Effect of Virus Infection on the Inflammatory Response. Depression of Macrophage Accumulation in Influenza-infected Mice

To better define the mechanisms by which viruses depress immune function, the effect of influenza infection on the ability of macrophages to accumulate at sites of inflammation was determined. Mice were inoculated with virus, and their inflammatory response measured in vivo by counting the number of leukocytes which accumulated in the peritoneal cavity 2 days after an intraperitoneal injection of phytohemagglutinin. Mice infected with influenza had a 57% and 65% depression of total leukocyte and macrophage accumulation, respectively, as compared to the response of uninfected mice. In contrast, bacterial pneumonia did not produce a decrease in the macrophage response. This indicated that the depression was produced by the virus infection rather than being a nonspecific phenomenon accompanying any inflammatory focus in the lung. The in vitro chemotactic responsiveness of normal peritoneal macrophages incubated with infectious influenza virus was 53% of normal. These experiments suggest that influenza infection may depress a host's ability to mobilize macrophages to inflammatory sites in vivo by inhibiting their chemotactic responsiveness.