Transformation of breast milk macrophages by HTLV-I: implications for HTLV-I transmission via breastfeeding

Human T cell leukemia virus type I (HTLV-I), a causative agent of adult T-cell leukemia (ATL), is transmitted from mother to child predominantly by breastfeeding. The source of HTLV-I-infected cells in breast milk has been thought to be T cells, however, the majority of cells in breast milk are CD14(+) macrophages but not CD3(+) T lymphocytes, and no data are available regarding HTLV-I transmission through breast milk macrophages (BrMM Phi). To explore the potential of BrMM Phi as a possible source of infection in mother to child transmission (MTCT) of HTLV-I, an immortalized cell line (HTLV-BrMM Phi) has been established from BrMM Phi by infection with HTLV-I. HTLV-BrMM Phi retained macrophage characteristics and did not express a complete dendritic cell (DC) phenotype; nevertheless, HTLV-BrMM Phi efficiently promoted T cell proliferation in primary allogeneic mixed lymphocyte reaction (MLR) like DC. Moreover, HTLV-I infection could be transmitted from HTLV-BrMM Phi to activated T cells in the peripheral blood. These findings suggested that BrMM Phi might be an appropriate HTLV-I reservoir involved in MTCT transmission via breastfeeding.