An increase in milk IgA correlates with both pIgR expression and IgA plasma cell accumulation in the lactating mammary gland of PRM/Alf mice

In mice, during late pregnancy and lactation, maternal precursors of IgA-containing cells (cIgA-cells) are primed in the gut and home to the mammary gland where they secrete IgA. In turn, the ensuing increase in milk IgA mediates immune protection of the newborn gastrointestinal tract. PRM/Alf is an inbred mouse strain which exhibits a substantial post-natal intestinal lengthening which develops throughout the neonatal suckling period, suggesting that the availability of cIg-A cells and the level of protective IgA in milk might also be increased. We confirmed that PRM/Alf milk contains higher amounts of IgA than C57BL/6J throughout lactation, concomitantly with an increase of pIgR on epithelial cells and a higher density of cIgA-cells in the PRM/Alf mammary gland. Furthermore, a search for variations in cellular and humoral factors implicated in regulating cIgA-cell migration towards the mammary gland, including the vascular addressins MAdCAM-1 (mucosal addressin cell adhesion molecule-1) and VCAM-1 (vascular cell adhesion molecule-1) as well as the mucosal epithelial chemokine CCL28, did not reveal any quantitative differences in expression between PRM/Alf and C57BL/6J mice strains. Thus our results indicate that these factors are not limiting in the recruitment of cIgA-cells released from the elongated gut of PRM/Alf mice. In the context of intestinal lengthening, these findings strengthen the notion of an entero-mammary gland link, where the neonatal gut is protected by the maternal gut through the immune function of the mammary gland.