IgG1 protects against renal disease in a mouse model of cryoglobulinaemia.

Immunoglobulins protect against disease to a considerable extent by activating complement and stimulatory immunoglobulin crystallizable fragment receptors (Ig FcRs), and aggregating microbial pathogens. Yet IgG1, the predominant murine serum Ig isotype, cannot activate complement by the classical pathway, binds more avidly to an inhibitory than to stimulatory FcRs, and has limited ability to aggregate pathogens. In these regards, it resembles human IgG4 (ref. 4). We hypothesized that limited ability to activate effector mechanisms might protect against immune complex immunopathology. Here we show that IgG1-deficient (γ1(-)) mice, immunized with a potent antigen, develop lethal renal disease soon after they begin to produce antigen-specific antibody, whereas similarly immunized wild-type mice remain healthy. Surprisingly, renal disease in this model is complement and FcR independent and results from immune complex precipitation in glomerular capillaries, as in some cryoglobulinaemic humans. IgG3, which self-associates to form large immune complexes, accounts for more than 97% of the mouse Ig in this cryoglobulin; furthermore, glomerular disease develops when mice are injected with IgG3 anti-trinitrophenyl (TNP) monoclonal antibody followed by a TNP-labelled protein. Renal disease is prevented in both active and passive immunization models by antigen-specific IgG1; other isotypes are less potent at preventing disease. These observations demonstrate the adaptive significance of Ig isotypes that poorly activate effector mechanisms, reveal an immune-complex-dependent, complement- and FcR-independent nephrotoxic mechanism, and suggest that isotypes that poorly activate effector mechanisms may be useful for inhibiting immune complex immunopathology.

FcαRI binding at the IgA1 CH2-CH3 interface induces long-range conformational changes that are transmitted to the hinge region.

IgA effector functions include proinflammatory immune responses triggered upon clustering of the IgA-specific receptor, FcαRI, by IgA immune complexes. FcαRI binds to the IgA1-Fc domain (Fcα) at the CH2-CH3 junction and, except for CH2 L257 and L258, all side-chain contacts are contributed by the CH3 domain. In this study, we used experimental and computational approaches to elucidate energetic and conformational aspects of FcαRI binding to IgA. The energetic contribution of each IgA residue in the binding interface was assessed by alanine-scanning mutagenesis and equilibrium surface plasmon resonance (SPR). As expected, hydrophobic residues central to the binding site have strong energetic contributions to the FcαRI:Fcα interaction. Surprisingly, individual mutation of CH2 residues L257 and L258, found at the periphery of the FcαRI binding site, dramatically reduced binding affinity. Comparison of antibody:receptor complexes involving IgA or its precursor IgY revealed a conserved receptor binding site at the CH2-CH3 junction (or its equivalent). Given the importance of residues near the CH2-CH3 junction, we used coarse-grained Langevin dynamics simulations to understand the functional dynamics in Fcα. Our simulations indicate that FcαRI binding, either in an asymmetric (1:1) or symmetric (2:1) complex with Fcα, propagated long-range conformational changes across the Fc domains, potentially impacting the hinge and Fab regions. Subsequent SPR experiments confirmed that FcαRI binding to the Fcα CH2-CH3 junction altered the kinetics of HAA lectin binding at the IgA1 hinge. Receptor-induced long-distance conformational transitions have important implications for the interaction of aberrantly glycosylated IgA1 with anti-glycan autoantibodies in IgA nephropathy.