TCR stimulation with modified anti-CD3 mAb expands CD8+ T cell population and induces CD8+CD25+ Tregs

Modified anti-CD3 mAbs are emerging as a possible means of inducing immunologic tolerance in settings including transplantation and autoimmunity such as in type 1 diabetes. In a trial of a modified anti-CD3 mAb [hOKT3gamma1(Ala-Ala)] in patients with type 1 diabetes, we identified clinical responders by an increase in the number of peripheral blood CD8+ cells following treatment with the mAb. Here we show that the anti-CD3 mAb caused activation of CD8+ T cells that was similar in vitro and in vivo and induced regulatory CD8+CD25+ T cells. These cells inhibited the responses of CD4+ cells to the mAb itself and to antigen. The regulatory CD8+CD25+ cells were CTLA4 and Foxp3 and required contact for inhibition. Foxp3 was also induced on CD8+ T cells in patients during mAb treatment, which suggests a potential mechanism of the anti-CD3 mAb immune modulatory effects involving induction of a subset of regulatory CD8+ T cells.     

Depletion of CD4+ CD25+ regulatory cells augments the generation of specific immune T cells in tumor-draining lymph nodes

Recent studies have identified a unique population of CD4+CD25+ regulatory T cells that is crucial for the prevention of spontaneous autoimmune diseases. Further studies demonstrated that depletion of CD4+CD25+ T cells enhances immune responses to nonself antigens. Because immune responses to malignant tumors are weak and ineffective, depletion of regulatory T cells has been reported to result in tumor regression. In the current study, using the weakly immunogenic MCA205 sarcoma and the poorly immunogenic B16/BL6/D5 (D5) melanoma, depletion of CD4+CD25+ T cells by the administration of anti-CD25 monoclonal antibodies (mAb), PC61 induced some tumor growth retardation, but all mice eventually succumbed to tumors. In our laboratory, immunotherapy by the transfer of tumor-immune T cells has demonstrated potent antitumor effects. A reliable source of tumor-reactive T cells has been lymph nodes (LN) draining progressive tumors. Therapeutic effector T cells can be generated by in vitro activation of draining LN cells with anti-CD3 mAb followed by culture in interleukin-2. In this system, PC61 mAb depletion of CD4+CD25+ T cells before or on day 8 of tumor growth resulted in increased sensitization in the draining LN. The therapeutic efficacy of activated tumor-draining LN cells from mAb depleted mice increased approximately three fold while maintaining specificity when tested in adoptive immunotherapy of established pulmonary metastases. Specific interferon-gamma secretion by LN T cells from mice treated with PC61 mAb 1 day before tumor inoculation increased significantly. However, this increase was not demonstrated with LN T cells from mice treated on day 8 despite their enhanced therapeutic reactivities. Our results indicate that although the antitumor immunity enhanced by the depletion of CD4+CD25+ T cells is insufficient to eradicate tumors, it augments the sensitization of immune T cells in the draining LN, thus, facilitating adoptive immunotherapy.     

B cell-deficient NOD.H-2h4 mice have CD4+CD25+ T regulatory cells that inhibit the development of spontaneous autoimmune thyroiditis

Wild-type (WT) NOD.H-2h4 mice develop spontaneous autoimmune thyroiditis (SAT) when given 0.05% NaI in their drinking water, whereas B cell-deficient NOD.H-2h4 mice are SAT resistant. To test the hypothesis that resistance of B cell-deficient mice to SAT was due to the activity of regulatory CD4+CD25+ T (T reg) cells activated if autoantigen was initially presented on non-B cells, CD25+ T reg cells were transiently depleted in vivo using anti-CD25. B cell-deficient NOD.H-2h4 mice given three weekly injections of anti-CD25 developed SAT 8 wk after NaI water. Thyroid lesions were similar to those in WT mice except there were no B cells in thyroid infiltrates. WT and B cell-deficient mice had similar numbers of CD4+CD25+Foxp3+ cells. Mice with transgenic nitrophenyl-specific B cells unable to secrete immunoglobulin were also resistant to SAT, and transient depletion of T reg cells resulted in severe SAT with both T and B cells in thyroid infiltrates. T reg cells that inhibit SAT were eliminated by day 3 thymectomy, indicating they belong to the subset of naturally occurring T reg cells. However, T reg cell depletion did not increase SAT severity in WT mice, suggesting that T reg cells may be nonfunctional when effector T cells are activated; i.e., by autoantigen-presenting B cells.     

