Peptide specificity of alloreactive CD4 positive T lymphocytes directed against a major histocompatibility complex class I disparity

Indirectly Activated Treg Allow Dominant Tolerance to Murine Skin-grafts Across an MHC Class I Mismatch After a Single Donor-specific Transfusion

BACKGROUND

Tolerance induced in stringent animal transplant models using donor-specific transfusions (DST) has previously required additional immunological manipulation. Here, we demonstrate a dominant skin-allograft tolerance model induced by a single DST across an major histocompatibility class I mismatch in an unmanipulated B6 host.

METHODS

C57BL/6 (H-2) (B6) mice were injected intravenously with splenocytes from B6.C.H-2 (H-2k) (bm1) or F1 (B6 × bm1) mice before skin transplantation. Mice were transplanted 7 days postinjection with donor (bm1 or F1) and third-party B10.BR (H-2) skin grafts.

RESULTS

B6 hosts acutely rejected skin grafts from B6.C.H-2 (bm1) and F1 (B6 × bm1) mice. A single transfusion of F1 splenocytes into B6 mice without any additional immune modulation led to permanent acceptance of F1 skin grafts. This graft acceptance was associated with persistence of donor cells long-term in vivo. The more rapid removal of DST bm1 cells than F1 cells was reduced by natural killer-cell depletion. Tolerant grafts survived an in vivo challenge with naive splenocytes. Both CD4CD25 and CD4CD25 T cells from F1 DST treated B6 mice suppressed alloproliferation in vitro. Tolerance was associated with expansion of peripheral Foxp3CD4CD25 regulatory T cells (Treg) and increased forkhead box P3 (Foxp3) expression in tolerant grafts. In tolerant mice, Foxp3 Treg arises from the proliferation of indirectly activated natural Foxp3 Treg (nTreg) and depletion of Foxp3 Treg abrogates skin-graft tolerance.

CONCLUSIONS

This study demonstrates that the persistence of transfused semiallogeneic donor cells mismatched at major histocompatibility class I can enhance tolerance to subsequent skin allografts through indirectly expanded nTreg leading to dominant tolerance without additional immunological manipulation.

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.

Epitope Prediction Assays Combined with Validation Assays Strongly Narrows down Putative Cytotoxic T Lymphocyte Epitopes

Tumor vaccine design requires prediction and validation of immunogenic MHC class I epitopes expressed by target cells as well as MHC class II epitopes expressed by antigen-presenting cells essential for the induction of optimal immune responses. Epitope prediction methods are based on different algorithms and are instrumental for a first screening of possible epitopes. However, their results do not reflect a one-to-one correlation with experimental data. We combined several in silico prediction methods to unravel the most promising C57BL/6 mouse-restricted Hepatitis C virus (HCV) MHC class I epitopes and validated these epitopes in vitro and in vivo. Cytotoxic T lymphocyte (CTL) epitopes within the HCV non-structural proteins were identified, and proteasomal cleavage sites and helper T cell (Th) epitopes at close proximity to these CTL epitopes were analyzed using multiple prediction algorithms. This combined in silico analysis enhances the precision of identification of functional HCV-specific CTL epitopes. This approach will be applicable to the design of human vaccines not only for HCV, but also for other antigens in which T-cell responses play a crucial role.

CD47 is required for suppression of allograft rejection by donor-specific transfusion

CD47 is a ligand of the inhibitory receptor, signal regulatory protein (SIRP)alpha, and its interaction with SIRPalpha on macrophages prevents phagocytosis of autologous hematopoietic cells. CD47-SIRPalpha signaling also regulates dendritic cell (DC) endocytosis, activation, and maturation. In this study, we show that CD47 expression on donor cells plays an important role in suppression of allograft rejection by donor-specific transfusion (DST). DST was performed by i.v. injection of splenocytes from C57BL/6 donors into MHC class I-disparate bm1 mice 7 d prior to donor skin grafting. Administration of wild-type (WT) C57BL/6 donor splenocytes markedly prolonged donor skin survival in bm1 mouse recipients. In contrast, bm1 mice receiving DST from CD47 knockout (KO) donors showed no inhibition or even acceleration of donor skin graft rejection compared with non-DST control (naive) bm1 mice. T cells from bm1 mice receiving CD47 KO, but not WT, DST exhibited strong anti-donor responses. The ability of DST to suppress alloresponses was positively correlated with the density of CD47 molecules on donor cells, as CD47(+/-) DST was able to prolonged donor skin survival, but to a significantly less extent than WT DST. Furthermore, DCs from CD47 KO, but not WT, DST recipients showed rapid activation and contributed to donor skin rejection. These results show for the first time that CD47 on donor cells is required to repress recipient DC activation and suppress allograft rejection after DST, and suggest CD47 as a potential target for facilitating the induction of transplant tolerance.

