Target cell infection occurred even after trypsin treatment of HIV-exposed and washed MEC (data not shown). ceramide (GalCer). While no evidence for direct infection of MEC was found, HIV-1 virions were observed in MEC endosomal compartments. Coculture of HIV-exposed MEC resulted Rabbit Polyclonal to NFYC in productive infection of activated CD4+T cells. In addition, MEC secretions increased HIV-1 replication and proliferation of infected target cells. Overall, our results indicate that MEC are capable of endosomal uptake of HIV-1 and can facilitate virus infection and Puromycin 2HCl replication in CD4+target cells. These findings suggest that MEC may serve as a viral reservoir for HIV-1 and may enhance infection of CD4+T lymphocytesin vivo. In the more than 2 decades since the discovery of human immunodeficiency virus (HIV) transmission via breast milk, little progress has been made in understanding the contribution made by the mammary gland to HIV replication in mammary tissue and breast milk. Mammary epithelial cells (MEC), the milk-secreting cells of the mammary gland, play a unique functional role within breast tissue. While most epithelial layers in the body serve a protective function, MEC selectively take up components of the Puromycin 2HCl plasma and manufacture constituents of breast milk to secrete a nutrient- and immune factor-rich fluid (22). In a nonpregnant or nonlactating woman, the mammary gland contains a slowly dividing Puromycin 2HCl population of mammary epithelial cell progenitors, known as mammary stem cells, which are present in the breast tissue from birth and persist throughout most of a woman’s premenopausal life (14). Hormonal changes associated with pregnancy and lactation induce rapid proliferation and terminal differentiation of these cells into mammary epithelial secretory alveolar cells, epithelial ductal cells, or myoepithelial cells (14,25,34,39,43). Stem cell division is primarily asymmetric, such that one daughter cell differentiates while the other retains the stem cell phenotype (11,13,43). Due to the steep rise in prolactin in the first half of pregnancy, hormone-induced proliferation and differentiation of mammary stem cells are largely complete by the second trimester (25). Whether mammary stem cells or their differentiated progeny are susceptible to human immunodeficiency virus type 1 (HIV-1) infection or sequestration of virions in endosomal compartments is largely unknown. Milk is a very cellular fluid throughout the lactation period, with more than 106leukocytes/ml in the colostrum (5,26), followed by a decline over time to approximately 105to 104leukocytes/ml in mature milk (9,15,16,26). To reach the milk, lymphocytes and monocyte/macrophage populations must traffic paracellularly or transcellularly through a normally impermeable MEC monolayer on their way into the milk duct. Epidemiologic studies show that HIV-1 RNA and proviral DNA in breast milk are significantly associated with virus Puromycin 2HCl transmission (19,28-30); however, little is known about the interaction between MEC and leukocytes in the mammary glands of HIV-infected women. Limited attention has been paid to the concept that MEC may contribute to the HIV-1 viral load in breast milk. This may be due, in part, to an early study of primary human MEC using a laboratory-adapted strain of HIV-1, which demonstrated low levels of viral replication in the MEC (40). However, even low-level HIV-1 replication or transcytosis of virions across MEC without direct infection could significantly impact the persistence and spread of virus within the mammary gland. MEC and leukocytes entering breast milk share close physical contact. Therefore, cell-to-cell contact between MEC, which either harbor HIV-1 or bind and transmit the virus to susceptible target cells in transit to the milk, and leukocytes could provide a potential mechanism for seeding newly infected cells into the breast milk. There is growing evidence that the mammary gland contributes.