MASP1 and systemic lupus erythematosus: Observing cleavages at the C-terminus of human C3 within our cross-sectional SLE cohort (Fig. 2D,E) as well as in vitro assays using our selected four proteases (Fig. EV2), we determined whether these cleavages in C3 can be generated in an assay using purified human C3 and recombinant active MASP-1 and MASP-3 (Fig. 3E), compared to their inactive form (Morgan, 2000; Pihl et al, 2017, 2021).