CD25+ regulatory T cell inhibition enhances vaccine-induced immunity to neuroblastoma

Evidence that CD4CD25 regulatory T (Treg) cells play a role in the progression of cancer continues to mount. There is a great deal of interest as to whether transient elimination or functional inhibition of these cells can improve the efficacy of immunotherapy for cancer. Our goals in this study were to test whether treatment of mice with anti-CD25 monoclonal antibody (mAb) (PC61) could induce rejection of a murine neuroblastoma, whether anti-CD25 treatment could increase tumor immunity when administered just before cell-based vaccination, and to learn how anti-CD25 treatment influences the vaccine-induced antitumor response. Treatment of mice with anti-CD25 mAb induced rejection of the mouse neuroblastoma, Neuro-2a, as 90% of anti-CD25-treated mice survived challenge with a lethal dose of tumor cells. In vivo anti-CD25 mAb treatment before the first of 2 weekly vaccines significantly improved the survival of tumor-vaccinated/challenged mice (75% vs. 33% survival), whereas antibody treatment before each of the 2 vaccines did not, suggesting that excessive treatment with anti-CD25 mAb interferes with activated antitumor effector cells. A detailed phenotypic analysis of tissues from anti-CD25-treated mice indicated that the antibody partially depletes CD4Foxp3 Treg cells (25% to 40%) in A/J mice, and that the antibody may inhibit the remaining cells by inducing loss of CD25 expression and blocking CD25 molecules, partially confirming recent data from other investigators. Importantly, we found that in vivo anti-CD25 mAb treatment significantly decreased the contribution of asialo GM1 cells in the antitumor response. As we did not see a direct effect of anti-CD25 mAb on in vitro assays of immune cell function in spleen cells from treated animals, this indicates that inhibition of Treg cells amplifies the immune response in vivo in a manner that bypasses the requirement for innate immune activation, potentially mediated by natural killer cells, and allows for protective CD4 and CD8 cells to expand directly in response to cell-based vaccines.     

OX40 triggering blocks suppression by regulatory T cells and facilitates tumor rejection

Regulatory T (T reg) cells are the major obstacle to cancer immunotherapy, and their depletion promptly induces conversion of peripheral precursors into T reg cells. We show that T reg cells can be functionally inactivated by OX40 triggering. In tumors, the vast majority of CD4(+) T cells are Foxp3(+) and OX40(bright). However, intratumor injection of the agonist anti-OX40 monoclonal antibody (mAb) OX86, but not anti-CD25 mAb, induces tumor rejection in 80% of mice, an effect that is abrogated by CD8 depletion. Upon intratumor OX40 triggering, increased numbers of infiltrating dendritic cells (DCs) migrate to draining lymph nodes and generate a new wave of tumor-specific cytotoxic T lymphocytes, as detected by tetramer and CD44 staining of node CD8(+) T lymphocytes. Tumor-bearing Rag1-knockout (KO) mice reconstituted with OX40-deficient T reg cells and wild-type (WT) effector T cells, or the reciprocal combination, showed that both T reg and effector T cells must be triggered via OX40 for the tumor to be rejected. Accordingly, WT but not OX40-KO mice receiving intratumor coinjection of OX86 and ovalbumin protein were able to revert tumor-induced tolerization of adoptively transferred OX40-competent OTII T lymphocytes. In conclusion, OX40-mediated inactivation of T reg cell function unleashes nearby DCs, allowing them to induce an adaptive immune response. In addition, the known OX40-dependent delivery of fitness signals to activated T cells is boosted by concurrent T reg cell inhibition. OX40 triggering thus has multiple effects that converge to mediate tumor rejection.     

Regulatory CD4+CD25+ T cells restrict memory CD8+ T cell responses

CD4+ T cell help is important for the generation of CD8+ T cell responses. We used depleting anti-CD4 mAb to analyze the role of CD4+ T cells for memory CD8+ T cell responses after secondary infection of mice with the intracellular bacterium Listeria monocytogenes, or after boost immunization by specific peptide or DNA vaccination. Surprisingly, anti-CD4 mAb treatment during secondary CD8+ T cell responses markedly enlarged the population size of antigen-specific CD8+ T cells. After boost immunization with peptide or DNA, this effect was particularly profound, and antigen-specific CD8+ T cell populations were enlarged at least 10-fold. In terms of cytokine production and cytotoxicity, the enlarged CD8+ T cell population consisted of functional effector T cells. In depletion and transfer experiments, the suppressive function could be ascribed to CD4+CD25+ T cells. Our results demonstrate that CD4+ T cells control the CD8+ T cell response in two directions. Initially, they promote the generation of a CD8+ T cell responses and later they restrain the strength of the CD8+ T cell memory response. Down-modulation of CD8+ T cell responses during infection could prevent harmful consequences after eradication of the pathogen.     

