Unusual biochemical features and follicular dendritic cell expression of human Fcα/μ receptor

The Fc receptor for IgA and IgM (Fc alpha-/mu R) is of particular interest because it can bind antibodies of both IgM and IgA isotypes and thus may play a pivotal role in systemic and mucosal immunity. Using IgM and IgA ligands and newly generated Fc alpha/mu R specific monoclonal antibodies we have defined biochemical features and cellular distribution of the human Fc alpha/mu R. Both recombinant and native forms of human Fc alpha/mu R are expressed on the cell surface as remarkably stable homodimeric transmembrane glycoproteins that can bind specifically polymeric IgM or IgA. The only human B cells to express Fc alpha/mu R, albeit at very low levels, are found in the pre-germinal center subpopulation defined by the IgD(+)/CD38(+) phenotype. Hence the expression pattern differs from that of the mouse wherein Fc alpha/mu R is expressed by both circulating and resident B cell populations. Significantly, the predominant cell type expressing the Fc alpha/ mu R in humans is the follicular dendritic cell of germinal centers. The Fc alpha/mu R may thus function in antigen presentation and B cell selection in the germinal center response.