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TNF receptor-associated factor 2 (TRAF2) is a signal-transducing protein associated with the CD40 cytoplasmic domain. It has been hypothesized that during signal transduction, TRAF2 must be released from CD40 in order for it to interact with downstream signaling molecules. We found that CD40 and TRAF2 were constitutively associated with each other in a human B cell line. Following stimulation with an anti-CD40 Ab, a decrease in the amount of CD40-associated TRAF2 was observed that could not be explained by a change in total level of either of the proteins. These results, as well as similar findings obtained with 293 cells overexpressing CD40 and TRAF2, suggested that CD40-mediated signals inhibited the CD40-TRAF2 interaction. We then conducted binding studies using CD40 cytoplasmic domain fusion proteins and TRAF2 derived from either control or CD40-stimulated cell lines. These in vitro studies also indicated that the binding of TRAF2 to the CD40 cytoplasmic domain was inhibited by CD40 stimulation. The results of these experiments, as well as differences between the in vitro and in vivo findings, indicated that multiple mechanisms were involved in the inhibition of the CD40-TRAF2 interaction by CD40 signals. Possible mechanisms of inhibition are discussed based on mapping of the TRAF2 binding site on the CD40 cytoplasmic domain.

Proteins that bind to discrete domains of the Blk, Fyn, Lyn, and Btk protein tyrosine kinases were examined in pre-B cells that had not been subjected to any external stimulation, as well as in nonstimulated and antigen-receptor-ligated B cells. Proteins that bind to the Src homology 2 domains of Blk and Fyn were identified in B cells that had been activated with anti-IgM but were not identified in unstimulated B cells. A number of Blk and Fyn Src homology 2 domain-binding phosphoproteins were also observed in pre-B cells that had not been stimulated in vitro. The phosphoproteins seen in activated B cells potentially represent substrates that play a role in the pathway of antigen-receptor-mediated signaling. Distinct signaling pathways involving distinguishable kinase substrates may be relevant in pre-B-cell-receptor-mediated cell survival during ontogeny. These results indirectly support models that predict constitutive ligand-independent signaling by the pre-antigen receptor during lymphoid ontogeny.

The membrane immunoglobulin heavy chain (micron) plays a feedback role during the pre-B stage of B lymphocyte differentiation. In pre-B cell lines, micron associates with two surrogate light chain proteins. The omega chain is disulfide linked to mu and was predicted to be the product of the lambda 5 gene. The iota chain is noncovalently associated with micron. We demonstrate that the omega protein is indeed the product of the lambda 5 gene and that mu, omega, and iota are coassociated in the same complex. Antibodies against the omega/lambda 5 protein demonstrate the existence of a subpopulation of "transitional" bone marrow B cells that express micron and omega on the cell surface. The majority of these cells also express surface kappa light chains, indicating that in B lymphoid ontogeny the lambda 5 gene is inactivated after the onset of kappa light chain expression.