basis of CDR3 diversity

In this study we have analyzed the TCR V alpha and V beta regions at the DNA level in the CD4+CD45RO+ memory T cell population of synovial tissue infiltrating T lymphocytes of three rheumatoid arthritis (RA) patients and one patient with chronic arthritis. Cell lines of CD4+CD45RO+, CD4+CD45RO-, CD8+CD45RO+ and CD8+CD45RO- T lymphocyte populations were generated following FACS cell sorting of freshly isolated synovial tissue mononuclear cell infiltrates (STMC) and of freshly isolated peripheral blood mononuclear cells (PBMC) of these patients. The phenotypic and molecular analyses have revealed the following. (i) The TCR repertoires of tissue infiltrating T lymphocytes in the various subsets were extensive on the basis of TCR V gene family usage. (ii) Furthermore, each patient displayed individual specific TCR V gene expression patterns in the various STMC and PBMC derived T cell subsets. However, the majority of these arthritis patients manifested increased expression of multiple TCR V gene families in the synovial tissue derived CD4+CD45RO+ T cell population when compared with the peripheral blood derived CD4+CD45RO+ subset. Of these gene families, we found enhanced expression of the TCR V alpha 7 and V beta 11 gene segments in synovial tissue to be shared by all four patients analyzed. (iii) Nucleotide sequence analysis of the CDR3 regions of a number of TCR V regions in the CD4+CD45RO+ T cell subsets has revealed that the CDR3 regions comprised within synovial tissue derived TCR V regions differed from those found in peripheral blood derived TCR V regions. These differences in CDR3 diversity might be the consequence of a specific interaction with particular MHC-peptide complexes expressed at the site of inflammation. (iv) The CDR3 region analysis also showed individual specific amino acid motifs within the N-D-N regions of all analyzed TCR V beta genes derived from PBMC as well as STMC.     

Age-related modifications of the human alphabeta T cell repertoire due to different clonal expansions in the CD4+ and CD8+ subsets

We have studied the effects of a life-long antigen stimulation on the clonal heterogeneity of human peripheral T cell subsets, as defined by their CD45 isoform expression. CD4+ or CD8+ T cells were obtained from healthy donors ranging in age from 20 to 100 years, and sorted into CD45RA+ and CD45RO+ populations. A modified PCR-heteroduplex analysis was then used to directly compare the TCR Vbeta clonal make up of either compartment pair. We find that the CD4+ T cell repertoire remains largely polyclonal throughout life, since CD4+ expanded clones are rare and accumulate predominantly in the CD45RO+ compartment of exceptionally old donors (100 years old). In contrast, the CD8+ T cell subset contains expanded clones which are already detectable in young adults and become very frequent in 70- to 75-year-old donors in both CD45RA+ and CD45RO+ compartments analyzed. Interestingly, some expanded clones are detectable in the CD45RA+ or in both CD45RA+ and CD45RO+ compartments of either CD4+ or CD8+ T cells. These results indicate that the age-dependent accumulation of expanded clones starts earlier and is more pronounced in the CD8+ than in the CD4+ T cell subset, reinforcing the concept that clonal expansion in the two subsets is controlled by substantially different mechanisms. Furthermore, whereas the finding of expanded CD45RO+ T cell clones is explained by antigen-driven proliferation, the detection of expanded clones in the CD45RA+ or in both CD45RA+ and CD45RO+ compartments would support the hypothesis of reversion from the CD45RO+ to the CD45RA+ phenotype after antigen encounter.     

Correlates of apoptosis of CD4+ and CD8+ T cells in tonsillar tissue in HIV type 1 infection

The immunopathogenesis of human immunodeficiency virus type 1 (HIV-1) infection has been associated with increased death by apoptosis of T cell subsets. In the present study, we have examined correlates of apoptosis of CD4+, CD8S+CD28+, and CD8+CD28- T cells in tonsillar lymphoid tissue in persons with HIV-1. Single-cell suspensions of tonsillar lymphocytes were analyzed by flow cytometry to determine the fraction of cells showing typical characteristics of apoptosis as well as the expression of activation markers within the live and the apoptotic cell populations. The proportion of cells carrying infectious provirus was quantified by limiting dilution analysis. Compared with uninfected controls, apoptosis of both CD4+ and CD8+ T cells was enhanced in HIV-1 infection and was higher among CD8+ than among CD4+ T cells. Apoptosis of CD28-cells was more prevalent than apoptosis of CD28+ cells for both CD4+ and CD8+ T cells. Occurrence of apoptosis of CD4+ T cells correlated with provirus levels and proportional expression of the activation marker HLA-DR. Apoptosis of CD8+CD28+ cells correlated with expression of the activation markers CD69 and HLA-DR while apoptosis within CD8+CD28- cells did not correlate with any of the studied parameters. Although apoptosis was much more prevalent among CD8+ than CD4+ T cells, CD8+ T cells still accumulated in tonsillar lymphoid tissue in persons with HIV-1. Our data may be interpreted to suggest that apoptosis of CD4+, CD8+CD28+, and CD8+CD28- cells in tonsillar tissue is regulated by different mechanisms and the results are of importance to our understanding of the immunopathogenesis of HIV-1 infection.     

