The specificity of receptors for IgA on human peripheral polymorphonuclear cells and monocytes

Abstract

The specificity of the receptor for IgA (RFcα) on human peripheral blood monocytes and polymorphonuclear (PMN) cells was evaluated by the ability of various human IgA preparations to inhibit rosette formation between these cells and IgA-sensitized ox erythrocytes. RFc̀α on PMNS and monocytes were blocked by both monomer and dimer IgA preparations indicating that multivalent expression of Fc regions does not play a major role in receptor binding and that neither secretory component nor J chain significantly influences the binding of RFcα to IgA. Immunoglobulins of both the IgA1 and IgA2 subclasses inhibited IgA rosette formation and were in fact quite similar in their efficiency of blocking of RFcα. An IgA paraprotein without a Cα 3 domain was an even better inhibitor of IgA rosette formation than the IgA1 or IgA2 immunoglobulins. This implicated the Cα 2 domain as the site on IgA which interacts with RFcα. Furthermore, that this Cα 3-deficient IgA, which exists as a half molecule, was very efficient at blocking RFcα also demonstrated that multivalent Fc expression is not important to binding of RFcα and moreover that the site on IgA which interacts with RFcα is not dependent on H-chain pairing. RFcα on both PMN cells and monocytes were susceptible to proteolysis by pronase at concentrations which did not affect the receptor for IgG on these cells. Within 18 hr after removal of RFcα these cells had resynthesized and displayed this receptor. Although it is unclear whether IgA alone can mediate the effector functions of PMNs and monocytes in mucosal areas, the present studies define more clearly the specificity and regenerative capacity of RFcα and provide a basis for understanding the role of receptors for IgA and the cells with which they are associated in immune defense especially on the mucosal surfaces.