Abstract
The cleavage of human serum monomeric immunoglobulin A1 (IgA1) and human secretory IgA1 (S-IgA1) by IgA1 proteinase of
Neisseria
meningitidis
and cleavage by the proteinase from
Proteus
mirabilis
have been compared. For serum IgA1, both proteinases cleaved only the α chain.
N
.
meningitidis
proteinase cleaved only in the hinge.
P
.
mirabilis
proteinase sequentially removed the tailpiece, the CH3 domain, and the CH2 domain. The cleavage of S-IgA1 by
N
.
meningitidis
proteinase occurred only in the hinge and was as rapid as that of serum IgA1.
P
.
mirabilis
proteinase predominantly cleaved the secretory component (SC) of S-IgA1. The SC of S-IgA1, whether cleaved or not, appeared to protect the α1 chain. Purified Fc fragment derived from the cleavage of serum IgA1 by
N
.
meningitidis
proteinase stimulated a respiratory burst in neutrophils through Fcα receptors, whereas the (Fcα1)
2
-SC fragment from digested S-IgA1 did not. The loss of the tailpiece from serum IgA1 treated with
P
.
mirabilis
proteinase had little effect, but the loss of the CH3 domain was concurrent with a rapid loss in the ability to bind to Fcα receptors. S-IgA1 treated with
P
.
mirabilis
proteinase under the same conditions retained the ability to bind to Fcα receptors. The results are consistent with the Fcα receptor binding site being at the CH2-CH3 interface. These data shed further light on the structure of S-IgA1 and indicate that the binding site for the Fcα receptor in S-IgA is protected by SC, thus prolonging its ability to activate phagocytic cells at the mucosal surface.