Low T cell reactivity to combined CD3 plus CD28 stimulation is predictive for progression to AIDS: correlation with decreased CD28 expression

In 219 HIV-1-infected men of the Amsterdam cohort we measured CD4+ T cell numbers and in vitro T cell responses to CD3 MoAbs with or without CD28 costimulation and phytohaemagglutinin (PHA). The value of these markers was estimated for disease progression within 4 years. CD28 expression on T cells has been related to T cell responses. CD28 costimulation considerably enhanced T cell reactivity (approximately 8-10-fold) with lower coefficients of variation compared with reactivity to CD3 MoAb alone (median 5 versus 20). T cell reactivity to CD3 plus CD28 MoAb was decreased during HIV-1 infection and was besides CD4+ T cell numbers the only independent predictor for progression to AIDS. Compared with the group with high CD4+ T cell numbers the relative risk (RR) for the group with intermediate levels was 2.28, with low levels 5.20. In the groups with intermediate and low CD3 plus CD28 responses the RR was 2.04 and 4.16, respectively. The combined RR for both was 4.65 and 21.63. The independence of this marker was confirmed when the group with low CD4+ T cell numbers was subdivided into groups with high, intermediate and low T cell responses. The expansion of CD8+CD28- T cells was already apparent in HIV- homosexual men, but CD8+CD28+ T cells specifically decreased in patients with AIDS. CD28 expression on T cells correlated moderately with T cell responses to CD3 plus CD28 MoAb. T cell reactivity to CD3 MoAb in the presence of CD28 MoAb is a stronger prognostic marker than T cell reactivity to CD3 MoAb alone.     

CD4+ type 1 and CD8+ type 2 T cell subsets in human leishmaniasis have distinct T cell receptor repertoires

The mechanism of protective immunity and immunologic resistance against intracellular pathogens is believed to involve the activation of Ag-specific T cells. The T cells involved in protection/resistance to Leishmania can be studied using localized American cutaneous leishmaniasis (LCL) as a model, because the disease is often self-healing. Our study was undertaken to identify specific T cell populations that had accumulated in LCL lesions on the basis of TCR V beta gene usage. RNA was derived from skin lesions and blood of eight LCL patients, as well as from purified CD4+ and CD8+ subsets from the lesions and blood of three patients. After synthesis of cDNA, V beta gene usage was assessed by polymerase chain reaction. In all eight patients, several V beta gene families were overrepresented in lesions compared to blood. More importantly, the TCR V beta repertoires of both lesional CD4+ and CD8+ subsets were skewed compared to the repertoire of the respective subsets in the blood of the same donor. The overrepresented V beta s in the CD4+ and CD8+ subsets from lesions were in most instances disparate, particularly with the V beta 6 TCR skewed in the lesional CD8+ subset. Not only were the TCR repertoires of the overrepresented V beta in the lesional CD4+ and CD8+ subsets generally distinct, but the cytokine mRNA expressed by these subsets were also discrete. Strikingly, the CD4+ subset was characterized by IFN-gamma mRNA expression and the CD8+ subset by IL-4 and IL-10 mRNA expression. These data indicate that the pathogenesis of human leishmaniasis may be explained by the balance of CD4+ type 1 and CD8+ type 2 T cells, which probably recognize distinct sets of Ag.     

