A new study has found that rodents infected with respiratory syncytial virus (RSV) soon after birth are more likely to develop asthma later in life. The idea that asthma is linked to viral infection is not new: numerous epidemiological studies have linked the two in humans. But it has not been clear whether the virus itself causes asthma, or whether children who are more susceptible to viruses and subsequently develop wheezing coughs are also more susceptible to asthma. To test this, Anuradha Ray and Prabir Ray of the University of Pittsburgh in Pennsylvania and colleagues turned to baby mice. The researchers exposed the mother mice to ovalbumin, a protein in egg whites that can trigger allergic reactions. They found that the young mice gained tolerance against ovalbumin through their mother’s milk. Once the baby mice had been weaned, the researchers repeatedly infected some of them with RSV. Three weeks later they exposed all the baby mice to ovalbumin again. The mice that had not been infected with RSV did not mount an allergic response, but the infected mice did. When the researchers examined these mice, they found that the lungs had become inflamed, and the lymph nodes that drain the lungs were swollen and full of immune cells known as T regulatory cells. These cells, which normally protect mice from infection, were instead secreting proteins and signalling factors that cause allergic responses. Although breast milk from the mothers is known to confer some immunity to viruses, the repeated viral infections were able to overwhelm even that protection, the researchers say. Tina Hartert of Vanderbilt University in Nashville, Tennessee, says the paper is an important step towards identifying a biological mechanism and determining a stage during development that doctors could focus on to prevent asthma. Prabir Ray says there are already drugs that can target T regulatory cells and block the allergic response. The researchers hope to test what effect these drugs have in primates that have been infected with RSV. But it remains to be seen whether the results in mice will translate to humans, Hartert adds. For instance, mice are not normally susceptible to RSV, so they had to be exposed multiple times to develop an infection. By contrast, a human infant who wheezes after a single infection with RSV is more likely to develop asthma, she says. Additionally, lab mice are genetically identical, whereas humans have a great deal of genetic variation that plays a major role in determining who develops asthma, says Anuradha Ray. Pollution, environmental factors and insufficient exposure to bacteria also contribute to the disease, she says.
New Scientist, 10 September 2012 ;http://www.newscientist.com/ ;