The molecular interactions that mediate signaling of T and B lymphocytes, both during development and in activation of mature T and B cells, are not fully understood. Genetic and biochemical approaches have therefore been taken to address these questions. Costimulatory B7 molecules (B7-1 or CD80; and B7-2 or CD86) are known to bind to T cell costimulatory receptors CD28 and CTLA4. Engagement of CD28 is know to transduce signals in T cells that play a critical role in T cell activation. TCR mediated activation of T cells to proliferation and IL2 secretion was enhanced in mice deficient in the adapter molecule cbl-b, suggesting a negative regulatory role for cbl-b under these conditions. Cbl-b deletion in fact reversed the inability of CD28-deficient mice to generate T cell-dependent Ig class switched primary and secondary antibody responses. Surprisingly, however, cbl-b deletion did not reverse the functional defect of B7 (CD80/CD86) deficient mice, indicating an unexpected difference between the requirement for CD28 and for CD28 ligand B7 molecules. Moreover, cbl-b inactivation substantially reversed the immune response defect in mice deficient in both CD28 and B7. Overall, these results suggest the unanticipated conclusion that the expression of CD28 has a strong negative regulatory effect on immune response in the absence of its B7 ligands. The mechanism underlying these effects is under study employing CD28 mutant transgenes. The relationship between members of the cbl family and other critical adapter molecules has been studied. Experiments have indicated that inactivation of c-cbl (but not cbl-b) completely reverses the lethal phenotype caused by SLP-76 deficiency. In addition, c-cbl inactivation partially reverses the defect in T cell development caused by deficiency in SLP-76 or LAT, resulting in fact in a marked hyperplasia of peripheral CD4 T cells. These findings indicate an unanticipated SLP-76 (and LAT) independent signaling pathway that is facilitated by c-cbl inactivation. The biochemical basis for these effects is under study, using SLP-76 mutants to dissect the structure-function relationships in this pathway. SLP-65 (also termed BLNK) is an adapter molecule that plays a central role in development of B cells, and mice deficient in BLNK manifest severe impairment in early as well as late stages of B cell development. We have observed that inactivation of c-cbl results in substantial rescue of early B cell development by facilitating signaling through the pre-BCR, demonstrating that c-cbl negatively regulates a BLNK-independent pathway of early B cell maturation.