Mice that lacked B cells and antibodies were completely unable to combat the virus. They developed serious brain and spinal-cord infection and ultimately died.
"These findings may help explain why the elderly and others with weakened immunity are most likely to develop serious disease when infected by the virus," says study leader Michael S. Diamond, M.D., Ph.D., assistant professor of medicine, of molecular microbiology and of pathology and immunology.
West Nile Virus first emerged in the eastern United States in 1999 and has spread steadily westward, reaching the West Coast last year. It is carried by mosquitoes and causes encephalitis, a brain inflammation. The virus affects mainly birds, especially crows and jays, but it also can cause disease in horses, humans and other mammals.
In humans, West Nile Virus causes serious illness in only a small proportion of infected people. Last year, doctors reported more than 3,500 cases of infection, with 5 to 10 percent of those resulting in serious illness or death.
Diamond and his colleagues infected a strain of immune-deficient mice that lacked two important components of the immune system -- T cells and B cells -- and compared the animals' response to mice with normal immunity. T cells coordinate immune responses and kill infected cells; B cells produce antibodies that attack viruses before they infect cells.
The immune-deficient mice became sick and died even with low doses of the virus. However, they could resist infection if given a dose of B cells after being injected with the virus.