Specific prolongation of MHC class II disparate skin allografts by in vivo administration of anti-IFN-gamma monoclonal antibody

In vivo rejection of MHC class II disparate skin allografts has been thought to involve IFN-gamma-induced expression of MHC class II alloantigens because less than 3% of skin epidermal cells express MHC class II alloantigens constitutively. In our study we directly tested this hypothesis by examining the effect of in vivo administered anti-IFN-gamma mAb on rejection of MHC class II disparate skin allografts, and comparing its effect on rejection of MHC class I disparate skin allografts placed on the same individual mice. We found that anti-IFN-gamma mAb blocked the rejection of MHC class II disparate skin allografts, but had no effect on the rejection of MHC class I disparate skin allografts. These results demonstrate that endogenously produced IFN-gamma is critical for rejection of MHC class II disparate skin allografts, but not for rejection of MHC class I disparate skin allografts. Thus, this study strongly supports the concept that MHC class II rejection responses require IFN-gamma induced MHC class II expression on keratinocytes of the allograft.     

The generation of cytolytic activity in mixed leukocyte cultures: cortisone-resistant thymocytes are deficient in helper T lymphocytes

Cortisone-resistant (CR) thymocytes did not generate cytolytic activity toward H-2 K or D alloantigen unless they were also stimulated by H-2 I or non-H-2 alloantigens, even though spleen cells generated brisk cytolytic activity toward H-2 K or D alone. Backstimulation by stimulating strain T lymphocytes accounted for neither the response of spleen cells toward H-2 K or D alloantigen nor the response of CR thymocytes to a full set of alloantigens. In addition, lack of non-T accessory cells did not account for the CR thymocyte pattern of reactivity. Rather, CR thymocytes appeared to be relatively deficient in helper T lymphocytes (HTL). CR thymocytes generated specific cytolytic activity toward H-2 D alloantigen when T cell growth factors (TCGF) or cloned alloreactive helper T lymphocytes were added to culture. CR thymocytes contained fewer HTL precursors detected at limit dilution than spleen cells did. Thus spleen cells generated cytolytic activity toward class I alloantigens alone, but under the same culture conditions CR thymocytes had to be stimulated by both class I and class II alloantigens. Class II alloantigens may be required to stimulate cytolytic activity only under culture conditions in which class I-reactive HTL are not sufficient to provide a minimal threshold of help.     

Rejection of skin allografts by CD4+ T cells is antigen-specific and requires expression of target alloantigen on Ia- epidermal cells

The effector mechanism of skin allograft rejection has been characterized as Ag specific, rejecting cells that express the target alloantigen but sparing those that do not. However, the rejection of MHC class II disparate skin grafts, in which very few cells (Langerhans cells) actually express the target Ia Ag could conceivably proceed by either one of two distinct rejection mechanisms. One possibility is that Ia- cells are destroyed by a sequence of events in which CD4+ T cells, activated by Ia+ LC, elaborate soluble factors that are either directly cytolytic or that recruit and activate non-specific effector cells. The alternative possibility is that activated CD4+ T cells elaborate soluble factors which induce Ia expression on Ia- cell populations, and that these Ia+ cells are subsequently destroyed by effector cells specific for the induced Ia alloantigens. We found that rejection of Ia+ LC was not of itself sufficient to cause rejection of skin grafts, indicating that skin allograft rejection is contingent on the destruction not only of LC but of other graft cell populations as well. We then investigated whether CD4+ T cells rejected allogeneic skin grafts in an antigen specific fashion. To do so, we engrafted immunoincompetent H-2b nude mice with trunk skin grafts from B6----A/J allophenic mice because such skin is composed of mutually exclusive cell populations expressing either H-2a or H-2b histocompatibility Ag, but not both. The engrafted mice were subsequently reconstituted with H-2b CD4+ T cells. The CD4+ T cells destroyed keratinocytes of A/J origin but spared keratinocytes of B6 origin, even though neither cell population constitutively expresses target IAk alloantigen. The targeted rejection of A/J keratinocytes but not of B6 keratinocytes indicates that the target Ia alloantigen must have been induced on Ia- A/J keratinocytes, rendering them susceptible to destruction by anti-Iak-specific CD4+ effector cells. These data demonstrate that CD4+ T cell rejection of skin allografts is mediated by Ag-specific CD4+ cytolytic T cells and hence, requires the induction of target Ia alloantigens on epidermal cells within the graft.     

