Localization of the binding site for the monocyte immunoglobulin (Ig) A-Fc receptor (CD89) to the domain boundary between Cα2 and Cα3 in human IgA1

Abstract

Immunoglobulin (Ig) A serves as the first line of humoral defense at all mucosal surfaces and is present in large quantities in blood. In playing its role in humoral immunity, IgA interacts with a variety of effector molecules present both in serum and on the surfaces of immune and inflammatory cells. To study these interactions, we previously established expression of human IgA1 in insect cells using recombinant baculoviruses and showed that the expressed antibody is a structurally and functionally intact polypeptide useful for examining the molecular properties of IgA. Indeed, since the Cα2 N-linked glycosylation site lies near the Fab-distal pole of Cα2, the inability of a mutant IgA1 lacking Cα2 N-glycosylation to bind its cognate receptor suggested that the monocyte Fcα receptor (mFcαR) recognizes IgA at a hinge-distal site encompassing the boundary between the Cα2 and Cα3 domains. In this report, we utilize both domain-swapped IgA/IgG and point-mutated IgA chimeras to verify the above hypothesis. Using an antigen-specific rosetting assay and a mFcαR-expressing cell line, we show that: (a) Cα2 and Cα3 together are necessary and sufficient for binding; (b) neither the IgA hinge nor the tailpiece is necessary for binding; (c) mutations away from the interdomain boundary do not affect binding; and (d) mutations located near the three-dimensional boundary between Cα2 and Cα3 completely disrupt binding. Taken together, these results localize the mFcαR recognition site on IgA to the boundary region between the second and third constant domains - a site analogous to that recognized by Staphylococcus aureus protein A on IgG. The use of this hingedistal site is, to date, unique among Fc receptors of the Ig superfamily.