The visualization of T cell responses

Transplanting allogeneic grafts is still significantly hampered by the rejection process, despite the use of powerful immunosuppressive agents. The T cell is recognized as playing a central role in the process of rejection, and it is believed that graft tolerance will ultimately be achieved by immunological manipulation of this cell (1, 2). As immunologists strive to define the role of the T cell in the fundamental processes of immunity and tolerance, new methods are emerging that will facilitate visualization of the T cells directly involved in the rejection response (3, 4). This overview addresses the visualization of T cell responses as made possible by these technological developments.

Alloreactive cytotoxic T-cell function, peptide nonspecific

The recognition requirements necessary for murine alloreactive cytotoxic T-cells to carry out their effector function has been investigated using target cells that express a unique class I major histocompatibility complex (MHC)-peptide pair. The human cell line T2 and the murine cell line RMA-S are defective in peptide transport components needed to effectively express stable MHC class I molecules at the cell surface. When T2 cells were infected with a vaccinia virus that encoded the Kd gene and provided with a Kd-motif peptide from the nucleoprotein of influenza virus (NPP), these cells could be lysed by polyclonal allo Kd-reactive cytotoxic T-lymphocytes (CTL). Similar results were obtained with the murine RMA-S-Kd cell line, transfected with cDNA able to express some 'empty' Kd that is heat-labile. Adding another Kd-motif peptide from influenza virus haemagglutinin (HAP) stabilized the surface expression of Kd and allowed the RMA-S-Kd cells to be lysed before or after heat shock. At 27 degrees C anti-Kd alloreactive CTL-lysed target cells in the presence and absence of HAP peptide. Alloreactive CTL appear to have a more stringent requirement for a high density of MHC class I on cell surfaces relative to peptide-specific MHC-restricted CTL. We conclude that while Kd-restricted CTL activity is strictly peptide-specific, anti-Kd-specific alloreactivity is MHC allele-specific, but peptide-nonspecific. This conclusion is at odds with the Standard Model of T-cell receptor (TCR) function, but consistent with the predictions of a Competing Model of TCR function.

Rejection of H-Y Disparate Skin Grafts by Monospecific CD4+ Th1 and Th2 Cells: No Requirement for CD8+ T Cells or B Cells

We wished to determine whether CD4+ T cells could reject a skin graft that was discordant for a single minor transplantation Ag in the absence of CD8+ T cells or Ab. Transgenic A1(M) mice were constructed that express the rearranged Vβ8.2 and Vα10 TCR genes from a T cell clone that is specific for the male Ag (H-Y) in the context of H2-Ek. In addition, the RAG-1−/− background was bred onto these mice to eliminate any endogenous TCR rearrangements. As expected, clonal deletion was found to be complete in the thymus of male A1(M)×RAG-1−/− mice, while only CD4+ T cells were positively selected and found in the periphery of females. Female A1(M)×RAG-1−/− mice were able to rapidly reject (in <14 days) male (but not female) skin grafts in a CD4-dependent fashion. After multiple grafts, it was confirmed that no CD8+ T cells or surface Ig+ B cells were present. An immunofluorescent analysis of spleen cells after grafting showed that the majority of T cells expressed activation markers (CD44, CD25, and intracytoplasmic IL-2) and a significant proportion were making IFN-γ and IL-4. Surprisingly, the transfer of either Th1 or Th2 CD4+ T cell lines from these mice into T cell-depleted recipients was sufficient to cause a specific rejection of male skin.

The mechanism of specific prolongation of class I-mismatched skin grafts induced by retroviral gene therapy

In the present study, we examine the mechanism of specific hyporesponsiveness to major histocompatibility complex (MHC) class I-mismatched skin allografts induced by retrovirus-mediated gene transfer of an allogeneic class I gene into syngeneic bone marrow (BM). Using appropriate congenic recombinant mouse strains, we have mapped MHC determinants that are capable of restoring rapid rejection of Kb-bearing skin grafts. Our results indicate that either a single class I or a single class II alloantigen expressed on skin in association with Kb is able to restore the rapid rejection of Kb-mismatched skin grafts. These data suggest that third-party alloantigens expressed on skin in association with Kb abrogate hyporesponsiveness by providing T cell help. Consistent with this interpretation, spleen cells from mice reconstituted with Kb-transduced BM were unable to elicit a significant anti-Kb cytotoxic T lymphocyte response in vitro unless interleukin-2 was added to the culture medium. Skin graft survival was also analyzed on B10. AKM mice thymectomized 3-4 weeks post-reconstitution with Kb-transduced BM. Thymectomy did not result in significantly prolonged survival of B10. MBR skin grafts compared to euthymic controls, suggesting that even early after reconstitution, intrathymic deletion of Kb-reactive T cells must have been incomplete. Taken together, these data suggest that prolongation of skin allograft survival in this model is controlled at the level of T cell help.