Neonatal tolerance of H-2 alloantigens. II. I region dependence of tolerance expressed to K and D antigens

Third-party skin allografts were employed to test the specificity of transplantation tolerance achieved by neonatal inoculation of cells bearing H-2 alloantigens. Tolerant animals rejected with normal vigour third-party grafts expressing strong Class I alloantigens foreign to the host and to the donor of the tolerance-conferring inoculum. However, these animals rejected with exceptional vigour third-party grafts expressing weak Class II alloantigens plus the tolerated Class I alloantigen; even third-party grafts comprised of the host's own Class II antigens in conjunction with the tolerated Class I alloantigen were acutely rejected. It is proposed, but there is no direct evidence to prove, that rejection of these third-party grafts is mediated by killer T cells directed at the tolerated Class I alloantigens and that these cells are activated by the presentation of the putative tolerogen in an inappropriate I region context. Inconsistency of these data with a clonal deletion mechanism is discussed.     

Tolerance to antigen-presenting cell-depleted islet allografts is CD4 T cell dependent

Pretreatment of pancreatic islets in 95% oxygen culture depletes graft-associated APCs and leads to indefinite allograft acceptance in immunocompetent recipients. As such, the APC-depleted allograft represents a model of peripheral alloantigen presentation in the absence of donor-derived costimulation. Over time, a state of donor-specific tolerance develops in which recipients are resistant to donor APC-induced graft rejection. Thus, persistence of the graft is sufficient to induce tolerance independent of other immune interventions. Donor-specific tolerance could be adoptively transferred to immune-deficient SCID recipient mice transplanted with fresh immunogenic islet allografts, indicating that the original recipient was not simply "ignorant" of donor antigens. Interestingly, despite the fact that the original islet allograft presented only MHC class I alloantigens, CD8+ T cells obtained from tolerant animals readily collaborated with naive CD4+ T cells to reject donor-type islet grafts. Conversely, tolerant CD4+ T cells failed to collaborate effectively with naive CD8+ T cells for the rejection of donor-type grafts. In conclusion, the MHC class I+, II- islet allograft paradoxically leads to a change in the donor-reactive CD4 T cell subset and not in the CD8 subset. We hypothesize that the tolerant state is not due to direct class I alloantigen presentation to CD8 T cells but, rather, occurs via the indirect pathway of donor Ag presentation to CD4 T cells in the context of host MHC class II molecules.     

Some cloned murine CD4+ T cells recognize H-2Ld class I MHC determinants directly. Other cloned CD4+ T cells recognize H-2Ld class I MHC determinants in the context of class II MHC molecules.

Murine T lymphocytes recognize nominal Ag presented by class I or class II MHC molecules. Most CD8+ T cells recognize Ag presented in the context of class I molecules, whereas most CD4+ cells recognize Ag associated with class II molecules. However, it has been shown that a proportion of T cells recognizing class I alloantigens express CD4 surface molecules. Furthermore, CD4+ T cells are sufficient for the rejection of H-2Kbm10 and H-2Kbm11 class I disparate skin grafts. It has been suggested that the CD4 component of an anti-class I response can be ascribed to T cells recognizing class I determinants in the context of class II MHC products. To examine the specificity and effector functions of class I-specific HTL, CD4+ T cells were stimulated with APC that differed from them at a class I locus. Specifically, a MLC was prepared involving an allogeneic difference only at the Ld region. CD4+ clones were derived by limiting dilution of bulk MLC cells. Two clones have been studied in detail. The CD4+ clone 46.2 produced IL-2, IL-3, and IFN-gamma when stimulated with anti-CD3 mAb, whereas the CD4+ clone 93.1 secreted IL-4 in addition to IL-2, IL-3, and IFN-gamma. Cloned 46.2 cells recognized H-2Ld directly, whereas recognition of Ld by 93.1 apparently was restricted by class II MHC molecules. Furthermore, cytolysis by both clones 46.2 and 93.1 was inhibited by the anti-CD4 mAb GK1.5. These results demonstrate that CD4+ T cells can respond to a class I difference and that a proportion of CD4+ T cells can recognize class I MHC determinants directly as well as in the context of class II MHC molecules.     

