C1q/MASP complexes - hybrid complexes of classical and lectin pathway proteins are found in the circulation

Complement pathways, traditionally regarded as separate entities in vitro, are increasingly noted for cross-communication and bypass mechanisms. Among these, the MBL/Ficolin/CL associated serine protease-3 (MASP-3) - a component of lectin pathway pattern recognition receptors (PRMs) - has shown the ability to process critical substrates like pro-factor D and insulin growth factor binding protein-5 (IGFBP-5). Given shared features between lectin pathway PRMs and C1q from the classical pathway, we hypothesized that C1q might be a viable in vivo binding partner for the MASPs. We used microscale thermophoresis, ELISA and immunoprecipitation assays to detect C1q/MASP complexes and functionally assessed the complexes through enzymatic cleavage assays. C1q/MASP-3 complexes were detected in human serum and correlated well with MASP-3 serum levels in healthy individuals. The binding affinity between MASP-3 and C1q in vitro was in the nanomolar range, and the interaction was calcium-dependent, as demonstrated by their dissociation in the presence of EDTA. Furthermore, most of the circulating C1q-bound MASP-3 was activated. Based on immunoprecipitatin, also C1q/MASP-2 complexes appereared to be present in serum. Finally, C1q/MASP-2 and C1q/MASP-3 in vitro complexes were able to cleave C4 and pro-factor D, respectively. Our study reveals the existence of C1q/MASP complexes in the circulation of healthy individuals and both C1q/MASP-2 and C1q/MASP-3 complexes display proteolytic activity. Hence, this study uncovers a crosstalk route between complement pathways not previously described.