Increased Immunoglobulin A Levels in Milk by Over-expressing the Murine Polymeric Immunoglobulin Receptor Gene in the Mammary Gland Epithelial Cells of Transgenic Mice

The polymeric immunoglobulin receptor (pIgR) transports dimeric immunoglobulin A (dIgA) across the epithelial cell layers into the secretions of various mucosal and glandular surfaces of mammals. At these mucosal sites, such as the gastrointestinal tract, respiratory tract, urogenital tract and the mammary glands, dIgA protects the body against pathogens. The pIgR binds dIgA at the basolateral side and transports it via the complex mechanism of transcytosis to the apical side of the epithelial cells lining the mucosa. Here, the extracellular part of the receptor is cleaved to form the secretory component (SC), which remains associated to dIgA, thereby protecting it from degradation in the secretions. One pIgR molecule transports only one dIgA molecule (1 : 1 ratio) and the pIgR is not recycled after each round of transport. This implies that the amount of available receptor could be a rate-limiting factor determining both the rate and amount of IgA transported per cell and therefore determining the total IgA output into the lumen or, in case of the mammary gland, into the milk. In order to test this hypothesis, we set up an in vivo model system. We generated transgenic mice over-expressing the murine pIgR gene under lactogenic control, by using a milk gene promoter, rather than under immunological control. Mice over-expressing the pIgR protein, in mammary gland epithelial cells, from 60- up to 270-fold above normal pIgR protein levels showed total IgA levels in the milk to be 1.5-2-fold higher, respectively, compared with the IgA levels in the milk of non-transgenic mice. This indicates that the amount of pIgR produced is indeed a limiting factor in the transport of dIgA into the milk under normal non-inflammatory circumstances.