The allograft defines the type of rejection (acute versus chronic) in the face of an established effector immune response

Transplanted organs fail due to either acute or chronic rejection. The prevailing view is that the nature or magnitude of the recipient's immune response to donor Ags determines the type of rejection. In variance with this view, we show in this study that the status of the graft itself plays a dominant role in defining the type of rejection even in the face of an established alloimmune response. Using adoptive transfer mouse models in which the graft is exposed to a constant number of effector lymphocytes, we found that newly transplanted heart allografts are rejected acutely, while healed-in allografts undergo chronic rejection. Acute rejection of healed-in allografts was largely recapitulated by subjecting the grafts to ischemia-reperfusion injury similar to that present in newly transplanted organs. Ischemia-Reperfusion injury altered the outcome of rejection by enhancing the accumulation of effector T cells within the graft. The accumulation of effector T cells in the graft was dependent on the presence of both ischemia-reperfusion injury (inflammation) and alloantigens. These findings demonstrate that the graft plays a dominant role in shaping the outcome of rejection by controlling the trafficking of effector T cells.     

Lymphocyte subsets differentially induce class II human leukocyte antigens on allogeneic microvascular endothelial cells

Increased expression of major histocompatibility complex class II (Ia) antigens on vascular endothelium is a common observation in allografts undergoing acute rejection. This phenomenon is generally ascribed to the host immune response directed against graft alloantigens, but its cellular and molecular basis are incompletely understood. In the present study we show that constitutively Ia-negative human microvascular endothelial cells (EC) can be induced to express surface class II human leukocyte antigens shortly after exposure to allogeneic lymphocytes in vitro. CD16+ (natural killer) and CD8+ (cytotoxic/suppressor) lymphocytes were efficient in triggering Ia antigen expression by EC, whereas CD4+ (helper/inducer) lymphocytes induced EC Ia expression only if cultured in the presence of autologous monocytes. Binding of lymphocytes to EC was shown to be essential for the subsequent induction of EC Ia, and anti-CD18 (LFA-1) antibody, which blocks lymphocyte-EC adhesion, was the only antibody of a panel of antilymphocyte antibodies that completely blocked the induction of EC Ia. Antibodies to interferon-gamma, which is a potent inducer of EC Ia, and to the CD3 T cell-surface antigen partly inhibited the induction of EC Ia by T cells, but neither antibody had any effect on Ia induction mediated by CD16+ cells, suggesting that T cells and natural killer cells utilize different mechanisms to induce Ia on EC. When combined with data from other laboratories indicating that Ia+ but not Ia- EC stimulate allogeneic T cell proliferation and cytotoxicity, our results suggest that the binding of EC by lymphocyte subpopulations followed by the induction of Ia antigen may represent the initial stage of incompatible allograft rejection.     

