Indirect T Cell Allorecognition and Alloantibody-Mediated Rejection of MHC Class I-Disparate Heart Grafts

The T cell allorecognition pathways responsible for the rejection of MHC class I disparate allografts remain poorly defined. The respective contributions of the 'direct' recognition of the allogeneic class I molecule by recipient CD8 T cells, and the 'indirect' recognition of processed allo-class I MHC peptide fragments by recipient CD4 T cells, requires clarification. Recent studies in the rat have established a role for T cell dependent antibody. MHC class I disparate PVG.R8 (RT1.Aa) heart grafts are rejected acutely in naive, and hyperacutely in sensitised, PVG.RTlu recipients by CD4 T cell dependent alloantibody. This thesis explores the T cell allorecognition pathways responsible and demonstrates that direct injection of plasmid DNA encoding a truncated, water-soluble sequence of the RT1.Aa heavy chain (pcmu-tAa) results in accelerated rejection of PVG.R8 heart grafts (MST 2 days). Pcmu-tAa injection did not generate of an allocytotoxic T cell response, but an anti-Aa IgG2b cytotoxic alloantibody response developed. That T cell recognition of the class I MHC alloantigen was restricted to the indirect pathway was confirmed by CD4 T cell depletion, which abrogated the alloantibody response and resulted in prolonged graft survival, rather than accelerated rejection. By comparison CD8 T cell depletion had no discernible effect. Priming CD4 T cells for indirect allorecogntion by the administration of synthetic 15-mer allopeptides spanning the ?1 and ?2 domains of the RT1.Aa antigen did not stimulate an alloantibody response against the intact Aa antigen but the antibody response to a subsequent PVG.R8 heart graft was accelerated. Graft rejection was, however, only modestly accelerated (MST 4 days). These results suggest that soluble class I MHC and allopeptides are equally efficient at priming CD4 T cells by the indirect pathway, but that soluble class I MHC is a more effective immunogen, not because it activates an additional subset of directly restricted T cells, but because its tertiary protein structure provides the appropriate B cell epitopes for the development of humoral immunity against the graft. This is analogous to the provision of cytotoxic T cell help by indirectly restricted T cells and the confirmation that the indirect pathway may be responsible for the development of effector mechanisms directed against intact MHC on donor cells provides the first demonstration of its role in effecting accelerated rejection. These results further imply that the potentially tolerogenic effects conferred by the ability of soluble donor class I MHC to induce apoptosis in the alloreactive CD8 cytotoxic T cell population may be masked by a detrimental indirect CD4 T cell response