The regulation of T cell responses by spontaneously active suppressor cells.

In studying T cell regulation, peripheral blood mononuclear cells from normal subjects were examined for 'spontaneous', rather than mitogen-induced, suppressor cell activity. Normal blood leucocytes from 30 subjects included a subpopulation of cells capable of suppressing the response of lymphocytes to the T cell mitogen phytohaemagglutinin by 21-35%. The indicator system for these studies consisted of fresh normal lymphocytes stimulated by three concentrations of PHA in the presence or absence of normal but mitomycin C treated peripheral blood lymphocytes. To measure accurately the spontaneous suppressor cell activity, additional cultures were needed to control for the suppressive effects of crowding and metabolic competition. Allogeneic, cryopreserved 'B' cell enriched populations, supplied satisfactory control cells for this purpose. While allogeneic culture systems could induce significant suppressor cell activity after 7 days of co-culture, they could not induce this activity in the 3 days required to assay spontaneous suppressor cell effects. In developing this assay we noted that (a) crowding became a factor in the cellular response to mitogens with concentrations higher than 2 X 10(4) cells/well, (b) spontaneous suppressor cell activity decreased rapidly once cells were placed in culture and (c) both spontaneous and concanavalin A (Con A) activated suppressor cells could significantly reduce the response to PHA even when added to cultures established with mitogens 72 hr earlier. The ability to measure spontaneous suppressor cell activity in vitro will allow more physiological studies of the membrane markers and functional characteristics of these cells than is possible in conventional studies utilizing Con A. In addition, this assay allows the detection of enhanced in vivo activity of suppressor cells not easily detected in assays relying on mitogen induction of suppression. Such increased activity is thought to be an important factor in the pathogenesis of a number of human diseases.