TUMOR-NECROSIS-FACTOR RELEASE BY HUMAN MONOCYTES STIMULATED WITH PLATELET-ACTIVATING-FACTOR

Platelet-activating factor (PAF) is a low molecular weight phospholipid which enhances human monocyte cytotoxicity for tumor cells. In the current studies, the capacity of PAF to stimulate release of tumor necrosis factor a (TNF-alpha) by human monocytes was assessed. PAF induced maximal TNF-alpha synthesis 2-3 hr after monocyte stimulation as assessed by dot blotting of cell-associated TNF-alpha using polyclonal anti-TNF antibody. Maximal net release of TNF-alpha occurred 5-16 hr after monocyte stimulation, as assessed by a specific ELISA. Dose-response studies revealed that a maximal two- to three-fold increase in release of TNF-alpha occurs at 10-100 pM PAF. LysoPAF and the optical isomer of PAF did not stimulate release of TNF-alpha, suggesting that stimulation is mediated by specific PAF receptors. Scatchard analysis of [H-3]PAF binding to monocyte membranes revealed 651 +/- 495 binding sites/monocyte with a Kd of 4.7 +/- 4.2 X 10(-10)M. PAF is a structurally unique activator of monocytes whose interactions with TNF-alpha and other cytokines may be critical to host defense against tumors.