The Generation of ‘Cytotoxic’ Macrophages in Mice during Infection with Influenza A or Sendai Virus

Injection of infectious but not of non‐infections influenza A virus or of infectious or non‐infectious Sendai virus intraperitoneally into mice induces the generation of plastic‐adherent cells that arc able to effect release of ⁵¹Cr from labelled virus‐infected target cells but not from labelled, uninfcctcd cells. Their activity is greatly diminished by exposure to silica or carrageenan but not by anti‐Thy I antibody and complement treatment. Similarly, the activity of the cell preparation cannot be explained by contamination with natural killer or ‘K’ cells. Thus, the effector cells were identified as macrophages and for convenience are called ‘cytotoxic macrophages’. The maximum cytotoxic activity was recovered from the peritoneal cavity 5 days after virus injection and declined thereafter. Although the effector cells are cross‐reactive in that cells activated by an influenza A strain virus lyse target cells infected with the same or other A strain viruses or with Sendai virus, there is preferential lysis of cells infected with the homologous virus. The action of the effector cells is not H‐2‐restrictcd. Preliminary experiments showed that similar effector cells can be recovered from the lungs of mice 5 days after intranasal inoculation of infectious influenza virus, so they may contribute to the host control of the disease.