An unknown component of breast milk kills HIV particles and virus-infected cells, as well as blocking HIV-transmission in mice with a human immune system, according to a new study.

Researchers at University of North Carolina (UNC; Chapel Hill, USA) created bone marrow, liver, and thymus (BLT) mice that underwent reconstitution of the oral cavity and upper gastrointestinal (GI) tract using humanized leukocytes, including the human cell types important for mucosal HIV transmission (i.e., dendritic cells, macrophages, and CD4+ T cells). The BLT mice were susceptible to oral transmission of cell-free and cell-associated HIV, which resulted in systemic infection of lymphoid and nonlymphoid tissues characterized by the presence of HIV RNA in plasma and a gradual decline of CD4+ T cells in peripheral blood.

The researchers found, however, that if the BLT mice were fed breast milk contaminated with HIV, the virus was not transmitted. The in vivo results demonstrated that breast milk had a strong inhibitory effect on oral transmission of both cell-free and cell-associated HIV. Finally, the researchers evaluated the effect of antiretrovirals on oral transmission of HIV, showing that systemic antiretrovirals administered prior to exposure could also efficiently prevent oral HIV transmission in BLT mice. The study was published on June 14, 2012, in PLoS Pathogens.

“These results are highly significant because they show that breast milk can completely block oral transmission of both forms of HIV that are found in the breast milk of HIV- infected mothers, virus particles and virus-infected cells,” said lead author Angela Wahl, MD, of the UNC Center for AIDS Research. “This refutes the 'Trojan horse' hypothesis which says that HIV in cells is more stubborn against the body's own innate defenses than HIV in virus particles.”

Pediatric HIV infection is associated with an accelerated course of disease and high mortality rate. In the absence of antiretroviral therapy, only 65% of HIV-infected children survive until their first birthday, and less than half will reach two years of age. The question arises as to why some breastfed babies born to HIV-positive mothers contract the virus, if breast milk does not transmit HIV? The answer might be that suckling on cracked nipples may expose babies to the HIV virus present in their mother's blood.

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