Inhibition of proliferation without affecting the generation of cytotoxicity in the human mixed lymphocyte reaction

Phosphoramide mustard (PM) is considered to be the major tumoricidal metabolite of cyclophosphamide in vivo. The effects of this metabolite in vitro on several immune functions of human lymphocytes have been investigated. Very low concentrations (10(-7) to 10(-9) M) of PM added to lymphocyte cultures inhibited proliferation of the lymphocytes in response to mitogens and alloantigens. At these concentrations, inhibition of proliferation appeared to be due to a direct action of PM on the proliferative cells. Thus, concanavalin A-stimulated lymphocytes still acquired IL-2 receptors (Tac antigen) normally in the presence of PM (10(-6) to 10(-9) M). Only exceedingly high concentrations of PM (10(-5) M or greater) prevented the acquisition of Tac antigen. Similarly, the inhibition of proliferation was probably not related to endogenous IL-2 levels: addition of exogenous IL-2 to PM-containing cultures did not result in any restoration of proliferation. Further evidence that PM directly affected proliferative cells was that low concentrations of PM inhibited the proliferation of T cells continuously growing in IL-2. The exposure time to PM necessary for inhibition was essentially identical to those for lymphoproliferative responses to mitogens and alloantigens. Paradoxically, however, the generation of cytotoxic lymphocytes in mixed lymphocyte reactions (MLRs) and mixed lymphocyte tumor cell cultures (MLTCs) was very resistant to PM. In parallel MLRs and MLTCs the cytotoxic responses were resistant to approximately 1000-fold more PM than were the proliferative responses. Only at 10(-5) M PM were these inhibited. These data suggest that clonal expansion of cytotoxic lymphocytes or their precursors by proliferation is not an absolute requirement for the generation of cytolytic activity.     

Effect of cyclosporin A on human lymphocyte responses in vitro. III. CsA inhibits the production of T lymphocyte growth factors in secondary mixed lymphocyte responses but does not inhibit the response of primed lymphocytes to TCGF

The mechanism whereby Cyclosporin A (CsA) inhibits secondary mixed lymphocyte responses was assessed. CsA added to secondary MLR cultures inhibited proliferation and induction of cytolytic lymphocyte activity. This inhibition was found to be associated with the inhibition of T lymphocyte stimulating growth factor(s) (TCGF) production in the supernatants of secondary MLR cultures. As little as 1.0 micrograms/ml of CsA added to secondary MLR cultures resulted in no measurable TCGF activity. In contrast, moderate doses of CsA (1.0, 2.5 micrograms/ml), which completely inhibited the secondary MLR response to alloantigen, did not inhibit the proliferative and CML response of alloantigen-primed lymphocytes to these stimulating growth factors. Even at high doses of CsA (20 micrograms/ml), substantial levels of proliferation (50% of control response) and CML induction (60% of control response) were observed when the primed cells were exposed to secondary MLR supernatants containing TCGF activity. It was concluded that inhibition of secondary mixed lymphocyte responses by CsA may be due in part to the inhibition of TCGF production rather than the inhibition of the effect of TCGF on mature cytotoxic T lymphocytes.     

Prostaglandin-mediated regulation of the mixed lymphocyte culture and generation of cytotoxic cells

We examined the role of prostaglandins or prostaglandin-producing cells in the regulation of proliferation and generation of specific cytotoxicity in one-way mixed lymphocyte cultures of mouse spleen cells. Cultures treated with indomethacin or other prostaglandin synthesis inhibitors resulted in enhanced proliferation and cytotoxicity. The level of prostaglandins produced in vitro, as measured by RIA, was 10^?8M and was found to be completely blocked by indomethacin. Adding back 10^?8M PG restored baseline (control) proliferative responses. Kinetics of the enhanced MLC response were unchanged from controls as were the specificities of the cytotoxic cells. Cells from indomethacin-treated cultures were more efficient at killing targets than those from control cultures. Prostaglandins appear to have a preferential effect on the induction of cytotoxic cells.     

