2. SLAT, a novel Th2-expressed adapter protein. Th2-derived cytokines play a central role in the pathophysiology of allergy and asthma. However, very little is known about proximal, TCR-coupled signaling events that regulate the Th1/Th2 differentiation process. Therefore, we initiated studies to elucidate the TCR-proximal signaling pathways that operate in Th2 cells, based on the notion that Th2-specific signaling elements represent potential targets for development of drugs that will selectively interfere with Th2 function. We previously reported the isolation and initial characterization of a novel protein, termed SWAP-70-Like Adapter of T cells (SLAT), which is selectively expressed at high levels in thymocytes and in differentiated Th2 cells. TCR signaling induces association of SLAT with ZAP-70 tyrosine kinase in Th2 cells, and this results in inhibition of ZAP-70 recruitment to the TCR complex and its activation. We hypothesize that SLAT favors the development and expansion of Th2 cells by selectively inhibiting the function of ZAP-70 in T cells. In addition, we found that the C-terminal region of SLAT is related to Dbl-homology (DH) domains, and it activates Rac1 and Cdc42, but not RhoA. We are currently studying whether SLAT also possesses a regulated GEF activity in T cells. In addition, we continued the backcrossing of SLAT-deficient mice onto the B6 and Balb/c backgrounds and begun to characterize SLAT-/- mice on a mixed background. Preliminary analysis indicates that SLAT-/- T cells cultured under Th2-inducing conditions display a bias towards Th1 development. Stimulation of SLAT-/- purified T cells with cross-linked anti-CD3 plus -CD4 antibodies revealed an increase in the basal phosphotyrosine content of SLAT-/- T cells and a ~2-fold increase in the inducible tyrosine phosphorylation of ZAP-70 on Tyr-319, a marker of its catalytic activity. SLAT-/- splenic CD4+ or CD8+ T cells also display a marked increase in the proportion of CD69high, CD44high and CD62Llow cells. These changes suggest that the SLAT mutation leads to increased proportion of mature/activated/memory T cells in the periphery, and are generally consistent with our hypothesis that SLAT is a negative regulator of TCR/costimulatory receptor signaling.