Immune regulation by monoclonal antibodies: failure to enhance mouse skin allografts

Monoclonal anti-H-2k alloantibodies were analyzed for their capacity to enhance the survival of B10.A skin grafted onto B10.D2 recipients. Included were five anti-class I and four anti-class II antibodies. In contrast to conventional B10.D2 anti-B10.A serum, none of the individual anti-class I or anti-class II monoclonal antibodies induced enhancement. The same negative results were obtained with various mixtures of anti-class I, anti-class II, or anti-class I + anti-class II antibodies. The failure to induce enhancement was not due to inefficient antigen binding in vivo, because monoclonal antibodies were as effective as conventional B10.D2 anti-B10.A serum in the induction of acute antibody-mediated graft rejection, and in the opsonization of 51Cr-labeled B10.A leukocytes injected into B10.D2 recipients pretreated with antibody. These results demonstrate that monoclonal antibodies cannot always substitute for conventional sera, at least not in immune regulation. They also show that although opsonization may be a prerequisite for the induction of enhancement, it does not guarantee that enhancement will invariably occur.     

Inhibition of antiskin allograft immunity induced by infusions with photoinactivated effector T lymphocytes (PET cells)

Induction of tolerance for skin allotransplantation requires selective suppression of the host response to foreign histocompatibility antigens. This report describes a new approach which employs pre-treatment with 8-methoxypsoralen (8-MOP) and ultraviolet A light (UVA) to render the effector cells of graft rejection immunogenic for the syngeneic recipient. Eight days after BALB/c mice received CBA/j skin grafts, their splenocytes were treated with 100 ng/ml 8-MOP and 1 J/cm2 UVA prior to reinfusion into naive BALB/c recipients. Recipient mice were tested for tolerance to alloantigens in mixed leukocyte culture (MLC), cytotoxicity (CTL), delayed-type hypersensitivity assays (DTH), and challenge with a fresh CBA/j graft. Splenocytes from BALB/c recipients of photoinactivated splenocytes containing the effector cells of CBA/j alloantigen rejection proliferated poorly in MLC and generated lower cytotoxic T-cell responses to CBA/j alloantigens in comparison with sensitized and naive controls and suppressed the MLC and CTL response to alloantigen from sensitized and naive BALB/c mice. In vivo, the DTH response was specifically suppressed to the relevant alloantigen in comparison with controls. BALB/c mice treated in this fashion retained a CBA/j skin graft for up to 42 days post-transplantation without visual evidence of rejection. These results showed that reinfusion of photoinactivated effector cells resulted in an immunosuppressive host response which specifically inhibited in vitro and in vivo responses that correlate with allograft rejection and permitted prolonged retention of histoincompatible skin grafts.     

Therapeutic implications of NK cell regulation of allogeneic CD8 T cell-mediated immune responses stimulated through the direct pathway of antigen presentation in transplantation

Natural killer (NK) cells are a population of innate type I lymphoid cells essential for early anti-viral responses and are known to modulate the course of humoral and cellular-mediated T cell responses. We assessed the role of NK cells in allogeneic CD8 T cell-mediated responses in an immunocompetent mouse model across an MHC class I histocompatibility barrier to determine its impact in therapeutic clinical interventions with polyclonal or monoclonal antibodies (mAbs) targeting lymphoid cells in transplantation. The administration of an NK cell depleting antibody to either CD8 T cell replete or CD8 T cell-depleted naïve C57BL/6 immunocompetent mice accelerated graft rejection. This accelerated rejection response was associated with an in vivo increased cytotoxic activity of CD8 T cells against bm1 allogeneic hematopoietic cells and bm1 skin allografts. These findings show that NK cells were implicated in the control host anti-donor cytotoxic responses, likely by competing for common cell growth factors in both CD8 T cell replete and CD8 T cell-depleted mice, the latter reconstituting in response to lymphopenia. Our data calls for precaution in solid organ transplantation under tolerogenic protocols involving extensive depletion of lymphocytes. These pharmacological biologics with depleting properties over NK cells may accelerate graft rejection and promote aggressive CD8 T cell cytotoxic alloresponses refractory to current immunosuppression.     