Contrasting effects of T cell growth factors on T cell responses to melanoma in vitro

 Determinants of T cell responses to tumor cells remain largely unknown. In the present study we have used long-term cultures of human melanoma cells and autologous peripheral blood lymphocytes to examine the influence of cytokines with T cell growth activity on the phenotype and cytotoxic and proliferative response of T cells to melanoma. It was found that addition of interleukin-4 (IL-4) inhibited the response of CD8⁺ T cells and promoted the response of the CD4 subset. IL-2 or IL-7 was effective in increasing melanoma-specific cytotoxic T lymphocyte (CTL) activity in cultures where CD8 T cells were predominant, whereas IL-4 followed by IL-2 was most effective in cultures where CD4 T cells predominated. IL-10 or IL-12 inhibited proliferation and CTL activity against melanoma in long-term cultures. The effects of IL-12 were reproduced in long-term cultures of T cells stimulated with mAb against CD3 and were shown to depend on prior exposure of T cells to IL-12 before IL-2. As yet unidentified factors, such as co-factor expression on melanoma, appear to be as important as exogenous cytokines in determining the nature of T cell responses to melanoma. These results suggest that analysis of responses in long-term culture may assist in defining the role of key cytokines and other determinants of immune responses to melanoma. Received: 4 June 1996 / Accepted: 12 November 1996     

Cyclosporin A inhibits IL 2-driven proliferation of human alloactivated T cells

The influence of cyclosporin A (CsA) on the interleukin 2 (IL 2)-driven proliferation of allo-activated human T lymphocytes has been studied. CsA (50, 100, and 250 ng/ml) appeared to affect the IL 2-driven proliferation. The impaired proliferation could not be reversed with exogenous interferon-gamma. The doubling time of the cell populations appeared to increase with higher CsA doses. The cytotoxic capacity of the cells was also strongly inhibited by CsA. The cells regained proliferative and cytotoxic properties after CsA has been removed and cells were additionally cultured in normal medium. The data strongly suggest that CsA has an inhibitory influence on essential basic processes in T cells.     

Immunologic effects of interleukin 2 in primary immunodeficiency diseases

Five children with primary deficiencies of T cell function were studied to assess the effects of highly purified exogenous Interleukin 2 (IL 2) on their in vitro T cell responses. The lymphocytes from one child with Nezelof's T cell deficiency demonstrated absence of endogenous IL 2 production and improved proliferative responses to mitogen or alloantigen in the presence of exogenous IL 2. Moreover, during in vitro mixed lymphocyte culture in the presence of exogenous IL 2, his lymphocytes were able to develop into cytotoxic effector cells. A second child with Nezelof's syndrome demonstrated a different type of defect. The lymphocytes from this child had less impairment of endogenous IL 2 production. Although IL 2 increased the proliferation of his cells in response to PHA, similar augmentation was not seen after stimulation with OKT3 or alloantigen. In cell-mediated cytotoxicity assays, after mixed lymphocyte culture, natural killer-like activity was strongly boosted in the cultures that contained IL 2, but T cell-mediated cytotoxicity was not. The lymphocytes from three patients with severe combined immunodeficiency did not show improved proliferative responses in the presence of IL 2. Thus, only one of the five patients demonstrated the combination of defective endogenous IL 2 production, but preservation of the ability to respond appropriately to exogenous IL 2. This child may therefore have suffered from a T cell defect pathophysiologically similar to that seen in nude or aged mice.     

Cyclosporin A does not inhibit the PHA-stimulated increase in intracellular Ca2+ concentration but inhibits the increase in E-rosette receptor (CD2) expression and appearance of interleukin-2 receptors (CD25)

The immunosuppressive drug cyclosporin A (CsA) inhibits mixed lymphocyte responses, blocks the generation of cytotoxic T lymphocytes, and inhibits the T lymphocyte proliferative response stimulated by polyclonal activators such as phytohemagglutinin (PHA). Nevertheless, there have been contradictory reports attempting to explain the mechanism(s) for this immunosuppressive activity. In the current studies, human peripheral blood mononuclear cells (PBM) were stimulated with PHA in the presence or absence of CsA. Flow cytometric examination of PBM loaded with the Ca2+-sensitive dye Indo-1 showed that concentrations of CsA sufficient to inhibit 90-100% of tritiated thymidine incorporation had no effect on the PHA-stimulated increase in the intracellular Ca2+ concentration ([Ca2+]i). Likewise, inhibitory amounts of CsA had virtually no effect on the increase in cell volume that occurs during T lymphocyte activation. These results were not altered by pretreating the PBM with CsA for 30 min at 37 degrees C prior to adding the PHA. On the other hand, inhibitory concentrations of CsA prevented the expression of receptors for T cell growth factor (interleukin-2, IL-2), as measured by monoclonal antibodies to CD25 after 16-24-hr incubation. In like manner, CsA also prevented the increase in the expression of the E-rosette receptor (CD2) on these same cells. If cultures containing PHA and inhibitory amounts of CsA were incubated for 40-72 h, there was partial recovery both of proliferative activity and of the expression of CD25 and CD2. Thus, CsA does not appear to affect the initial activation signal(s), but does interfere with one or more subsequent events necessary to initiate the appearance of "activation antigens."     

