Accessory cell paradox: monocytes enhance or inhibit lectin-mediated human T-lymphocyte proliferation depending on the choice of mitogen.

Monocytes suppressed the mitogenic response of human T lymphocytes to the lectin from Datura stramonium but enhanced the mitogenic response of the same lymphocytes to wheat germ agglutinin under identical culture conditions. The inhibitory action was probably cell-mediated, but was not the result of cytotoxicity. The stimulative activity could be replaced by cell-free conditioned medium.