The role of stomatin-like protein 2 in T cell activation

Activation of the T cell through the T cell receptor (TCR) by antigen-presenting cells (APC) results in the coordinated redistribution of cell surface proteins into a morphological structure known as the immunological synapse (IS). Before IS formation and T cell activation, protein-protein microclusters form as the initial signaling units. The coordinated redistribution of these signaling microclusters during IS formation occurs concomitantly with a reorganization of the membrane skeleton and an accumulation of lipids rafts at the IS. We hypothesized that a molecular machinery exists that would anchor TCR microclusters to the cytoskeleton in the IS that would facilitate recruitment of signaling molecules to the growing TCR signalosome and ensure the sustainability of signaling from multimolecular complexes. To test this hypothesis, we performed a proteomic analysis of the lipid raft compartments of activated T cells, and discovered a previously unknown protein of the stomatin family, known as stomatin-like protein 2 (SLP-2). Here, we report that SLP-2 is part of the machinery that bridges the TCR signalosome in the peripheral supramolecular activation cluster (pSMAC) to the actin cytoskeleton and contributes to sustain signaling. This is shown biochemically by assessing the interactions of SLP-2 with signaling molecules from the TCR signalosome and morphologically by tracking SLP-2 redistribution towards the IS. Consistent with this proposed role of SLP-2 in T cells, we observed increased SLP-2 expression in primary activated T cells in vivo and in vitro. Increased SLP-2 expression correlated with the differentiation of T cells into effector cells. Furthermore, modulation of SLP-2 levels altered the temporal profile of T cell signaling and activation, with down-regulation resulting in decreased T cell activation and increased expression resulting in increased T cell activation. Finally, we found that there are two pools of SLP-2 in T cells, one in the plasma membrane and one associated with mitochondria and that both pools coalesce in the IS as activation proceeds. Overall, our data suggest that SLP-2 plays an important role in T cell activation by mediating sustained TCR signaling, which is required for effector T cell function. The results generated through this research provide us with a deeper understanding of how TCR signalosomes are assembled and how signaling is sustained in the T cell.;KEY WORDS. Immunology, T lymphocyte, T cell activation, T cell receptor, signalosomes, immunological synapse, lipid rafts, cytoskeleton, signal transduction, Stomatin-like protein-2, Mitochondria.