Characterization and localization of secretory component in the chicken

A component found free in intestinal contents and caecal contents of conventional and germ-free chickens (lacking IgA producing cells) was found to have similar characteristics to mammalian secretory component (SC). Free secretory component (FSC) showed a classic reaction of partial identity with secretory IgA (SIgA) from bile, intestinal contents and cystic oviduct fluid. Furthermore, there was demonstrable cross-reactivity between FSC and a low molecular weight component released from SIgA by mild reductive dissociation, confirming the presence of a disulphide-linked accessory polypeptide chain. Fractionation of serum IgA revealed two molecular classes of IgA, a high molecular weight 15S IgA which possessed SC and could not be differentiated antigenically from SIgA and a low molecular weight 7S IgA which showed a reaction of partial identity with 15S IgA and non-identity with FSC. Fluorescent localization of SC in young germ-free chicks demonstrated its presence in the supranuclear golgi zone, apical cytoplasm and basement membrane of crypt epithelial cells. It is concluded that the characteristics of chicken SIgA are closely aligned with those of its mammalian counterpart and are consistent with a system in which SIgA is the wynthetic product of two distinct cells, final assembly occurring in the crypt epithelium.