Induction and amplification of T-lymphocyte proliferative responses to periodate and soybean agglutinin by human adult vascular endothelial cells

Human mononuclear phagocyte (M phi) populations were compared to adult human endothelial cells (HEC) for their respective abilities to influence the proliferative responses of purified human T lymphocytes to the mitogenic agents Na-m-periodate (IO-4), soybean agglutinin (SBA), or allogeneic cells. HEC and M phi were both capable of inducing proliferative responses of allogeneic T lymphocytes in mixed-lymphocyte culture. Under low cell density culture conditions, purified T-lymphocyte proliferative responses to IO-4 or SBA could be restored by addition of syngeneic M phi or HEC. At higher cell density culture conditions, proliferation of T cells to IO-4 could be amplified more by HEC than M phi. T-lymphocyte proliferative responses to SBA were amplified by addition of HEC but were suppressed by addition of M phi. These findings indicate that human adult HEC are unique and potent accessory cells for T lymphocytes. Furthermore, these findings demonstrate that accessory cell functions of HEC can be discriminated from those of M phi.     

Blockade of physiologically secreted IFN-gamma inhibits human T lymphocyte and natural killer cell activation

The role of physiologically secreted human IFN-gamma in T lymphocyte and NK cell activation has been probed with a panel of mouse mAb directed against various epitopes of the human IFN-gamma molecule, or human IFN-gamma R. Addition to the culture medium of those mAb that neutralize the antiviral activity of IFN-gamma or interact with its receptor inhibited proliferative and cytotoxic responses elicited in PBL by HLA alloantigens, anti-CD3 mAb, and IL-2, but not the proliferative response to PHA. The IFN-gamma blockade also inhibited IFN-gamma, IL-2, and TNF-alpha release during MLC. Kinetic experiments showed that reduction of proliferative and cytotoxic responses to HLA alloantigens is maximal when IFN-gamma is blocked within the first 48 h. Exogenous rIFN-gamma restored the proliferative response only when added at the beginning. Moreover, when IFN-gamma was blocked, T lymphocytes recovered from 6-day MLC displayed a profound decrease in their expression of p55 and p75 chains of the IL-2R, as well as in the number of high-affinity IL-2 binding sites. These findings strongly suggest that IFN-gamma is required in the early phases of induction of the oligo- and polyclonal proliferative and cytotoxic responses of lymphocytes.     

The interaction of recombinant IL-2 with human resting lymphocytes: blocking effects of monoclonal antibodies to IL-2 receptors

Several lines of evidence suggest that subsets of resting lymphocytes naturally express interleukin-2 receptors (IL-2.R). Recombinant IL-2 (rIL-2) induced the enhancement of natural killer (NK) activity, the generation of activated killer (AK) cells, the proliferation of resting lymphocytes, and the production of interferon-gamma (IFN-gamma) in lymphocyte cultures. The subsets of lymphocytes mediating these responses appeared to be heterogeneous, but reside predominantly in nylon wool-passed non-T, non-B cells ("null cells" or T3- cells); in response to rIL-2, only Leu 11+T3- cells showed enhanced NK activity, and both Leu 11+T3- and Leu11-T3- cells showed predominantly AK activity, proliferation and production of IFN-gamma. These findings suggest that the T3- fraction (null cell fraction) contains predominantly cells expressing IL-2.R at the resting state. Unlike the case with activated T cells, however, none of these responses was blocked by any of three monoclonal antibodies to IL-2.R, including anti-Tac antibody at any dilution. These results indicate that IL-2.R on the resting T3- cells possess unique biological features compared to those on activated T or B cells. A most likely explanation is that T3- cells possess higher affinity IL-2.R than activated T or B cells. Other possibilities are also discussed.     

