The contribution of phagocytes to the early protection of mice inoculated intravenously with influenza virus was investigated in phagocyte-depleted mice. Following the inoculation of a sublethal dose of influenza virus, virus titres in the liver and lung of both untreated and carrageenan-treated mice were reduced rapidly by day 1 and decreased slowly to reach an undetectable level by day 7. The titres in γ-irradiated mice decreased transiently by day 1 and increased progressively thereafter to kill all of the hosts by day 8. The clearance of virus from blood at the early stage of infection was retarded by γ-irradiation but not by carrageenan treatment. In addition, increase in virus titres in the liver and lung in the early stage of the infection was prevented by adoptive transfer with syngeneic polymorphonuclear leukocytes into γ-irradiated mice. No significant rise of neutralizing antibody was detectable by day 3 after the inoculation, in any of the three groups of mice. These observations imply that γ-sensitive and carrageenan-resistant polymorphonuclear leukocytes play a protective role at the early stage in the infection, whereas fixed macrophages or natural killer cells, both of which are carrageenan-sensitive and γ-resistant, scarcely participate in the early phase.


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