Activation of alloreactive T cells by allogeneic nonprofessional antigen-presenting cells and interleukin-12 from bystander autologous professional antigen-presenting cells

BACKGROUND: After solid organ transplantation most alloantigens are presented to the recipient's immune system by normal tissue cells, which can be considered to act as nonprofessional antigen-presenting cells (APC). It is well accepted that such nonprofessional APC fail to activate recipient resting T cells due to their inability to deliver costimulatory activity. In our study, we tested a hypothesis that such costimulatory activity may be provided by "bystander" recipient professional APC. METHODS: We set up mixed lymphocyte cultures (MLC) of purified T cell responders and T cell stimulator cells, the latter as nonprofessional APC carrying allogeneic MHC class I and II, and tested if responder-type autologous APC could facilitate responder T cell proliferation. In this assay also the effects of anti-CD28 antibody and interleukin- (IL) 1beta, IL-6, or IL-12 mediated costimulation on responder T cell proliferation and IL-2 production were investigated. RESULTS: Autologous APC, i.e., monocytes, were found to facilitate the proliferative response of resting T cells stimulated by allogeneic nonprofessional APC. IL-12 was identified as the most important costimulatory factor for induction of proliferation. IL-1beta enhanced IL-2 production and proliferation of allostimulated resting T cells but its presence was not essential. Although CD28 triggering alone was ineffective, this costimulatory pathway enhanced IL-2 production and proliferation when combined with IL-12 or IL-1beta. CONCLUSIONS: We conclude that costimulatory activity for activation of resting human T cells by nonprofessional donor APC can be delivered through activity of bystander recipient-type autologous APC. This mechanism of allostimulation may contribute to the induction and perpetuation of alloreactivity "in vivo" in a time frame when intragraft professional donor-type APC have been replaced with professional recipient-type APC.     

Accessory cell functions in mononuclear cell cultures uremic patients

Mononuclear cells (PBMC) were isolated from dialyzed patients and healthy control subjects. Lymphocyte responses to stimulation with optimal and suboptimal concentrations of lectin (PHA), or stimulation with T cell receptor antibody (Leu 4) were found decreased in the patient cultures. The separate and the combined effects of exogenous interleukin-1 (IL-1) and interleukin-2 (IL-2) were examined in PHA and Leu 4 stimulated cell cultures. Addition of IL-1 did not normalize the decreased proliferation response of the patient cultures. In contrast, addition of IL-2 alone clearly enhanced and almost normalized the response of patient cultures stimulated with suboptimal concentrations of PHA. The combined addition of IL-1 and IL-2 gave no evidence of an additive effect of IL-1 and IL-2. Cell cultures from uremic and normal HLA-identical relative were examined. Substitution of uremic adherent monocytes with normal adherent monocytes as accessory cells did not improve the uremic T cell responses to stimulation with PHA. Furthermore, uremic adherent cells did not suppress the normal T cell responses. These results suggest that uremic accessory cells support T cell activation and, in particular, do not suppress T cell responses. The effect of IL-2 in the present study as well as previous findings of decreased IL-2 production in patients cultures may indicate that uremia primarily influences the proliferation of T cells.     

B7.1 costimulation increases T-cell proliferation and cytotoxicity via selective expansion of specific variable alpha and beta genes of the T-cell receptor

BACKGROUND: Optimal T-cell activation requires not only ligation of the T-cell receptor (TcR) but also delivery of costimulatory signals by various accessory molecules. The interaction of the costimulatory molecule B7.1 (CD80) with its receptor CD28 provides a strong positive signal to T cells. METHODS: The B7.1 gene was transduced into cultured human ovarian, breast, and pancreatic tumor cells by using a retroviral vector. Autologous as well as allogeneic naive T-cells were stimulated with either wild-type or B7.1-transduced tumor cells in a mixed lymphocyte tumor cell culture (MLTC). In addition to cytolytic activity, T-cell proliferation, T-cell subset composition, and the frequencies of TcR variable (V) alpha and beta genes were compared in T cells from both types of MLTC. RESULTS: Introduction of the B7.1 gene into tumor cells was successful in all tumors to a varying degree. Those tumors expressing high levels of B7.1 induced significantly higher levels of T-cell proliferation than wild-type tumor cells. T-cell subset composition did not markedly differ between T cells stimulated with wild-type tumor cells or B7.1-expressing tumor cells. However, T cells stimulated with B7.1-expressing tumor cells showed a significantly increased cytolytic potential. The increased cytotoxic T lymphocyte activity was associated with a higher frequency of specific TcR V alpha and V beta genes. In addition, B7.1 costimulation promoted oligoclonality among the responding T cells. CONCLUSIONS: These data suggest that costimulation through B7.1 promotes T-cell proliferation and cytotoxic activity through clonal expansions of T cells bearing antigen-specific TcR V alpha and V beta genes and through promotion of oligoclonality. The data also suggest that promoting B7.1-mediated costimulation is an important aspect of immune therapies.     

