Rat secretory component binds poorly to rodent IgM.

Our previous studies and those of others indicated that human secretory component (SC), the five domain extracellular portion of the poly Ig receptor, binds avidly to both pIgA and IgM. In this study we report that in rodents, SC binds primarily to pIgA. Rat secretory component was isolated from bile and radiolabeled to known specific activity with 125I. Radiolabeled rat SC was incubated with rat and mouse monoclonal proteins for 1 h at room temperature and overnight at 4 degrees D. Binding of 125I-rat SC to Ig was determined in two ways: 1) immunoprecipitation of putative 125I-rat SC-Ig complexes with anti-L chain antibodies; 2) HPLC gel filtration on an analytical TSK 4000 column that separated free 125I-rat SC from 125I-rat SC bound to Ig. Both methods of analysis yielded similar results. Rat and mouse polymeric (p) IgA bound rat SC with high avidity, although the binding activity of the IgM from either species was virtually nil. The number of SC-binding sites on rat polymeric Ig was determined by immunoprecipitation of mixtures of rat pIg with saturating concentrations of 125I-rat SC and yielded values of 1.0 and 0.05 for rat pIgA and IgM, respectively. The significance of these findings with respect to the biologic function of the pIg R in rodents and the nature of the pIg R-binding site on pIg is discussed.