@Department of Experimental Medicine and Biochemical Sciences, University of Rome "Tor Vergata", *Department of Experimental Medicine, University of L'Aquila and #Department of Experimental Medicine, University of Rome "La Sapienza".

Corresponding author: Roberto Testi

The expression and function of CD69, a member of the Natural Killer cell gene Complex family of signal transducing receptors, was investigated on human monocytes. CD69 was found expressed on all peripheral blood monocytes, as a 28 and 32 kD disulfide-linked dimer. Molecular crosslinking of CD69 receptors induced extracellular Ca2+ influx, as revealed by flow cytometry. CD69 crosslinking resulted also in PLA2 activation, as detected by in vivo AA release measurement from intact cells and by direct in vitro measurement of enzymatic activity using radiolabeled phosphatidylcholine vesicles. PGE2alpha, 6-keto-PGF1alpha, and LTB4 were detected by RIA in supernatants from CD69-stimulated monocytes, suggesting the activation of both cyclooxygenase and lipoxygenase pathways following CD69 stimulation. CD69 crosslinking, moreover, was able to induce strong NO production from monocytes, as detected by accumulation of NO oxydixed derivatives, and cGMP. Importantly, NO generation was responsible for CD69-mediated increase in spontaneous cytotoxicity against L929 murine transformed fibroblast cell line and induction of redirected cytotoxicity towards P815 FcRII+ murine mastocytoma cell line. These data indicate that CD69 can act as a potent stimulatory molecule on the surface of human peripheral blood monocytes.