Regulatory mechanisms in cytotoxic T lymphocyte development. III. Induction, specificity, and genetic restriction of an in vitro suppressor T cell

We addressed questions pertaining to the immunogenetics of an in vitro alloinduced suppressor T cell (Ts) previously shown to inhibit cytotoxic T-lymphocyte (CTL) development by suppressing CTL precursor proliferation. Using intra-MHC recombinant strains of B10 congenic mice, the requirements for H-2 differences to induce Ts activity, the antigen specificity of the Ts, and the genetic restriction of Ts function were studied. It was found that differences at the K, D, or I regions alone can induce strong suppressor activity. Suppression of CTL development does not appear to be genetically restricted since the Ts inhibit CTL from responder cells disparate at K, K and D, I, or K and I. The alloinduced Ts is specific for the antigen stimulating its induction, but also inhibits CTL responses against immunologically unrelated determinants, even between class I and class II antigens, provided those determinants are carried on cells expressing the original inducing antigen. Ts can be triggered by antigens present on the responder cells but absent on the stimulator cells, indicating that the suppressive signal may be exerted directly on the responder population without specific interaction with stimulator cells.     

Antigen presentation determines the fate of the T memory response in vivo after sublethal gamma-irradiation.

The survival of memory T cells is critical to vaccination strategies for infectious diseases and cancer, whereas their elimination may be crucial for treatment of autoimmune states. We examined the consequences of gamma-irradiation, which induces apoptosis of memory T cells in vitro, on the memory response to MHC class I alloantigen in vivo. Sublethal gamma-irradiation of primed mice eliminated accelerated rejection of skin allografts but failed to induce tolerance. Accelerated rejection was restored in irradiated mice by infusion of bone marrow cells expressing the priming alloantigen on immunostimulatory APCs (dendritic cells), whereas the memory response was not restored by infusion of bone marrow cells expressing the priming alloantigen on nonstimulatory APCs (B cells). Strikingly, irradiated mice infused with nonstimulatory bone marrow APCs exhibited long-term survival or tolerance to skin grafts expressing the priming MHC class I alloantigen. The mechanism of tolerance in this setting is explored.     

Cutting edge: atopy promotes Th2 responses to alloantigens and increases the incidence and tempo of corneal allograft rejection

A large body of evidence suggests that corneal allograft rejection is mediated by a type 1 Th cell response and that deviation toward type 2 immunity favors graft survival. However, clinical observations indicate that patients with severe ocular allergies have increased risk of corneal allograft rejection. We used a mouse model of atopic conjunctivitis to evaluate the effects of Th2 immune deviation on corneal allograft survival and possible mechanisms of graft rejection. Our results reveal the following novel findings: 1) atopic conjunctivitis promotes systemic Th2 immune responses to corneal graft donor alloantigens; 2) corneal allografts in atopic host eyes have an increased incidence and swifter tempo of rejection; 3) increased rejection is associated with alterations in systemic T cell-mediated responses to donor alloantigens; and 4) corneal allograft rejection in atopic hosts does not require the direct involvement of infiltrating eosinophils.     

Heterogeneity of CD4+ T cells involved in anti-allo-class I H-2 immune responses. Functional discrimination between the major proliferating cells and helper cells assisting cytotoxic T cell responses.

MLR in various combinations with class I H-2 disparity revealed that there are three patterns of MLR in the aspect of responding T subset (CD4 vs CD8) dominance. Irrespective of the CD8 vs CD4 dominance, a single i.v. administration of class I-disparate allogeneic spleen cells resulted in almost complete abrogation of anti-class I proliferative capacity of both CD4+ and CD8+ T cells in six combinations. The suppression of proliferative responses was correlated with the striking reduction in the ability to produce IL-2 upon stimulation with the relevant class I alloantigens. In contrast, i.v. presensitized recipient mice exhibiting only marginal MLR/Il-2 production could generate comparable magnitudes of anti-allo class I CTL as well as graft rejection responses to those induced by normal unpresensitized mice. The administration in vivo of anti-CD4 antibody along with the i.v. presensitization not only suppressed the generation of CTL responses by spleen cells but also induced appreciable prolongation of allo-class I-disparate skin grafts under conditions in which neither alone did it. These results demonstrate that 1) the suppression of graft rejection responses is not necessarily reflected on the reduction of MLR; 2) CD8+ CTL precursors responsible for graft rejection can be activated by either allo-class I-reactive CD8+ or CD4+ Th cells; 3) i.v. presensitization induces functional elimination of CD8+ and CD4+ proliferative/IL-2-producing T cells but not of CD8+ CTL precursors and CD4+ Th whose capacity is expressed by assistance of CTL induction but not by their own proliferation. Thus, this study illustrates the heterogeneity of class I alloantigen-reactive CD4+ T cells in the aspect of their capacity to proliferate themselves vs contribute to CTL induction as well as graft rejection.     

