CELLULAR MECHANISMS THAT MAINTAIN NEONATALLY-INDUCED TOLERANCE OF CLASS-II ALLOANTIGENS - EVIDENCE THAT FACTOR-MEDIATED SUPPRESSION SILENCES CYTOTOXIC T-CELL ACTIVITY

Virtually all neonatal mice of the A strain background are rendered pr ofoundly and permanently tolerant of test skin allografts if they rece ive an i.v. inoculation of semiallogeneic hematopoietic cells expressi ng class II disparate alloantigens. After neonatally injected mice rea ch immunologic maturity, their lymphoid organs have been found to cont ain 1) tolerogen-specific CD4(+) T cells that proliferate and secrete IL-4 when stimulated in vitro with class II tolerogens and 2) toleroge n-specific CD8(+) T cells that fail to differentiate into cytotoxic ef fector cells. In this study, experiments are described that investigat e the possibility that tolerance is maintained by regulatory T cells o f the Th2-type. When A.TH T cells were stimulated in vitro with A.TL a lloantigens in the presence of lymphoid cells from tolerant mice, tole rogen-specific cytotoxic T cells responses were absent or greatly dimi nished. When regulatory cells from tolerant mice were fractionated and tested, the cell type responsible for suppression proved to be CD4(+) class II tolerogen-specific and gamma irradiation sensitive. Moreover , suppression of tolerogen-speciiic cytotoxic T cell generation was ac hieved when regulatory cells and naive responder cells were separated by a transwell barrier and supernatants harvested from cultures in whi ch tolerant cells were stimulated specifically with class II tolerogen s also inhibited cytotoxic T cell generation. Thus, suppression appear s to be mediated by a soluble factor(s) produced by regulatory T cells . We conclude that tolerance of class II alloantigens is maintained, a t least in part, by regulatory cells of the Th2-type that secrete fact ors that suppress the generation of tolerogen-specific effector cells required for rejection of solid tissue allografts.