Cellular mechanisms that maintain neonatally-induced tolerance of class II alloantigens. Evidence that precursor cytotoxic T cells are present but silenced

Clonal deletion of alloantigen-specific lymphocytes was the first explanation advanced to explain why neonatal mice that receive injections of alloantigenic hematopoietic cells mature into adults that accept donor-derived skin allografts indefinitely, i.e., they are immunologically tolerant. However, numerous other passive and active regulatory mechanisms have been invoked to explain neonatal transplantation tolerance. In A strain mice (A.TH, las, A.TL, lak) rendered tolerant of class II-only alloantigens, formal evidence exists demonstrating that tolerogen-reactive T cells are not eliminated. In fact, tolerogen-reactive T cells are present in peripheral lymphoid organs and can secrete lymphokines (IL-2/IL-4) when stimulated with tolerogen-bearing cells in vitro. Despite the presence of cytokines in these cultures, class II-specific T cells are usually not generated, raising the possibility that selective deletion of these cells may contribute to the tolerant state. To examine this issue, limiting dilution analysis was performed and revealed that tolerant mice possess significantly diminished precursor cytotoxic T cell frequencies. Virtually all cytotoxic T cells generated by normal A.TH mice in response to A.TL class II Ags are CD8+ cells. Moreover, the frequency of donor I-E reactive V beta 5 cells among CD4+ and CD8+ subpopulations among tolerant mice was comparable to naive mice. This suggests that the peripheral lymphoid organs of tolerant mice are functionally deleted of tolerogen-specific cytotoxic T cells and that tolerogen-specific CD8+ T cells are present in normal numbers of tolerant mice. Therefore, this circumstantial evidence implies that tolerogen-specific T cells have not been physically eliminated in class II tolerant mice. Instead, either tolerogen-specific CD8+ T cells have been rendered anergic or active suppression prevents their activation in vitro and presumably in vivo.