Assays for total and antigen-specific polymeric IgA in serum based on binding to secretory component

Abstract

Binding assays with secretory component (SC) were used to detect polymeric IgA antibody to E. coli lipopolysaccharide and to estimate total polymeric IgA in sera from 14 patients with alcoholic liver disease and eight normal controls. Radioiodinated human SC was shown to bind to polymeric IgA and IgM but not to monomeric IgA, secretory IgA or IgG. Serum aliquots (0.5 ml) were totally depleted of IgM using 2 ml anti-IgM affinity columns and the effluent sera were titrated in microtitre plates coated with lipopolysaccharide, the binding of polymeric IgA being detected by adding 10 ng radiolabelled SC. Total polymeric IgA was measured via its capacity to inhibit the binding of 5 ng labelled SC to IgM coated wells, quantitation being achieved by comparison with the inhibition produced by purified polymeric IgA. Total lipopolysaccharide-specific IgA antibody was detected by ELISA in sera from both patients and controls, 1185 ± 793 and 56 ± 19 U/100 μl (mean ± SD), respectively; but polymeric IgA antibody was detected only in patients' sera (131 ± 214 U/100 μl). The concentration of total polymeric IgA was higher in patients' sera than in control sera (488 ± 333 and < 120 μg/ml respectively).