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 (MASP)-3, a component of lectin pathway pattern recognition molecules, has shown the ability to process critical substrates such as pro-factor D and insulin growth factor binding protein-5. Given shared features between lectin pathway pattern recognition molecules and C1q from the classical pathway, we hypothesized that C1q might be a viable in vivo binding partner for the MASPs.