Identification of novel blood-borne soluble binding partners of factor H-related proteins

The deposition of circulating complement factor H-related (FHR) proteins in tissues around the body has been implicated in a series of complement-mediated diseases. However, the array of blood-borne binding partners with which they interact remains unclear. Here, we identify novel blood-borne binding partners of FHR proteins, firstly through preliminary untargeted immunoprecipitation and mass spectrometry, and subsequently validating direct interactions through solid-phase binding assays. We uncover direct interactions between FHRs and soluble immune mediators including complement C4 (C4), cathepsin G (CTSG), mannose-binding lectin 2 (MBL2), and platelet basic protein (PPBP). Functional assays show that FHR-1 and FHR-2 attenuate CTSG-mediated C3b degradation, while FHR-5 and FHL-1 appear to affect lectin pathway activation via MBL2 binding. These interactions suggest that FHRs perhaps confer activity not only through surface competition with factor H, but also via selective engagement with circulating ligands. Our findings expand the known FHR interactome and reveal potential new avenues for understanding FHR biology and targeting complement dysregulation in disease.