Mixed lymphocyte cultures produce effector cells: model in vitro for allograft rejection

Transplant Tolerance, Not Only Clonal Deletion

The quest to understand how allogeneic transplanted tissue is not rejected and how tolerance is induced led to fundamental concepts in immunology. First, we review the research that led to the Clonal Deletion theory in the late 1950s that has since dominated the field of immunology and transplantation. At that time many basic mechanisms of immune response were unknown, including the role of lymphocytes and T cells in rejection. These original observations are reassessed by considering T regulatory cells that are produced by thymus of neonates to prevent autoimmunity. Second, we review "operational tolerance" induced in adult rodents and larger animals such as pigs. This can occur spontaneously especially with liver allografts, but also can develop after short courses of a variety of rejection inhibiting therapies. Over time these animals develop alloantigen specific tolerance to the graft but retain the capacity to reject third-party grafts. These animals have a "split tolerance" as peripheral lymphocytes from these animals respond to donor alloantigen in graft versus host assays and in mixed lymphocyte cultures, indicating there is no clonal deletion. Investigation of this phenomenon excludes many mechanisms, including anti-donor antibody blocking rejection as well as anti-idiotypic responses mediated by antibody or T cells. This split tolerance is transferred to a second immune-depleted host by T cells that retain the capacity to effect rejection of third-party grafts by the same host. Third, we review research on alloantigen specific inhibitory T cells that led to the first identification of the CD4+CD25+T regulatory cell. The key role of T cell derived cytokines, other than IL-2, in promoting survival and expansion of antigen specific T regulatory cells that mediate transplant tolerance is reviewed. The precise methods for inducing and diagnosing operational tolerance remain to be defined, but antigen specific T regulatory cells are key mediators.

Zinc Levels Affect the Metabolic Switch of T Cells by Modulating Glucose Uptake and Insulin Receptor Signaling

SCOPE

T cell activation requires a metabolic reprogramming from oxidative phosphorylation to aerobic glycolysis to rapidly provide substrates for biosynthesis. An individual's zinc status plays an important role in balancing the activation of T cells and is required for a proper function of immune cells. Furthermore, zinc plays an important role during effector T cell polarization to T helper cell subsets or regulatory T cells, with effector T cells relying on glycolysis and regulatory T cells on oxidative phosphorylation. Therefore, we aimed to analyze if zinc also impacts on T cell activation on the level of intracellular metabolism.

METHODS AND RESULTS

We used mixed lymphocyte culture and anti-CD3/CD28 stimulation as in vitro models for T cell activation to investigate the effect of zinc supplementation and deprivation on metabolic switching. We observed promoted glucose uptake, insulin receptor expression and signaling in both zinc conditions, whereas key metabolic enzymes were stimulated mainly by zinc deprivation. Alterations in cytokine production suggest an immune-activating effect of zinc deprivation and a balancing effect of zinc supplementation.

CONCLUSION

Our results suggest a supportive effect of both zinc supplementation and deprivation on the metabolic switch during T cell activation, adding another level of immune regulation by zinc. This article is protected by copyright. All rights reserved.

Differential Roles of Dendritic Cells in Expanding CD4 T Cells in Sepsis

Sepsis is a systemically dysregulated inflammatory syndrome, in which dendritic cells (DCs) play a critical role in coordinating aberrant immunity. The aim of this study is to shed light on the differential roles played by systemic versus mucosal DCs in regulating immune responses in sepsis. We identified a differential impact of the systemic and mucosal DCs on proliferating allogenic CD4 T cells in a mouse model of sepsis. Despite the fact that the frequency of CD4 T cells was reduced in septic mice, septic mesenteric lymph node (MLN) DCs proved superior to septic spleen (SP) DCs in expanding allogeneic CD4 T cells. Moreover, septic MLN DCs markedly augmented the surface expression of MHC class II and CD40, as well as the messaging of interleukin-1β (IL-1β). Interestingly, IL-1β-treated CD4 T cells expanded in a dose-dependent manner, suggesting that this cytokine acts as a key mediator of MLN DCs in promoting septic inflammation. Thus, mucosal and systemic DCs were found to be functionally different in the way CD4 T cells respond during sepsis. Our study provides a molecular basis for DC activity, which can be differential in nature depending on location, whereby it induces septic inflammation or immune-paralysis.

