The Basis of Immunogenicity of Endocrine Allografts

Abstract

Two signals are required for optimal T-cell activation: the engagement of the antigen-specific receptor and the provision of a second non-antigen-specific inductive signal, or costimulator (CoS). Regarding allograft immunity, two primary pathways of donor antigen presentation can fulfill this two-signal requirement, resulting in cellular immunity to a transplant: (1) "direct" (donor MHC-restricted) presentation in which the antigen-presenting cells (APCs) resident within the transplant directly activate host T lymphocytes and (2) "indirect" (host MHC-restricted) presentation in which host-derived APCs acquire donor antigens that are then presented to host T lymphocytes. It appears that endocrine allografts, such as pancreatic islets and thyroid, are highly dependent on donor-derived APCs, or "passenger leukocytes," to trigger acute graft rejection. Tissue pretreatment aimed at selectively eliminating APCs within endocrine tissues can result in indefinite allograft survival in immune-competent recipients. Although such results implicate the "direct" pathway as the predominant route of host sensitization, the role of donor APCs in rejection appears to be more complex. Recently, we have found that indirect, CD4 T-cell-dependent reactivity can contribute to islet allograft rejection. However, such indirect recognition nevertheless requires donor-derived APCs as a source of antigen. Thus, whereas the donor-type APC is a critical limiting step for initiating islet allograft rejection, such cells can trigger both direct and indirect forms of immune responses that can result in graft rejection. That is, donor hematopoietic cells, rather than tissue parenchymal cells, probably play a major role in providing antigens that stimulate cellular immunity.