Anti-MHC class I antibody activation of proliferation and survival signaling in murine cardiac allografts

Anti-MHC class I alloantibodies have been implicated in the process of acute and chronic rejection because these Abs can bind to endothelial cells and transduce signals leading to the activation of cell survival and proliferation pathways. To characterize the role of the MHC class I-signaling pathway in the pathogenesis of Ab-mediated rejection, we developed a mouse vascularized heterotopic cardiac allograft model in which B6.RAG1 KO hosts (H-2K(b)/D(b)) received a fully MHC-incompatible BALB/c (H-2K(d)/D(d)) heart transplant and were passively transfused with anti-donor MHC class I Ab. We demonstrate that cardiac allografts of mice treated with anti-MHC class I Abs show characteristic features of Ab-mediated rejection including microvascular changes accompanied by C4d deposition. Phosphoproteomic analysis of signaling molecules involved in the MHC class I cell proliferation and survival pathways were elevated in anti-class I-treated mice compared with the isotype control-treated group. Pairwise correlations, hierarchical clustering, and multidimensional scaling algorithms were used to dissect the class I-signaling pathway in vivo. Treatment with anti-H-2K(d) Ab was highly correlated with the activation of Akt and p70S6Kinase (S6K). When measuring distance as a marker of interrelatedness, multidimensional scaling analysis revealed a close association between members of the mammalian target of rapamycin pathway including mammalian target of rapamycin, S6K, and S6 ribosomal protein. These results provide the first analysis of the interrelationships between these signaling molecules in vivo that reflects our knowledge of the signaling pathway derived from in vitro experiments.     

In vivo priming of mouse CTL precursors directed to product of a newly defined minor H-42 locus is under a novel control of class II MHC gene

In a previous study, we discovered a new mouse minor histocompatibility antigen encoded by a locus at 8.5 cM apart from the H-2 complex, and we have since named the locus H-42. One allele of H-42, which is named H-42a, had been elucidated, but the other alleles, which we tentatively named H-42b, have not been elucidated. In the present study, we explored MHC control on the anti-H-42a cytotoxic T lymphocyte (CTL) responsiveness in H-42b mice. In vivo immunization (i.v. injection) of H-42b mice with 5 to 30 X 10(6) spleen cells (SC) bearing allogeneic H-42a antigen but carrying H-2 complex (mouse MHC) matched with the H-42b mice failed to prime anti-H-42a CTL but induced stable and specific anti-H-42a CTL unresponsiveness, i.e., tolerance, in the H-42b recipient mice. In contrast, H-2 heterozygous H-42b F1 mice injected with SC bearing H-42a alloantigen on either of the parental H-2 haplotypes were effectively primed to generate anti-H-42a CTL. Exploration of the region or subregion in the H-2 complex of H-42a donor SC that should be compatible with H-42b recipient mice for the induction of their anti-H-42a CTL tolerance demonstrated that the compatibility at I region, most probably I-A subregion, but not at K, S, or D region, determined the induction of the tolerance. MHC class II compatible H-42a skin graft (SG) to H-42b mice, however, consistently primed the anti-H-42a CTL in the H-42b recipients. These results were discussed in several aspects, including uniqueness of MHC class II control on the CTL response to minor H-42a antigen, possibility of inactivation of responding anti-H-42a precursor CTL or helper T cells in H-42b mice by encountering the veto cells present in MHC class II-matched H-42a SC population, and significance of the present observations as a mechanism of CTL tolerance to self-components.     

