Purification and characterization of secretory IgA from baboon colostrum

In this report, we describe a method for purifying secretory immunoglobulin A (sIgA) from baboon (Papio anubis) colostrum. The colostrum was first clarified by centrifugation and then analyzed with various anti-human Ig-specific immunologic reagents. Cross-reactive IgA in the baboon colostrum was identified by ELISA. Western blot analysis also demonstrated cross-reactive epitopes associated with human IgA1, IgA2, secretory component (SC), and joining (J) chain. To purify the sIgA, colostrum was separated into 4 distinct fractions by gel filtration chromatography. Analysis of the individual fractions by ELISA indicated that the IgA elutes over one peak. The IgA fraction was compared with purified human sIgA on SDS-PAGE, and exhibited heavy (H) chains, light (L) chains, SC, and J chain. The baboon colostrum was also analyzed by ELISA for specific IgG H and L chain epitopes utilizing monoclonal antibodies (MAbs). No significant quantity of IgG was detected in the baboon colostrum or in the individual 4 fractions, while L chain reactivity was observed in the sIgA fraction. The sIgA fraction was pooled, concentrated, and was found to contain approximately 7 mg/ml sIgA. To determine if the baboon sIgA was dimeric like human sIgA, the purified sIgA was sized by molecular sieve chromatography. The molecular size of the sIgA preparation (350 kDa) was determined empirically by comparison to known molecular species used to calibrate the column. In addition, native SDS-PAGE indicated that baboon sIgA, like human sIgA, migrates between IgG and IgM, suggesting it has a dimeric form. The purified baboon sIgA preparation should prove useful in the future study of mucosal immune responses induced in non-human primate species and for the generation of sIgA-specific immunological reagents.