Immune suppression or enhancement by CD137 T cell costimulation during acute viral infection is time dependent

CD137 is expressed on activated T cells and ligands to this costimulatory molecule have clinical potential for amplifying CD8 T cell immunity to tumors and viruses, while suppressing CD4 autoimmune T cell responses. To understand the basis for this dichotomy in T cell function, CD4 and CD8 antiviral immunity was measured in lymphocytic choriomeningitis virus (LCMV) Armstrong- or A/PR8/34 influenza-infected mice injected with anti-CD137 mAbs. We found that the timing of administration of anti-CD137 mAbs profoundly altered the nature of the antiviral immune response during acute infection. Antiviral immunity progressed normally for the first 72 hours when the mAb was administered early in infection before undergoing complete collapse by day 8 postinfection. Anti-CD137-injected LCMV-infected mice became tolerant to, and persistently infected with, LCMV Armstrong. Elevated levels of IL-10 early in the response was key to the loss of CD4(+) T cells, whereas CD8(+) T cell deletion was dependent on a prolonged TNF-alpha response, IL-10, and upregulation of Fas. Blocking IL-10 function rescued CD4 antiviral immunity but not CD8(+) T cell deletion. Anti-CD137 treatment given beyond 72 hours after infection significantly enhanced antiviral immunity. Mice treated with anti-CD137 mAb 1 day before infection with A/PR8/34 virus experienced 80% mortality compared with 40% mortality of controls. When treatment was delayed until day 1 postinfection, 100% of the infected mice survived. These data show that anti-CD137 mAbs can induce T cell activation-induced cell death or enhance antiviral immunity depending on the timing of treatment, which may be important for vaccine development.     

Roles of CD4+ and CD8+ cells, and the effect of administration of recombinant murine interferon gamma in listerial infection

Studies were made on the effects of in vivo administration of anti-CD4 mAb, anti-CD8 mAb, or a combination of both mAbs on multiplication of bacteria, the levels of serum transaminases, and mortality in mice infected with Listeria monocytogenes. Results showed that in sublethal infection, CD8+ cells enhanced the peak of bacterial multiplication and liver cell necrosis, and CD4+ cells suppressed CD8+ cell-mediated enhancement. Results also showed that either CD4+ or CD8+ cells were necessary for, and capable of, mediating clearance of the bacteria. CD8+ cells were more efficient than CD4+ cells, but for optimal clearance both were necessary. In lethal listeriosis, treatment of mice with anti-CD8 mAb or a combination of both anti-CD4 and anti-CD8 mAbs, but not anti-CD4 mAb only, protected mice from death by decreasing multiplication of bacteria in the liver and spleen after a peak of approximately 10(8) CFU, and lowering the elevated serum levels of transaminases. These findings indicated that CD8+ cells were responsible for causing irreversible systemic Listeria infection and severe liver necrosis. In lethal listeriosis, administration of rMuIFN-gamma markedly prolonged survival by decreasing multiplication of bacteria and promoting recovery from liver necrosis.     

CD4+CD25+ regulatory T cells inhibit natural killer cell functions in a transforming growth factor-beta-dependent manner

Tumor growth promotes the expansion of CD4+CD25+ regulatory T (T reg) cells that counteract T cell-mediated immune responses. An inverse correlation between natural killer (NK) cell activation and T reg cell expansion in tumor-bearing patients, shown here, prompted us to address the role of T reg cells in controlling innate antitumor immunity. Our experiments indicate that human T reg cells expressed membrane-bound transforming growth factor (TGF)-beta, which directly inhibited NK cell effector functions and down-regulated NKG2D receptors on the NK cell surface. Adoptive transfer of wild-type T reg cells but not TGF-beta-/- T reg cells into nude mice suppressed NK cell-mediated cytotoxicity, reduced NKG2D receptor expression, and accelerated the growth of tumors that are normally controlled by NK cells. Conversely, the depletion of mouse T reg cells exacerbated NK cell proliferation and cytotoxicity in vivo. Human NK cell-mediated tumor recognition could also be restored by depletion of T reg cells from tumor-infiltrating lymphocytes. These findings support a role for T reg cells in blunting the NK cell arm of the innate immune system.     