Characterization of human umbilical cord lining-derived epithelial cells and transplantation potential

In this study we describe the derivation and immunological characterization of a primary epithelial cell type from the human umbilical cord membrane. These cord lining epithelial cells (CLECs) expressed and/or secreted isoforms of the nonclassical human leukocyte antigen class I (HLA-1b) glycoproteins, HLA-G and E. Conditioned media from CLECs inhibited mitogen-stimulated T-lymphocyte responses, and in a mixed leukocyte reaction (MLR) assay, cocultured CLECs inhibited allogeneic responses with a concomitant reduction in proinflammatory cytokines. Using a transwell coculture system, it was demonstrated that these immunoregulatory effects were mediated by soluble factors secreted by CLECs, in a dose-dependent manner. Functional studies using HLA-G blocking antibody showed that the effects of CLEC-secreted products could be inhibited, thus demonstrating a significant and important role for soluble HLA-G. In vivo, we show that transplanted CLECs could be maintained for extended periods in immunocompetent mice where xenorejection rapidly destroyed primary keratinocytes, a control human epithelial cell type. Additionally, CLECs delayed the rejection of keratinocytes and extended their survival when cotransplanted, indicating an ability to protect adjacent human cell types that would otherwise be rejected if transplanted alone. We also show that CLECs transduced with a modified human proinsulin gene were transplanted intraperitoneally into streptozotocin (STZ)-induced diabetic mice, resulting in significantly lower levels of serum glucose compared to control mice. This study has characterized the immunological properties of CLECs and tested a potential therapeutic application in the treatment of a type 1 diabetes mouse model.

In vivo responses of alloreactive lymphocytes stimulated in vitro. Helper-cell activity of MLR-primed lymphocytes

Populations of mouse lymphocytes enriched in specific alloreactive cells by priming in a mixed lymphocyte response (MLR) include cells which, when injected into congenic nude mice, enable them to make alloantibody after immunization. Helper cells for the priming H-2 alloantigens (H-2b or H-2k) were enriched relative to helper cells for the other H-2 type. Furthermore, the alloantibody responses of nude mice reconstituted with lymphocytes primed twice in vitro were virtually monospecific for the priming alloantigens. These studies suggest that lymphocytes that proliferate in MLR include lymphocytes capable of giving specific help for H-2 antigens in vivo. Nude mice reconstituted with MLR-primed lymphocytes made less antibody to bacteriophage T4 and phix than mice reconstituted with unprimed cells, and fewer mice responded. Priming of cells a second time in MLR further depleted the population of phage helper cells. Similar results were sometimes, but not always, obtained when testing reconstituted nude mice for their ability to make anti-sheep erythrocyte (SRBC) responses. These results suggest that lymphocytes primed against H-2b or H-2k alloantigens do not have specificity for antigens of T4 or phix. These alloreactive cells may also lack specificity for SRBC. However, the results do not allow a definitive conclusion to be drawn.

In vivo responses of alloreactive lymphocytes stimulated in vitro. Skin graft rejection mediated by MLR-Primed lymphocytes

Mouse lymphocytes that have been primed in vitro against alloantigens show a specific increase in cells reactive to the priming antigens in mixed lymphocyte response (MLR) and include cells that are specifically cytotoxic in vitro. The primed population also contains cells capable of causing rejection of skin grafts when injected into nude mice. Functional enrichment of cells capable of rejecting skin grafts bearing specific alloantigens and depletion of cells capable of rejecting a third-party graft have been shown. Priming the cells a second time in vitro may result in a moderate enrichment of cells capable of rejecting the specific graft and depletion of cells reactive to third-party skin compared with once-primed cells. These findings support the prediction that the MLR is an in vitro model of allograft responses in vivo.

