HUMAN MACROPHAGES SECRETE A TUMORICIDAL ACTIVITY DISTINCT FROM TUMOR-NECROSIS-FACTOR-ALPHA AND REACTIVE NITROGEN INTERMEDIATES

Human macrophages, differentiated in vitro from blood monocytes, can be induced to secrete tumouricidal activity when activated by combined treatment with recombinant interferon-gamma and bacterial lipopolysaccharide. We have analysed conditioned culture supernatants of activated human monocytes and in vitro differentiated macrophages cultivated under serum-free conditions for cytolytic activity against a TNF-alpha-insensitive human tumour cell line and characterized this activity with respect to its relationship to TNF-alpha and reactive nitrogen intermediates. Cytolytic activity was recovered in the high molecular weight fraction of culture supernatants conditioned by terminally differentiated macrophages, whereas conditioned culture supernatants of freshly isolated blood monocytes, processed under identical conditions, were devoid of significant cytolytic activity. This activity was tumour-specific, strongly affecting the human lymphoma cell line JMP, whereas freshly isolated human peripheral blood lymphocytes were not affected to a significant extent. It was inactivated by heat or trypsin treatment, but only partially inhibited by a monoclonal antibody against recombinant human TNF-alpha, which completely neutralized all of the TNF-alpha activity detectable in the supernatants tested. Cytolytic activity could not be reduced further even by a 1000-fold excess of anti-TNF-alpha antibody, suggesting that TNF-alpha has some synergistic effect on the tumouricidal activity observed, rather than being the central effector molecule. This notion was supported by enhancement of low levels of cytolytic activity by addition of recombinant human TNF-alpha at concentrations not having any direct cytotoxic effect on the tumour target cells used. We thus conclude that terminally differentiated human macrophages, but not freshly isolated blood monocytes, have the capacity to secrete a tumouricidal activity, which is distinct from TNF-alpha or reactive nitrogen intermediates, but comprises a novel, different type of macrophage-derived tumouricidal effector molecule(s) of protein nature, which may synergize with TNF-alpha in the selective destruction of tumour cells by human macrophages.