Expression of relB transcripts during lymphoid organ development: specific expression in dendritic antigen-presenting cells.

We have studied the expression of the relB gene during mouse development using in situ hybridization and immunocytochemical analysis. The results show that the expression of the relB gene is highly restricted to a subpopulation of cells that colonize the lymphoid tissues and that appear very late during the process of hematopoietic diversification. RNA transcripts of relB are very low or undetectable in early and late embryos. Low relB expression is observed in the thymus at late stages of embryogenesis but rapidly increases after birth. In adult lymphoid tissues, relB is detected in the medullary region of the thymus, the periarterial lymphatic sheaths of the spleen, and the deep cortex of the lymph nodes, which correspond to the regions where T cells of mature phenotype and interdigitating dendritic cells are present. Using double immunofluorescent labeling of thymic cell suspensions, we have identified the interdigitating dendritic cells as the target of RelB expression. These cells are part of a system of antigen-presenting cells that function in the induction of several immune responses, such as, tolerance, sensitization of MHC-restricted T cells, rejection of organ transplants and formation of T-dependent antibodies. Our observations indicate that RelB may play a particular role in the signal transduction pathway that regulate dendritic cell differentiation and its cellular responses.