Role of endotoxin in the expression of human monocyte cytotoxicity

The role of bacterial endotoxin in the expression of human monocyte cytotoxicity was studied. Endotoxin contamination of all reagents and steps of the experimental procedure were assessed by the limulus amebocyte lysate (LAL) assay. Peripheral blood monocytes were separated by adherence, and cytotoxicity was measured as [3H]-thymidine release from prelabeled mKSA-TU5 murine target cells in a 48-hr assay. Human monocytes expressed appreciable levels of spontaneous cytotoxicity under LAL- conditions irrespective of the serum source employed (LAL- or LAL+ fetal bovine serum, FBS, or LAL- human cord serum, HCS). HCS gave the highest cytotoxicity levels and LAL- FBS the lowest. Polymyxin B (10 micrograms/ml), which inhibited endotoxin-induced gelation of LAL and activation of mononuclear cells for procoagulant activity, did not affect the expression of spontaneous monocyte cytotoxicity with all three sera used. Three preparations of endotoxin used in the present study (Escherichia coli, S. typhosa, S. minnesota) caused small increases in monocyte killing in micrograms per milliliter concentrations only in 5 of 19 experiments performed. Human lymphoblastoid interferon (LAL-) and phytohemagglutinin-elicited lymphokine supernatants (LAL-) enhanced the tumoricidal activity of human monocytes under LAL- conditions and were not affected by Polymyxin B. It is concluded that exposure to endotoxin is not a prerequisite for the expression of spontaneous cytotoxicity by human blood monocytes.