Exogenous control of I-A expression in fetal thymus explants.

With the use of a system in which 14 day fetal thymus lobes were cultured in vitro with bone marrow or other lymphoid cells, evidence was obtained that entry of macrophage/dendritic cells (M phi/DC) into the thymus causes a marked rise in the density of endogenous I-A molecules expressed in the cortex, presumably on epithelial cells. High cortical I-A expression also occurred when thymus lobes were cultured with supernatants containing IFN-gamma; addition of anti-IFN-gamma antibody blocked I-A induction. The working hypothesis for these findings is that cellular interactions occurring in the medullary region between M phi/DC and a subset of thymocytes leads to production of IFN-gamma, which then diffuses into the cortex and promotes epithelial I-A expression. The possible relevance of this scheme to the process of thymic "education" is discussed.