Direct mitogenic properties of interleukin-1

Optimal lymphocyte activation generally requires two concurrent signals, one involving the T cell receptor, and another supplied by an accessory cytokine such as interleukin-1. We investigated the conditions under which partial cellular activation occurs in the absence of signal to the T cell receptor. Both murine thymocytes and the T cell clone D10.G4.1 responded to recombinant interleukin-1 in the absence of a mitogenic signal, although the magnitude of these responses was smaller and required higher concentrations of interleukin-1 than if a comitogen had been present. Interleukin-1 and interleukin-6 in combination were synergistic and induced tritiated thymidine incorporation in D10.G4.1 cells equal to 15% of that obtained with optimal concentrations of concanavalin A. Such synergy suggests that a significant degree of nonspecific activation of lymphocytes may occur in the presence of combinations of monocyte-derived cytokines.