Model for studying virus attachment. II. Binding of biotinylated human T cell leukemia virus type I to human blood mononuclear cells potential targets for human T cell leukemia virus type I infection.

Purified human T cell leukemia virus type I (HTLV-I) was biotinylated and used to study its attachment to human PBMC. The use of biotinylated HTLV-I (biot-HTLV-I) in conjunction with mouse mAb specific for selected cell-surface molecules and flow cytometric analysis allowed us to positively identify virus-binding cells among a heterogeneous blood mononuclear cell population. Biot-HTLV-I efficiently bound not only to T cells, but also to B cells and monocytes. Preincubation of monocytes with excess of unlabeled HTLV-I significantly reduced the attachment of biot-HTLV-I. HTLV-I not only bound to, but also infected, B cells, as suggested by: i) in situ hybridization of a 35S-labeled full length HTLV-I DNA probe with EBV-transformed B cells, previously cocultured with HTLV-I-producing (G11MJ) T cells, and ii) hybridization of the same nick-translated 32P-labeled DNA probe with blotted DNA from similar HTLV-I-infected EBV-transformed B cells. HTLV-I infection did not affect the ability of B cells to secrete IgG. These findings suggest that HTLV-I cannot only infect cells of the T lineage, but can also infect B cells.     

Human T-cell leukemia virus type I infection of CD4+ or CD8+ cytotoxic T-cell clones results in immortalization with retention of antigen specificity.

The human T-cell leukemia virus type I (HTLV-I) is capable of chronically infecting various types of T cells and nonlymphoid cells. The effects of chronic infection on the specific functional activities and growth requirements of mature cytotoxic T lymphocytes (CTL) have remained poorly defined. We have, therefore, investigated the results of HTLV-I infection of both CD4+ and CD8+ human CTL clones. HTLV-I infection resulted in the establishment of functional CTL lines which propagated indefinitely in culture many months longer than the uninfected parental clone. The infected cells became independent of the need for antigen (target cell) stimulation as a requirement for proliferation and growth. Like their uninfected counterparts, however, these HTLV-I-infected clones remained strictly dependent on conditioned medium from mitogen-stimulated T lymphocytes for their growth. This growth factor requirement was not fulfilled by recombinant interleukin-2 alone. Furthermore, the infected lines remained functionally identical to their uninfected parental CTL clones in their ability to specifically recognize and lyse the appropriate target cells. Our findings indicate that the major effects of HTLV-I infection on mature CTL consist of (i) the capacity for proliferation in the absence of antigen stimulation and (ii) a prolonged or immortal survival in vitro, but they also indicate that the fine specificity and cytolytic capacity of these cells remain unaffected.     

In vitro infection of natural killer cells with different human immunodeficiency virus type 1 isolates.

Natural killer (NK) cells are a discrete subset of leukocytes, distinct from T and B lymphocytes. NK cells mediate spontaneous non-MHC-restricted killing of a wide variety of target cells without prior sensitization and appear to be involved in initial protection against certain viral infections. Depressed NK cell-mediated cytotoxicity, one of the many immunological defects observed in AIDS patients, may contribute to secondary virus infections. Here we report that clonal and purified polyclonal populations of NK cells, which expressed neither surface CD4 nor CD4 mRNA, were susceptible to infection with various isolates of human immunodeficiency virus type 1 (HIV-1). Viral replication was demonstrated by detection of p24 antigen intracellularly and in culture supernatants, by the presence of HIV DNA within infected cells, and by the ability of supernatants derived from HIV-infected NK cells to infect peripheral blood mononuclear cells or CD4+ cell lines. Infection of NK cells was not blocked by anti-CD4 or anti-Fc gamma RIII monoclonal antibodies. NK cells from HIV-infected and uninfected cultures were similar in their ability to lyse three different target cells. Considerable numbers of cells died in HIV-infected NK cell cultures. These results suggest that loss of NK cells in AIDS patients is a direct effect of HIV infection but that reduced NK cell function involves another mechanism. The possibility that NK cells serve as a potential reservoir for HIV-1 must be considered.     

