Analysis of IgA1 N-Glycosylation and Its Contribution to FcαRI Binding

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Abstract
The IgA isotype of human antibodies triggers inflammatory responses via the IgA-specific receptor FcαRI (CD89). Structural studies have suggested that IgA1 N-glycans could modulate the interaction with FcαRI. We have carried out detailed biophysical analyses of three IgA1 samples purified from human serum and recombinant IgA1-Fc and compared their binding to FcαRI. Analytical ultracentrifugation revealed wide variation in the distribution of polymeric species between IgA1 samples, and Fourier transform ion cyclotron resonance mass spectrometry showed overlapping but distinct populations of N-glycan species between IgA1 samples. Kinetic and equilibrium data from surface plasmon resonance experiments revealed that variation in the IgA1 CH2 N-glycans had no effect on the kinetics or affinity constants for binding to FcαRI. Indeed, complete enzymatic removal of the IgA1 N-glycans yielded superimposable binding curves. These findings have implications for renal diseases such as IgA nephropathy.