Regulation of transplantation immunity in vivo by monoclonal antibodies recognizing host class II restriction elements. I. Genetics and specificity of anti-Ia immunotherapy in murine skin allograft recipients

Antibodies reactive with host class II restriction elements exert profound regulatory effects on the immune response to a variety of antigenic stimuli, including tumor, autoantigens, and alloantigens. In the present studies, monoclonal reagents specific for host I-A and I-E glycoproteins were evaluated for their capacity to modulate transplantation immunity in a murine tail skin allograft system. It was found that treatment of A/J mice with 200 micrograms 10-3.6 hybridoma-derived anti-I-Ak antibody daily for 10 days resulted in an average twofold increase in the survival time of B10.A minor antigen-incompatible allografts. Similar results were achieved by using class I, but not H-2-mismatched, donor tissue. Specificity of antibody activity was demonstrated by the failure of isotype-matched reagents recognizing irrelevant class II or relevant class I H-2 antigens to influence rejection under these conditions. Although antibodies reactive with graft as well as host alloantigens provided the greatest degree of prolongation, interaction with host restriction elements alone was sufficient for the in vivo expression of regulatory activity. As in previous studies, anti-I-A treatment was associated with the development of antigen-specific suppressor T cells that serve to dampen allograft immunity without altering secondary responses to unrelated antigens encountered after the initial treatment interval. These data suggest that anti-Ia immunotherapy may provide a clinically relevant approach toward the specific regulation of transplantation immunity in the appropriate donor-recipient combinations.     

Neonatal tolerance of H-2 alloantigens. I. I region modulation of tolerance potential of K and D antigens

By utilizing H-2 congenic strains of mice and appropriate recombinants, it has been possible to determine that Class I alloantigens (of K and D region genes) function poorly as tolerogens when inoculated into neonatal recipients. Alternatively, Class II alloantigens (encoded by I region genes) are excellent tolerogens. Moreover, Class I alloantigens are converted to good tolerogens when conjoined in the tolerizing inoculum with Class II alloantigens. The I subregions in which genes reside that mediate this tolerance-promoting effect are IJ and/or IE. It is suggested that elicitation of a suppressor mechanism, perhaps related to IJ region alloantigens, is responsible.     

Regulatory T cells in γ irradiation-induced immune suppression

Sublethal total body γ irradiation (TBI) of mammals causes generalized immunosuppression, in part by induction of lymphocyte apoptosis. Here, we provide evidence that a part of this immune suppression may be attributable to dysfunction of immune regulation. We investigated the effects of sublethal TBI on T cell memory responses to gain insight into the potential for loss of vaccine immunity following such exposure. We show that in mice primed to an MHC class I alloantigen, the accelerated graft rejection T memory response is specifically lost several weeks following TBI, whereas identically treated na?ve mice at the same time point had completely recovered normal rejection kinetics. Depletion in vivo with anti-CD4 or anti-CD25 showed that the mechanism involved cells consistent with a regulatory T cell (T reg) phenotype. The loss of the T memory response following TBI was associated with a relative increase of CD4+CD25+ Foxp3+ expressing T regs, as compared to the CD8+ T effector cells requisite for skin graft rejection. The radiation-induced T memory suppression was shown to be antigen-specific in that a third party ipsilateral graft rejected with normal kinetics. Remarkably, following the eventual rejection of the first MHC class I disparate skin graft, the suppressive environment was maintained, with markedly prolonged survival of a second identical allograft. These findings have potential importance as regards the immunologic status of T memory responses in victims of ionizing radiation exposure and apoptosis-inducing therapies.