Nearly 800,000 new infant HIV infections occur each year, and it is estimated that one-third to one-half of these infections are attributable to breastfeeding ( 3). However, HIV is vertically transmitted via breast milk and mother to child transmission via breast milk remains a significant mode of HIV transmission in the developing world. Moreover, poor access to clean water in the developing world limits the safety of infant replacement feeding. The benefits of breastfeeding, including optimal nutrition and protection against gastrointestinal and respiratory infections, are well-established and significantly improve infant morbidity and mortality in the developing world ( 1, 2). This robust viral-specific cellular immune response in breast milk may contribute to control of breast milk virus replication. Furthermore, the appearance of the SIV-specific CD8 + T lymphocyte response in breast milk was associated with a reduction in breast milk viral load, and this response remained higher than that in the blood after viral set point. Surprisingly, while the kinetics of the SIV-specific cellular immunity in breast milk mirrored that of the blood, the peak magnitude of the SIV-specific CD8 + T lymphocyte response in breast milk was more than twice as high as the cellular immune response in the blood. The breast milk viral load was one to two logs lower than plasma viral load through peak and set-point of viremia. Hormone-induced lactating monkeys were inoculated intravenously with SIVmac251 and CD8 + T lymphocytes specific for two immunodominant SIV epitopes, Gag p11C and Tat TL8, and SIV viral load were monitored in peripheral blood and breast milk during acute infection. The composition of lymphocyte subsets in hormone-induced lactation (HIL) breast milk was found to be similar to that in natural lactation (NL) breast milk. To model HIV-specific immunity in breast milk, lactation was pharmacologically induced in Mamu-A*01 + female rhesus monkeys.

Remarkably, only a small minority of breastfeeding infants born to HIV-infected mothers contract HIV via breast milk exposure, raising the possibility that immune factors in the breast milk confer protection to the infants who remain uninfected. Breast milk transmission of HIV is a leading cause of infant HIV/AIDS in the developing world.

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