Structural requirements for the interaction of human IgM and IgA with the human Fcα/μ receptor

Abstract

Here we unravel the structural features of human IgM and IgA that govern their interaction with the human Fca/l receptor (hFca/lR). Ligand polymerization status was crucial for the interaction, because hFca/lR binding did not occur with monomeric Ab of either class. hFca/lR bound IgM with an affinity in the nanomolar range, whereas the affinity for dimeric IgA (dIgA) was tenfold lower. Panels of mutant IgM and dIgA were used to identify regions critical for hFca/lR binding. IgM binding required contributions from both Cl3 and Cl4 Fc domains, whereas for dIgA, an exposed loop in the Ca3 domain was crucial. This loop, comprising residues Pro440-Phe443, lies at the Fc domain interface and has been implicated in the binding of host receptors FcaRI and polymeric Ig receptor (pIgR), as well as IgA-binding proteins produced by certain pathogenic bacteria. Substitutions within the Pro440-Phe443 loop resulted in loss of hFca/lR binding. Furthermore, secretory component (SC, the extracellular portion of pIgR) and bacterial IgA-binding proteins were shown to inhibit the dIgA-hFca/lR interaction. Therefore, we have identified a motif in the IgA-Fc inter-domain region critical for hFca/lR interaction, and highlighted the multi-functional nature of a key site for protein-protein interaction at the IgA Fc domain interface.Key words: Human Fca/m receptor . IgA . IgM IntroductionA receptor for IgM and IgA, termed Fca/m receptor (Fca/mR), was first described in the mouse where it is constitutively expressed on the majority of B cells and macrophages [1]. Mouse Fca/mR (mFca/mR) can mediate endocytosis of IgM-coated targets, and may contribute to the primary stages of the immune response to microbes. It has been suggested that mFca/mR, due to its high affinity for IgM and intermediate affinity for IgA, may assist also in the maintenance of plasma concentrations of IgM and IgA [2]. Although there is no significant homology between Fca/mR and other known proteins, the N-terminal Ig-like domain of Fca/mR Ã These authors contributed equally to this work. Certain mAb raised against mFca/mR have been found to bind to a peptide encompassing the region from mFca/mR's Ig-like domain that corresponds to the Ig-binding motif in domain 1 of pIgR [6]. Interestingly, these mAb can block binding of IgA and IgM to mFca/mR. The fact that these same mAb cross-react with hFca/mR, which carries the conserved motif in its Ig-like domain, suggests that the site for IgM and IgA on hFca/mR lies in this same region. In contrast, the site(s) on human IgM and IgA that interacts with hFca/mR is unknown. To gain a better understanding of the structural requirements for the interaction of human IgA and IgM with hFca/mR, we have produced recombinant dIgA and pentameric IgM and have adopted a mutational approach to identify individual residues on the Fc involved in interaction with hFca/mR. We demonstrate that both the Cm3 and Cm4 domains of IgM are required for binding to the receptor, and that residues at the Fc inter-domain region of dIgA are critical for hFca...

Aggregation of Streptococcus pneumoniae by a Pneumococcal Capsular Polysaccharide-Specific Human Monoclonal IgM Correlates with Antibody Efficacy In Vivo

“…In contrast to A7, a protective human IgM MAb to Cryptococcus neoformans GXM, 2E9, was opsonic and promoted fungal killing in vitro (17,54). Given that there is limited evidence for a phagocytic IgM Fc receptor in mice (18,30,47), 2E9-induced phagocytosis was thought to stem from MAb-dependent C3 deposition on GXM, enabling binding to host phagocyte complement receptors (54).…”

Fc Receptors in Mucosal Immunology

“…Binding to hFcα/μR requires ligand polymerization (IgM and IgA polymers) but the presence of the J chain does not seem to be required for binding (Ghumra et al, 2009;Yoo et al, 2011). A site at the Cα2-Cα3 domain interface, overlapping with that of FcαRI, has been shown to be critical for interaction with Fcα/μR (Kikuno et al, 2007;Ghumra et al, 2009). Although the human receptor does not contain a dileucine motif, deletion of the cytoplasmic tail resulted in the inability to internalize cross-linked IgM (Yang et al, 2012).…”

“…CDR1-and CDR2-like loops of hFcα/μR EC2 are both essential for binding IgA and IgM, whereas the CDR3-like loop of EC2 is less important. Binding to hFcα/μR requires ligand polymerization (IgM and IgA polymers) but the presence of the J chain does not seem to be required for binding (Ghumra et al, 2009;Yoo et al, 2011). A site at the Cα2-Cα3 domain interface, overlapping with that of FcαRI, has been shown to be critical for interaction with Fcα/μR (Kikuno et al, 2007;Ghumra et al, 2009).…”

“…The extracellular domain consists of a short EC1, an Ig variable (V) region-like EC2, and a stalklike EC3 domain (Yang et al, 2012). hFcα/μR also has dual specificity for IgM and IgA, with high affinity (K d = 1.2 nM) for IgM and approximately 10-fold lower affinity for IgA (K d = 10-18 nM) (Ghumra et al, 2009;Yoo et al, 2011). Various isotypes and allotypes of human IgA bind to hFcα/μR, as well as IgA from which N-linked carbohydrates have been removed (Yoo et al, 2011).…”

Translocalized IgA mediates neutralization and stimulates innate immunity inside infected cells

“…In turn, the weaker binding affinity may be the reason why IgA-mediated ADIN is less efficient than IgG-mediated ADIN. Interestingly, the Fcα PLAF loop is critical in mediating interactions with multiple proteins including the human Fc receptors FcαRI, Fcα/μR, the polymeric Ig receptor (pIgR), the pathogenproduced antigens SSL7 from Staphylococcus aureus, M proteins, Sir22, and ARP4 from group A streptococci, β-protein from group B streptococci, and the type 2 IgA1 protease produced by Neisseria meningitidis (15,19,(22)(23)(24)(25)(26)(27). Unlike TRIM21, however, these IgA receptors are more isotype specific (although Fcα/μR interacts with both IgA and IgM).…”