Natalie Anosova

Tolerance to Noninherited Maternal MHC Antigens in Mice

The phenomenon of tolerance to noninherited maternal Ags (NIMA) is poorly understood. To analyze the NIMA effect C57BL/6 (H-2b/b) males were mated with B6D2F1 (H-2b/d) females, whereby 50% of the offspring are H-2b/b mice that have been exposed to maternal H-2d alloantigens. Controls were H-2b/b offspring of C57BL/6 mothers, either inbred C57BL/6 mice or F1 backcross mice from breedings with H-2b/d fathers. We found that 57% of the H-2b/b offspring of semiallogeneic (H-2b/d) mothers accepted fully allogeneic DBA/2 (H-2d/d) heart grafts for >180 days, while similar transplants were all rejected by day 11 in controls (p < 0.0004). Foster nursing studies showed that both oral and in utero exposure to NIMA are required for this tolerogenic effect. An effect of NIMA was also found to extend the survival of skin grafts from a semiallogeneic donor (p < 0.02). Pretransplant analysis of splenocytes showed a 40–90% reduction of IL-2-, IL-5-, and IFN-γ-producing T cells responding to H-2d-expressing APC in NIMAd-exposed vs control mice. Injection of pregnant BALB/c-dm2 (H-2Ld-negative) female mice i.v. with H-2Ld61–80 peptide profoundly suppressed the offspring’s indirect pathway alloreactive CD4+ T cell response to H-2Ld. These results suggest that the natural exposure of the fetus and newborn to maternal cells and/or soluble MHC Ags suppresses NIMA-allospecific T cells of the offspring, predisposing to organ transplant tolerance in adult mice.

CD28-independent Costimulation of T Cells in Alloimmune Responses

T cell costimulation by B7 molecules plays an important role in the regulation of alloimmune responses. Although both B7-1 and B7-2 bind CD28 and CTLA-4 on T cells, the role of B7-1 and B7-2 signaling through CTLA-4 in regulating alloimmune responses is incompletely understood. To address this question, we transplanted CD28-deficient mice with fully allogeneic vascularized cardiac allografts and studied the effect of selective blockade of B7-1 or B7-2. These mice reject their grafts by a mechanism that involves both CD4+ and CD8+ T cells. Blockade of CTLA-4 or B7-1 significantly accelerated graft rejection. In contrast, B7-2 blockade significantly prolonged allograft survival and, unexpectedly, reversed the acceleration of graft rejection caused by CTLA-4 blockade. Furthermore, B7-2 blockade prolonged graft survival in recipients that were both CD28 and CTLA-4 deficient. Our data indicate that B7-1 is the dominant ligand for CTLA-4-mediated down-regulation of alloimmune responses in vivo and suggest that B7-2 has an additional receptor other than CD28 and CTLA-4 to provide a positive costimulatory signal for T cells.

Role of CD4+ and CD8+ T Cells in Allorecognition: Lessons from Corneal Transplantation

Corneal transplantation represents an interesting model to investigate the contribution of direct vs indirect Ag recognition pathways to the alloresponse. Corneal allografts are naturally devoid of MHC class II+ APCs. In addition, minor Ag-mismatched corneal grafts are more readily rejected than their MHC-mismatched counterparts. Accordingly, it has been hypothesized that these transplants do not trigger direct T cell alloresponse, but that donor Ags are presented by host APCs, i.e., in an indirect fashion. Here, we have determined the Ag specificity, frequency, and phenotype of T cells activated through direct and indirect pathways in BALB/c mice transplanted orthotopically with fully allogeneic C57BL/6 corneas. In this combination, only 60% of the corneas are rejected, while the remainder enjoy indefinite graft survival. In rejecting mice the T cell response was mediated by two T cell subsets: 1) CD4+ T cells that recognize alloantigens exclusively through indirect pathway and secrete IL-2, and 2) IFN-γ-producing CD8+ T cells recognizing donor MHC in a direct fashion. Surprisingly, CD8+ T cells activated directly were not required for graft rejection. In nonrejecting mice, no T cell responses were detected. Strikingly, peripheral sensitization to allogeneic MHC molecules in these mice induced acute rejection of corneal grafts. We conclude that only CD4+ T cells activated via indirect allorecognition have the ability to reject allogeneic corneal grafts. Although alloreactive CD8+ T cells are activated via the direct pathway, they are not fully competent and cannot contribute to the rejection unless they receive an additional signal provided by professional APCs in the periphery.

