Journal of Leukocyte Biology 38:495-507 (1985) Accessory Cells Provide More Than One Signal for Lectin Mitogen-Stimulated

A culture system was developed in which human peripheral blood mononu-clear cells (PBMC) depleted of la-expressing cells did not proliferate in re-sponse to the lectin mitogen phytohemagglutinin (PHA). These cells were able to respond to mitogen if purified autologous accessory cells were added back to the culture, thus showing an absolute requirement for la-expressing acces-sory cells in mitogen-driven T-cell proliferation. The identity of these accessory cells was shown to be not only monocytes but also la-expressing B cells and possibly other unidentified la-bearing cell types. Human interleukin-i (IL-i) in the lectin mitogen-unresponsive culture sys-tem was found unable to reconstitute normal human T-celI proliferation. This suggests that those cells with Ia surface antigen acting as accessory cells must deliver more than an IL-i signal for T-celI proliferation. The phorbol ester 12-O-tetradecanoyl-phorbol-i3-acetate (TPA) was tested for its ability to replace necessary la-expressing accessory cells. TPA was able to replace accessory cells in culture, thus mimicking the Ia-expressing acces-sory cells and all their delivered signals in the triggering of human lymphocytes. Key words: accessory cells, interleukin-1, TP