CD8 T Cells Specific for a Donor-Derived, Self-Restricted Transplant Antigen Are Nonpathogenic Bystanders after Vascularized Heart Transplantation in Mice

CD8 T cell cross-priming, an established mechanism of protective antiviral immunity, was originally discovered during studies involving minor transplantation Ags. It is unclear whether or how cross-primed CD8 T cells, reactive to donor-derived, but recipient class I MHC-restricted epitopes, could injure a fully MHC-disparate, vascularized transplant. To address this question we studied host class I MHC-restricted, male transplantation Ag-reactive T cell responses in female recipients of fully MHC-disparate, male heart transplants. Cross-priming to the immune-dominant determinant HYUtyp occurred at low frequency after heart transplantation. CD8 T cell preactivation through immunization with HYUtyp mixed in CFA did not alter the kinetics of acute rejection. Furthermore, neither HYUtyp immunization nor adoptive transfer of HYUtyp-specific TCR-transgenic T cells affected outcome in 1) a model of chronic rejection in the absence of immunosuppression or 2) a model of allograft acceptance induced by costimulatory blockade. The results support the contention that CD8 T cells reactive to host-restricted, but donor-derived, Ags are highly specific and are nonpathogenic bystanders during rejection of MHC-disparate cardiac allografts.

Cross-primed CD8(+) T cells mediate graft rejection via a distinct effector pathway

To prevent bystander destruction of healthy host tissues, cytotoxic CD8(+) T lymphocytes are fitted with specific receptors that direct their destructive forces specifically against chosen targets. We show here, however, that anti-H-Y monospecific, H-2(b-restricted MataHari CD8(+) T cells reject H-2(k) male skin grafts, with which they cannot directly interact. Such rejection is interferon-gamma-dependent and only occurs if the recipient endothelium expresses H-2(b). The findings suggest an alternate indirect effector pathway that requires processing and presentation of the donor H-Y antigen by recipient endothelium and have implications for both transplantation and autoimmune disease.

Cell-mediated cytotoxicity: a predictor of chronic rejection in pediatric HLA haploidentical renal transplants

BACKGROUND

Recipient antidonor cytotoxic T-cell activity has been associated with graft loss and acute rejection in renal allograft recipients. The role of immunologic mechanisms in the development of chronic graft rejection is controversial. We analyzed all living related renal transplants performed at Children's Hospital (Boston, MA) from 1983 to 1995 to assess whether cell-mediated cytotoxicity, determined in vitro and measured before transplantation, was predictive of chronic rejection.

METHODS

Eighty-three patients were studied retrospectively. Fifty-seven patients with one haplotype-matched renal transplants from living related donors were studied to determine the association between cell-mediated lympholysis (CML) level, acute rejection, chronic rejection, and graft failure. Acute rejection was defined by the decision to treat. Chronic rejection was defined by histology and/or the absolute serum creatinine value using an increasing serum creatinine level >1.0 mg/dl for children less than 3, a creatinine level >1.5 mg/dl for children between 3 and 10 years of age, and a creatinine level >2.0 mg/dl for children above 10 years of age. Return to dialysis or retransplantation was considered graft failure.

RESULTS

Of the 57 haploidentical patients, there were 33 males and 24 females. The mean age at transplant was 11.1 years (SD=6.7). Twelve patients developed chronic rejection, 24 patients developed acute rejection, and 7 patients had graft failure. Pretransplant cytotoxic T lymphocyte activity was associated with chronic rejection (P=0.001) and graft failure (P=0.013) but only marginally with acute rejection (P=0.058). Controlling for age and sex, Cox's proportional hazards model revealed that CML level was predictive of time to chronic rejection (P<0.01) but not acute rejection (P=0.11). It was estimated that every 1-unit increase in CML level raises the monthly risk of chronic rejection by 7%. Ten children received HLA-identical kidneys from their siblings. There were no episodes of chronic rejection after 5 years. Two patients with high CML levels had episodes of acute rejection; both patients responded to treatment.

CONCLUSION

Our data demonstrate an association between pretransplant cell-mediated cytotoxicity and the occurrence of chronic rejection in living related one-haploidentical renal transplants in pediatric patients.