Role of complement in the immune response

Evidence has accumulated that shows that fragments of C3 are potent inhibitors of immune responses. A low-molecular-weight fragment of C3 and fragments possessing leukocyte-mobilizing activity have been shown to block both antigen- and mitogen-induced human T cell proliferation, and to block mixed lymphocyte culture responses and the generation of cytotoxic lymphocytes. The same fragments inhibit the development of secondary in vitro antibody responses of rat lymphocytes. C3b can be shown to inhibit the polyclonal activation of human lymphocytes by pokeweed mitogen, but it has no effect on T cell proliferation or on the generation of cytotoxic T cells. We now propose that different C3 fragments selectively act on various lymphocyte subsets and thus play a profound role in regulating both immune effector functions and the intensity of the immune response.     

Generation of MHC-nonrestricted and restricted oncolytic subsets from human bone marrow.

We analyzed the cytotoxicity and characterized the phenotype of oncolytic bone marrow (BM) lymphocyte subsets generated in vitro by interleukin-2 (IL-2) and stimulator cells (SC). Two irradiated B-lymphoblastoid cell lines (Daudi and EBV-transformed BSM) and fresh human acute myelogenous leukemia (AML) were used as SC. Stimulation with Daudi and IL-2 resulted in a substantial increase in cytotoxic activity (100- to 1000-fold) against a broad range of tumor targets, and total cellular expansion was higher compared to stimulation with IL-2 alone. The most prominent increase was observed in the CD16+ and CD56+/CD3- natural killer (NK) cell subset; however, a significant increase was also observed in CD56+/CD3+ T cells. Functional analysis of Daudi- and IL-2-generated subsets using fluorescence-activated cell sorting (FACS) revealed that most of the lytic activity was mediated by NK cells. Significant potentiation of oncolytic activity and cell growth was also seen in the cultures stimulated with BSM or fresh AML and IL-2. The highest oncolytic activity in the latter cultures was mediated primarily by CD8+, CD3+, and CD56- T cells, although NK cells also participated in cytotoxic activity. The T cell-mediated cytotoxicity was restricted by the major histocompatibility complex (MHC), since most cytotoxicity could be blocked by HLA I antibodies. Additionally, we observed that optimum stimulation of cytotoxicity required effector cell-stimulator cell contact. These data indicate that depending on the tumor used for stimulation, different lymphocyte subsets may be generated in IL-2 cultures. These different approaches may be useful in both specific and nonspecific immunotherapy.     

Suppressive effects of cyclosporin A on the induction of alloreactivity in vitro and in vivo

The purpose of the present study was the investigation of the effect of cyclosporin A (CsA) on the induction of alloreactivity in vitro and in vivo. Addition of CsA to mouse mixed lymphocyte cultures (MLC) not only inhibited lymphocyte proliferation but also prevented the generation of alloreactive cytolytic lymphocytes (CL). It was necessary to add CsA within the first 3 days of a 5-day MLC in order to achieve a significant suppressive effect. Lymphocytes, after being cultured in MLC with CsA for 4 days or longer, were incapable of being activated upon re-exposure to the same alloantigens although their responses to unrelated antigens remained intact, indicating antigen specificity of the suppression induced by CsA and its long-lasting effect. Furthermore, lymphocytes from mice treated with CsA after allosensitization failed to manifest primary cytotoxicity and could not be reactivated in a secondary MLC. Finally, CsA had no effect on those CL already generated, suggesting that CsA acts upon the induction of CL rather than the effector phase.     

Synergy between interleukin-2 and prothymosin α for the increased generation of cytotoxic T lymphocytes against autologous human carcinomas

 Peripheral blood mononuclear cells (PBMC) from cancer patients were cultured in vitro with irradiated autologous tumor cells isolated from malignant effusions (mixed lymphocyte tumor cultures, MLTC) and low-dose (50 IU/ml) recombinant interleukin-2 (IL-2). The combination of IL-2 and prothymosin α (ProTα) resulted in a greater PBMC-induced response to the autologous tumor than that brought about by IL-2 alone. In particular, ProTα specifically enhanced the CD4⁺ T-cell-mediated proliferation against the autologous tumor. CD4⁺ T cells seemed to recognize tumor antigens presented by HLA-DR molecules expressed on the autologous monocytes, since preincubation of the latter with an anti-HLA-DR monoclonal antibody (mAb) abrogated the response. In addition, MLTC set up with IL-2 and ProTα also generated more MHC-class-I-restricted cytotoxic T lymphocytes (CTL) against the autologous tumor than did MLTC set up with IL-2 alone. The MLTC-induced CTL contained high levels of cytoplasmic perforin and their development was strictly dependent on the presence of both autologous CD4⁺ T cells and monocytes. In the absence of either population there was a strong impairment of both proliferative and cytotoxic responses which was not restored by the presence of ProTα. In contrast, when both cell populations were present, ProTα exerted optimal enhancement of CD4⁺ T cell proliferation, which was associated with potentiated CTL responses. Our data emphasize the role of ProTα for the enhancement of IL-2-induced CTL responses against autologous tumor cells. Such responses require collaborative interactions between CD4⁺, CD8⁺ T cells and monocytes as antigen-presenting cells. Our data are relevant for adoptive immunotherapeutic settings utilizing IL-2 and ProTα-induced autologous-tumor-specific CTL. Received: 2 March 2000 / Accepted: 1 June 2000     

