High frequency of spontaneous interferon-gamma-producing cells in human tonsils: role of local accessory cells and soluble factors

Abstract

SUMMARY The frequency of mononuclear cells (MNC) spontaneously secreting interferon-gamma (IFN-γ) has been examined in freshly isolated cell suspensions from human palatine tonsils. Two-site reverse enzyme-linked immunospot (ELISPOT) analyses, involving short term (20 h) incubation of MNC in the absence of any added exogenous stimulus, revealed that tonsillar MNC suspensions contain exceptionally large numbers of cells secreting IFN-γ. No significant differences were observed when comparing the frequency of IFN-γ-producing cells between cell suspensions obtained from hyperplastic and tonsillitis specimens. Cell-sorting experiments disclosed that spontaneous tonsillar IFN-y production was essentially contributed by CD4+ T cells, and required the presence of accessory cells and/or soluble factors to be detected. Thus, depletion of plastic adherent cells or monocytcs from the tonsillar MNC suspensions resulted in reduced numbers of detectable IFN-γ-secreting cells. Addition of very small numbers of autologous monocytes restored spontaneous IFN-γ production in tonsillar MNC cultures depleted of monocytes. Neutralization of endogenous IL-1β and IL-2, as well as blocking of the IL-2 receptor, also decreased IFN-γ production from unfractionated tonsillar cells. Addition of exogenous IL-1β restored IFN-γ production in cultures of tonsillar MNC depleted of plastic adherent cells. Furthermore, IL-1β synergized with IL-2 by increasing intracellular as well as cell-free levels of IFN-γ in cultures of unfractionated tonsillar MNC. This study further establishes that the tonsils are highly active immunological organs containing large numbers of T cells spontaneously producing IFN-γ whose detection is contingent upon the presence of functional accessory cells. It also demonstrates that concomitant production of IL-1β and IL-2 occurs in tonsils and is necessary to maintain ongoing synthesis and extracellular accumulation of IFN-γ in these organs.