B and T lymphocytes develop normally in mice lacking the guanine nucleotide exchange factor Vav-2. However, the immune responses to type II thymus-independent antigen as well as the primary response to thymus-dependent (TD) antigen are defective. Vav-2-deficient mice are also defective in their ability to switch immunoglobulin class, form germinal centers and generate secondary immune responses to TD antigens. Mice lacking both Vav-1 and Vav-2 contain reduced numbers of B lymphocytes and display a maturational block in the development of mature B cells. B cells from Vav-1(-/-)Vav-2(-/-) mice respond poorly to antigen receptor triggering, both in terms of proliferation and calcium release. These studies show the importance of Vav-2 in humoral immune responses and B cell maturation.

We show here that Vav-2 is tyrosine phosphorylated following antigen receptor engagement in both B-and T-cells, but potentiates nuclear factor of activated T cells (NFAT)-dependent transcription only in B cells. Vav-2 function requires the N-terminus, as well as functional Dbl homology and SH2 domains. Moreover, the enhancement of NFAT-dependent transcription by Vav-2 can be inhibited by a number of dominant-negative GTPases. The ability of Vav-2 to potentiate NFAT-dependent transcription correlates with its ability to promote a sustained calcium¯ux. Thus, Vav-2 augments the calcium signal in B cells but not T cells, and a truncated form of Vav-2 can neither activate NFAT nor augment calcium signaling. The CD19 co-receptor physically interacts with Vav-2 and synergistically enhances Vav-2 phosphorylation induced by the B-cell receptor (BCR). In addition, we found that Vav-2 augments CD19-stimulated NFATdependent transcription, as well as transcription from the CD5 enhancer. These data suggest a role for Vav-2 in transducing BCR signals to the transcription factor NFAT and implicate Vav-2 in the integration of BCR and CD19 signaling.

RhoG, a member of the Rho family of GTPases, has been implicated as a regulator of the actin cytoskeleton. In this study, we show a novel function for the small GTPase RhoG on the regulation of the interferon-c promoter and nuclear factor of activated T cells (NFAT) gene transcription in lymphocytes. Optimal function of RhoG for the expression of these genes requires a calcium signal, normally provided by the antigen receptor. In addition, RhoG potentiation of NFAT requires the indirect activity of Rac and Cdc42; however, pathways distinct from those activated by Rac and Cdc42 mediate RhoG activation of NFAT-dependent transcription. Using effector domain mutants of RhoG we found that its ability to potentiate NFAT-dependent transcription correlates with its capacity to increase actin polymerization, supporting the suggestion that NFAT-dependent transcription is an actin-dependent process. RhoG also promotes T-cell spreading on fibronectin, a property that is independent of its ability to enhance NFAT-dependent transcription. Hence, these results implicate RhoG in leukocyte trafficking and the control of gene expression induced in response to antigen encounter.

RhoG is a low-molecular-weight GTPase highly expressed in lymphocytes that activates gene transcription and promotes cytoskeletal reorganization in vitro. To study the in vivo function of RhoG, we generated mice homozygous for a targeted disruption of the RhoG gene. Despite the absence of RhoG, the development of B and T lymphocytes was unaffected. However, there was an increase in the level of serum immunoglobulin G1 (IgG1) and IgG2b as well as a mild increase of the humoral immune response to thymus-dependent antigens. In addition, B-and T-cell proliferation in response to antigen receptor cross-linking was slightly increased. Although RhoG deficiency produces a mild phenotype, our experiments suggest that RhoG may contribute to the negative regulation of immune responses. The lack of a strong phenotype could indicate a functional redundancy of RhoG with other Rac proteins in lymphocytes.

There is overwhelming evidence for the importance of GEMS in membrane signalling, in particular for cells of the haematopoietic system. Although the presence of GEMS in platelets has been already described their functional relevance is not known. The present study was therefore undertaken to adress this with specific relevance to signalling by the collagen receptor GPVI. Platelet GEMs were highly enriched in the GPI-anchored protein CD59, the Src family kinases Lyn and Fyn and the adaptor molecules LAT and PAG-85 (Cbp). Phosphorylation of LAT following platelet stimulation with the GPVI specific agonist convulxin occurs predominantly in GEMs. Furthermore, SLP-76 and PLC y2 are rcruited to GEMS and critical interactions between these molecules and LAT are restricted to this membrane microdomain. Disruption of GEM integrity by methyl-P-cyclodextrin treatment inhibited tyrosine phosphorylation, calcium mobilisation and platelet aggregation by GPVI agonists. Importantly, there was no effect seen in platelets incubated with cholesterol loaded methyl-P-cyclodextrin, consistent with the effect being mediated by cholesterol depletion and GEM disruption. The results suggest, that membrane microdomains participate in platelet activation by GPVI.Septic shock is a pathological inflammatory response to bacterial lipopolysaccharide (LPS). Mononuclear phagocytes are key players because they produce TNF-a and can be primed by IFN-y. We have shown that this involves changes in membrane phospholipid acyl composition, which is controlled by acyltransferases. In the present study IFN-y was found to activate lysophosphatidylcholine acyltransferase (LPCAT)by 22% (p=O.Ol l), but not lysophosphatidic acid acyltransferase, although the latter is known to participate in inflammatory responses. The production of TNFa and IL-6 was approximately 5 times greater in cells primed with IFN-y than unprimed cells. A specific inhibitor of LPCAT reduced both TNFa and IL-6 production by about 90%. By quantifying TNFa mRNA levels we suggest that this inhibition may be controlled at the transcriptional level. We have shown that activation of monocytes, under conditions relevant to endotoxic shock, is associated with increased LPCAT activity. Together with the finding that an inhibitor of the enzyme drastically reduces cytokine production, this suggests an important role for LPCAT in inflammatory responses and that inhibitors of the enzyme may be beneficial for therapeutic intervention.Vav2 is a guanine nucleotide exchange factor that is phosphorylated and activated by antigen receptor stimulated tyrosine kinases. In B cells Vav2 associates with CD19 and potentiates CD19 stimulated NFAT-dependent transcription. We have shown that Vav-2 potentiates antigen receptor triggered calcium flux and NFAT-dependent transcription in B cells but not T cells. To understand the role of this molecule in the immune response we have established mice deficient in Vav2. The development of both B cells and T cells appears normal in the absence of Vav2. Vav2 deficient mice ...