A baby with a cold is never fun, but the long-term effects of a viral infection in the young could be far more serious – in mice, at least. 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.