Inhibitory effect of haptoglobin on granulocyte chemotaxis, phagocytosis and bactericidal activity

Human haptoglobin (Hp) is synthesized at hepatic and extrahepatic sites as an acute-phase reactant protein (APP). We investigated the effects of Hp on granulocyte function. The chemotaxis of freshly isolated human granulocytes and differentiated HL-60 cells in response to the bacterial tripeptide, f-met-leu-phe, was inhibited in the presence of a physiological concentration of Hp, but chemotaxis in the presence of the proinflammatory cytokine interleukin-8 (IL-8) was not inhibited. Phagocytosis of viable Escherichia coli, as well as fluoresceinated nonviable E. coli, was inhibited. Hp also reduced granulocyte intracellular bactericidal activity against E. coli. The observed inhibitory effects of Hp on granulocyte function are similar to those reported for C-reactive protein and suggest that APPs dampen the acute inflammatory response.