Platelet leukocyte interactions. II. In-vivo correction of Chediak-Higashi leukocyte function with serotonin or normal platelets

Oxidative stress in phagocytes--"the enemy within"

Phagocytes represent a powerful defense system against invading microorganisms that threaten the life or functional integrity of the host. The capacity to generate and release substantial amounts of reactive oxygen species is a unique property of activated polymorphonuclear and mononuclear phagocytes. The crucial role of these molecules in killing microorganisms and their consecutive contribution to tissue damage during injury and inflammation is widely known. Although much research has been done to explore the molecular events involved in the interaction of oxygen intermediates with microbes or host tissue, surprisingly little attention has been paid to the effect of reactive metabolites on the phagocyte itself. This fact is especially surprising, since it is apparent that the activated phagocyte is directly exposed to its own toxic metabolites. The potential damage occurring during excessive radical formation might notably alter the vital functions of these primarily immunocompetent cells. Moreover, the critical role of oxygen radicals in apoptosis of leukocytes has been recently revealed. Apoptosis is now supposed to represent a key mechanism in neutrophil deactivation and resolution of inflammation. Therefore, this review will focus on the delicate balance between released oxidants and antioxidative protection within the phagocytes themselves. General and phagocyte-specific antioxidative mechanisms, which have co-evolved with the radical generating machinery of phagocytes, are discussed, since the outcome of local inflammation can directly depend on this antioxidative capacity and might range from adequate elimination of the pathogen with minimal acute tissue damage to progression towards a systemic inflammatory response syndrome.

Antioxidative properties of serotonin and the bactericidal function of polymorphonuclear phagocytes

We investigated the antioxidative properties of platelet-released serotonin on the bactericidal function of polymorphonuclear neutrophils (PMN) since there is a surprising co-incidence of low blood serotonin and an increased rate of infections. The antioxidative properties of serotonin were demonstrated by its suppressive effects on phagocytosis-associated, luminol-enhanced chemiluminescence (CL). The bactericidal activity of PMN was determined by a microbiological assay using opsonized Staphylococcus aureus. Serotonin suppresses luminol-enhanced chemiluminescence in a dose-dependent manner indicating an interaction with reactive oxygen species, which are responsible for effective bacterial killing during the phagocytosis-associated "respiratory burst". The modulation of the bactericidal function of PMN by serotonin is complex and depends upon the amount of serotonin: at concentrations normally present at sites of tissue injury and consecutive thrombus formation (10(-6) to 10(-5) M) bacterial killing increases by about 50%. In contrast, at pharmacological concentrations (10(-3) to 10(-2) M) an adverse effect can be observed: the elimination of opsonized S. aureus is reduced by 30 to 90%. Exogenous serotonin is capable of modulating important biological functions of human PMN in vitro. At appropriate concentrations, the antibacterial defence improves significantly probably due to reduced autooxidation, whereas higher concentrations counteract an efficient bacterial killing.