Breast Milk Yields Clues to Natural Inhibitors of HIV
Most infants breastfed by an HIV-positive woman do not become infected. It has long been known that components of human breast milk from HIV-negative mothers can inhibit the growth of HIV in the test tube, indicating that HIV-specific antibodies in milk are not responsible for this activity. But until Dr. Angela Wahl, an amfAR Krim Fellow, began her studies, no one had explored such a phenomenon in HIV-positive women.
Writing in the November issue of the Journal of Infectious Diseases, Dr. Wahl, working at the University of North Carolina in Chapel Hill, along with colleagues from Duke, Harvard, Columbia, UC-San Diego, and the University of Southern California, explored the activity of milk derived from HIV-positive women from Zambia who had breastfed their infants and either transmitted the virus or not. Rather than limiting experiments to test-tube studies and oral transmission, Dr. Wahl used "humanized mice" carrying components of a human immune system, and explored vaginal and intravenous HIV transmission as well.
All humanized mice exposed orally, vaginally or intravenously to small doses of HIV became infected. In contrast, only a quarter of the animals became infected when the virus was introduced orally along with human breast milk. It was 25% regardless of whether the milk came from known virus transmitters, or those Zambian women who breastfed and did not transmit. Significant protection was also obtained using virus administered with breast milk by the vaginal and intravenous routes.
The nature of the active components in human breast milk that are responsible for this protective effect remains unclear. The activity was not found in milk from cow, camel, goat, or monkey. It was not destroyed by pasteurization or by manipulations designed to remove or destroy protein, sugars, or fat. The authors concluded that the activity most likely involved an interaction among those three components.
Given the nutritional and immunologic benefits of breast milk, and the dangers of preparing infant formula when clean water cannot be guaranteed, these data bolster current recommendations that HIV-positive women in resource-poor nations exclusively breastfeed in combination with antiretroviral therapy. The promise is that identification of the components responsible for the anti-HIV activity might form the basis for the development of natural microbicides -- substances that could be applied topically to prevent acquisition of HIV infection -- or other anti-HIV medications.
Dr. Laurence is amfAR's senior scientific consultant.