Purine nucleoside modulation of functions of human lymphocytes.

The accumulation of endogenous substrates in patients with adenosine deaminase deficiency or purine nucleoside phosphorylase deficiency is believed to be responsible for the immunodeficiency observed in these patients. To identify the lymphocyte populations that are most susceptible to these substrates, we investigated the effect of their nucleoside analogs on a number of T and B cell functions of human lymphocytes. We found that tubercidin (Tub), 2-chloro 2'deoxyadenosine (2CldA), 2-fluoro adenine arabinoside-5'phosphate (FaraAMP), and 9-beta-D-arabinosyl guanine (AraGua) inhibited the proliferative responses of human peripheral blood mononuclear cells (PBMC) to polyclonal activators (PHA, OKT3 mab) or to allogeneic PBMC in mixed lymphocyte cultures (MLC). Addition of recombinant IL-2 from the beginning of the culture did not alter the inhibition by Tub of the proliferative responses of PBMC. These purine nucleoside analogs also inhibited the proliferative responses of purified human peripheral blood CD4+ and CD8+ T cells to PHA and of purified B cells to SAC. The concentrations of these nucleosides required to achieve a given degree of inhibition of proliferative responses of T lymphocyte subpopulations or B cells was similar, suggesting that these analogs do not exhibit any selectivity for these purified lymphocyte populations. Tub and FaraAMP, respectively, inhibited and enhanced, at the effector phase, both NK cytotoxicity and specific T cell-mediated cytotoxicity. In contrast to these findings, LAK cytotoxicity at the effector phase was not significantly inhibited by Tub, and was not enhanced by FaraAMP. Both analogs inhibited rIL-2-induced proliferative responses of PBMC, but did not affect the generation of LAK cytotoxicity (induction phase) against the K562 targets when added at the beginning of the culture. This suggests that DNA synthesis is not required for LAK cell induction. Both Tub and FaraAMP inhibited immunoglobulin production (IgG and IgM) by PBMC in the PWM-induced system. These results demonstrate that purine nucleoside analogs significantly inhibited a number of functions of human lymphocytes. Although selectivity for T lymphocyte subpopulations and B cells was not observed, a differential effect of Tub and FaraAMP on LAK cytotoxicity versus NK cytotoxicity and specific T cell cytotoxicity was found.     

H-2 I alloantigens and recall of memory cytotoxic responses

AQR mice were immunized with H-2K and H-2 I encoded alloantigens presented by (Ax6R)F₁ splenocytes. Spleen cells from these alloimmune mice were subsequently restimulated in vitro with B10.A lymphocytes and/or B10.T(6R) lymphocytes, thus presenting them with the immunizing H-2K and H-2 I alloantigens independently. When stimulated with B10.A lymphocytes, alloimmune lymphocytes develop significant cytotoxicity against the immunizing H-2K target antigens. When stimulated with a similar number of B10.T(6R) spleen cells, alloimmune lymphocytes undergo a prominant proliferative response, but develop little, if any, cytotoxicity against the immunizing H-2 K target antigens. The most efficient restimulation of cytotoxicity occurs when the alloimmune spleen cells are simultaneously restimulated by B10.A and B10.T(6R) lymphocytes. Stimulation with the immunizing H-2 I alloantigens alone is not sufficient for regeneration of detectable cytotoxic responses from alloimmune spleen populations. Stimulation with the immunizing H-2K alloantigens alone appears to be both necessary and sufficient to stimulate alloimmune cytotoxic responses. Although the immunizing H-2 I alloantigens are apparently not required to generate alloimmune cytotoxic responses, they markedly potentiate the cytotoxic responses induced by the immunizing H-2K alloantigens.     

