Taurine chloramine down-regulates the generation of murine neutrophil inflammatory mediators

Abstract

We previously reported that taurine chloramine (TauCl), a product of activated neutrophils, inhibits the generation of macrophage inflammatory mediators such as nitric oxide (NO), TNF-α, and PGE₂. Taurine, the most abundant free amino acid in the cytosol of neutrophils, is chlorinated to form TauCl by the halide-dependent myeloperoxidase (MPO) system. Under physiological conditions, TauCl reduces HOCl toxicity. In this study, we investigated the influence of TauCl on generation of oxygen free radicals, cytokines and eicosanoids by activated murine peritoneal neutrophils. We found that TauCl, but not taurine alone, inhibited the production of NO, prostaglandin E₂, interleukin-6 and tumor necrosis factor-α, in a dose-dependent manner. In contrast, the products of the respiratory burst, as measured by luminol-dependent chemiluminescence (LCL), were reduced by both taurine and TauCl. However, taurine affected LCL at higher concentrations and to a lesser extent than TauCl. The results of these studies suggest that TauCl decreases production of tissue-damaging inflammatory mediators and may regulate the balance between protective, microbicidal and toxic effect of neutrophils.

Introduction

Neutrophils (PMN), the major cells of acute inflammation, play an important role in host defence against various infectious agents but, paradoxically, are also involved in damage of host tissues (Smith, 1994; Lloyd and Oppenheim, 1992). During phagocytosis, neutrophils are activated and microbicidal agents such as reactive oxygen intermediates (ROI) and reactive nitrogen intermediates (RNI) are generated intracellularly to kill pathogens (Klebanoff and Hamon, 1992; Thomas, 1979; Ding et al., 1988). However, tissue damage may occur when neutrophil cytotoxic microbicidal products (e.g., HO, HOCl, NO) are released extracellularly to such extent that host defences are locally overwhelmed (Cantin, 1994; Claesson et al., 1996). Moreover, activated neutrophils release a number of proinflammatory mediators including cytokines (TNF-α, IL-1, IL-6) and eicosanoids (PGE₂, LTB₄) which also contribute to the host tissue injury (Smith, 1994; Rappole and Werb, 1988). This pathological process may be attenuated by several regulatory mechanisms. For example, neutrophils contain large reserves of endogenous antioxidants such as glutathione, ascorbate and taurine (Learn et al., 1990; Voetman et al., 1980).

Taurine, 2-aminoethanesulfonate (Tau), was reported to be the most abundant free amino acid in the cytosol of leukocytes, especially in neutrophils (10–30 mM) (Fukkuda et al., 1982; Wright et al., 1986). It was proposed that Tau acts as a scavenger for hypochlorous acid (HOCl), a microbicidal agent produced by the myeloperoxidase–hydrogen peroxide–chloride (MPO–H₂O₂–Cl⁻) system of activated monocytes and neutrophils (Wright et al., 1986). The reaction of Tau with HOCl forms taurine chloramine (TauCl), the long-lived oxidant, which is less reactive and much less toxic than HOCl (Zgliczynski et al., 1971; Marquez and Dunford, 1994). Therefore, Tau in the cytosol of neutrophils may protect these cells against attack by chlorinated oxidants that escape from phagolysosomes. In addition, Tau released extracellularly may protect the tissue at a site of inflammation (Cantin, 1994). TauCl itself, however, may exert prolonged oxidative and chlorinating effects long after the initiation of the respiratory burst and at some distance from the cell of origin (Zgliczynski et al., 1971). Recently, a few studies have suggested that TauCl could act to down-regulate the inflammatory response (Park et al., 1993; Marcinkiewicz et al., 1995a, Marcinkiewicz et al., 1995b). For example, TauCl, at non-toxic concentrations, inhibits in vitro the production of various macrophage inflammatory mediators such as TNF-α, nitric oxide (NO) and prostaglandins (Marcinkiewicz et al., 1995a, Marcinkiewicz et al., 1995b; Park et al., 1995). TauCl chloramine is transported into macrophages and if it is present either before or during activation, production of the above inflammatory mediators is inhibited. In contrast to the well-documented suppression of macrophage functions by TauCl, much less is known about how this agent influences neutrophils, the cells which closely cooperate with macrophages at the site of inflammation.

The aim of this study was to determine whether TauCl and its precursor Tau, can down-regulate the generation of microbicidal and proinflammatory agents by neutrophils, and thus minimise host tissue damage by these cells.