Response of circulating immune complexes to food challenge in relapsing IgA nephropathy

The response of circulating immune complexes (CIC) to food challenge was assessed in 15 subjects with IgA nephropathy (IgAN) and recurrent macroscopic haematuria. CIC were measured by solid-phase C1q binding assay (SP-C1q), immunoglobulin class-specific polyethylene glycol (PEG) precipitation assays (PEG-G, PEG-A, PEG-M) and by an antigen (ovalbumin)-specific radioimmunoassay after acid dissociation (OA-IC). CIC were measured when the subjects were fasting and hourly for 6 h after a test meal containing eggs. All 15 subjects were tested while clinically quiescent (remission) and 6 were tested again during episodes of macroscopic haematuria (relapse). The PEG-A CIC response to food challenge was significantly exaggerated in IgAN remission compared with controls at 3–6 h after food. There were also non-significant increases in PEG-G, though not in PEG-M. Paired data showed further exaggeration of PEG-G, PEG-A and PEG-M responses to food during IgAN relapse, but significance was not attained if the findings in 1 subject were separated. In this individual a florid clinical relapse with transient decline in renal function was associated with very high levels of PEG-IC, and only in this patient in relapse was OA-IC detectable, confirming that some PEG-precipitated material represented antigen-antibody complexes containing food antigen.