The relative contribution of direct and indirect antigen recognition pathways to the alloresponse and graft rejection depends upon the nature of the transplant.

In this study, we measured direct and indirect T-cell alloresponses mediated by CD4(+) and CD8(+) T cells in three mouse transplantation models: skin, cornea, and retina. We show that the contribution of direct and indirect antigen recognition pathways to the alloresponse to fully allogeneic grafts varies depending upon the nature of the tissue/organ transplanted. The implications of this finding for understanding the cellular mechanisms by which rejection is mediated in different transplant models are discussed.

Lack of Role for CsA‐Sensitive or Fas Pathways in the Tolerization of CD4 T Cells Via BMT and Anti‐CD40L

Anti‐CD40L mAb plus bone marrow transplantation (BMT) and recipient CD8 T‐cell depletion permits long‐term mixed hematopoietic chimerism and systemic donor‐specific tolerance to be achieved across full MHC barriers. Initial tolerance is characterized by peripheral deletion of donor‐reactive CD4 cells. In regimens using costimulatory blockade without BMT to achieve allograft survival, cyclosporine inhibited graft survival, suggesting that the combination may not be clinically applicable. We assessed the role of cyclosporine‐sensitive mechanisms and the mechanisms of T‐cell apoptosis involved in the induction of early peripheral CD4+ T‐cell tolerance by BMT with anti‐CD40L. Neither a short course of cyclosporine (14 days) nor the absence of FAS‐mediated activation‐induced cell death (AICD) blocked the induction or maintenance of donor‐specific tolerance. IL‐2 production was not associated with tolerance induction, consistent with the lack of a role for Fas‐mediated AICD. Mice in which passive T‐cell death was impaired because of constitutive expression of a Bcl‐xL transgene did not develop tolerance with this protocol. These data confirm that deletion of donor‐reactive T cells is critical for the induction of mixed chimerism and tolerance. However, the mechanisms involved may differ from those involved in costimulatory blockade regimens that do not include BMT.

Induction of T-cell response to cryptic MHC determinants during allograft rejection.

The presentation of MHC peptides by recipient and donor antigen presenting cells is an essential element in allorecognition and allograft rejection. MHC proteins contains two sets of determinants: the dominant determinants that are efficiently processed and presented to T cells, and the cryptic determinants that are not presented sufficiently enough to induce T-cell responses in vivo. In transplanted mice, initial T-cell response to MHC peptides is consistently limited to a single or a few immunodominant determinants on donor MHC molecule. However, in this article we show that under appropriate circumstances the hierarchy of determinants on MHC molecules can be disrupted. First, we observed that gamma IFN can trigger de novo presentation of cryptic self-MHC peptides by spleen cells. Moreover, we showed that allotransplantation is associated with induction of T-cell responses to formerly cryptic determinants on both syngeneic and allogeneic MHC molecules. Our results suggest that cross-reactivity and inflammation are responsible for the initiation of these auto- and alloimmune responses after transplantation.

Dual effects of the alloresponse by Th1 and Th2 cells on acute and chronic rejection of allotransplants.

