Impaired mammary gland T cell population during early lactation in hypoprolactinemic lactation-deficient rats.

Mammary stroma is composed of various cell types, including migratory leukocytes. Although mammary antibody-secreting cells have been extensively studied, reports focusing on mammary T cells are scarce. It is thought that the recruitment mechanism of leukocytes to the mammary gland (MG) is controlled by pregnancy- and lactation-specific stimuli. But whether prolactin (PRL) modulates the T-cell population in MG is still unknown. Our aim was to study the relationship between PRL levels and T and B cells during early lactation (L2, day 2 post partum) and mid-lactation (L12, day 12 of lactation). In order to investigate whether PRL is associated with homing events to MG, female Sprague Dawley (SD) and SD-derived desmoglein 4(-/-) hairless (phenotype with lactation deficit, OFA hr/hr) rats were killed during estrus, pregnancy, and post partum, and blood, MG, and corpora lutea were obtained to perform fluorescent-activated cell sorting (FACS), real-time PCR, and histological and RIA studies. Serum PRL levels were lower in OFA hr/hr rats than in SD rats during early lactation. MG of OFA hr/hr rats showed less secretory material compared with SD rats. FACS analysis showed lower percentage of MG CD3+ cells in OFA hr/hr rats compared with SD rats on L2 and L12. OFA hr/hr rats showed higher absolute numbers of circulating CD3+ cells compared with SD rats on L2 but not on L12. These results show that T-cell population in MG is affected in early lactating OFA hr/hr rats and strongly suggest that serum PRL levels may be involved in the homing events to MG, probably helping antibody-secreting cells and protecting the gland during lactation development.