Primary HIV-1 infection of human CD4+ T cells passaged into SCID mice leads to selection of chronically infected cells through a massive fas-mediated autocrine suicide of uninfected cells

We have recently shown that a human CD4+ T cell line (CEM-SS) acquires the permissiveness to M-tropic strains and primary isolates of HIV-1 after transplantation into SCID mice. This permissiveness was associated with the acquisition of a memory (CD45RO+) phenotype as well as of a functional CCR5 coreceptor. In this study, we have used this model for invest-igating in vivo the relationships between HIV-1 infection, apoptosis and T cell differentiation. When an in vivo HIV-1 infection was performed, the CEM cell tumors grew to a lower extent than the uninfected controls. CEM cells explanted from uninfected SCID mice (ex vivo CEM) underwent a significant level of spontaneous apoptosis and proved to be CD45RO+, Fas+ and Fas-L+, while Bcl-2 expression was significantly reduced as compared to the parental cells. Acute HIV-1 infection markedly increased apoptosis of uninfected ex vivo CEM cells, through a Fas/Fas-L-mediated autocrine suicide/fratricide, while parental cells did not undergo apoptosis following viral infection. The susceptibility to apoptosis of ex vivo CEM cells infected with the NSI strain of HIV-1, was progressively lost during culture, in parallel with the loss of Fas-L and marked changes in the Bcl-2 cellular distribution. On the whole, these results are strongly reminiscent of a series of events possibly occurring during HIV-1 infection. After an initial depletion of bystander CD4+ memory T cells during acute infection, latently or chronically infected CD4+ T lymphocytes are progressively selected and are protected against spontaneous apoptosis through the development of an efficient survival program. Studies with human cells passaged into SCID mice may offer new opportunities for an in vivo investigation of the mechanisms involved in HIV-1 infection and CD4+ T cell depletion.     

Coexistence of fusion receptors for human T-cell leukemia virus type-I (HTLV-I) and human immunodeficiency virus type-1 (HIV-1) on MOLT-4 cells.

Human immunodeficiency virus type-1 (HIV-1) and human T-cell leukemia virus type-I (HTLV-I) have a similar tropism for target cell types, especially for CD4+ T cells. In this study, we provide evidence that receptors of these two viruses exist independently on the target cell. We established an HTLV-I-producing CD8+ T cell line (ILT-8M2) with a remarkable cell fusion capacity. When cocultured with MOLT-4 cells, ILT-8M2 cells induced giant syncytia more efficiently than any other tested HTLV-I-producer cell lines. In contrast to other HTLV-I-producers, ILT-8M2 cells were minimally susceptible to cytopathic effects of HIV-1 due to very low expression of CD4, although they were able to be persistently infected by HIV-1. The indicator MOLT-4 cells are known to respond well to HIV-1-induced cell fusion, but they lose this ability if they become persistently infected with HIV-1 because of the reduction of CD4 receptor expression. ILT-8M2 was, however, still capable of inducing syncytia with the MOLT-4 cells persistently infected by HIV-1 (MOLT-4/IIIB). This syncytium formation was dependent on the HTLV-I-envelope, as it was inhibited by HTLV-I-positive human sera or a monoclonal antibody to HTLV-I gp46 but not by monoclonal antibodies to HIV-1 gp120 or CD4. Moreover, ILT-8M2 cells persistently infected by HIV-1 (ILT-8M2/IIIB) induced both HTLV-I- and HIV-1-mediated syncytia with uninfected MOLT-4 cells. These results suggest that HTLV-I induces cell fusion utilizing receptors on the target cells independent of HIV-1-receptors.     

The mitogenic activity of human T-cell leukemia virus type I is T-cell associated and requires the CD2/LFA-3 activation pathway.

The presence of a high number of activated T cells in the bloodstream and spontaneous proliferation of peripheral blood mononuclear cells in vitro are striking characteristics of human T-cell leukemia virus type I (HTLV-I) infection. The HTLV-I regulatory protein Tax and the envelope protein gp46 have been implicated in mediating the activation process. In this study, HTLV-I-producing cell lines and purified virus from the cell lines were examined for the ability to activate peripheral blood lymphocytes (PBLs) and Jurkat cells. Antisera and monoclonal antibodies against several cellular adhesion proteins involved in T-cell activation and against viral proteins were used to identify which molecules may be participating in the activation process. First, neither virus from a T-cell line, MT2, nor virus produced from the human osteosarcoma cell line HOS/PL was able to induce PBLs to proliferate. In contrast, both fixed and irradiated HTLV-I-producing T-cell lines induced proliferation of PBLs; HOS/PL cells did not activate PBLs. Second, HTLV-I-positive T-cell lines were capable of activating interleukin-2 mRNA expression in Jurkat cells. Induction of interleukin-2 expression was inhibited by anti-CD2 and anti-lymphocyte function-associated antigen 3 (LFA-3) monoclonal antibodies but not anti-human leukocyte antigen-DR, anti-CD4, anti-LFA-1, or anti-intercellular adhesion molecule 1. Similar results were obtained with PBLs as the responder cells. Furthermore, monoclonal antibodies and antisera against various regions of the HTLV-I envelope proteins gp46 and gp21 as well as p40tax did not block activation. These data indicate that HTLV-I viral particles are not intrinsically mitogenic and that infection of target T cells is not necessary for activation. Instead, the mitogenic activity is restricted to virus-producing T cells, requires cell-to-cell contact, and may be mediated through the LFA-3/CD2 activation pathway.     

