Characteristics of Fc-independent human antimannan antibody-mediated alternative pathway initiation of C3 deposition to Candida albicans

The complement system has an important role in host resistance to systemic candidiasis but regulation of complement activation by Candida albicans remains poorly defined. Previous studies have identified a requirement for naturally occurring antimannan IgG antibody in initiation of C3 opsonization of C. albicans through either the classical or alternative pathway. This study characterized antibody-dependent initiation of the alternative pathway using the recombinant human monoclonal antimannan Fab fragment M1 and its full-length IgG1 antibody M1g1. Kinetic analysis of C3b deposition onto C. albicans with flow cytometry demonstrated the ability of M1g1 to restore the activity of either the classical or alternative pathway to the yeast-absorbed normal human serum, but the Fc-free M1 Fab restored only the activity of the alternative pathway. This Fc-independent, antimannan Fab-mediated C3 deposition through the alternative pathway was also observed in a serum-free assay containing the six alternative pathway proteins and in C1q- or C2-depleted serum but not in factor B-depleted serum. M1- or M1g1-dependent alternative pathway initiation of C3b deposition occurred in an asynchronous manner at discrete sites that expanded to cover the entire cell surface over time as revealed with immunofluorescence microscopy, in contrast to a uniform appearance of initial C3 deposition through the classical pathway. Furthermore, antimannan Fab M1 promoted the assembly of the alternative pathway convertase on the cell surface seen as colocalization of C3 and factor B with immunofluorescence microscopy. Thus, human antimannan antibody has a distinct Fc-independent effector function in regulation of C3 deposition to C. albicans.