T cell costimulatory blockade: new therapies for transplant rejection.

Optimal T cell responses occur when T cells receive both antigen-specific signals through the T cell receptor and non-antigen-specific costimulatory signals through accessory cell surface molecules. The best understood costimulatory receptor is CD28. Signals through the T cell receptor and CD28 cooperatively induce cytokine gene expression and promote T cell proliferation and survival. Negative signals delivered through a related cell surface receptor, cytotoxic T lymphocyte antigen (CTLA-4), act to terminate immune responses and are required for normal immune homeostasis. This article reviews T cell costimulation, including the CD28/CTLA-4 system and other potential costimulatory pathways (such as CD40/CD154), the role of these pathways in normal immune responses, and the potential for the inhibition of these pathways to induce transplantation tolerance.     

Dysregulation of IL-2/IL-2R system alters proliferation of early activated CD4+ T cell subset in patients with end-stage renal failure

BACKGROUND/AIM: Although CD4+ T cells are preactivated in patients with end-stage renal failure (ESRF), these patients present an impairment of T cell immune response, which is partly responsible for the higher incidence of infection in this population. The aim of the present study was to analyze the mechanisms underlying the altered function of activated CD4+ T cells in patients with ESRF. METHODS: Thirty patients undergoing chronic hemodialysis (HD) and 20 patients with ESRF were compared with 15 sex- and age-matched controls. CD4+ T cell early activation (CD69, CD25), interleukin-2 (IL-2)/IL-2 receptor (IL-2R) system, and proliferation capacity of CD69+/CD4+ T cells were assessed ex vivo after blood draw sampling, in culture conditions and after phytohemagglutinin (PHA) stimulation. RESULTS: Although the CD4+ T cell count was lower in chronic HD patients than in predialysis patients and controls (p = 0.007), CD4+ T cells showed a pre-activation state as demonstrated by higher percentage of CD69+/CD4+ T cells and CD25+/CD4+ T cells in chronic HD patients compared with the other groups ex vivo. Furthermore, CD69+/CD4+ T cells from chronic HD patients spontaneously released more IL-2 (22 +/- 6 pg/ml) than those from pre-dialysis patients (12 +/- 4 pg/ml, p = 0.005) and controls (5 +/- 3 pg/ml, p = 0.001). However, after PHA stimulation, CD69+/CD4+ T cells from chronic HD patients expressed lower cell surface CD25 density, and were unable to show further activation. Indeed, these cells produced less IL-2 and released more soluble IL-2R, and correlatively with IL-2 production, they showed lower proliferation capacity compared with predialysis patients (p = 0.001) and controls (p < 0.001). They also displayed decreased responsiveness to exogenous human recombinant IL-2. The restoration of the PHA stimulation index of CD69+/CD4+ T cells from chronic HD patients in the presence of normal human serum as well as the decreased stimulation index of CD69+/CD4+ T cells from control subjects incubated with HD serum, strongly suggest that uremic toxins and mediators induced by HD affect the IL-2/IL-2R pathway. CONCLUSION: These findings demonstrate the presence, in chronic HD patients, and to lesser extent, in predialysis patients, of abnormally high proportion of spontaneously preactivated CD4+ T cells whose proliferation and further activation are blunted due to dysregulation of the IL-2/IL-2R system.     

Evidence that stimulator cell-derived IL-6 and IL-1 are released in the mixed lymphocyte culture but are not requisite for responder T cell proliferation.

