Respiratory diseases in cyclophosphamide-treated mice. II. Decreased virulence of PR8 influenza virus.

Mice infected intranasally with the PR8 strain of influenza virus were treated with cyclophosphamide, a potent immunologic suppressor. During the first week of infection, mortality in the unmodified influenza infection averaged 65%, whereas in those animals also treated with cyclophosphamide it averaged 22.5%. After the first week, the mortality rate in the infected cyclophosphamide-treated animals rose to that seen during the first week in the animals only infected. This decreased mortality in the first week was found despite the fact that the cyclophosphamide-treated and infected animals had higher virus titers which persisted longer, decreased circulating antibody, and a decreased interferon response. This delayed mortality appeared to be related to the finding of decreased cellular infiltration in the lungs of infected cyclophosphamide-treated animals.