Human T cell leukemia virus type I and neurologic disease: events in bone marrow, peripheral blood, and central nervous system during normal immune surveillance and neuroinflammation.

Human T cell lymphotropic/leukemia virus type I (HTLV-I) has been identified as the causative agent of both adult T cell leukemia (ATL) and HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Although the exact sequence of events that occur during the early stages of infection are not known in detail, the initial route of infection may predetermine, along with host, environmental, and viral factors, the subset of target cells and/or the primary immune response encountered by HTLV-I, and whether an HTLV-I-infected individual will remain asymptomatic, develop ATL, or progress to the neuroinflammatory disease, HAM/TSP. Although a large number of studies have indicated that CD4(+) T cells represent an important target for HTLV-I infection in the peripheral blood (PB), additional evidence has accumulated over the past several years demonstrating that HTLV-I can infect several additional cellular compartments in vivo, including CD8(+) T lymphocytes, PB monocytes, dendritic cells, B lymphocytes, and resident central nervous system (CNS) astrocytes. More importantly, extensive latent viral infection of the bone marrow, including cells likely to be hematopoietic progenitor cells, has been observed in individuals with HAM/TSP as well as some asymptomatic carriers, but to a much lesser extent in individuals with ATL. Furthermore, HTLV-I(+) CD34(+) hematopoietic progenitor cells can maintain the intact proviral genome and initiate viral gene expression during the differentiation process. Introduction of HTLV-I-infected bone marrow progenitor cells into the PB, followed by genomic activation and low level viral gene expression may lead to an increase in proviral DNA load in the PB, resulting in a progressive state of immune dysregulation including the generation of a detrimental cytotoxic Tax-specific CD8(+) T cell population, anti-HTLV-I antibodies, and neurotoxic cytokines involved in disruption of myelin-producing cells and neuronal degradation characteristic of HAM/TSP.     

The relationship of CD16 (Leu-11) and Leu-19 (NKH-1) antigen expression on human peripheral blood NK cells and cytotoxic T lymphocytes

We examined the antigenic and functional characteristics of human peripheral blood lymphocytes that differentially express the CD16 (Leu-11) and Leu-19 (NKH-1) antigens. Leu-19 is a approximately 220,000 daltons protein expressed on approximately 15% of freshly isolated peripheral blood lymphocytes. Within the Leu-19+ subset, three distinct populations were identified: CD3-,CD16+,Leu-19+ cells; CD3+,CD16-,Leu-19+ cells; and CD3-,CD16-,Leu-19bright+ cells. Both the CD3+,CD16-,Leu-19+ and CD3-,CD16+,Leu-19+ populations mediated non-major histocompatibility complex (MHC)-restricted cytotoxicity against the NK-sensitive tumor cell K562 and were large granular lymphocytes. CD3-,CD16+,Leu-19+ NK cells were the most abundant (comprising approximately 10% of peripheral blood lymphocytes) and the most efficient cytotoxic effectors. The finding that CD3+,Leu 19+ lymphocytes mediated cytotoxicity against K562 unequivocally demonstrates that a unique subset of non-MHC-restricted cytotoxic CD3+ T lymphocytes are present in the peripheral blood of unprimed, normal individuals. However, CD3+,CD16-,Leu-19+ cells comprised less than 5% of peripheral blood lymphocytes, and the cytotoxic activity of this subset was significantly less than CD3-,CD16+,Leu-19+ NK cells. Most CD3+,Leu-19+ T cells co-expressed the CD2, CD8, and CD5 differentiation antigens. The antigenic and functional phenotype of peripheral blood CD3+,Leu-19+ cytotoxic T lymphocytes corresponds to the interleukin 2-dependent CD3+ cell lines that mediate non-MHC-restricted cytotoxicity against NK-sensitive tumor cell targets. A small population of Leu-19bright+ lymphocytes lacking both CD3 and CD16 was also observed. This population (comprising less than 2% of peripheral blood lymphocytes) contained both large agranular lymphocytes and large granular lymphocytes. CD3-,CD16-,Leu-19bright+ lymphocytes also mediate non-MHC-restricted cytotoxicity. The relationship of these CD3-CD16-,Leu-19bright+ lymphocytes to CD3+ T cells or CD16+ NK cells is unknown.     

