New insights into the dual recruitment of IgA+ B cells in the developing mammary gland

In monogastric mammals, transfer of passive immunity via milk and colostrum plays an important role in protecting the neonate against mucosal infections. Here we analyzed the hypothesis that during gestation/lactation IgA+ plasmablasts leave the intestinal and respiratory surfaces towards the mammary gland (MG). We compared the recruitment of lymphocytes expressing homing receptors α4β1 and α4β7 to expression of their vascular counter-receptors, VCAM-1 and MAdCAM-1. Furthermore, the expression of the chemokines responsible for the recruitment of IgA+ plasmablasts was analyzed. Data confirmed that expressions of CCL28 and MAdCAM-1 in the MG increased during pregnancy and α4β1+ and α4β7+/IgA+ cell recruitment in lactation correlated with increase of CCL28 expression. Interestingly, VCAM-1 expression was found in small blood vessels of the lactating porcine MG, while in mice VCAM-1 was expressed in large blood vessels within the MG. Thus, our results indicate that the recruitment of IgA+ plasmablasts to MG is mediated by VCAM-1/α4β1 and MAdCAM-1/α4β7 in conjunction with CCL28/CCR10. They support the existence of a functional link between entero- and upper respiratory surfaces and MG, thereby, conferring protection against aero-digestive pathogens in the newborn.