The Src-Like Adaptor Proteins, SLAP and SLAP2, Regulate Flt3-Dependent Dendritic Cell Development.

The Src-like Adaptor Proteins, SLAP and SLAP2, are hematopoietic adaptor proteins that have been previously shown to act as negative regulators of T- and B-cell signaling. SLAP and SLAP2 work in conjunction with the E3 ubiquitin ligase, c-Cbl, to down regulate the T-cell receptor and other components of the T- and B-cell receptor signaling pathways including ZAP-70 and Syk. SLAP and SLAP2 are expressed in many hematopoietic cell types, including progenitor cells that give rise to cells of both myeloid and lymphoid lineages. Recent evidence indicates a role for SLAP and SLAP2 in regulating hematopoietic receptor tyrosine kinase (RTK) signaling. We have shown that SLAP and SLAP2 interact with CSF-1R/Fms and demonstrated that SLAP2 negatively regulates CSF-1 dependent differentiation of FD-Fms cells. Our recent work demonstrates that both SLAP and SLAP2 interact with the Flt3 receptor in an SH2 domain dependent manner and the interaction was mapped to pY589 and pY591 within the juxtamembrane region of the receptor. To examine the role of SLAP and SLAP2 in Flt3 regulation and signaling in vivo, we utilized Flt3 ligand (Flt3L) to generate dendritic cells from mice lacking both SLAP and SLAP2. Slap1−/− Slap2−/− mice have impaired development of Flt3L-dependent CD11c⁺ bone marrow-derived dendritic cells (BMDC). In contrast to wild-type mice, Slap1−/− Slap2−/− mice produce 47% ± 9.7% less CD11c⁺ BMDC after 10 days in culture with Flt3L. Whether the reduction in BMDC numbers is indicative of a defect in the proliferation of Slap1−/−Slap2−/− progenitor cells in response to Flt3L is currently being explored. Although the absolute number of Flt3L-generated BMDC from Slap1−/−Slap2−/− mice is reduced, there are no major differences in the subtype, myeloid (70–90% CD11b⁺) and lymphoid (4–8% B220⁺), of DC produced between wild-type and double knock-out mice. To determine whether the reduction in dendritic cell numbers is specific to those generated with Flt3L, we generated BMDC using GM-CSF and IL-4. Both wild-type and double knock-out mice produce similar numbers of CD11c⁺ BMDC when this combination of cytokines is used. The maturation and activation of Flt3L-generated BMDC from Slap1−/−Slap2−/− mice is being investigated. These data indicate a novel role for SLAP and SLAP2 in the differentiation of Flt3L-dependent dendritic cells.