Abnormal thymocyte subset distribution and differential reduction of CD4+ and CD8+ T cell subsets during peripheral maturation in diabetes-prone BioBreeding rats

In this study we quantified CD8+ and CD4+ T cells in T lymphocytopenic BB rats as compared with control rats at given stages along the maturational pathway from immature thymocytes to mature peripheral T cells. Our results show that BB rats exhibit abnormal thymocyte subset distribution. Numbers of mature TCRhigh/CD4-8+ thymocytes, and also their TCRhigh/CD4+8+ precursors were decreased, as were levels of CD8 expression on all thymocyte subsets investigated. By analogy with mouse thymocyte development, these findings suggest a decreased efficiency for positive selection of CD8 precursors in BB rats. Furthermore, as related to the number of available mature TCRhigh single positive thymocytes, numbers of CD4+ and CD8+ T cells most recently migrated from the thymus were severely decreased in BB blood, indicating either reduced thymic output or rapid cell death after migration. Subsequently, in peripheral blood and cervical lymph nodes, a 95% decrease of CD8+ and a 50 to 80% decrease of CD4+ T cells were demonstrated upon maturation from recent thymic migrants to mature peripheral T cells, leaving the BB rat with a severely reduced T cell population, consisting of CD4+ T cells and a minute population of CD8+ T cells. The vast majority of the latter was found to have an immature peripheral phenotype. Possible consequences of our findings for the generation of autoreactive CD8+ T cells are discussed.     

CD45RA(bright)/CD11a(bright) CD8+ T cells: effector T cells

An important aspect of peripheral T cell development is the differentiation from naive into memory cells. To distinguish naive from memory cells, CD45RA and CD11a are commonly used: CD45RA+ or CD11a(dim) T cells are regarded as naive, while CD45RA- or CD11a(bright) T cells are thought to be of memory type. There is, however, a CD8+ T cell subset which is CD45RA+ and at the same time CD11a(bright). It increases with age and in patients with systemic viral infections, though its functional role in the immune response is unknown. In the present study, we give evidence that this subset is related to memory-like T cells as it produces IFN-gamma and tumor necrosis factor-alpha, contains high levels of perforin, and expresses CD95 in the same way as memory-type CD45RA-/CD11a(bright) CD8+ T cells. Since it contains a high percentage of CD28- and CD57+ cells, is increased in size and granularity, and is transiently expressed following in vitro stimulation of naive CD8+ T cells, we speculate that this subset mainly represents recently activated effector T cells that are able to interact with CD80 and CD86 (B7-1 and B7-2 respectively) negative tissue cells.     

Oligoclonal CD4+ CD57+ T-cell expansions contribute to the imbalanced T-cell receptor repertoire of rheumatoid arthritis patients

A peculiar feature of rheumatoid arthritis patients is that they carry clonally expanded CD4+ and CD8+ cells in the peripheral blood. While the distortion of the repertoire of CD8+ cells has been ascribed to the increase of CD8+ CD57+ large granular lymphocytes, often detected in these patients, the mechanism responsible for the clonal expansion of CD4+ cells remains unexplained. Here, we report that CD4+ CD57+ cells, that in healthy individuals represent a small subset of peripheral CD4+ lymphocytes, are significantly expanded in the peripheral blood of a considerable percentage of rheumatoid arthritis patients. Furthermore, the expansion of these lymphocytes appears to correlate with the presence of rheumatoid factor. The molecular analysis of the T-cell receptor variable beta segments expressed by the CD4+ CD57+ cells enriched in rheumatoid arthritis patients showed that they use restricted repertoires, that partially overlap with those of their CD4- CD57+ counterpart. The structural feature of the receptor ligand expressed by these cells revealed that their expansion is most likely mediated by strong antigenic pressures. However, since we also found that CD4+ CD57+ and CD4- CD57+ cells can share the same clonal specificity, it is likely that their selection is not mediated by conventional major histocompatibility complex restricted mechanisms. Thus, while our data demonstrate that CD4+ CD57+ cells play an important role in establishing the imbalance of the CD4+ cell repertoire observed in rheumatoid arthritis patients, they also suggest that these cells have common features with mouse CD4+ CD8- NK1.1+/T cells.     

Increased expression of CD80, CD86 and CD70 on T cells from HIV-infected individuals upon activation in vitro: regulation by CD4+ T cells

T cells express CD28 and CD27 which transduce co-stimulatory signals after interaction with their ligands on antigen-presenting cells (APC). These ligands, CD80, CD86 and CD70, are also expressed to some extent on activated T cells. Here, we show that in human immunodeficiency virus (HIV)-infected individuals, CD28 and CD27 expression is decreased on CD8+ T cells. On the other hand, T cell stimulation in vitro induced high CD80, CD86 and CD70 expression on T cells from HIV-infected individuals. It appeared that an inverted CD4:CD8 T cell ratio could explain this enhanced expression of co-stimulatory ligands. Indeed, high expression levels of CD80, CD86 and CD70 were found on activated CD8+ T cells from HIV- individuals cultured in the absence of CD4+ T cells. Addition of CD4+ T cells prevented this up-regulation. However, in HIV-infected individuals, addition of excess autologous or healthy control CD4+ T cells did not completely counteract up-regulation of co-stimulatory ligand expression on CD8+ T cells. Thus, to some extent, CD8+ T cells in HIV-infected individuals appeared to be refractory to CD4+ T cell-mediated regulation of ligand expression in vitro. Activated T cells from HIV-infected individuals and activated CD8+ T cells from healthy controls were able to act as accessory cells in CD3-induced T cell proliferation, which was dependent on cell-cell contact. Thus, we showed that T cells from HIV-infected individuals express enhanced levels of co-stimulatory ligands upon activation, which provides them with accessory cell properties. Enhanced stimulatory potential of these nonprofessional APC may contribute to persistently high levels of immune activation in HIV infection related to disease progression.     