We investigated whether IL-6 and (or) IL-1 are crucial costimulatory signals in the human MLC with purified responder T cells. With allogeneic PBMC as stimulators, IL-6 and IL-1 were rapidly produced, and reached plateau values of 100-300 U/ml and 200-500 pg/ml after 24 hr, respectively. Irradiated or mitomycin-c treated PBMC could easily be induced (with LPS) to produce IL-6 and IL-1 while no activity was measured after 48 hr in the supernatant of PHA-stimulated T cells, suggesting that in the MLC the monokines were entirely produced by stimulator PBMC. In cultures of responder T cells and stimulator B cells, no IL-6 and IL-1 activity was measured in the supernatant, and only a marginal proliferative response was found. Exogenous IL-6 and IL-1 increased in a dose-dependent way the B-cell-induced alloresponse and induced significant cytotoxicity in the responder cells. Antisera to IL-6 and IL-1 totally inhibited the induced response. The proliferation was accompanied by increased IL-2 production and IL-2R expression. Preincubation of B cells with IL-6 and IL-1 did not improve the proliferation, suggesting direct effects of IL-6 and IL-1 on the T cells. The proliferative responses induced by B cells and exogenous IL-6 and IL-1 represented a fraction of those induced by PBMC. Moreover, in PBMC-stimulated cultures exogenous IL-6 and IL-1 or antisera to these lymphokines did not significantly alter proliferative responses, cytotoxicity, IL-2 levels in the supernatant, or IL-2R expression on responder T cells. We conclude that a role for IL-6 and IL-1 in allogeneic T cell stimulation can be demonstrated in conditions of suboptimal stimulation with B cells. With PBMC, neutralizing antisera to these cytokines do not seem to inhibit the proliferative response, suggesting that these cells are superior in alloantigen presentation either by producing various costimulatory signals or by the fact that due to cell-cell contact stimulator cell-derived monokines cannot be blocked. This finding makes it unlikely that antimonokine therapy will be useful in transplantation.     

Inhibition of the accessory function of murine macrophages in vitro by cyclosporine

The accessory function of macrophages, which is strictly related to the induction of T cell activation, has been studied to determine whether it is affected by cyclosporine (CsA). Irradiated spleen cells, used as a source of macrophages, were pulsed overnight with beta-galactosidase (GZ) in the presence of CsA. After washing of the pulsed macrophages, cells from a GZ-specific T cell line were added to cultures and 3H-thymidine incorporation was measured 72 hr later. We found that 500 ng/ml CsA present during macrophage pulsing with GZ reduced T cell proliferation to 5%. On the other hand, 100 ng/ml CsA almost completely abrogated the proliferative response when present for the duration of the culture. Similar results were also obtained using antigen-pulsed peritoneal-adherent macrophages to stimulate the T cell line to proliferate, or a T hybridoma clone to produce interleukin-2 (IL-2). The possibility that CsA actually affects interleukin-1 (IL-1) production by macrophages by inhibiting uninvolved T cells could be ruled out. We conclude that CsA-induced inhibition of T cell functions (proliferation and IL-2 production) is partially due to the effect of the drug on the accessory function of macrophages. This immunosuppressive mechanism of action of CsA on macrophages has not been previously described.     

Uremic serum effects on peripheral blood mononuclear cell and purified T lymphocyte responses.

We have studied the effects of uremic serum on the activation state and function of normal lymphocytes in vitro, by examining both accessory cell-dependent and accessory cell-independent responses. Uremic serum was obtained from patients on conservative treatment and from the same patients after they have undergone six months of maintenance hemodialysis. Uremic serum inhibited the proliferative responses to mitogens and to recombinant IL-2 (rIL-2) of both peripheral blood mononuclear cells (PBMC) and purified T cell populations. However, the responsiveness to IL-2 of pre-formed lymphoblasts, obtained from both PBMC and purified T cells, in the presence of uremic serum was similar to that obtained in the presence of normal serum, or was even enhanced. Uremic serum did not affect the cellular IL-2 receptor alpha (IL-2R) generation though it inhibited significantly the release of soluble IL-2 receptor (sIL-2R) and the production of IL-2 after mitogenic stimulation. Uremic serum from patients after six months of hemodialysis enhanced, but did not completely restore, proliferative responses and IL-2 production by control PBMC. Neither IL-1 nor IL-2R, which are present at elevated concentrations in uremic serum, appeared to be responsible for serum effects on in vitro responses of control lymphocytes. In conclusion, our results indicate that uremic serum affects both accessory cell-mediated and accessory cell-independent normal T cell responses. Uremic serum inhibition of T cell proliferation is associated with down-regulation of IL-2 synthesis by lymphocytes and the induction of an abnormal state of activation of lymphoblasts which is further enhanced following chronic hemodialysis.     

