Abstract 001: Skap2 Regulates Atherosclerosis through Macrophage Polarization and Efferocytosis

Abstract

Background: Atherosclerosis causes more deaths than any other pathophysiologic process. It has a well-established inflammatory, macrophage-mediated component, but potentially protective intracellular macrophage processes in atherosclerosis remain enigmatic. Src Kinase-Associated Phosphoprotein 2 (Skap2) is a macrophage-predominant adaptor protein critical for cytoskeletal reorganization, and thereby, for macrophage migration and chemotaxis. The role of macrophage Skap2 in atherosclerosis is unknown and deserves exploration. Results: Human macrophages express Skap2, and in human arterial gene expression analysis, Skap2 expression is enriched in macrophage-containing areas of human atheroma; the transcript level varies with plaque characteristics, with higher levels in more stable plaques. In ApoE -/- mice on a standard diet, deletion of Skap2 accelerates atherosclerosis by threefold at 18 and 24 weeks. We find that Skap2 expression is switched on only as monocytes differentiate into adherent macrophages, so Skap2 -/- monocytes do not have a defect in infiltrating the atheroma, as reflected by abundant macrophages in the Skap2 -/- plaques. On the other hand, once they fully differentiate, Skap2-deficient macrophages cannot polarize efficiently into alternatively-activated, regulatory macrophages, and instead they preferentially polarize toward the classical pro-inflammatory phenotype both ex vivo and within the developing atheroma. This defect extends to polarized effector functions, as ex vivo analysis of macrophage phagocytosis of dying foam cells indicates that Skap2 is required for the regulatory process of efferocytosis. There are no differences in macrophage proliferation or apoptosis attributable to Skap2. Finally, Skap2 binds to Sirpα, whose interaction with CD47 has been shown to be important for efferocytosis, and we observe colocalization of actin, Sirpα, and CD47 at leading edges of macrophages in a Skap2-dependent manner. Conclusions: Taken together, our findings support a model in which Skap2 drives a regulatory, efferocytic mode of behavior to quell atherosclerosis.