A New Serum Macrophage Checkpoint Biomarker for Innate Immunotherapy: Soluble Signal-Regulatory Protein Alpha (sSIRPα)

Background and Aims: The macrophage "don't eat me" pathway CD47/SIRP alpha is a target for promising new immunotherapy. We hypothesized that a soluble variant of SIRP alpha is present in the blood and may function as a biomarker. Methods: Monocyte derived macrophages (MDMs) from human buffy-coats were stimulated into macrophage subtypes by LPS and IFN-gamma (M1), IL-4 and IL-13 (M2a), IL-10 (M2c) and investigated using flow cytometry. Soluble SIRP alpha (sSIRP alpha) was measured in cell cultures and serum by Western blotting and an optimized ELISA. Serum samples were obtained from 120 healthy individuals and from 8 individuals challenged by an LPS injection. Results: All macrophage phenotypes expressed SIRP alpha by flowcytometry, and sSIRP alpha was present in all culture supernatants including unstimulated cells. M1 macrophages expressed the lowest level of SIRP alpha and released the highest level of sSIRP alpha (p < 0.05). In vivo, the serum level of sSIRP alpha increased significantly (p < 0.0001) after an LPS challenge in humans. The median concentration in healthy individuals was 28.7 mu g/L (19.8-41.1, 95% reference interval), and 20.5 mu g/L in an IFCC certified serum reference material. The protein was stable in serum for prolonged storage and repeated freeze/thawing. Conclusions: We demonstrate that sSIRP alpha is produced constitutively and the concentration increases upon macrophage activation both in vitro and in vivo. It is present in human serum where it may function as a biomarker for the activity of tumor-associated macrophages (TAMs), and for monitoring the effect of immunotherapy.