Depression of early protection against influenza virus infection by cyclophosphamide and its restoration by protein-bound polysaccharide.

Relationship between depression of early protection against influenza virus infection and the decrease in the number of peripheral polymorphonuclear leukocytes in cyclophosphamide-treated mice was investigated using protein-bound polysaccharide (PSK), which had been shown to exert a potent restorative effect on leukocytopenia in immunocompromised hosts. Following intranasal inoculation with influenza virus (1.5 x 10(3) PFU) into untreated mice, the pulmonary virus titer progressively increased during 3 days and decreased gradually from the day 7 after infection. The treatment of mice with cyclophosphamide (150 mg/kg) 2 days before infection markedly enhanced the pulmonary virus multiplication from the early phase of infection, and the higher virus titer was maintained thereafter. When mice were given cyclophosphamide after PSK-treatment, virus titers from the early to late phases of infection were lower than those in untreated mice.