本资料来自于资源最齐全的２１世纪教育网www.21cnjy.com 请认真阅读下列短文，从短文后各题所给的A、B、C、D四个选项中，选出最佳选项，并在答题卡上将该项涂黑。 For decades, public health officials have puzzled over a surprising fact about HIV: Only about 10-20 percent of infants who are breastfed by infected mothers catch the virus. Tests show, though, that HIV is indeed present in breast milk, so these children are exposed to the virus multiple times daily for the first several months of their lives. Now, a group of scientists and doctors from Duke University has figured out why these babies don’t get infected. Human breast milk naturally contains a protein called Tenascin C that neutralizes HIV and, in most cases, prevents it from being passed from mother to child. Eventually, they say, the protein could potentially be valuable as an HIV-fighting tool for both infants and adults that are either HIV-positive or at risk of contracting the infection. The research, published in Proceedings of the National Academy of Sciences, was inspired by previous work by other researchers showing that, both in tissue cultures (组织培养) and live mice, breast milk from HIV-negativemothers was naturally endowed with HIV-fighting properties. Scientists suggested that a few different proteins in the milk could potentially be responsible, but no one knew which one. As part of the study, the researchers divided breast milk into smaller fractions (部分) made up of specific proteins via a number of filters (过滤) — separating the proteins by size, electrical charge and other characteristics—and tested which of these fractions, when added to a tissue culture, prevented the cells from being infected by HIV. Eventually, they found that one particular protein was present in all the HIV-resistant fractions but in none of the others: Tenascin C. Tenascin C works by blocking a key protein on HIV’s envelope that normally ties up to a receptor on a T cell’s membrane called CCR5. In doing so, Tenascin C prevents HIV from mixing with the T cell and injecting its RNA inside. Still, the researchers say that other natural elements in milk might play a role in fighting HIV as well. “It’s clearly not the whole story, because we do have samples that have low amounts of this protein but still have HIV-neutralizing activity,” the study’s lead author Permar says. “So it may be acting in concert with other antiviral and antimicrobial factors in the milk.” Whatever those other factors are, though, the finding vindicates (证明……的正确) recent changes to UN guidelines that recommend even HIV-positive mothers in resource-poor countries should breastfeed, if they’re taking anti-retroviral drugs to combat their own infection. The next steps, Permar says, are determining which area of Tenascin C is active and whether it can effectively prevent transmission in a live animal. If it works, it could potentially be incorporated (合并) into an HIV drug with broader applications. It’s even possib ... ...
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