Sensitivity of baboon lymphocytes to cyclosporin A and FK 506: relative resistance of alloactivated cells to CyA

The interspecies differences in CyA pharmacokinetics necessitate the establishment of optimal immunosuppressive doses in the baboon, especially as its use as host for preclinical xenografts is anticipated. We assessed the immunosuppressive effects of CyA and FK 506 on lymphocytes from chacma baboons, using human cells for comparison. At concentrations up to 100 mumol/l, neither drug was toxic to lymphocytes. FK 506 inhibited baboon and human lymphocyte proliferation and IL-2 synthesis equally. In contrast, approximately four times higher doses of CyA were needed to inhibit baboon lymphocytes responding to alloantigens. This may explain the inadequate immunosuppression of baboon graft recipients treated with clinically acceptable doses of CyA. We propose that CyA whole blood target levels of +/- 1500 ng/ml should be used in this species and we provide evidence that chacma baboons are able to tolerate such doses without nephrotoxicity.

Cell-mediated immune responses in renal transplant recipients treated with cyclosporin and prednisolone with or without azathioprine

The cell-mediated immune response was studied, using mixed lymphocyte reactivity (MLR) and cell-mediated lympholysis (CML) tests, in patients with well-functioning kidney grafts from living donors at 6 and 12 months and at 2-5 years after transplantation. The patients were allocated to treatment with cyclosporin A (CyA) and prednisolone (group A) or with CyA, prednisolone and azathioprine (group B). The MLR towards a third party were in the range of that of untreated controls while the anti-donor activities were reduced after 6 months in both groups. The CML activities in group A towards a third party were in the range of that of untreated controls at all times, while the anti-donor activities were decreased. By contrast, the CML activities in group B towards a third party were decreased during the first year and were in the range of that of normal controls at 2-5 years. The anti-donor CML activities were low at all times in group B. In summary, the two CyA protocols allowed the induction of donor-specific unresponsiveness within the first post-transplant year. The anti-third party activities were low during the first post-transplant year in recipients with triple therapy but not in those without azathioprine.

The biology of acute transplant rejection

An intriguing and increasingly understood facet of immune responses is the ability of a recipient to destroy a foreign tissue or organ graft. The phenomenon of acute rejection of an allograft involves a series of complex and inter-related cellular and humoral events, culminating in graft death. Some of the current thinking surrounding this phenomenon is reviewed.

Cultured rat kidney parenchymal components are damaged by cytotoxic lymphocytes produced in MLC

Donor-specific cytotoxic lymphocytes (CTL) can be recovered from rejecting allografts both in animal models and in humans. These allograft infiltrating CTL expressing specific cytotoxicity to relevant MHC-restricted target cells in vitro most likely constitute the major effector arm during rejection. Effector cells with similar phenotype, specificity and function can be produced in mixed lymphocyte cultures (MLC). Despite the pivotal importance of CTL in allograft rejection, there is limited information about the susceptibility of different parenchymal graft cells to CTL-mediated damage. We used cultured rat glomerular epithelial cells (GEPC), glomerular mesangial cells (GMC), tubular cells (TC) and heart endothelial cells (HEC) as distinct parenchymal target components in 4 h Chromium release assays. All of these graft components were damaged by CTL produced in 6-day MLC (specific release: 10-25%). Three-day gamma-interferon (IFN-G) treatment enhanced MHC class I and II surface expression on all our targets and increased their susceptibility for CTL-mediated lysis in our experiments (specific release: 20-60%).

HLA-DP antigens provide the proliferative impetus for the generation of cytotoxic T lymphocytes

Investigating HLA-A, -B, -C, -DR and -Dw antigens and MLR, CML, and PLT reactivity in two unrelated persons, it was found that, despite their HLA-D/DR identity, cytotoxic T lymphocytes (CTL) could be induced in the CML assay. The HLA-DP antigens proved to provide the proliferative impetus for the generation of CTL.

