Human T-cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia/lymphoma and HTLV-1-associated myelopathy/tropical spastic paraparesis. In addition to blood transfusion and sexual transmission, HTLV-1 is transmitted mainly through prolonged breastfeeding, and such infection represents a major risk for the development of adult T-cell leukemia/lymphoma. Although HTLV-1-infected lymphocytes can be retrieved from maternal milk, the mechanisms of HTLV-1 transmission through the digestive tract remain unknown. In the present study, we assessed HTLV-1 transport across the epithelial barrier using an in vitro model. Our results show that the integrity of the epithelial barrier was maintained during coculture with HTLV-1-infected lymphocytes, because neither morphological nor functional alterations of the cell monolayer were observed. Enterocytes were not susceptible to HTLV-1 infection, but free infectious HTLV-1 virions could cross the epithelial barrier via a transcytosis mechanism.Such virions were able to infect productively human dendritic cells located beneath the epithelial barrier. Our data indicate that HTLV-1 crosses the tight epithelial barrier without disruption or infection of the epithelium to further infect target cells such as dendritic cells. The present study provides the first data pertaining to the mode of HTLV-1 transport across a tight epithelial barrier, as can occur during mother-to-child HTLV-1 transmission during breastfeeding. IntroductionApproximately 5-20 million persons worldwide are infected by the human retrovirus human T-cell leukemia virus type 1 (HTLV-1), 1 and this virus is the causative agent of severe adult T-cell leukemia/lymphoma 2 and inflammatory syndromes such as tropical spastic paraparesis/HTLV-1-associated myelopathy, a slowly progressing neurodegenerative disease that occurs in 0.5%-3% of infected persons. 3 HTLV-1 is transmitted via sexual intercourse, by transfusion with contaminated blood, and from mother to child. The latter constitutes 15%-25% of overall transmission, so it is a major method of viral spread. 4,5 Although intrauterine or perinatal transmission during birth cannot be excluded, breastfeeding is the main pathway of HTLV-1 transmission in highly endemic areas such as intertropical Africa, the Caribbean, or regions of South America. 6,7 Milk-borne transmission is supported by the presence of high levels of infected lymphocytes in maternal milk, 10 5 -10 7 of these being transferred to the child at each feed, 8,9 in addition to infected macrophages and breast epithelial cells. Furthermore, leukocytes in the breast milk remain viable after ingestion for up to 4 hours because of the buffering capacity of breast milk and the low acidity of the neonatal child's stomach. 10,11 The risk of transmission increases with the time that children are breastfed, especially when this exceeds 6 months. In addition to the duration of breastfeeding, the mother's age, Ab titers directed against HTLV-1 antigens, proviral load in the PBMCs and milk, a...