Inhibition of leukocyte phagocytosis by serotonin and its possible role in tumor cell destruction

Intraportal tumour cell (TC) injection activates platelets which release serotonin (5-HT). Thrombocytopenia or 5-HT blockade decrease the hepatic lodgement of the injected TCs. A hypothetical explanation of this is 5-HT inhibition of TC killing by phagocytes (e.g. leukocytes and Kupffer cells). In this study, leukocyte phagocytosis was analysed by a chemiluminescence technique at different 5-HT concentrations. Significant inhibition of phagocytosis occurred at 5-HT concentrations of 10(-4) M to 10(-8) M, with maximum inhibition at 10(-4) M. Comparable concentrations of 5-HT may be found locally in the liver during TC infusion, supporting the hypothesis that 5-HT has an inhibitory effect on phagocyte-mediated TC killing.