Functional and phenotypic analysis of thymic B cells: role in the induction of T cell negative selection

The phenotype of mouse thymic B cells and their capacity to induce T cell n egative selection in vitro were analyzed. Thymic B cells expressed B cell m arkers such as IgM, Fc gamma receptor, CD44, heat-stable antigen, LFA-I and CD40. In addition, they were positive for the activation molecule CD69 and displayed high levels of B7-2. Although thymic B cells expressed CD5 on th eir surface, no CD5-specific mRNA was detected. Moreover thymic B cells ind uced a stronger deletion of TCR-transgenic (TG) thymocytes than splenic B c ells, which had low CD69 and B7-2 levels. Interestingly, CD40-activated spl enic B cells up-regulated CD69 and B7-2 and acquired a capacity to induce T cell deletion comparable to that of thymic B cells. Moreover thymic B cell s from CD40-deficient mice displayed lower CD69 and B7-2 levels than contro l thymic B cells, and lower capacity to induce the deletion of TCR TG thymo cytes. These results support the hypothesis that CD40-mediated activation o f thymic B cells determines a high efficiency of antigen presentation, sugg esting that within the thymus B cells may play an important role in the eli mination of autoreactive thymocytes.