Cellular mechanisms that maintain neonatally-induced tolerance of class II alloantigens. Evidence that factor-mediated suppression silences cytotoxic T cell activity.

Abstract

Virtually all neonatal mice of the A strain background are rendered profoundly and permanently tolerant of test skin allografts if they receive an i.v. inoculation of semiallogeneic hematopoietic cells expressing class II disparate alloantigens. After neonatally injected mice reach immunologic maturity, their lymphoid organs have been found to contain 1) tolerogen-specific CD4+ T cells that proliferate and secrete IL-4 when stimulated in vitro with class II tolerogens and 2) tolerogen-specific CD8+ T cell that fail to differentiate into cytotoxic effector cells. In this study, experiments are described that investigate the possibility that tolerance is maintained by regulatory T cells of the Th2-type. When A.TH T cells were stimulated in vitro with A.TL alloantigens in the presence of lymphoid cells from tolerant mice, tolerogen-specific cytotoxic T cells responses were absent or greatly diminished. 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-specific cytotoxic T cell generation was achieved when regulatory cells and naive responder cells were separated by a transwell barrier and supernatants harvested from cultures in which tolerant cells were stimulated specifically with class II tolerogens also inhibited cytotoxic T cell generation. Thus, suppression appears to be mediated by a soluble factor(s) produced by regulatory T cells. We conclude that tolerance of class II alloantigens is maintained, at least in part, by regulatory cells of the Th2-type that secrete factors that suppress the generation of tolerogen-specific effector cells required for rejection of solid tissue allografts.