CD4+ T-cell and monocyte interdependence during discordant xenoimmune responses

This study was designed to examine our hypothesis that human monocytes provide missing constimulatory signals to host CD4⁺ cells during interactions with porcine endothelial cells (PECs). PECs were isolated from the aorta. Human CD4⁺ T cells and monocytes were purified from peripheral blood mononuclear cells (PBMCs). A xenogeneic mixed lymphocyte-PEC reaction (xMLER) was performed to determine the proliferation of PBMCs or CD4⁺ cells in response to PEC. Monocyte-PEC cocultures with or without CD4⁺ cells were followed by analysis using fluorescence-activated cell scanning (FACS). We evaluated the CD4⁺ cells proliferation induced by PEC-conditioned monocytes with or without costimulation blockade. xMLER demonstrated strong lymphocyte proliferation in response to PECs. However, purified CD4⁺ cells showed reduced proliferative responses to PECs when compared with PBMCs. FACS analysis found that CD14⁺ monocytes up-regulated CD40 and CD80 expressions in the presence of CD4⁺ cells. PEC-activated but not resting monocytes induced CD4⁺-cell proliferation, which was inhibited by anti-CD154, anti-CD80, or anti-CD86 antibodies. In summary, human monocytes exposed to PECs are conditioned to up-regulate costimulatory molecules upon exposure to T cells. PEC-conditioned monocytes induced T-cell proliferation by indirect presentation. Costimulation blockade inhibited T-cell proliferation induced by PEC-conditioned monocytes. Our findings suggested that monocytes play an important role in indirect xenoantigen presentation, providing costimulation to T cells. This interaction can occur distant from the initial site of xenoantigen, but monocytes remaide void of costimulatory signals until their interaction with T cells.     

Suppression of allogeneic T-cell proliferation by human marrow stromal cells: implications in transplantation

BACKGROUND: Marrow stromal cells (MSC) can differentiate into multiple mesenchymal tissues. To assess the feasibility of human MSC transplantation, we evaluated the in vitro immunogenicity of MSC and their ability to function as alloantigen presenting cells (APC). METHODS: Human MSC were derived and used in mixed cell cultures with allogeneic peripheral blood mononuclear cells (PBMC). Expression of immunoregulatory molecules on MSC was analyzed by flow cytometry. An MSC-associated suppressive activity was analyzed using cell-proliferation assays and enzyme-linked immunoassays. RESULTS: MSC failed to elicit a proliferative response when cocultured with allogeneic PBMC, despite provision of a costimulatory signal delivered by an anti-CD28 antibody and pretreatment of MSC with gamma-interferon. MSC express major histocompatibility complex (MHC) class I and lymphocyte function-associated antigen (LFA)-3 antigens constitutively and MHC class II and intercellular adhesion molecule (ICAM)-1 antigens upon gamma-interferon treatment but do not express CD80, CD86, or CD40 costimulatory molecules. MSC actively suppressed proliferation of responder PBMC stimulated by third-party allogeneic PBMC as well as T cells stimulated by anti-CD3 and anti-CD28 antibodies. Separation of MSC and PBMC by a semipermeable membrane did not abrogate the suppression. The suppressive activity could not be accounted for by MSC production of interleukin-10, transforming growth factor-beta1, or prostaglandin E2, nor by tryptophan depletion of the culture medium. CONCLUSIONS: Human MSC fail to stimulate allogeneic PBMC or T-cell proliferation in mixed cell cultures. Unlike other nonprofessional APC, this failure of function is not reversed by provision of CD28-mediated costimulation nor gamma-interferon pretreatment. Rather, MSC actively inhibit T-cell proliferation, suggesting that allogeneic MSC transplantation might be accomplished without the need for significant host immunosuppression.     

Induction of cytotoxic T lymphocyte development from murine thymocytes by IL-1 and IL-6

Cytotoxic T lymphocytes (CTL) are believed to play an important role in the regression of advanced malignancies in response to adoptive immunotherapy with interleukin-2 (IL-2) and lymphokine-activated killer cells or tumor-infiltrating lymphocytes. Because the current limitations to the use of adoptive immunotherapy are the IL-2 dose-dependent toxicities and the difficulty in expanding the effector cell population, recent investigations have focused on the development of newer methods for generating CTL in vitro. IL-1 and IL-6 have been shown to synergistically promote thymocyte proliferation; however, their effect on CTL development has not been studied. We investigated the ability of these two cytokines to induce CTL development from immature thymocytes. Thymocytes from 5-week-old BALB/c mice were cultured for 72 hours in the presence of Con A and recombinant IL-1, IL-6, or IL-1 plus IL-6. Cytotoxicity against 51Cr-labeled P815 target cells was then measured in the presence of submitogenic doses of PHA. Neither IL-1 nor IL-6 induced a significant number of CTL from immature thymocytes. However, these two cytokines synergistically induced maximal CTL development. The monoclonal antibody to IL-4 completely abrogated CTL development induced by IL-1 and IL-6, but antibody to the IL-2 receptor had no effect. The data suggest that IL-1 and IL-6 can provide an additional method for in vitro CTL generation in adoptive immunotherapy of advanced tumors.     

