Posted by: Glow-worm PJ6 NOV 2013
Healthcare agencies are keen to maintain the nutritional and immunological benefits of breastfeeding in developing countries, even in infants whose mothers are HIV positive. Infant retroviral drug prophylaxis can reduce the rate of infection, but other issues such as cost, toxicity and drug resistance can limit its effectiveness.
However, even in the absence of antiretroviral prophylaxis, less than 10 per cent of infants breastfed by HIV-infected mothers contract the virus. It has also been shown that breast milk from uninfected women neutralises HIV, and prevents its transmission in animal models.
Research has now identified the responsible agent as tenascin-C, an extracellular matrix glycoprotein more usually associated with inflammation states, rapidly induced at sites of tissue injury or infection. Persistent tenascin-C expression is associated with conditions such as chronic, non-healing wounds, autoimmune diseases and cancer.
In breast milk the protein has been shown to prevent fusion of the HIV envelope with that of the host cell, and subsequent transfer of the viral DNA to the host. Tenascin-C binds to receptors on the HIV envelope at the point where fusion would occur, greatly inhibiting viral effectiveness. It is thought that other factors in breast milk may also play a part in inhibiting HIV, because even samples low in tenascin-C display some HIV neutralising activity.
These findings have led to a change in UN advice, with HIV-positive mothers in developing countries now encouraged to breastfeed. Scientists were puzzled as to why milk should contain a protein active against a virus that has evolved only recently. The answer is more likely to be that tenascin-C possesses a more generalised antimicrobial activity, and the fact that it can inhibit HIV transmission is a happy coincidence.