Mechanisms of breast-milk transmission

Although HIV has been detected in breast milk, (Nduati, 1995; Ruff, 1994; Van de Perre et al., 1993) mechanisms of breast-milk transmission are not yet fully understood. The respective roles of cell-free and cell-associated virus in breast-milk transmission are not known, nor is the association between plasma and milk virus levels understood. The portal of entry for the virus via the infant mucosa also merits further investigation.

Animal models have been used to explore potential mechanisms of transmission. It is possible to infect neonatal rhesus monkeys with simian immunodeficiency virus (Baba et al., 1994) and kittens with feline immunodeficiency virus (Sellon et al., 1994) by applying cell-free virus on the mucosa. This suggests that cell-free HIV in breast milk could infect cells of the intestinal mucosa. M-cells, which are specialized epithelial cells found in the Peyer's patches of the intestinal mucosa, may be a mechanism allowing infectious agents such as HIV to cross the intact mucosa. M-cells engulf and transport the pathogen and present it to macrophages that indent the serosal surface of the M-cell (Featherstone, 1997). Results from in vitro studies on rabbit M cells suggest that HIV-1 particles could use M cells to cross the intestinal barrier (Amerongen et al., 1991). A recent in vitro study indicated that HIV-infected cells themselves may also play an important role by stimulating ordinary enterocytes to engulf HIV particles presented by HIV-infected cells in the intestinal lumen (Bomsel, 1997). Moreover, HIV RNA has been detected in the oropharyngeal and gastric aspirates of a substantial proportion of infants born to HIV-infected mothers (Nielsen et al., 1996, Ait-Khaled et al., 1998).