Cellular basis of skin allograft rejection across a class I major histocompatibility barrier in mice depleted of CD8+ T cells in vivo.

The present study was undertaken to define the cellular mechanisms involved in the rejection of major histocompatibility complex (MHC) class I disparate skin grafts by mice depleted of CD8+ T cells in vivo. Mice were effectively depleted of CD8+ T cells by adult thymectomy followed by in vivo administration of anti-CD8 monoclonal antibody (mAb) and then engrafted with allogeneic skin. We found that CD8 depleted mice did reject MHC class I disparate skin grafts, but only when the grafts also expressed additional alloantigens. Despite the marked depletion of CD8+ T cells in these mice, we found that their rejection of MHC class I disparate grafts was mediated by CD8+ cytolytic T lymphocyte (CTL) effectors that had escaped depletion. These CD8+ CTL effectors were unique in that: (a) their generation was dependent upon the injected anti-CD8 mAb and upon exposure to class I MHC alloantigens expressed on the engrafted skin, and (b) their effector function was resistant to blockade by anti-CD8 mAb. We observed that the additional alloantigens coexpressed on MHC class I disparate grafts that triggered graft rejection in CD8-depleted mice could be MHC-linked or not and that they functioned in these rejection responses to activate third party specific CD4+ T helper (Th) cells to provide helper signals for the generation of CD8+ anti-CD8 resistant CTL effector cells. Thus, mice depleted of CD8+ T cells by thymectomy and in vivo administration of anti-CD8 mAb harbor a unique population of anti-CD8 resistant, CD8+ effector cells that mediate anti-MHC class I responses in vivo and in vitro, but require help from third party specific Th cells to do so.     

Dendritic cell-expanded, islet-specific CD4+ CD25+ CD62L+ regulatory T cells restore normoglycemia in diabetic NOD mice

Most treatments that prevent autoimmune diabetes in nonobese diabetic (NOD) mice require intervention at early pathogenic stages, when insulitis is first developing. We tested whether dendritic cell (DC)-expanded, islet antigen-specific CD4+ CD25+ suppressor T cells could treat diabetes at later stages of disease, when most of the insulin-producing islet beta cells had been destroyed by infiltrating lymphocytes. CD4+ CD25+ CD62L+ regulatory T cells (T reg cells) from BDC2.5 T cell receptor transgenic mice were expanded with antigen-pulsed DCs and IL-2, and were then injected into NOD mice. A single dose of as few as 5x10(4) of these islet-specific T reg cells blocked diabetes development in prediabetic 13-wk-old NOD mice. The T reg cells also induced long-lasting reversal of hyperglycemia in 50% of mice in which overt diabetes had developed. Successfully treated diabetic mice had similar responses to glucose challenge compared with nondiabetic NOD mice. The successfully treated mice retained diabetogenic T cells, but also had substantially increased Foxp3+ cells in draining pancreatic lymph nodes. However, these Foxp3+ cells were derived from the recipient mice and not the injected T reg cells, suggesting a role for endogenous T reg cells in maintaining tolerance after treatment. Therefore, inoculation of DC-expanded, antigen-specific suppressor T cells has considerable efficacy in ameliorating ongoing diabetes in NOD mice.     

CD8 is required during positive selection of CD4-/CD8+ T cells

Interactions between self-MHC molecules and T cells are necessary for the proper development of mature T cells, in part due to an absolute requirement for self-MHC-TCR interactions. Recently, we showed that CD4-mediated interactions also participate in shaping the T cell repertoire during thymic maturation. We now examine the possible role of the CD8 molecule during in vivo T cell development. Our results demonstrate that perinatal thymi treated with intact anti-CD8 mAb fail to generate CD8 single-positive T cells, while the generation of the other main phenotypes remains unchanged. Most importantly, the use of F(ab')2 anti-CD8 mAb fragments gave identical results, i.e., lack of generation of CD4-/CD8+ cells, with no effect on the generation of CD4+/CD8+. Furthermore, selective blocking of one CD8 allele with F(ab')2 mAbs in F1 mice expressing both CD8 alleles did not interfere with the development of CD4-/CD8+ cells, demonstrating that the absence of CD8+ T cells in homozygous mice is not due to depletion, but rather is caused by a lack of positive selection. This is most likely attributable to a deficient CD8-MHC class I interaction. Our findings strongly advocate that CD8 molecules are vital to the selection process that leads to the development of mature single-positive CD8 T cells.     