Induction of anomalous killing activity from antigen-specific CTL clones by adding high doses of human recombinant interleukin 2

Four out of six long-term murine cytotoxic T lymphocyte (CTL) clones specific for trinitrophenyl (TNP)-modified spleen cells could develop an anomalous cytotoxicity against syngeneic and allogeneic tumor cells upon stimulation with TNP-modified spleen cells and high doses of human recombinant interleukin 2 (rIL-2). On FACS analysis, hyperactivated CTLs were positive for Thy-1, Ly 2 and LFA-1, but negative for L3T4 and asialo GM1. The staining profile of the cells with each antibody indicated that the CTL clones consisted of just one cell type. Monoclonal anti-Ly 2.2 and anti-LAA (lymphokine-activated cell-associated antigen) antibodies inhibited cytolysis of CTL and hyperactivated CTL clones against TNP-modified spleen cells, but failed to inhibit the anomalous killing of the hyperactivated CTL. The cold target competition test suggested the degeneracy of antigen specificity. The present study demonstrated that the CTL clone acquired a new specificity for tumor target cells upon stimulation with a high dose of rIL-2.

Effects of calmodulin antagonists Cd2+ and trifluoperazine on mixed lymphocyte culture cell-mediated lysis and antibody-dependent cell cytolysis

The effects of two substances with calmodulin-antagonistic properties, cadmium and trifluoperazine, were studied on mixed lymphocyte culture-cell mediated lysis (MLC-CML) and antibody-dependent cell cytolysis (ADCC). Both cadmium and trifluoperazine readily inhibited MLC-CMC, while ADCC was inhibited only to a small extent or not at all. Trifluoperazine almost completely inhibited phagocytosis in cells whose ability to lyse antibody-coated red blood cells was unimpaired. The results suggests a striking difference in the extent in which calmodulin mediated processes take part in MLC-CML on the one hand and ADCC on the other. This indicates a profound dissimilarity in the mechanisms of these two types of cell-mediated lysis.

Immunopathology of renal allograft rejection analyzed with monoclonal antibodies to mononuclear cell markers

The composition of the mononuclear cell infiltrate in rejecting renal allografts was determined on 96 renal biopsies and 22 nephrectomy specimens by the use of monoclonal antibodies to mononuclear cell surface markers and an indirect immunoperoxidase staining technique. During rejection the composition of the infiltrate was heterogeneous, with T cells (T11), monocytes (OKM1) and HLA-DR expressing mononuclear cells the most frequent sub-populations. B cells (B1) and activated T cells, identified by OKT10, were always in the minority. The T cells infiltrate usually included the helper/inducer (T4) and cytotoxic (T8) subclasses, which suggests that both may contribute to the mediation of rejection. Whether T4 or T8 predominated in the graft did not relate to the ratio of T4:T8 in blood, the HLA A, B or DR incompatibilities of the graft, or the immunosuppressive used. The frequency of T11, T4, T8, HLA-DR positive cells and monocytes, but not B cells, increased with the severity of rejection and was similar in biopsies from patients immunosuppressed with Cyclosporine (CSA) to those given a combination of azathioprine, prednisone and antilymphocyte globulin (AZA). Severe rejection episodes which did not respond to treatment with corticosteroids were more often characterized by a predominance of T8 over T4 cells and T cells infiltrating the glomeruli. In grafts with evidence of cellular rejection, renal tubular cells were shown to have a marked increase in their expression of HLA-DR antigens compared to normal kidneys or grafts with minimal rejection. The expression of HLA-DR antigens on graft tubular cells correlated with the presence of T cells in the interstitium and the severity of rejection, except for moderate rejection in CSA treated biopsies, in which HLA-DR expression was lower than in AZA biopsies. These immunopathological studies have demonstrated that a variety of potential effector cells exist within the graft, and several features have been identified which may assist in assessing the prognosis of the rejection episode.

