Further Evidence for a Role of Secretory Component (SC) and J Chain in the Glandular Transport of IgA

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Abstract
Human secretory epithelia transmit preferentially dimers and larger polymers of IgA and pentameric IgM. During their passage through the serous glandular cell, the secretory immunoglobulins become associated with an epithelial glycoprotein called SC. Several models have been proposed for their mode of external transport (1). The balance of evidence indicates that SC acts as an epithelial receptor for J chain-containing IgA and IgM for which it shows specific non-covalent affinity in vitro (2). Polymeric IgA and IgM with incorporated J chains are formed in plasma cells adjacent to glandular epithelia (3), but there has been no direct evidence that these immunoglobulins indeed combine with SC on the surface of the epithelial cell. Moreover, although in vitro experiments have demonstrated that incorporation of J chains is mandatory for the specific non-covalent SC-binding site of the Ig polymers (3), a requirement for J chain in their selective glandular transport has not been directly shown. The purpose of the studies reported here was to establish more clearly the involvement of SC and J chain in the external translocation of IgA.