SIRPα/CD172a Regulates Eosinophil Homeostasis

Eosinophils are abundant in the lamina propria of the small intestine, but they rarely show degranulation in situ under steady-state conditions. In this study, using two novel mAbs, we found that intestinal eosinophils constitutively expressed a high level of an inhibitory receptor signal regulatory protein alpha (SIRP alpha)/CD172a and a low, but significant, level of a tetraspanin CD63, whose upregulation is closely associated with degranulation. Cross-linking SIRP alpha/CD172a on the surface of wild-type eosinophils significantly inhibited the release of eosinophil peroxidase induced by the calcium ionophore A23187, whereas this cross-linking effect was not observed in eosinophils isolated from mice expressing a mutated SIRP alpha/CD172a that lacks most of its cytoplasmic domain (SIRP alpha Cyto(-/-)). The SIRP alpha Cyto(-/-) eosinophils showed reduced viability, increased CD63 expression, and increased eosinophil peroxidase release with or without A23187 stimulation in vitro. In addition, SIRP alpha Cyto(-/-) mice showed increased frequencies of Annexin V-binding eosinophils and free MBP(+)CD63(+) extracellular granules, as well as increased tissue remodeling in the small intestine under steady-state conditions. Mice deficient in CD47, which is a ligand for SIRP alpha/CD172a, recapitulated these phenomena. Moreover, during Th2-biased inflammation, increased eosinophil cell death and degranulation were obvious in a number of tissues, including the small intestine, in the SIRP alpha Cyto(-/-) mice compared with wild-type mice. Collectively, our results indicated that SIRP alpha/CD172a regulates eosinophil homeostasis, probably by interacting with CD47, with substantial effects on eosinophil survival. Thus, SIRP alpha/CD172a is a potential therapeutic target for eosinophil-associated diseases. The Journal of Immunology, 2011, 187: 2268-2277.