The contribution of direct and indirect alloresponses by CD4+ Th1 and Th2 cells in acute and chronic rejection of allogeneic transplants remains unclear. In the present study, we addressed this question using a transplant model in a single MHC class I‐disparate donor–recipient mouse combination. BALB/c‐dm2 (dm2) mutant mice do not express MHC class I Ld molecules and reject acutely Ld+ skin grafts from BALB/c mice. In contrast, BALB/c hearts placed in dm2 mice are permanently accepted in the absence of chronic allograft vasculopathy. In this model, CD4+ T cells are activated following recognition of a donor MHC class I determinant, Ld 61–80, presented by MHC Class II Ad molecules on donor and recipient APC. Pre‐transplantation of recipients with Ld 61–80 peptide emulsified in complete Freunds adjuvant induced a Th1 response, which accelerated the rejection of skin allografts, but it had no effect on cardiac transplants. In contrast, induction of a Th2 response to the same peptide abrogated the CD8+ cytotoxic T cells response and markedly delayed the rejection of skin allografts while it induced de novo chronic rejection of heart transplants. This shows that Th2 cells activated via indirect allorecognition can exert dual effects on acute and chronic rejection of allogeneic transplants.

CD4+ T Cell Responses to Self- and Mutated p53 Determinants During Tumorigenesis in Mice

We analyzed CD4+ T helper responses to wild-type (wt) and mutated (mut) p53 protein in normal and tumor-bearing mice. In normal mice, we observed that although some self-p53 determinants induced negative selection of p53-reactive CD4+ T cells, other p53 determinants (cryptic) were immunogenic. Next, BALB/c mice were inoculated with J774 syngeneic tumor cell line expressing mut p53. BALB/c tumor-bearing mice mounted potent CD4+ T cell responses to two formerly cryptic peptides on self-p53. This response was characterized by massive production of IL-5, a Th2-type lymphokine. Interestingly, we found that T cell response was induced by different p53 peptides depending upon the stage of cancer. Mut p53 gene was shown to contain a single mutation resulting in the substitution of a tyrosine by a histidine at position 231 of the protein. Two peptides corresponding to wt and mutated sequences of this region were synthesized. Both peptides bound to the MHC class II-presenting molecule (Ed) with similar affinities. However, only mut p53.225–239 induced T cell responses in normal BALB/c mice, a result strongly suggesting that high-affinity wt p53.225–239 autoreactive T cells had been eliminated in these mice. Surprisingly, CD4+ T cell responses to both mut and wt p53.225–239 peptides were recorded in J774 tumor-bearing mice, a phenomenon attributed to the recruitment of low-avidity p53.225–239 self-reactive T cells.

Influence of noninherited maternal antigens on alloimmunity and allotransplant rejection

Purpose of reviewA series of recent papers has demonstrated the powerful and long lasting impact of noninherited maternal antigens on the offspring’s immune system. Noninherited maternal antigens are transmitted as membrane-bound and soluble proteins via transplacental passage during pregnancy and during breast-feeding. The vast majority of maternal cells found in the adult offspring are of hematopoietic origin, including stem cells. Some recent papers describing potential mechanisms underlying the influence of noninherited maternal antigens in transplantation are reviewed here. Recent findingsExposure to noninherited maternal antigens decreases the frequency of corresponding inflammatory T cells recognizing alloantigens in a direct fashion. This phenomenon observed in both naïve and transplanted offspring presumably accounts for transplantation tolerance, that is, long-term survival of allografts displaying noninherited maternal antigens. In addition, trafficking of soluble maternal major histocompatibility noninherited maternal antigen proteins and/or peptides can induce long-term tolerization of offspring T cells recognizing this alloantigen via indirect allorecognition. SummaryThere are implications of the effect of noninherited maternal antigens on pediatric transplantation when the child receives a kidney from his or her mother. In this setting, pretransplantation donor transfusion is likely to trigger the noninherited maternal antigen effect and promote transplant survival. There are numerous other applications, including cord blood stem cell transplantation and cadaveric organ transplantation, as well as in nontransplantation fields such as autoimmunity and development of antitumor vaccination approaches.