Characterization of IgA and IgM binding and internalization by surface-expressed human Fc[alpha]/ mu receptor

The Fc[alpha]/ mu receptor (Fc[alpha]/ mu R) is an unusual Fc receptor in that it binds to two different antibody isotypes, IgA and IgM. This receptor is of interest because it is thought to be involved in the capture of IgA- and IgM-immune complexes and antigen presentation. To further characterize this receptor, we were able to stably express human Fc[alpha]/ mu R on the surface of the 293T cell line. Using this system, we determined the affinity of the interactions of the receptor with IgA and IgM, which led to novel insights including the important finding that IgM polymers can bind to human Fc[alpha]/ mu R in the absence of J chain. This is in contrast to the polymeric immunoglobulin receptor (pIgR), which requires the presence of J chain to bind to polymeric IgA and IgM. The dissociation constants (K sub(d) of all of the different human IgA isotypes and allotypes for human Fc[alpha]/ mu R were determined, and we show that the N-linked glycans on IgA1 are not required for binding to the receptor. In addition, we demonstrate that IgA can be rapidly internalized by human Fc[alpha]/ mu R in the presence of cross-linking antibody.)