NFATc2 and NFATc3 transcription factors play a crucial role in suppression of CD4+ T lymphocytes by CD4+ CD25+ regulatory T cells

The phenotype of NFATc2(-/-) c3(-/-) (double knockout [DKO]) mice implies a disturbed regulation of T cell responses, evidenced by massive lymphadenopathy, splenomegaly, and autoaggressive phenomena. The population of CD4(+) CD25(+) T cells from DKO mice lacks regulatory capacity, except a small subpopulation that highly expresses glucocorticoid-induced tumor necrosis factor receptor family-related gene (GITR) and CD25. However, neither wild-type nor DKO CD4(+) CD25(+) regulatory T cells (T reg cells) are able to suppress proliferation of DKO CD4(+) CD25(-) T helper cells. Therefore, combined NFATc2/c3 deficiency is compatible with the development of CD4(+) CD25(+) T reg cells but renders conventional CD4(+) T cells unresponsive to suppression, underlining the importance of NFAT proteins for sustaining T cell homeostasis.     

Selective activation of gamma/delta + T cell clones by single anti-CD2 antibodies

The CD2 antigen is the target for an "alternative" T cell activation pathway. Numerous studies have demonstrated that pairs of monoclonal antibodies (mAbs) directed toward two different epitopes are required for activation of T cell receptor (TCR)-alpha/beta + T cells via CD2. We have now explored the activation of human TCR-gamma/delta + T cell clones by a panel of anti-CD2 mAbs directed against the sheep erythrocyte-binding (T11.1) epitope of CD2. Seven of seven gamma/delta + clones expressing different molecular forms of the TCR-gamma/delta responded to stimulation by a single anti-CD2 mAb (OKT11, 9E8, BW0110, M-T910) with IL-2 secretion and/or proliferation. Immobilization of anti-CD2 mAbs in microculture plates was essential for activation of gamma/delta + clones, which occurred in the absence of feeder cells. In addition to interleukin 2 (IL-2) production and proliferation, anti-CD2 mAbs also triggered cytotoxic effector activity in gamma/delta + clones as measured against FcR+ P815 target cells. In contrast to gamma/delta + clones (but in line with established data), none of five CD4+ or CD8+ TCR-alpha/beta + clones were activated by any of the tested individual anti-CD2 mAbs. Taken together, our results reveal a striking difference between cloned gamma/delta + and alpha/beta + T cells in that gamma/delta + T cells are selectively activated by a single anti-CD2 (T11.1) mAb, without need for the simultaneous signal of a second anti-CD2 mAb directed against another (T11.2 or T11.3) CD2 epitope.     

CD27 cooperates with the pre-T cell receptor in the regulation of murine T cell development

CD27 is a lymphocyte-specific member of the TNF receptor family and has a TNF-related transmembrane ligand, CD70. The CD27/CD70 receptor-ligand pair cooperates with the TCR in the regulation of the peripheral T cell response. The study presented here reveals that CD27 may play a similar role in thymic pre-T cell development. We have previously cloned the cDNA encoding murine CD27, prepared specific mAbs and observed that murine CD27 is expressed on virtually all thymocytes, with the exception of a subpopulation of CD4-8- precursor T cells. It is shown here that induction of murine CD27 expression occurs at the transition from the CD4-8-25+ to the CD4-8-25- precursor T cell stage and is regulated by the pre-TCR. Therefore, we investigated whether CD27 contributes to pre-TCR-mediated thymocyte development. Pre-TCR function was mimicked by the induction of CD3 signaling in thymocytes of recombination activating gene (RAG)-deficient mice. This in vivo anti- CD3 epsilon mAb treatment induces an about fifty fold numerical expansion of CD4-8-25+ thymocytes and their differentiation to the CD4+8+25- stage. Co-injection of anti-CD27 mAb inhibited the CD3- mediated expansion and differentiation of the CD4-8-25+ precursor population. Also, injection of anti-CD27 mAb in TCR alpha-/- mutant mice led to a reduction in the absolute number of CD4+8+25- thymocytes. We present evidence that in these in vivo systems, anti-CD27 mAb inhibits CD27-ligand interaction. Therefore, we conclude that CD27 may contribute to normal murine T cell development by synergizing with the pre-TCR-mediated signal.     

