Cytokines and particle-induced inflammatory cell recruitment.

The inflammatory response is a key component of host defense. However, excessive or persistent inflammation can contribute to the pathogenesis of disease. Inflammation is regulated, in part, by cytokines, which are small, typically glycosylated proteins that interact with membrane receptors to regulate cellular processes such as proliferation, differentiation, and secretion. During the past 10 years studies in humans and experimental animals have demonstrated that a cytokine called tumor necrosis factor alpha (TNF-alpha) plays a key role in the initiation of inflammatory responses in the lung and other tissues, including inflammation resulting from inhalation of noxious particles. There is now compelling evidence that one of the pathways by which inhaled particles stimulate the recruitment and subsequent activation of inflammatory cells is through the activation of lung macrophages to release TNF-alpha. TNF-alpha then acts via paracrine and autocrine pathways to stimulate cells to release other cytokines known as chemokines, which are directly chemotactic to leukocytes and other cells that participate in inflammatory and wound healing responses. In addition to a TNF-alpha-mediated pathway, there is growing evidence that some particles such as quartz and crocidolite can directly activate lung epithelial cells to release chemokines such as macrophage inflammatory protein-2, cytokine-induced neutrophil chemoattractant, and interleukin-8. A direct stimulatory effect of particles on lung epithelium may represent an additional or alternate pathway by which inhaled particles may elicit inflammation in the lung. Recent studies have suggested that oxidative stress may be a component of the mechanism by which particles activate cytokine production in cells such as macrophages and epithelial cells. The contribution of oxidative stress to particle-induced cytokine gene expression appears to be mediated, at least in part, through activation of the transcription factor nuclear factor kappa B.