A comparison of the binding of secretory component to immunoglobulin A (IgA) in human colostral S-IgA1 and S-IgA2

A detailed investigation of the binding of secretory component to immunoglobulin A (IgA) in human secretory IgA2 (S-IgA2) was made possible by the development of a new method of purifying S-IgA1, S-IgA2 and free secretory component from human colostrum using thiophilic gel chromatography and chromatography on Jacalin-agarose. Sodium dodecyl sulphate–polyacrylamide gel electrophoresis of unreduced pure S-IgA2 revealed that, unlike in S-IgA1, a significant proportion of the secretory component was bound non-covalently in S-IgA2. When S-IgA1 was incubated with a protease purified from Proteus mirabilis the secretory component, but not the α-chain, was cleaved. This is in contrast to serum IgA1, in which the α-chain was cleaved under the same conditions – direct evidence that secretory component does protect the α-chain from proteolytic cleavage in S-IgA. Comparisons between the products of cleavage with P. mirabilis protease of free secretory component and bound secretory component in S-IgA1 and S-IgA2 also indicated that, contrary to the general assumption, the binding of secretory component to IgA is different in S-IgA2 from that in S-IgA1

Mapping the interaction between murine IgA and murine secretory component carrying epitope substitutions reveals a role of domains II and III in covalent binding to IgA

We have identified sites for epitope insertion in the murine secretory component (SC) by replacing individual surface-exposed loops in domains I, II, and III with the FLAG sequence (Crottet, P., Peitsch, M. C., Servis, C., and Corthesy, B. (1999) J. Biol. Chem. 274, 31445-31455). We had previously shown that epitope-carrying SC reassociated with dimeric IgA (IgA(d)) can serve as a mucosal delivery vehicle. When analyzing the capacity of SC mutants to associate with IgA(d), we found that all domain II and III mutants bound specifically with immobilized IgA(d), and their affinity for IgA(d) was comparable to that of the wild type protein (IC(50) approximately 1 nM). We conclude that domains II and III in SC are permissive to local mutation and represent convenient sites to antigenize the SC molecule. No mutant bound to monomeric IgA. SC mutants exposing the FLAG at their surface maintained this property once bound to IgA(d), thereby defining regions not required for high affinity binding to IgA(d). Association of IgA(d) with SC mutants carrying a buried FLAG did not expose de novo the epitope, consistent with limited, local changes in the SC structure upon binding. Only wild type and two mutant SCs bound covalently to IgA(d), thus implicating domains II and III in the correct positioning of the reactive cysteine in SC. This establishes that the integrity of murine SC domains II and III is not essential to preserve specific IgA(d) binding but is necessary for covalency to take place. Finally, SC mutants existing in the monomeric and dimeric forms exhibited the same IgA(d) binding capacity as monomeric wild type SC known to bind with a 1:1 stoichiometry

Secretory immunoglobulin A: from mucosal protection to vaccine development.

Immune responses taking place in mucosal tissues are typified by secretory immunoglobulin A (S-IgA) molecules, which are assembled from proteins expressed in two cell lineages. The heavy and light chains as well as the J chain are produced in plasma cells, whereas the secretory component (SC) is associated to the immunoglobulin complex during transcytosis across the epithelial layer. S-IgA antibodies represent the predominant immunoglobulin class in external secretions, and the best defined entity providing specific immune protection for mucosal surfaces by blocking attachment of bacteria and viruses. S-IgA constitutes greater than 80% of all antibodies produced in mucosa-associated lymphoid tissues in humans. The existence of a common mucosal immune system permits immunization on one mucosal surface to induce secretion of antigen-specific S-IgA at distant sites. In addition, S-IgA antibodies not only function in external secretions, but also exert their antimicrobial properties within the epithelial cell during transport across the epithelium. Passive mucosal delivery of monoclonal IgA molecules neutralizes pathogens responsible for gastrointestinal and respiratory infections. Mucosal and systemic immunity can be achieved by orally administered recombinant S-IgA molecules carrying a protective bacterial epitope within the SC polypeptide primary sequence