Increase of circulating CD3+CD4-CD8-CD19+ cells in the latent phase of HIV-1 infection

Having reported that HIV-1-infected T cell lines are rescued as CD4- from cytolysis by human complement factor B, we now show the presence of an in vivo counterpart of such CD4- T cells by demonstrating the circulating CD3+ CD4- CD8- CD29+ cells in the blood of seropositive subjects (n = 91, classified by the immunologic scale scores 0, 1, 2 and 3). The cell population was found to be significantly increased in the early phase of infection in score 0: 195/mm3 (p < 0.005) and in score 1:376/mm3 (p = 0.001). With the infection progressing to score 2, the cells decreased to 220/mm3 (p < 0.001) and finally to the same range: 101/mm3, as that of uninfected subjects. Further elucidation of the mechanism of the appearance and disappearance of that population in vivo could help to elucidate protective immunologic processes.     

CDw60: a marker for human CD8+ T helper cells

The existence of helper cells among the CD8+ T cell subset has been recognized for a long time. However, the phenotype of these cells has remained elusive. In this study, we provide evidence that the expression of the CDw60 antigen on human CD8+ T cell allows one to distinguish between CD8+ T helper cells and CD8+ T cells with cytotoxic and suppressor capacity. CDw60 monoclonal antibodies (mAb) recognize the 9-O-acetylated disialosyl group on ganglioside GD3 expressed on 20-40% of CD8+ cells. By use of the direct and indirect mAb-rosetting technique, we were able to isolate the CDw60+CD8+ and CDw60-CD8+ cells at high purity. The alloantigen-specific cytotoxic activity of CD8+ cells resided entirely in the CDw60- population. Helper and suppressor capacity of both CD8 subsets was assayed by the pokeweed mitogen-induced differentiation of B cells into immunoglobulin-secreting cells. These studies clearly indicate that the CDw60+CD8+ subset provided substantial help to B lymphocytes, whereas the CD8+ cells with the CDw60- phenotype were suppressing B cell differentiation. Both subsets produced similar amounts of interleukin 2 (IL-2) after stimulation with phytohemagglutinin. Activation with phorbol myristate acetate in combination with Ca-ionophore induced IL-4 secretion in both populations, but preferentially in the CDw60+ subset, whereas the vast majority of interferon gamma was produced by the CDw60-CD8+ cells. When used in combination with other markers, CDw60 may prove to be useful in defining CD8+ subsets with reciprocal functional activities.     

Maturation of human neonatal CD4+ and CD8+ T lymphocytes into Th1/Th2 effectors

The increased susceptibility of neonates to infections has been ascribed to the immaturity of their immune system. More particularly, T cell-dependent responses were shown to be biased towards a Th2 phenotype. Our studies on the in vitro maturation of umbilical cord blood T cells suggest that the Th2 bias of neonatal response cannot be simply ascribed to intrinsic properties of neonatal T cells. Phenotypically, neonatal CD4+ T cells are more immature than their adult CD45RO-/RA+ naive counterparts and they contain a subset (10-20%) of CD45RO-/RA+ CD31- cells which is very low in adults and displays some unique functional features. The activation and maturation of neonatal CD4+ T cells is particularly dependent upon the strength of CD28-mediated cosignal which dictates not only the cytokine profile released upon primary activation but also the response to IL-12. Activation of adult as well as neonatal CD4+ T cells in the context of low CD28 costimulation yields to the production of low levels of only one cytokine, i.e. IL-2. In contrast, strong CD28 costimulation supports the production of high levels of type 1 (IL-2, IFN gamma and TNF beta) and low levels of type 2 (IL-4 and IL-13) cytokines by neonatal T cells. The low levels of naive T cell-derived IL-4 are sufficient to support their development into high IL-4/IL-5 producers by an autocrine pathway. The ability of IL-12 to prime neonatal CD4+ T cells for increased production of IL-4 (in addition to IFN gamma) is observed only when CD28 cosignal is minimal. Under optimal activation conditions (i.e. with anti-CD3/B7.1 or allogenic dendritic cells) the response and the maturation of neonatal and adult naive T cells are similar. Thus the Th2 bias of neonatal immune response cannot be simply ascribed to obvious intrinsic T cell defect but rather to particular conditions of Ag presentation at priming. Unlike CD4+ T cells, neonatal CD8+ T cells strictly require exogenous IL-4 to develop into IL-4/IL-5 producers. Most importantly, anti-CD3/B7-activated neonatal CD8 T cells coexpress CD4 as well as CCR5 and CXCR4 and are susceptible to HIV-1 infection in vitro.     

