Size-dependent effect of IgA on the IgA Fc receptor (CD89)

The IgA Fc receptor (FcR; CD89) is expressed on several types of cells of the myeloid cell lineage. We investigated whether different sizes of heat-aggregated IgA (aIgA) bind to CD89 and subsequently induce cellular activation. As a model we used the murine B cell line IIA1.6 transfected with CD89 or IIA1.6 cells transfected with CD89 as well as with the FcR γ chain to study the binding of IgA to CD89. When these cells expressing CD89 were incubated with monomeric IgA, no significant binding of IgA to the cells was detectable by fluorescence-activated cell sorter analysis; however, incubation of the cells with aggregated IgA resulted in 93 ± 2% positive cells. Incubation of the cells with different sizes of IgA-containing aggregates revealed optimal binding with aggregates containing five to six molecules of IgA per aggregate. No difference was observed between the binding to CD89 of both IgA1- or IgA2-containing aggregates. Furthermore, the binding of aIgA was found to be CD89-specific, since the binding of IgA was completely inhibited by the CD89-specific monoclonal antibody My43 and no detectable binding occurred to the IIA1.6 parent cell line. Activation studies using interleukin-2 (IL-2) production as a marker, showed that the FcR γ chain is necessary to induce cellular activation. Only cells transfected with both CD89 and the FcR γ chain (CD89⁺/γ⁺) enhance the IL-2 production 10–12-fold upon stimulation with aggregates of IgA. Furthermore, triggering of CD89 only results in increase of intracellular calcium concentration (Ca²⁺]_i) in cells co-expressing FcR γ chain. Mutation of the tyrosine residues in the FcR γ chain immunoreceptor tyrosine-based activation motif of the FcR γ chain abolishes this increase in Ca²⁺]_i, indicating association and involvement of the FcR γ chain in CD89-mediated signaling.