Signal regulatory protein α ligation induces macrophage nitric oxide production through JAK/STAT- and phosphatidylinositol 3-kinase/Rac1/NAPDH oxidase/H2O2-dependent pathwayst

Signal regulatory protein alpha (SIRP alpha) is a glycoprotein receptor that recruits and signals via the tyrosine phosphatases SHP-1 and SHP-2. In macrophages SIRP alpha can negatively regulate the phagocytosis of host cells and the production of tumor necrosis factor alpha. Here we provide evidence that SIRPa can also stimulate macrophage activities, in particular the production of nitric oxide (NO) and reactive oxygen species. Ligation of SIRPa by antibodies or soluble CD47 triggers inducible nitric oxide synthase expression and production of NO. This was not caused by blocking negative-regulatory SIRP alpha-CD47 interactions. SIRP alpha-induced NO production was prevented by inhibition of the tyrosine kinase JAK2. JAK2 was found to associate with SIRP alpha in macrophages, particularly after SIRP alpha ligation, and SIRP alpha stimulation resulted in JAK2 and STAT1 tyrosine phosphorylation. Furthermore, SIRP alpha-induced NO production required the generation of hydrogen peroxide (H2O2) by a NADPH oxidase (NOX) and the phosphatidylinositol 3-kinase (PI3-K) -dependent activation of Racl, an intrinsic NOX component. Finally, SIRP alpha ligation promoted SHP-1 and SHP-2 recruitment, which was both JAK2 and PI3-K dependent. These findings demonstrate that SIRP ligation induces macrophage NO production through the cooperative action of JAK/STAT and PI3-K/Racl/NOX/H2O2 signaling pathways. Therefore, we propose that SIRPot is able to function as an activating receptor.