Identification of human immunodeficiency virus envelope gene sequences influencing viral entry into CD4-positive HeLa cells, T-leukemia cells, and macrophages.

Infectious recombinant viruses were constructed from three molecularly cloned human immunodeficiency virus (HIV) strains varying in cell tropism. All recombinants showed a high infectivity titer on phytohemagglutinin-stimulated normal T lymphocytes. However, a 120-bp region of the envelope gene including the area of the V3 hypervariable loop was found to influence infectivity titer on both clone 1022 CD4-positive HeLa cells and CD4-positive CEM leukemia cells. Infectivity for macrophages was more complex. All viruses replicated in macrophages to a low level, but viral sequences both inside and outside the V3 loop region influenced the efficiency of replication. Two experiments showed that the mechanism of restriction of infection of 1022 cells by HIV strain JR-CSF was related to lack of virus entry. First, productive virus infection occurred after transfection of 1022 cells with viral plasmid DNA. Second, the nonpermissive HIV strain JR-CSF could infect 1022 cells when pseudotyped with the envelope of other retroviruses, including human T-cell leukemia virus type I (HTLV-I), HTLV-II, and amphotropic murine leukemia virus. These results demonstrate the possibility that unexpected cell types might be infected with HIV in human patients coinfected with HIV and HTLV-I or HTLV-II.     

Emergence of CD8+ T cells expressing NK cell receptors in influenza A virus-infected mice

Both innate and adaptive immune responses play an important role in the recovery of the host from viral infections. In the present report, a subset of cells coexpressing CD8 and NKR-P1C (NK1.1) was found in the lungs of mice infected with influenza A virus. These cells were detected at low numbers in the lungs of uninfected mice, but represented up to 10% of the total CD8(+) T cell population at day 10 postinfection. Almost all of the CD8(+)NK1.1(+) cells were CD8alphabeta(+)CD3(+)TCRalphabeta(+) and a proportion of these cells also expressed the NK cell-associated Ly49 receptors. Interestingly, up to 30% of these cells were virus-specific T cells as determined by MHC class I tetramer staining and by intracellular staining of IFN-gamma after viral peptide stimulation. Moreover, these cells were distinct from conventional NKT cells as they were also found at increased numbers in influenza-infected CD1(-/-) mice. These results demonstrate that a significant proportion of CD8(+) T cells acquire NK1.1 and other NK cell-associated molecules, and suggests that these receptors may possibly regulate CD8(+) T cell effector functions during viral infection.     

RAPID SYNCYTIUM FORMATION BETWEEN HUMAN T-CELL LEUKAEMIA VIRUS TYPE-I (HTLV-I)-INFECTED T-CELLS AND HUMAN NERVOUS SYSTEM CELLS: A POSSIBLE IMPLICATION FOR TROPICAL SPASTIC PARAPARESIS/HTLV-I ASSOCIATED MYELOPATHY