删除CD4^＋CD25^＋T细胞对OVA诱导的小鼠体液免疫应答的影响

目的观察删除CD4＋CD25＋T细胞对OVA免疫小鼠体液免疫应答的影响。方法小鼠腹腔注射抗CD25单克隆抗体,分别于3天、10天、27天采血流式检测外周血中CD4＋CD25＋T细胞的比例;注射抗CD25单克隆抗体3天后,OVA加铝佐剂免疫未删除和删除CD4＋CD25＋T细胞小鼠,7天后加强免疫一次,分别于初次免疫后7天,加强免疫后14天采血制备血清,ELISA法检测血清中OVA特异性IgE和IgG1的浓度。结果注射抗CD25单克隆抗体3天和10天,外周血中无CD4＋CD25＋T细胞;27天后CD4＋CD25＋T细胞的比例部分恢复。初次免疫7天后,删除CD4＋CD25＋T细胞小鼠总IgE、OVA特异性IgE和IgG1浓度较未删除组升高;加强免疫14天后,删除CD4＋CD25＋T细胞小鼠OVA特异性IgE较未删除组升高。结论删除CD4＋CD25＋T细胞能够影响小鼠OVA特异性体液免疫应答。     

Requirement for CD4+ cells in resistance to Pneumocystis carinii pneumonia in mice

The importance of CD4+ cells in resistance to Pneumocystis carinii (PC) in PC-susceptible severe combined immunodeficient (SCID) mice that were made resistant to PC by immunocompetent spleen cell transfer, and in conventional PC-resistant mice, was investigated. SCID mice with naturally acquired PC pneumonia (PCP) were given infusions of spleen cells from immunocompetent donors. This reconstitution caused the recipients to resolve their PCP. Treatment of reconstituted SCID mice with anti-CD4 monoclonal antibodies (mAbs) to deplete them of CD4+ cells eliminated their ability to resolve PCP, whereas treating them with anti-CD8 mAb to deplete CD8+ cells had no effect. The findings indicate, therefore, that resistance to PCP in immunologically reconstituted SCID mice is dependent on CD4+ cells. To determine whether CD4+ cells enable conventional mice to resist PCP, B6D2 mice were treated with anti-CD4 mAb to deplete them of CD4+ cells in an attempt to induce PCP. After 10-11 wk of treatment, these mice developed progressive PCP. Taken together, these results indicate that loss of CD4+ cells predisposes mice to PC infection.     

CD69-mediated pathway of lymphocyte activation: anti-CD69 monoclonal antibodies trigger the cytolytic activity of different lymphoid effector cells with the exception of cytolytic T lymphocytes expressing T cell receptor alpha/beta

The effect of anti-CD69 monoclonal antibodies (mAbs) on the induction of the cytolytic activity in different types of lymphoid effector cells has been investigated. Three anti-CD69 mAbs, including the reference mAb MLR3 and two new mAbs (c227 and 31C4), have been used. All cloned CD3-CD16+ natural killer (NK) cells belonging to different subsets (as defined by the surface expression of GL183 and/or EB6 antigens) were efficiently triggered by anti-CD69 mAbs and lysed P815 mastocytoma cells in a redirected killing assay. Triggering of the cytolytic activity could also be induced in CD3-CD16- NK clones, which fail to respond to other stimuli (including anti-CD16, anti-CD2 mAbs, or phytohemagglutinin). A similar triggering effect was detected in T cell receptor (TCR) gamma/delta+ clones belonging to different subsets. On the other hand, anti-CD69 mAbs could not induce triggering of the cytolytic activity in TCR alpha/beta+ cytolytic clones. Since all thymocytes are known to express CD69 antigen after cell activation, we analyzed a series of phenotypically different cytolytic thymocyte populations and clones for their responsiveness to anti-CD69 mAb in a redirected killing assay. Again, anti-CD69 mAb triggered TCR gamma/delta+ but not TCR alpha/beta+ thymocytes. Anti-CD69 mAb efficiently triggered the cytolytic activity of "early" thymocytes lines or clones (CD3-4-8-7+), which lack all other known pathways of cell activation. Thus, it appears that CD69 molecules may initiate a pathway of activation of cytolytic functions common to a number of activated effector lymphocytes with the remarkable exception of TCR alpha/beta+ cytolytic cells.