Interferon-gamma production by specific lung lymphocyte phenotypes in silicosis in mice

We recently described overproduction of interferon (IFN)-gamma by lung lymp hocytes in mice with silicosis (11% of cells in air-control versus 19% of c ells from silica-exposed mice; Davis and colleagues, Am. J. Respir. Cell Mo t Biol. 1999;20:813-824), We hypothesized that the increased IFN-gamma prod uction might be due to selective enrichment of one lymphocyte phenotype. To test this hypothesis, small mononuclear cells from lung digest preparation s of mice exposed 4 mo previously to cristobalite silica (70 mg/m(3), 12 d, 5 h/d) or to sham-air were stained for intracellular cytokines and surface antigen phenotypes, and examined by flow cytometry, Air-sham mouse lung di gests included CD4(+) (16%) and CD8(+) (6%)T cells, gamma delta T-cell anti gen receptor (TCR)(+) CD4(-)CD8(-)T cells (3%), natural killer (NK) cells ( 15%), B cells (6%), and macrophages (12%). The total number of lung lymphoc ytes was increased 1.7-fold in silicosis, but the phenotype frequencies did not change significantly. In the control lungs IFN-gamma was produced by t hree major phenotypes of lymphocytes: 5% of CD4(+) T cells, 5% of gamma del ta-TCR+ CD4(-)CD8(-) T cells, and 2% of NK cells. The percentage of each ty pe producing IFN-gamma was increased 2- to 3-fold in silicosis, When multip lied by cell number, the increased percentages yielded a 3- to 5-fold incre ase in the total number of each IFN-gamma-producing phenotype in the lung. Our results demonstrate no selective phenotype enrichment but upregulated I FN-gamma production by at least three lymphocyte phenotypes, IFN-gamma may be an important signal driving lymphocyte differentiation and macrophage ac tivation in silicosis.