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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-negative mothers was naturally endowed with HIV-fighting properties.Scientists suggested that a few different proteins in the milk could potentially be responsible, but noone knew which one.
As part of the study. the researchers divided breast milk into smaller fractions made up of specific proteins via a number of filters-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 HIWs 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. "lt'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 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 into an HIV drug with broader applications. It's even possible that it could someday be adapted to reduce the risk of HIV transmission in adults as well as infants.
43. What is the main idea of Paragraph l?
A. The breast milk from an infected mother doesn't contain HIV at all.
B. Only a small percentage of infants breastfed by infected mothers catch HIV
C. The possibilities of infants catching HIV have nothing to do with infected mothers.
D. Children feeding on infected mothers' milk won't catch HIV though exposed to it frequently.
44. Why did the researchers divide breast milk into smaller fractions?
A. To find out Tenascin.
B. To make up specific proteins.
C. To add them to a tissue culture.
D. To test what prevents cells being infected.
45. Which of the foUowing about Tenascin C is true?
A. It helps prevent mother-to-child transmission of HIV.
B. It is the key protein on HIWs envelop.
C. It can prevent transmission in a live animal effectively.
D. It has become an effective tool in fighting HIV.
43．【答案】B。解析：主旨题。第一段中的“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“是本段的中心句。从感染了艾滋病病毒的母亲那里获得母乳的婴儿只有一小部分感染上了病毒。故本题选B。
44．【答案】D，解析：细节题。根据题干关键词定位到原文第四段中的第一句话“…the researchers divided breast milk into smaller fractions…and tested which of these fractions…prevented the cells from being infected by HIV”：这段话点明了研究人员按照不同的蛋白质成分将乳汁进行分离是为了检测出是什么成分使细胞免于艾滋病病毒的感染，由此可知，本题选D。
45．【答案】A。解析：细节题。根据第四段中的“Eventually, they found that one particular protein was present in all the HIV-resistant fractions but in none of the others: Tenascin C”和第五段整段
可知Tenascin C对艾滋病病毒有抑制功能。故本题选A。B、C两项原文没有体现；D项与文章最后一句话“lt's even possible that it could someday be adapted to reduce the risk of HIV transmission in adults as well as infants”矛盾。

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