Interleukin-4. A regulatory protein

Since its discovery in 1982, numerous biological activities of interleukin-4 (IL-4) have been described. Like other cytokines, IL-4 is highly pleiotropic, both with respect to the number of different target cells that are responsive to it and with respect to the number of different biological responses it elicits. Interleukin-4 was initially described as a costimulant for the proliferation of B lymphocytes stimulated with anti-IgM antibody. Synonyms for this cytokine are B cell growth factor-1 (BCGF-1) and B cell stimulatory factor-1 (BSF-1). After cloning of both the murine and human IL-4, the use of recombinant IL-4 enabled detailed studies of its biological functions. Many cell types, mainly of hematological origin, express receptors for IL-4. Accordingly, effects of IL-4 have been described on B lymphocytes, T lymphocytes, NK cells, mononuclear phagocytes, mast cells, fibroblasts and hematopoietic progenitor cells. Currently, there are three major areas in which IL-4 appears to play an important role: 1) regulation of B cell growth and of antibody isotype expression. In this context, a possible role for IL-4 in allergic reactions is of special interest. 2) Stimulation of T cell growth and the generation of cytotoxic T lymphocytes. In addition to the suppressive effects on the induction of non HLA-restricted cellular cytotoxicity by natural killer- (NK) and lymphokine-activated killer (LAK) cells, this suggests a role for IL-4 in the regulation of cellular immune responses. 3) Regulation of the growth and differentiation of hematopoietic bone marrow stem cells. IL-4 itself does not induce proliferation of hematological progenitor cells but it can modulate the growth-factor dependent proliferation of these cells. In this review the biological functions of IL-4, reported until present, are discussed.     

IL-4 (B cell-stimulatory factor 1) regulates multiple aspects of influenza virus-specific cell-mediated immunity

The effect of endogenous and exogenous IL-4 on the generation of influenza virus-specific cell-mediated immunity was examined. When added at the onset of the culture, IL-4 augmented both cluster of differentiation (CD)8+ lymphoproliferation and MHC-restricted, influenza virus-specific cytotoxicity. When added 5 or 6 days after initiation of cultures, IL-4 was highly effective at augmenting cytotoxicity, whereas no augmentation of proliferation was observed. This disassociation of the effect of IL-4 on lymphoproliferation and cytotoxicity indicated that IL-4 was providing a late signal in CTL generation. Studied at the level of CTL precursor maturation in microcultures, IL-4 was found not to increase cytotoxicity but to be required, in some cases, for the generation of cytotoxicity. Endogenous IL-4 production was observed and demonstrated to be important because neutralizing antiserum to IL-4 suppressed CTL development. In contrast to the effects of IL-4 when added later to the cultures, pulsing the lymphocytes with IL-4 before, or shortly after, exposure to antigen resulted in suppression of the CTL response. These results indicate that IL-4 has differentiative, proliferative, and suppressive effects on cell-mediated immune responses.     

Stimulation with autologous lymphoblastoid cell lines: lysis of Epstein-Barr virus-positive and -negative cell lines by two phenotypically distinguishable effector cell populations

Human lymphocytes, stimulated in vitro for 6 days with x-irradiated or glutaraldehyde-treated autologous Epstein-Barr (EB) virus-transformed lymphoblastoid cell lines (LCL), are cytotoxic for autologous and allogeneic EB+ LCLs as well as for several EB- cell lines that are also susceptible to lysis by interferon-activated natural killer (NK) cells. To determine whether the apparent nonspecific lysis mediated by LCL-stimulated cells is due to a mixture of effector cells directed against different target cells, advantage was taken of our recent finding that monoclonal antibody OKT8 reacts with human cytotoxic T lymphocytes but not with NK cells or NK-like cells generated in mixed leukocyte cultures. The depletion of OKT8+ cells from LCL-stimulated cultures by treatment with OKT8 and complement abolished or markedly depleted cytotoxicity against all EB+ target cells tested, whereas cytotoxicity against EB-, NK-sensitive cell lines including K562, MOLT-4 and HSB-2 was not or only minimally reduced. These results indicate that stimulation with autologous LCL results in the generation of OKT8+ cytotoxic T lymphocytes that lyse EB virus-transformed LCL and OKT8- NK-like cells that lyse EB-, NK-sensitive cells.