PIGR

Polymeric immunoglobulin receptor (pIgR) is a type I transmembrane glycoprotein expressed on the basolateral surface of epithelial cells lining mucosal tissues. It is encoded by the PIGR gene in humans. pIgR binds polymeric immunoglobulins, notably IgA in its dimeric form and IgM in its pentameric form, via the J chain present on these antibodies.

During transcytosis, pIgR is endocytosed at the basolateral surface, transported across the cell, and released at the apical surface. Proteolytic cleavage releases a large extracellular fragment, the secretory component, which remains bound to the immunoglobulin to form secretory IgA (SIgA) or secretory IgM (SIgM). The secretory component protects the immunoglobulins from proteolysis in secretions and facilitates passage through mucosal barriers.

PIGR is expressed in epithelial cells of many mucosal sites, including the respiratory, gastrointestinal, and urogenital tracts, as well as in mammary glands where it contributes to IgA in milk. By delivering IgA and IgM to mucosal surfaces, pIgR plays a key role in mucosal immunity, helping to neutralize pathogens and limit microbial adherence.

Defects or altered expression of pIgR can affect mucosal antibody transport and susceptibility to infections. pIgR function is also a focus of mucosal immunology research and has implications for vaccine design and maternal transfer of antibodies via breast milk.