Comparison of human, bovine and rabbit secretory component-immunoglobulin interactions

The ability of free secretory component of several species to hind to homologous and heterologous polymeric IgA and IgM was studied. Experiments were performed in which whole milk whey from man, cow, sheep, rabbit and rat was used as a source of free secretory component (SO to competitively inhibit the binding of¹²⁵I-human SC to either human polymeric IgA or IgM.

When competitive inhibition of¹²⁵I-human SC to human IgM was tested, greater differences were observed among the different secretory components than was observed when similar experiments were carried out with human IgA. The relative ability of each of the wheys to displace¹²⁵I-human SC from human IgM was: human > bovine > sheep > rabbit > rat. The relative ability of each of the wheys to displace¹²⁵I-human SC from human IgA dimer was: rabbit > human > rat ∼- sheep > cow.

Direct and relative binding studies with purified human and rabbit SC and purified human and rabbit polymeric IgA and IgM demonstrated that in man. SC bound to IgM with two-five-fold greater affinity than to IgA dimer, while in rabbit, SC bound to rabbit IgM with seven-fold lower affinity than to rabbit IgA polymers.

Binding of SC (either human or rabbit) to polymeric rabbit IgA was largely non-covalent. In contrast, binding of SC (either human or rabbit) to human IgA dimer involved covalent bond formation.

The relationship of the observed affinity differences to the relative proportion of polymeric IgA and IgM in the secretions as well as the nature of the SC-binding site, is discussed.