Leukemia X fibroblast hybrid cells prolong the lives of leukemic mice

ASL-1 leukemia X LM(TK-) fibroblast hybrid cells prolong the livers of leukemic (A/JXC3H/HeJ)F1 mice. The hybrid cells, like the fibroblast cells used in forming the hybrid, have lost malignant growth properties in immunocompetent recipients and are rejected. Mice receiving hybrid cells along with ASL-1 cells exhibit immunity toward the leukemia cells; approximately 50% of the animals injected with 10(6) or more hybrid cells along with ASL-1 cells survive more than 60 days; animals in the control group injected with leukemia cells alone invariably die in shorter intervals. The immunity generated is persistent for at least 6 months. Some leukemic mice receiving doses of combination chemotherapy which are insufficient to cure them of the disease survive for prolonged and at times indefinite periods if they are injected with hybrid cells. The immunity generated in mice receiving hybrid cells is directed toward a leukemia-associated antigen of leukemia cells expressed by hybrid cells as well. In mixed lymphocyte culture a heightened stimulation of spleen cells from hybrid cell-injected mice toward ASL-1 cells is observed.

Immunology of corneal allograft rejection. Associated human leucocyte populations

Human serum samples and peripheral blood mononuclear leucocytes (PBML) were collected from ten corneal transplant recipients over a two-year period at intervals before and after surgery, and during and after episodes of immunologic (rejection) reaction. Serum was monitored for the development of antibodies to histocompatibility antigens and PBML were stained with a panel of monoclonal reagents specific for T cell and for monocyte populations using indirect immunofluorescence. Patients who demonstrated an elevation in the Leu 2a+ (suppressor/cytotoxic) T cell population following an immunologic reaction ultimately accepted their grafts, whereas patients whose Leu 2a+ population was unaffected developed irreversible graft clouding. Additionally, a direct positive correlation between the development of elevated levels of lymphocytotoxic antibodies and helper T cells was noted in some patients during immunologic reactions. Neither DR-positive nor monocyte population changes demonstrated any consistent correlation with rejection episodes or final graft outcome.

Corneal allograft rejection. The role of the major histocompatibility complex

The greater success of corneal transplantation compared to other organ transplants has led to the concept that the cornea is a site of "immunological privilege." Corneal cells possess the antigens of the major histocompatibility complex responsible for allograft rejection in other tissues (i.e., HLA antigens). The avascularity of the cornea accounts for the relative protection of the donor cornea from the immunological surveillance of the recipient. As the roles and functions of the major histocompatibility complex are unravelled, the mechanisms responsible for host sensitization, lymphocyte activation and allograft rejection are becoming better understood. In particular, the HLA-DR antigen in humans is believed to play an integral part in allograft rejection. Langerhans cells in human corneal epithelium have been shown to bear this antigen. Evidence suggests that these cells or similar HLA-DR-bearing cells in the cornea play a major role in corneal allograft rejection. In light of these advances in transplantation immunobiology, new methods of suppressing and possibly preventing allograft rejection in corneal transplantation are presented.

Generation of cytotoxic T lymphocytes against modified self in the absence of antigen by interleukin 2-containing preparations

Spleen cells obtained from normal mice were cultured with interleukin 2 (IL2); no antigen was added. After 4-5 days, these cultures contained effector cells which lysed autologous spleen target cells that were modified with 2,4,6-trinitrobenzene sulfonic acid or fluorescein isothiocyanate. No killing was seen on unmodified spleen target cells. These effector cells were Thy 1+, Lyt 1-, 2+ and were derived from Thy 1+ precursor cells. IL2 preparations induced the generation of such cytotoxic T lymphocytes (CTL) in a dose-dependent manner. IL2-induced CTL were shown to be different from the natural killer (NK) cells augmented by IL2 by virtue of their time of appearance in culture, by cold-target competition, and by different cell-surface markers. These results demonstrate that the IL2 signal may be sufficient for the induction of the differentiation of CTL precursors in the absence of an antigenic signal.

