Cytotoxicity of human peripheral blood monocytes

Native tumoricidal activity of human peripheral blood mononuclear cells was examined before and after their separation by counterflow centrifugation elutriation (CCE). Tumoricidal activity was found in the subpopulation of small mononuclear cells but not within the relatively pure subpopulation of large monocytes. Addition of lymphokine and/or lipopolysaccharide demonstrated that large monocytes were resistant to activation for tumor killing, in contrast to small mononuclear cells. However, cryopreservation or simply exposure to dimethyl sulfoxide (DMSO) rendered the large monocytes sensitive to activating agents without altering their unstimulated tumoricidal activity. Cryopreservation was not detrimental to small or large monocytes either in number or tumoricidal function but did decrease the number of large granular lymphocytes (LGL). The small mononuclear cell fraction was enriched for small monocytes to 80% by combining CCE with Percoll gradient separation. HNK-1 mouse monoclonal antibody against human LGL was used with complement to remove virtually all LGL from cryopreserved cells as judged by morphology and tumoricidal activity against K-562 human lymphoblastoid cells. Such treatment actually augmented rather than suppressed tumoricidal activity against P-815 mastocytoma cells. Therefore, we conclude that small monocytes but not large monocytes possess native tumoricidal activity distinct from that attributed to LGL or natural killer lymphocytes. Further, small monocytes are readily activated for tumor killing and can be cryopreserved without loss of tumoricidal activity.