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 expl anation advanced to explain why neonatal mice that receive injections of alloantigenic hematopoietic cells mature into adults that accept do nor-derived skin allografts indefinitely, i.e., they are immunological ly tolerant. However, numerous other passive and active regulatory mec hanisms have been invoked to explain neonatal transplantation toleranc e. In A strain mice (A.TH, la(s), A.TL, la(k)) rendered tolerant of cl ass II-only alloantigens, formal evidence exists demonstrating that to lerogen-reactive T cells are not eliminated. In fact, tolerogen-reacti ve T cells are present in peripheral lymphoid organs and can secrete l ymphokines (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 th at selective deletion of these cells may contribute to the tolerant st ate. To examine this issue, limiting dilution analysis was performed a nd revealed that tolerant mice possess significantly diminished precur sor cytotoxic T cell frequencies. Virtually all cytotoxic T cells gene rated 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 am ong CD4(+) and CD8(+) subpopulations among tolerant mice was comparabl e 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 nor mal numbers of tolerant mice. Therefore, this circumstantial evidence implies that tolerogen-specific T cells have not been physically elimi nated in class II tolerant mice. Instead, either tolerogen-specific CD 8(+) T cells have been rendered anergic or active suppression prevents their activation in vitro and presumably in vivo.