Tropical spastic paraparesis/HTLV-I associated myelopathy (TSP/HAM), is characterized by infiltration of human T cell leukaemia virus type-I (HTLV-I)-infected T-cells, anti-HTLV-I cytotoxic T cells and macrophages into the patients’ cerebrospinal fluid and by intrathecally formed anti-HTLV-I antibodies. This implies that the disease involves a breakdown of the blood—brain barrier. Since astrocytes play a central role in establishing this barrier, the authors investigated the hypothesis that the HTLV-I infected T cells disrupt this barrier by damaging the astrocytes. The present study revealed the HTLV-I-producing T cells conferred a severe cytopatic effect upon monolayers of astrocytoma cell line in co-cultures. Following co-cultivation, HTLV-I DNA and proteins appeared in the monolayer cells, but after reaching a peak their level gradually declined. This appearance of the viral components was proved to result from a fusion of the astrocytic cells with the virus-producing T cells, whereas their subsequent decline reflected the destruction of the resulting syncytia. This fusion could be specifically blocked by anti HTLV-I Env antibodies, indicating that it was mediated by the viral Env proteins expressed on the surface of the virus-producing cells. Similar fusion was observed between the HTLV-I-producing cells and certain other human nervous system cell lines. If such fusion of HTLV-I-infected T cells occurs also with astrocytes and other nervous system cells in TSP/HAM patients, it may account, at least partially, for the blood—brain barrier breakdown and some of the neural lesions in this syndrome.     

Human T cell leukemia/lymphoma virus type I infection of a CD4+ proliferative/cytotoxic T cell clone progresses in at least two distinct phases based on changes in function and phenotype of the infected cells

The effect of human T cell leukemia/lymphoma virus type I (HTLV-I) infection on the function and the phenotype of a human proliferating/cytotoxic T cell clone, specific for tetanus toxin, was investigated. During the period after infection, two distinct phases were observed, based on growth properties, phenotype, and functional activity of the infected cells. Phase I HTLV-I infected cells (0 to about 150 days after infection) proliferated in an IL-2-dependent way, but without the requirement for repetitive antigenic stimulation. No differences in expression of the CD2, CD3, CD4, Tp103, and CD28 Ag between these cells and the parental cells could be demonstrated, with the exception of the expression of IL-R p55 and HLA-DR Ag, which were constitutively expressed on the phase I cells. The phase I HTLV-I-infected cells, as well as the parental 827 cells reacted with a mAb specific for an epitope on the variable part of the TCR beta-chain, indicating that the TCR was not altered after HTLV-I infection. Like the parental clone, the phase I cells proliferated in response to tetanus toxin, but the tetanus toxin-specific response of the phase I cells did not require the presence of APC. Results of experiments, in which the levels of intracellular Ca2+ were measured, indicated that HTLV-I cells can acquire the capability to process Ag and present that to themselves. Phase I HTLV-I-infected T cells had lost their cytotoxic activity which was likely to be due to an effect on the lytic machinery rather than on Ag recognition by the TCR, inasmuch as it was found that phase I HTLV-I-infected T cells did no longer contain N-alpha-benzyloxy-L-lysine thiobenzylester-serine esterase activity. Furthermore, it was found that phase I HTLV-I-infected T cells had a diminished capacity to form conjugates with target cells. From a period of about 200 days after HTLV-I infection, phase II cells emerged that proliferated strongly in the absence of IL-2 and that had lost all functional activity. These cells did not express the CD3/T cell receptor complex on their surface. Phase I as well as phase II HTLV-I-infected cells were targets for CTL raised in the autologous donor.     

Monocyte-driven activation-induced apoptotic cell death of human T-lymphotropic virus type I-infected T cells.

We attempted apoptotic cell death induction of T cells infected with human T lymphotropic virus type I (HTLV-I) which induces HTLV-I-associated myelopathy/tropical spastic paraparesis and adult T cell leukemia. T cells acutely infected and expressing HTLV-Igag Ags were killed by cross-linking their TCR with anti-CD3 mAb. Cells in apoptotic process were found by staining with annexin V. The apoptosis was not affected by costimulation through CD28 molecules and was resistant to ligation of Fas molecules. Whereas the virus-infected T cells expressed higher levels of HLA-DR, CD25, CD80, and CD86 Ags than apoptosis-resistant PHA-blasts, the T cell apoptosis was enhanced by addition of exogenous IL-2. Furthermore, in this apoptosis, monocytes played an important role because T cells infected in the absence of monocytes were resistant to the death signals. The apoptosis-sensitive T cells responded to TCR signaling more strongly by proliferating than those apoptosis-resistant cells. Monocytes weakly affected the expression levels of viral Ags on T cells. However, HTLV-I-infected monocytes primed T cells to die by subsequent TCR signaling. T cells primed with the monocytes, subsequently infected in the absence of monocytes, were killed by TCR signaling. These observations suggest that primed and infected T cells could be killed by activation-induced cell death.     

