Objectives: Definition of the cellular constituents of breast milk and infant saliva that are involved in milk-borne transmission of HIV infectivity.
Study design/methods: Productively infected cells in colostrum and milk of HIV-1-seropositive mothers were identified by in situ hybridization and immunocytochemistry. Additionally, normal cells from mature milk were infected in vitro to determine which cell types were capable of supporting productive HIV-1 infection. Cellular interactions and transfer of HIV-1 in saliva-milk mixtures were studied to monitor the viability of milk cells and the potential for transfer of infectious virus during ingestion of milk.
Results: Colostrum and early milk from HIV-1-seropositive mothers contained 0.1% to 1% productivity infected macrophages and T cells. Macrophages and epithelial cells from mature milk were susceptible to productive HIV infection in vitro. When milk was mixed with saliva, milk cells became disrupted or were bound and endocytized by salivary epithelial cells.
Conclusions: Productively infected milk cells may contribute directly to transmission of HIV infectivity in breastfed infants during both early and late lactation. Macrophages are the principal cellular carriers of productive HIV-1 infection in milk. Cellular complexes produced during milk-saliva interactions may play a key role in oral transmission of HIV.