A reliable method for the preparation of bovine secretory immunoglobulin a (SIgA) which circumvents problems posed by IgG1 dimers in colostrum

Abstract

Quantitative data are presented which show that the IgA-rich fraction of bovine colostrum contains large amounts of dimeric IgG1. The IgG1 and IgA dimers are incompletely resolved by chromatography in 1 M NaCl on Sepharose 6B columns, 300cm in length, by DEAE-chromatography or by differential ZnSO 4 solubility. Contamination of IgA by IgG1 may be as high as 65% even after ion-exchange chromatography. This contamination can be substantially reduced by the property of this IgG1 to bind Protein-A while most IgA does not. These colostral IgG-dimers are only partially deaggregated by treatment with 1% SDS. Bronchio-alveolar washings (BAW) from slaughterhouse animals contains 10% IgA protein; slightly less than the combined total of IgG2 and IgG1. Furthermore, BAW prepared in the manner we describe lacks IgG1 dimers, free secretory component and is relatively free of mucus. Highly purified IgA can be obtained from BAW by a three step method involving the removal of contaminating α 2-macroglobulin by ZnSO 4 precipitation. IgA prepared in this manner is heterogeneous in its content of secretory component but free of contamination when assayed by a battery of immunochemical techniques, by ultracentrifugation, SDS-PAGE electrophoresis and by its immunogenicity in rabbits. Data on the physicochemical characteristics of this purified IgA as well as its unique association with mucus are presented.