Recombinant interleukin-2-induced polyclonal proliferation of in vitro unstimulated human peripheral blood lymphocytes. 
Human peripheral blood mononuclear cells as well as T-cell-enriched or T-cell-depleted populations were found to proliferate in response to recombinant interleukin-2 (IL-2) in vitro in the absence of a lectin preactivation signal.  This proliferative response was detected at Day 2, peaked at Day 5, and was dependent on the concentration of IL-2 used.  At the initiation of culture, these cells did not appear to be activated as determined by the expression of Tac antigens.  In cultures of unfractionated T-cell-enriched suspensions, high concentrations of IL-2 resulted in preferential expansion of the OKT8+ population, although both OKT4+ and OKT8+ cells proliferated in response to IL-2 when cultured alone.  These studies demonstrate that human lymphocytes obtained by standard fractionation procedures from peripheral blood are capable of proliferation in response to IL-2 without in vitro preactivation signals given by the addition of mitogens or antigens to cultures.  These findings suggest that in vivo IL-2, in the absence of other exogenous stimuli, may directly influence immune responses and thus may have a potential role as a clinical immunopharmacologic agent.