Requirement for monocytes in the spontaneous cytotoxic effects of human lymphocytes against non-lymphoid target cells

Human lymphocytes have been shown to lyse in vitro a large variety of target cells derived from tumour as well as normal tissues. The effector cells involved in this spontaneous cell-mediated cytotoxicity (SCMC) have generally been characterised as lymphocytes which lack surface membrane immunoglobulin (sIg) and lack or express only low-affinity receptors for sheep red blood cells (SRBC), but possess receptors for the Fc portion of IgG (refs 3--7). We have previously shown that monocytes may be involved in SCMC, as strong depletions of adherent cells always resulted in a reduction in SCMC. In the present study, experiments were undertaken to determine the role of monocytes in SCMC. Evidence is presented that monocytes have a helper function in the lymphocyte-mediated SCMC against target cells growing in monolayer cultures, whereas effector cells in SCMC against lymphoid target cells growing in suspension cultures were lysed by lymphocytes in the absence of monocytes.