Homing Receptors and Vascular Addressins Differential Recruitment of T- and IgA B-lymphocytes in the Developing Mammary Gland in Relation to

SUMMARY The mammary gland (MG) develops new vasculature and is colonized by lym-phocytes, primarily T-cells, during pregnancy. In contrast, during lactation it is colonizedprimarily by IgA-containing B-cells (c-IgA cells). To explain this difference, we analyzed thespatiotemporal relationships between lymphocytes that expressed peripheral or mucosalhoming receptors (HR) and the location of their vascular counterreceptors using quantita-tive immunohistochemical techniques. We observed that the density of 7 / CD3T-cellsvaried with the amount of the mucosal addressin cell adhesion molecule-1 (MAdCAM-1)-stained area. Both increased during pregnancy to peak at delivery, decreased rapidly inearly lactation to a steady level in mid- and late lactation, and returned to resting values af-ter weaning. Although 60% of these 7 / CD3T-cells scattered in the epithelium co-expressed E 7 , whereas the remaining 40% in association with blood vessels were 4 7 , these resultsare consistent with a role of MAdCAM-1 in the localization of