Role of the Src-like Adaptor Proteins, SLAP and SLAP-2, in Dendritic Cell Development and SIgnaling

Dendritic cells (DC) are critical to an immune response by functioning as sensors of foreign antigen and presenting antigen to direct T cell responses. Under the influence of Flt3 ligand this heterogeneous group of cells originate from hematopoietic stem cells (HSCs) in the bone marrow and takes up residence in lymphoid organs, skin as well as mucosal surfaces where they are most likely to encounter pathogens. GM-CSF is another cytokine involved in DC development that is specifically involved in the maintenance of skin resident DC. Together Flt3L and GM-CSF as well as their respective receptors, Flt3 and GM-CSFR, are the most important factors identified to date for maintenance of DC homeostasis. The Src-like adaptor proteins, SLAP and SLAP-2, are negative regulators of antigen receptor signaling. SLAP and SLAP2 contain SH3 and SH2 protein interaction modules that facilitate interaction with proline-rich sequences and phosphotyrosine motifs, respectively. Through a unique C-terminal region, SLAP and SLAP2 also interact with the E3 ubiquitin ligase, c-Cbl. SLAP and SLAP2 enhance c-Cbl-mediated ubiquitylation and down-regulation of antigen receptors by binding both activated receptors and c-Cbl. SLAP and SLAP2 have also been shown to function as negative regulators of receptor tyrosine kinases (RTKs) and our group has shown that SLAP and SLAP2 bind to and inhibit the colony-stimulating factor-1 receptor (CSF-1R). In the work presented here, we identify a novel role for SLAP in regulation of the GM-CSF receptor of bone marrow (BM)-derived DC. We show that potentiated GM-CSF signaling, in the absence of SLAP and SLAP2, impairs BM-DC maturation such that these cells express minimal MHCII, secrete low amounts of IL-12 and are functionally impaired in their ability to stimulate T cell responses. SLAP and SLAP2 deficiency also has an effect on Flt3L-derived BM-DC development. For example, SLAP/SLAP2-/- BM-DC numbers are reduced in the presence of Flt3L as compared to wild-type BM-DC. To investigate the mechanism of reduced DC numbers, we examined splenic DC and found that DC numbers were similar in wild-type and SLAP/SLAP2-/- mice. In fact, SLAP/SLAP2-/- mice had proportionally more CD8α+ splenic DC in vivo than wild-type mice. Thus there may be cytokines affected by SLAP/SLAP2-deficiency in our cultures that are either dispensable or compensated for in vivo DC development. The work presented in this thesis has implications for the role of SLAP and SLAP2 in immune response to infections by the regulation of GM-CSFR and Flt3 in maintaining dendritic cell homeostasis.Ph