No abstract

RasGRP1 is a guanine nucleotide exchange factor that activates Ras GTPases and is activated downstream of antigen receptors on both T and B lymphocytes. Ras-GRP1 provides signals to immature T cells that conferRasGRP1 is a guanine nucleotide exchange factor that transduces intracellular signaling by converting Ras GTPases to their active GTP-bound states (1-6). RasGRP1 is expressed most prominently in thymocytes and mature T cells but is also expressed in some immature B cell lines (2, 3, 5, 6). RasGRP1 is positively regulated by membrane translocation, which is driven by binding of its C1 domain to diacylglycerol (3, 4, 6 -9). This provides a mechanism for RasGRP1 activation by any receptor that couples to diacylglycerol-generating phospholipase Cs, including the T cell receptor (TCR) 1 (4) and the B cell receptor (BCR) (8).Recent work with genetically modified mice has demonstrated that RasGRP1 provides signals that enhance the survival and proliferation of T cells at several stage during their development and immunological selection. RasGRP1 is transcriptionally induced by pre-TCR signaling, and the resulting increase in RasGRP1 expression is sufficient to confer survival and proliferation in the absence of other pre-TCR-derived signals (10). RasGRP1 expression augments the effectiveness with which ␣␤TCR/major histocompatibility complex interactions confer survival and maturation through the process of positive selection (11) and increases the proliferative responses of thymic T cells to TCR ligation (10 -12). RasGRP1 deficiency does not noticeably alter the efficiency of TCR-dependent deletion (negative selection) of thymic T cells by a self-antigen (11) but impedes apoptosis induction in mature splenic T cells following TCR ligation (13). These experiments portray RasGRP1 as a promoter of survival and proliferation responses to pre-TCR or TCR during intrathymic T cell development but with the converse role of mediating TCR-induced deletion in mature peripheral T cells.B cell development in the bone marrow is also governed by a series of selections for immunologically competent but not selfreactive antigen receptors. To avoid auto-immunity, immature B cells carrying BCR that recognize self-antigen need to be suppressed either by anergization, by respecification through receptor editing, or by deletion through BCR-induced apoptosis (14 -16). Much of our knowledge of the signal transduction pathways that mediate BCR-induced deletion of immature B cells has been obtained from experiments with WEHI-231 cells, a B lymphoma-derived murine cell line that resembles immature B cells both phenotypically and functionally (17). WEHI-231 cells express surface IgM of unknown antigen specificity, but cross-linking with anti-IgM mimics the interaction of this BCR with a high affinity antigen. Anti-IgM-initiated signaling imposes cell cycle arrest on WEHI-231 cells within a few hours, and maintenance of BCR ligation beyond 1 day results in apoptosis. BCR-induced deletion of WEHI-231 cells shares many of the regulatory mechan...

Specific members of the Rho family of GTPases exert unique influences on thymocyte proliferation, differentiation and deletion. Dbs is a guanine nucleotide exchange factor which is expressed throughout thymocyte development and is able to activate the Rho family GTPases CDC42, RhoA and RhoG. Transgenic mice expressing an activated form of Dbs had increased numbers of double-negative thymocytes. The Dbs transgene promoted expansion of double-negative thymocytes in the absence of pre-TCR, but had no effect on pre-TCR-dependent differentiation of double-negative thymocytes into double-positive thymocytes. Transgenic double-positive thymocytes were proliferative in vivo, but were also susceptible to apoptosis in vivo and in vitro. The transgenic single-positive thymocytes had attenuated proliferative responses following TCR ligation, and were depleted rather than expanded during culture in the presence of anti-CD3. When expressing a positively selectable TCR, transgenic double-positive thymocytes were increased in number and activated, but the output of single-positive thymocytes was reduced. Transgenic double-positive thymocytes were acutely sensitive to deletion by TCR ligation in vivo. These results indicate that activation of Dbs has the potential to promote proliferation throughout thymocyte development, but also sensitizes double-positive and single-positive thymocytes to deletion.