An essential role for gp39, the ligand for CD40, in thymic selection

The interactions between CD40 on B cells and its ligand gp39 on activated T helper cells are known to be essential for the development of thymus-dependent humoral immunity. However, CD40 is also functionally expressed on thymic epithelial cells and dendritic cells, suggesting that gp39-CD40 interactions may also play a role in thymic education, the process by which self-reactive cells are deleted from the T cell repertoire. Six systems of negative selection were studied for their reliance on gp39-CD40 interactions to mediate negative selection. In all cases, when the antigen/superantigen was endogenously expressed (in contrast to exogenously administered), negative selection was blocked by loss of gp39 function. Specifically, blockade of gp39-CD40 interactions prevented the deletion of thymocytes expressing Vβ3, Vβ11, and Vβ12, specificities normally deleted in BALB/c mice because of the endogenous expression of minor lymphocyte-stimulating determinants. Independent verification of a role of gp39 in negative selection was provided by studies in gp39-deficient mice where alterations in T cell receptor (TCP,) Vβ expression were also observed. Studies were also performed in the AND TCR. transgenic (Tg) mice, which bear the Vti11, Vβ3 TCR. and recognize both pigeon cytochrome c (PCC)/IE k and H-2A s. Neonatal administration ofanti-gp39 to AND TCP, Tg mice that endogenously express H-2A s or endogenously produce PCC prevented the deletion of TCR Tg T cells. In contrast, deletion mediated by high-dose PCC peptide antigen (administered exogenously) in AND TCI< mice was unaltered by administration ofanti-gp39. In addition, deletion by Staphylococcus enterotoxin B in conventional mice was also unaffected by antigp39 administration, gp39 expression was induced on thymocytes by mitogens or by antigen on TCR Tg thymocytes. Immunohistochemical analysis of B7-2 expression in the thymus indicated that, in the absence of gp39, B7-2 expression was substantially reduced. Taken together, these data suggest that gp39 may influence negative selection through the regulation of costimulatory molecule expression. Moreover, the data support the hypothesis that, for negative selection to some endogenously produced antigens, negative selection may be dependent on TCR. engagement and costimulation.