Production of a factor, or factors, suppressing IL-2 production and T cell proliferation by Sertoli cell-enriched preparations. A potential role for islet transplantation in an immunologically privileged site

Isolated islet allografts survive indefinitely in the abdominal testis of nonimmunosuppressed diabetic rats. The predominant feature of these testes is that the presence of Sertoli cells, but not Leydig cells, is required for extended survival of the islet allografts. Sertoli cells cultures were therefore established in vitro and we examined the effects of the conditioned media on Con A--stimulated spleen lymphocyte proliferation. These studies revealed that a product(s) secreted by Sertoli cells inhibits Con A-stimulated lymphocyte proliferation in a dose-dependent manner. The synthesis of this product is both temperature-dependent, occurring predominantly at 37 degrees C, and hormone-dependent, requiring the presence of follicle stimulating hormone, in the culture medium. We further examined the mechanism of inhibition of lymphocyte proliferation and showed that Sertoli cell-enriched media inhibit the production of IL-2 in a dose-dependent manner. Furthermore, the finding that the addition of exogenous IL-2 is not able to reverse this inhibition indicates that the Sertoli cell-enriched media inhibit both IL-2 production and IL-2 responsiveness of T lymphocytes.     

Simultaneous ligation of CD5 and CD28 with monoclonal antibodies restores impaired immunostimulatory function in human renal cell carcinoma

Tumor cells, including renal cell carcinoma (RCC) cells, do not effectively stimulate T lymphocyte responses against specific antigens presented on their surface. Reasons for this low immunogenicity may include low or absent expression of MHC class I and/or class II molecules, as well as accessory and costimulatory molecules. We used tumor cell pretreatment with cytokines, together with monoclonal antibodies (mAbs) directed at receptors for costimulatory molecules, to render RCC cells immunostimulatory. Interferon-gamma or tumor necrosis factor-alpha pretreatment enhanced expression of MHC class I and class II molecules, as well as CD54, but had only minimal effects on T cell activation. A CD28 mAb, or an even more effective combination of CD28 and CD5 mAb, induced strong primary proliferative responses of allogeneic resting T lymphocytes. Cytokine pretreatment further augmented this T cell response in vitro and allowed T cell expansion and establishment of T cell lines. Stimulation of T cells with autologous RCC cells resulted in a similar T cell activation but with the expansion of cytolytic T cells directed at autologous MHC class II molecules. These experiments demonstrate that cytokines combined with costimulatory mAbs are useful for increasing the immunogenicity of tumor cells. They also indicate. however, that autologous MHC class II expression on tumor cells, together with strong costimulation, may lead to the activation of autoreactive T cells.     

Intercellular adhesion molecule-1/leukocyte function associated antigen-1 blockade inhibits alloantigen specific human T cell effector functions without inducing anergy.

BACKGROUND: Intercellular adhesion molecule (ICAM-1) is important in leukocyte adhesion-dependent events and some data suggest that ICAM-1 provides T cell costimulation. We anlayzed the role of the ICAM-1 and leukocyte function associated antigen-1 (LFA-1) interaction in human T cell alloreactivity in vitro. METHODS: Allo-antigen-induced T cell proliferation and cytotoxic T lymphocyte lytic activity were assessed by mixed lymphocyte reaction assay and 51 Chromium release assay, respectively. Immunostaining and flow cytometry were used to assess the expression of receptors on activated T cells. RESULTS: Alloantigen-induced T cell proliferation and cytotoxic T lymphocyte activity were markedly inhibited by antibodies to ICAM-1 and LFA-1. These antibodies had to be present at the time of initial T cell receptor/antigen engagement to inhibit proliferation. Neither IL-2 nor IL-4 were involved in the observed inhibition by antibodies. Inhibition was not associated with altered cell surface expression of receptors such as CD3, CD4, ICAM-1, LFA-1, CD25, and HLA-DR however, these antibodies did impede the ability of generation of functionally active T cells. Interestingly, these antibodies inhibited soluble, but not immobilized OKT3-induced proliferation of peripheral blood leukocytes. Antibody-mediated inhibition of proliferation failed to impair the ability of T cells to subsequently proliferate in response to stimulation by the original or third party alloantigen or mobilize [Ca++]i in response to CD3 or LFA-1 receptor ligation. CONCLUSIONS: These data demonstrate that blockade of ICAM-1/LFA-1 binding at the time of allorecognition potently blocks initial T cell effector functions that could be due to lack of effective T cell/APC engagement.     