Lectin-dependent cell-mediated cytotoxicity following in vitro culture of normal lymphocytes in medium containing 2-mercaptoethanol

Cell-mediated cytotoxic reactivity resulting from the in vitro incubation of normal lymphocytes was assessed using nonspecific lectin-dependent cell-mediated cytotoxicity (LDCC) as a measure of overall reactivity. Spleen cells from non-immune C57BL/6 mice were incubated in vitro in RPM1-1640 supplemented with 10% fetal calf serum and 2-mercaptoethanol (2ME). Cytotoxicity was assayed against syngeneic Cr51-labeled EL-4 cells in the presence of Con A or PHA. Optimal LDCC was observed after 8 days of culture in the presence of 5 X 10(-5) M 2ME. Cytotoxicity was mediated by an activated T-lymphocyte population whose development did not appear to require macrophages. Usually LDCC in the presence of PHA was significantly greater than that obtained in the presence of Con A. The presence of 2ME during the initial phase of culture was crucial for the development of cytotoxicity, since early removal of 2ME after 1 or 3 days of culture did not alter the subsequent development of cytotoxicity, whereas delayed addition of 2ME on day 1 or 3 failed to produce cytotoxic reactivity. This rapid conversion from a 2ME sensitive state to a 2ME insensitive state may be related to a rapid loss of accessory cell viability during the early phase of culture. Together the results indicate that this system may provide a useful model for the investigation of the events leading to the development of CTL in vitro.

Direct analysis of individual killer T cells: susceptibility of target cells to lysis and secretion of hydrolytic enzymes by CTL

Direct identification, enumeration and biological characterization of cytolytic T lymphocytes (CTL) complementing the chromium release test had allowed us to propose a mechanism for T-cell mediated cytotoxicity (T-CMC). One CTL without accessory cells is able to lyse a specific target. Binding which allows cell contact (E-T doublets), should activate effector CTLs and render target cells susceptible to lysis. Secretion of hydrolytic enzymes localized at the CTL-target junction accounts for the lethal hit given by CTLs to susceptible targets. This hypothesis that the resulting cell lesion is identified with an alteration of the lipid bilayer membrane had been confirmed by single killer cell study. Furthermore, we have proposed that phospholipase enzymes are involved in the membrane alterations, since susceptible targets bound to effector cells (conjugates) were lysed in a much higher yield in the presence of phospholipase. Membrane cell lesion determined by hydrolytic enzyme (phospholipase) could represent a basic general mechanism for other cellular or molecular mediated processes.

Comparison between the specificity of primary and secondary killer cells against alloantigens and hapten-modified syngeneic lymphoid cells

Cytotoxic responses of lymphoid cells from different mouse strains against syngeneic cells modified with the haptens fluorescein isothiocyanate and trinitrophenyl were investigated. Mic of the H-2k strain demonstrated strong primary in vitro hapten-specific cytotoxicity reactions, which were H-2 restricted and involved the Kk specificity. However, cells from H-2d and H-2b mice developed hapten-specific cytotoxic reactions that showed H-2 cross-reactivity. This cross-reactivity, with regard to the restriction element, was particularly evident with cells from mice that had been immunized in vivo. No cross-reaction was observed between the two haptens, and Dd target cell antigens. Similar cross-reactions were demonstrated in vitro experiments in which secondary in vitro responses were induced by stimulation with cross-reacting H-2 antigens. This finding was also investigated in allogeneic cytotoxicity. In vitro induced responses resulting in relatively weak specific cell-mediated lympholysis reactions were H-2 specific, whereas secondary in vitro responses demonstrated cross-reactivity between Dd and Db antigens. In these test systems, cross-stimulation was also demonstrated in secondary in vitro responses. These results are discussed in terms of similarities of T cell recognition of hapten-modified self antigens and of alloantigens.

Immunoregulation

The immune system of higher vertebrates is a complex network of separate, interacting cell populations, each ontogenetically endowed with specific regulatory (inductive or suppressive) or effector functions. The products of at least two major gene clusters-the immunoglobulin structural genes and the genes of the major histocompatibility complex-are expressed as active and passive recognition structures on cells of the immune system and at least some of their secreted products. Macrophages play a critical role in the initiation of immune responses. Regulatory subsets of thymus-derived lymphocytes interact with macrophages and with each other in the control of immune effector cells. At every level of the immune response, cell interactions require that these regulatory cells recognize gene products of the major histocompatibility complex. Due to recent technical advances, rapid progress is being made in identifying subsets of human immunoregulatory cells; those identified to date show strong functional homology to previously well characterized murine cell subsets.