Spontaneous production of interferon-gamma in adult and newborn humans.

By using a hemolytic plaque assay for IFN-gamma secretion by single cells, we found that one to three IFN-gamma secreting cells per 1 X 10(3) mononuclear cells were present in the blood from normal adult and newborn humans, without in vitro culture or stimulation by antigens or mitogens. The frequency of such cells was higher in newborns. These spontaneous IFN-gamma-secreting cells were predominantly T cells of the helper/inducer phenotype, as defined by treatment with monoclonal antibody and complement. By using indirect immunofluorescence staining, it was seen that 1% of blood mononuclear cells from adults and 3.5% from newborns contained cytoplasmic IFN-gamma. These cytoplasmic IFN-gamma-positive cells belonged to a heterogeneous population by morphology and by staining with monoclonal antibody. The majority of these cells were DR-positive T cells. We propose that these IFN-gamma-producing cells were activated in vivo, and that they possibly mediate antiviral or lymphokine activities.