Characterization of interferons induced by bacteria and interferon-producing leukocytes in human peripheral blood

All of 23 different preparations of formaldehyde-fixed and heat-killed bacteria induced the appearance of high levels of interferon (IFN) in cultures of human peripheral blood mononuclear leukocytes. Some bacteria induced peak IFN titers after 24 h of culture, whereas other bacteria showed maximal titers on culture days 2 to 3. The IFN displayed various properties. One type, which appeared early during the cultures, had characteristics of IFN-alpha, being resistant to pH 2 treatment but neutralized by antibodies to IFN-alpha. A second type, which appeared later, on culture days 2 to 3, resembled IFN-gamma in being sensitive to pH 2 treatment but resistant to anti-IFN-alpha antibodies. A third type, which appeared to be sensitive to both pH 2 and antibody treatment, was interpreted as atypical IFN-alpha. The application of cell fractionation procedures indicated that nonadherent, predominantly Fc receptor-bearing, non-T, non-B cells were producers of IFN-alpha as defined by its antigenic properties. They copurified approximately with cells carrying natural killer activity toward human erythroid leukemia K562 cells. Some bacteria apparently also stimulated T lymphocytes to produce material with properties of IFN-gamma.