IgG antibodies to secretory component in normal human serum.

While isolating free secretory component (FSC) by monoclonal antibody affinity chromatography, we demonstrated FSC-IgG complexes in human milk. We hypothesized that IgG antibody to secretory component (SC) might be transported into the milk from the serum. We therefore examined sera from 10 normal adults and 10 infants for IgG capable of binding to FSC in an enzyme-linked immunosorbent assay. Eight of 10 normal adult sera and nine of 10 infant sera demonstrated IgG binding to FSC with titers ranging from 1:54 to 1:4096. Quantitation of the IgG bound to FSC was hampered in adult sera by the binding of IgM and polymeric IgA to the FSC. Quantitation in five infant sera ranged from 0.5 to 6.4 micrograms/ml. A pepsin digest of an IgG fraction of serum demonstrated binding of the F(ab')2 fragments to the FSC. The specificity of the antibodies in human serum was evaluated by examining the binding to secretory IgA (sIgA) and FSC isolated from pooled human milk and polymeric IgA isolated from the ascitic fluid of a patient with an IgA myeloma. Eight of the 10 adults had antibody specific for FSC. Three of the eight, all female, also had antibody specific for sIgA. Two of the eight had antibody either to FSC and sIgA or to FSC plus an antibody that could bind to an epitope shared by sIgA and FSC. Competition experiments with monoclonal antibodies to human secretory component and sIgA were used to confirm and further define these specificities. The results of this study indicate that antibody to SC is common in normal adult and infant sera. The majority of antibodies seem to be directed against epitopes present on FSC but not on sIgA, which suggests sensitization to circulating or membrane-bound SC. The significance of these antibodies in normal human sera remains to be elucidated.