不同细胞因子诱导对树突状细胞体外分化的影响

目的 观察不同细胞因子诱导对于体外培养的树突状细胞(DC)免疫刺激活性的影响.方法 通过1000 U/ml人粒细胞-巨噬细胞集落刺激因子(rhGM-CSF)和500 U/ml白细胞介素(IL)-4体外诱导的人单个核细胞来源的DC,经1000 U/ml肿瘤坏死因子(TNF)-α、1 mg/L脂多糖(LPS)、1 mmol/L丁酸钠、细胞因子鸡尾酒法诱导成熟,分别以流式细胞仪、FITC-Dxtran内吞检测、混合淋巴细胞反应(MLR)、酶联免疫吸附试验(ELISA)检测DC的表面标志、内吞能力、刺激淋巴细胞增殖能力和IL-12分泌的变化.结果 鸡尾酒法诱导下的DC成熟标志显著上调,内吞能力减弱186.74±38.66,刺激淋巴细胞增殖能力增强18.23 ±2.22,并且IL-12的分泌能力增加(656.18±38.52)ng/L,说明其显著促进DC成熟.而丁酸钠诱导下的各项成熟指标均下调,导致DC免疫刺激源性减弱.结论 细胞因子鸡尾酒法是诱导DC成熟的最佳方法;而丁酸钠可以抑制DC成熟,改变DC的免疫状态.

Abstract：

Objective To investigate the immune effect of different cytokines on immature dendritic cells (DC) in vitro. Methods The human monocyte-derived DCs were induced in the presence of recombinant human GM-CSF (rhGM-CSF, 1000 U/ml) and interleukin (IL)-4 (500 U/ml). The effects of tumor necrosis factor (TNF) -α (1000 U/ml) , lipopolysaccharide (LPS) (1 mg/L), the cocktail of cytokines (TNF-α, IL-6, IL-1β, PGE2) , and sodium butyrate (1 mmol/L) on the DCs were detected by flow cytometry (FCM), endocytic activity, T cells stimulatory proliferation capacity, and IL-12 production. Results The cocktail of cytokines could up-regulate the major histocompatibility complex (MHC) class Ⅱ and costimulatory molecules of DCs, decrease the endocytic activity 186. 74 ±38. 66, induce a stage of Tcell stimulation 18. 23 ± 2. 22 and promote the T helper cell type 1-skewing factor IL-12 production (656. 18 ±38. 52) ng/L. But the sodium butyrate induced reverse result on DCs compared to cocktail of cytokines. Conclusion The most effective method for inducing dendritic cells maturation was cocktail of cytokines. The sodium butyrate could inhibit the development of mature DCs, resulting in impaired DC function, and modify the outcome of the subsequent immune response.     

Induction of HLA-specific CTL to nonimmunogenic, heat-inactivated lymphocytes by interleukin 2

Human lymphocytes that have been heat-inactivated (1 hr, 45 degrees C) were used as stimulator cells in a model system to study the requirements of allogeneic T cell activation in vitro. Cytotoxic T lymphocytes were not generated in either primary or secondary mixed lymphocyte cultures after exposure to heated stimulator cells. Successful reconstitution of cytolytic activity in primary cultures was achieved by the addition of rIL-2. Further, cytotoxic T cell lines could be maintained in culture for several weeks by stimulation with heated allogeneic cells and periodic addition of exogenous IL-2. The cytotoxic T cells generated in primary cultures or in the T cell lines were specific for the HLA class I antigens of the stimulating cells. Thus, the combination of heated cells and IL-2 stimulated antigen-specific cytotoxic cells, and not merely lymphokine-activated killers. Although IL-2 production appeared to be a crucial missing component of MLCs with heated lymphocytes, the addition of IL-1, a factor known to act as a second signal for stimulating IL-2 production, did not reconstitute cytolytic activity. These results indicate that (1) heat treatment does not appreciably affect class I structure; (2) HLA class I/T cell receptor interactions are intact, resulting in responsiveness to IL-2 but not IL-1; and (3) heating creates a defect that has a minimal effect on CTL precursor activation but does disrupt a T helper cell/stimulator cell interaction critical for IL-2 production.