Mechanisms of mannose-binding lectin-associated serine proteases-1/3 activation of the alternative pathway of complement

► MBL−/−/FCN A−/− mice were fully susceptible to collagen antibody induced arthritis. ► MBL−/−/FCN A−/− mice have intact alternative pathway of the complement. ► MASP-1 protein does not require binding to MBL A, MBL C and ficolin A to cleave pro-Df. ► FCN-B and MASP-1 are present in the sera of MBL−/−/FCN A−/− mice and FCN-B can bind to MASP-1.