Loss of CD4+ T cells in human immunodeficiency virus type 1-infected chimpanzees is associated with increased lymphocyte apoptosis

Supportive evidence that apoptosis contributes to loss of CD4+ lymphocytes in human immunodeficiency virus type 1 (HIV-1)-infected humans comes from an apparent lack of abnormal apoptosis in apathogenic lentivirus infections of nonhuman primates, including HIV-1 infection of chimpanzees. Two female chimpanzees were inoculated, one cervically and the other intravenously, with HIV-1 derived from the LAI/LAV-1b strain, which was isolated from a chimpanzee infected with the virus for 8 years. Within 6 weeks of infection, both recipient chimpanzees developed a progressive loss of CD4+ T cells which correlated with persistently high viral burdens and increased levels of CD4+ T-cell apoptosis both in vitro and in vivo. Lymph nodes from both animals also revealed evidence of immune hyperactivation. Intermediate levels of T-cell apoptosis in both peripheral blood and lymph nodes were seen in a third chimpanzee that had been infected with the LAI/LAV-1b strain for 9 years; this animal has maintained depressed CD4/CD8 T-cell ratios for the last 3 years. Similar analyses of cells from 4 uninfected animals and 10 other HIV-1-infected chimpanzees without loss of CD4+ cells revealed no difference in levels of apoptosis in these two control groups. These results demonstrate a correlation between immune hyperactivation, T-cell apoptosis, and chronic loss of CD4+ T cells in HIV-1-infected chimpanzees, providing additional evidence that apoptosis is an important factor in T-cell loss in AIDS. Furthermore, the results show that some HIV-1 strains are pathogenic for chimpanzees and that this species is not inherently resistant to HIV-1-induced disease.     

Differential susceptibility of naive and memory CD4+ T cells to the cytopathic effects of infection with human immunodeficiency virus type 1 strain LAI

CD4+ T lymphocytes of individuals infected with human immunodeficiency virus type 1 (HIV-1) exhibit a qualitative defect in their ability to mount memory responses to previously encountered antigens although their responses to mitogens remain normal. T cells responsible for memory responses can be distinguished from naive T cells based on differential expression of isoforms of the tyrosine phosphatase CD45. It has been suggested that memory CD4+ T cells from infected individuals have a greater virus burden than naive CD4+ T cells and that this accounts for the loss of recall responses in infected individuals. However, it has been unclear whether naive and memory T cells are equally susceptible to infection and to the cytopathic effects of the virus. We therefore infected highly purified resting naive and memory CD4+ T cells from HIV-1-seronegative individuals with HIV-1(LAI). Infected cells were then stimulated with phytohemagglutinin to render them permissive for viral replication. Cell viability and growth rate were monitored for 8 to 10 days as indicators of cytopathic effects induced by HIV-1(LAI). Our results indicated that naive and memory CD4+ T cells display marked differences in susceptibility to the cytopathic effects induced by HIV-1(LAI), infection. The cytopathic effects induced by HIV-1(LAI) were much more severe in memory CD4+ T cells than in naive CD4+ T cells. Differential cytopathic effects in naive and memory T cells were not due to differences in virus entry into and replication in these cell populations. Rather, memory cells were more susceptible to cytopathic effects. Pronounced cytopathic effects in memory cells were clearly detectable at 7 day postinfection. Cell death occurred at the single-cell level and was not accompanied by syncytium formation. The growth rate of infected memory CD4+ T cells was also severely compromised compared to that of naive CD4+ T cells, whereas the growth rates of both uninfected naive and memory CD4+ T cells were approximately the same. At least a portion of the dying cells exhibited biochemical changes characteristic of apoptosis. These results suggest that the selective functional defects present in the memory CD4+ T-cell subset of HIV-1-infected individuals may in part be the result of the greater susceptibility of memory T cells to cytopathic effects induced by HIV-1.