Turnover rates of B cells,T cells,and NK cells in simian immunodeficiency virus-infected and uninfected rhesus macaques

We determined average cellular turnover rates by fitting mathematical models to 5-bromo-2'-deoxyuridine measurements in SIV-infected and uninfected rhesus macaques. The daily turnover rates of CD4(+) T cells, CD4(-) T cells, CD20(+) B cells, and CD16(+) NK cells in normal uninfected rhesus macaques were 1, 1, 2, and 2%, respectively. Daily turnover rates of CD45RA(-) memory T cells were 1%, and those of CD45RA(+) naive T cells were 0.5% for CD4(+) T cells and approximately 1% for CD4(-)CD45RA(+) T cells. In SIV-infected monkeys with high viral loads, the turnover rates of T cells were increased approximately 2-fold, and that of memory T cells approximately 3-fold. The turnover of CD4(+)CD45RA(+) naive T cells was increased 2-fold, whereas that of CD4(-)CD45RA(+) naive T cells was marginally increased. B cells and NK cells also had increased turnover in SIV-infected macaques, averaging 3 and 2.5% per day, respectively. For all cell types studied here the daily turnover rate increased with the decrease of the CD4 count that accompanied SIV infection. As a consequence, the turnover rates of CD4(+) T cells, CD4(-) T cells, B cells, and NK cells within each monkey are strongly correlated. This suggests that the cellular turnover of different lymphocyte populations is governed by a similar process which one could summarize as "generalized immune activation." Because the viral load and the CD4 T cell count are negatively correlated we cannot determine which of the two plays the most important role in this generalized immune activation.     

Lysis of human cytomegalovirus infected fibroblasts by natural killer cells: demonstration of an interferon-independent component requiring expression of early viral proteins and characterization of effector cells

We investigated the susceptibility of cells infected with human cytomegalovirus (HCMV) to lysis by human natural killer (NK) cells, examining in particular its relationship to sequential viral protein expression, interferon (IFN), and the nature of the effector cells. HCMV-infected fibroblasts were lysed by peripheral blood mononuclear cells from normal seronegative individuals. The effector cells were large granular lymphocytes of Leu-7+, Leu-11+, and to a lesser extent Leu-7- phenotype. Depletion studies suggested they were the same population of NK cells that lyse uninfected fibroblasts, but a subset of NK cells that lyse K562 cells. HCMV-infected cells treated with phosphonoformate and cells infected for 16 hr that only express the nonstructural HCMV immediate early and early proteins and not the late (structural) proteins were susceptible to lysis by IFN-pretreated effector cells, whereas cells expressing immediate early antigens alone were not. This enhanced susceptibility to lysis was associated with increased effector:target binding in target cell binding assays, and was competitively inhibited by uninfected fibroblasts in cold target competition assays. It was independent of IFN release from the infected target cells or effector cells. These results suggest that the increased susceptibility to lysis by NK cells produced by a human herpes virus HCMV i) is manifest when early viral proteins are expressed, ii) is related to enhanced expression of a target structure likely to be present on uninfected fibroblasts, and iii) has a major component that is independent of IFN.     

HIV-infected humans, but not chimpanzees, have circulating cytotoxic T lymphocytes that lyse uninfected CD4+ cells

It has been suggested that autoimmune phenomena contribute to the depletion of CD4+ T cells and the development of AIDS in HIV-1 infected humans based, in part, on observations that some HIV-1-infected humans have autoantibodies reactive with Ag expressed on uninfected CD4+ cells. In this study, 11 of 14 asymptomatic HIV-1-infected homosexuals and hemophiliacs, but none of 17 uninfected homosexuals or heterosexuals, were found to have cytotoxic lymphocytes in blood that can lyse uninfected CD4+ T cells from humans and chimpanzees but not human B lymphoblastoid cells or mouse T cells. The cytotoxic PBL were concluded to be CTL rather than NK cells, with the phenotype being CD3+, TCR-1 alpha beta+, CD8+, CD4-, CD16- based on findings that PBL-mediated lysis of uninfected CD4+ cells was 1) blocked by a mAb to CD3, which inhibits CTL but not NK activity; 2) diminished by treatment of PBL with a mAb to CD8 and C, but not by treatment with mAb to CD4 or CD16 and C; and 3) blocked by mAb WT31 directed against the TCR-1 alpha beta. In contrast, PBL from HIV-1-infected chimpanzees, which to date have not developed AIDS, lacked detectable CTL lytic for uninfected CD4+ cells.