Human T cell leukemia viruses are predominantly transmitted from mother to child by breastfeeding. Endemic levels of HTLV infection are associated with ethnic groups that have traditionally practised long-term breastfeeding. In the course of long-term lactation, we have found that human milk contains leukocytes and epithelial cells and that mixed primary cultures of these milk cells are susceptible to HTLV-I infection in vitro. We have established and characterized an immortalized line of milk epithelial cells, HTLV-LEC, that are productively infected and transformed with HTLV-I. This is the first reported case of human cells, other than T cells, that are transformed with HTLV-I. Cultures of HTLV-LEC are distinctive because of the synthesis of an extensive extracellular matrix that appears to support in vitro morphogenesis. HTLV-I infection can be transmitted from HTLV-LEC into normal epithelial cells and leukocytes. Our results suggest that infected epithelial cells could be involved in the persistence and transmission of virus infection in HTLV-I carriers.
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We thank S. McCormick, K. Peterson, and C. Manivel for advice on cell identification by immunohistochemistry, samples of antibodies, and invaluable discussions; M. L. Becker, A. T. Haase, K. Staskus, and S. Wietgrefe for critical reading of the manuscript; D. Heieren for expert assistance with the photomicrography; and T. Leonard for skillful preparation of the artwork. Special thanks to our children Derek and Carolyn Southern, who inspired this work. This work was supported in part by grants from the NIH and the Minnesota Medical Foundation.