Regulation of T cell receptor and integrin function by the adaptor protein SH2-containing leukocyte phosphoprotein of 76 kDa SLP-76

Successful adaptive immunity requires that T cells respond to antigens encountered at the appropriate location within the body. Signaling from the T cell antigen receptor (TCR) regulates all arenas of T cell biology, directing T cell proliferation, migration, cytokine production, lysis of target cells and activation of humoral immune responses. Signaling originating by integrins binding adhesion molecules on other cells or the extracellular matrix also regulates T cell function, T cell migration in the periphery and induction of T cell effector functions. The adaptor protein SH2-containing leukocyte phosphoprotein of 76 kDa (SLP-76) is known to be an essential mediator of signaling from the TCR. Experiments in this dissertation show that SLP-76 is also required for signaling downstream of integrins in T cells resulting in cell adhesion. In response to both TCR and integrin stimulation, SLP-76 relocalizes to surface microclusters that colocalize with phosphorylated signaling proteins. Disrupting SLP-76 recruitment to the adaptor linker for activation of T cells (LAT) inhibits SLP-76 clustering downstream of the TCR but not downstream of integrins. Conversely, disrupting SLP-76 association with the adhesion and degranulation promoting adaptor protein (ADAP) inhibits clusters following TCR but not integrin engagement, and integrin-mediated T cell adhesion. Together, these findings demonstrate that SLP-76 relocalizes to integrin-initiated signaling complexes by a different mechanism than during TCR signaling, and that SLP-76 relocalization corresponds with SLP-76-dependent integrin function in T cells. To further investigate how localization to the membrane directs SLP-76 function, we examined a novel mouse in which wild type SLP-76 was replaced with a SLP-76 variant constitutively targeted to the cell membrane. Membrane-targeted SLP-76 (MTS) supports immunoreceptor signaling in platelets and immature thymocytes. Signaling from the mature TCR, however, is defective in MTS thymocytes, resulting in failed T cell differentiation. Defective MTS thymic selection is not rescued by a SLP-76 mutant whose localization is restricted to the cytosol. Thus, fixing SLP-76 localization reveals differential requirements for signaling complexes downstream of the preTCR versus mature TCR. Together, these findings demonstrate the importance of SLP-76 localization for coordinating its adaptor function downstream of both integrins as well as the TCR