Insulin-regulated aminopeptidase contributes to setting the intensity of FcR-mediated inflammation

The function of intracellular trafficking in immune-complex triggered inflammation remains poorly understood. Here, we investigated the role of Insulin-Regulated Amino Peptidase (IRAP)-positive endosomal compartments in Fc receptor (FcR)-induced inflammation. Less severe FcγR-triggered arthritis, active systemic anaphylaxis and FcεRI-triggered passive systemic anaphylaxis were observed in IRAP-deficient versus wild-type mice. In mast cells FcεRI stimulation induced rapid plasma membrane recruitment of IRAP-positive endosomes. IRAP-deficient cells exhibited reduced secretory responses, calcium signaling and activating Syk^Y519/520 phosphorylation albeit receptor tyrosine phosphorylation on β and γ subunits was not different. By contrast, in the absence of IRAP, SHP1-inactivating phosphorylation on Ser⁵⁹¹ that controls Syk activity was decreased. Ex-vivo cell profiling after FcγR-triggered anaphylaxis confirmed decreased phosphorylation of both Syk^Y519/520 and SHP-1^S591 in IRAP-deficient neutrophils and monocytes. Thus, IRAP-positive endosomal compartments, in promoting inhibition of SHP-1 during FcR signaling, control the extent of phosphorylation events at the plasma membrane and contribute to setting the intensity of immune-complex triggered inflammatory diseases.