Morphological and Cytochemical Characterization of Cells Infiltrating Mouse Lungs After Influenza Infection

To initiate evaluation of the cell-mediated immunological response to influenza virus in a major site of disease, lung cells were obtained by transpleural lavage from lungs of uninfected mice and from those infected 3 or 6 days previously with 5 50% mouse infectious doses (MID50) of avirulent (P3) or virulent (P9) influenza A Hong Kong (H3N2) virus. The number of cells recovered by lavage was dependent on the dose, time after inoculation, and the type of virus used for inoculation. Although lavage pools were shown to contain peripheral blood leukocytes, this contamination was shown to be consistently less than 5% of the total leukocytes harvested. Among the ca. 0.75 × 106lavage cells obtained from each uninfected mouse, about 90% were macrophages or lymphocytes in approximately equal proportion. T, B, and null (lyphocytes lacking theta or surface immunoglobulin markers) lymphocytes averaged 23, 9, and 7% of cells in these suspensions, respectively. After infection with either P3 or P9 virus, increased numbers of activated macrophages and lymphoblasts were observed. The major change during P3 infection was an increase in absolute numbers of null lymphocytes. In contrast, during P9 infection, T and B lymphocytes and macrophages progressively increased in absolute numbers while null cells decreased. These data suggest that cell-mediated immunological responses to influenza virus occur in the lung during infection, but that the responses to virulent and avirulent variants may differ both qualitatively and quantitatively.