g. Zymosan), and other stimulants such as phorbol 12-myristate 13-acetate (PMA) (Zughaier et al., 2005). The phorbol ester PMA is a soluble chemical mitogen that acts through a protein kinase C cell signaling pathway (Babior, 1992) and activates reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (Dusi and Rossi, 1993). In contrast, Zymosan is an insoluble cell wall polysaccharide of Saccharomyces cerevisiae that activates macrophages via Toll-like receptor 2, and that is recognized by macrophages and dendritic cells by dectin-1, a pattern recognition receptor Rigosertib order important in antifungal innate immunity ( Brown et al., 2003). Furthermore, Zymosan activates

NADPH oxidase through the cell membrane receptors type 3 complement receptor and mannosyl-fucosyl receptor ( Ezekowitz et al., 1985). The cell wall component of Gram-negative bacteria, LPS primes macrophages through a receptor-dependent mechanism that involves acute-phase plasma protein, LPS binding protein, CD14 cell surface receptor, and transmembrane receptor Toll-like receptor 4 ( Ulevitch, 1999 and Wright Tanespimycin et al., 1990). Stimulation of alveolar macrophages with LPS has also been linked to mitogen-activated protein kinase signaling pathways, including c-Jun N-terminal kinase, extracellular

signal-regulated kinase, and p38 ( Carter et al., 1999 and Monick et al., 1999). The kinases affect apoptosis, chemotaxis, cytoskeletal rearrangement, cytokine gene expression, degranulation and respiratory burst

( Davis, 1993). Whereas particles can induce respiratory burst in phagocytic cells, the oxidant response of cells to other stimuli such as microbes or microbial components is diminished by exposure to particulate matter. For instance, particle exposures have resulted in a decreased release of reactive oxygen species in alveolar macrophages induced with oxidant-generating stimuli such as phorbol esters, or opsonized yeast (Becker and Soukup, 1998 and Fabiani et al., 1997). Another study has shown that insoluble Epothilone B (EPO906, Patupilone) components of urban air particles play a role in the inhibition of oxidant release and phagocytosis in activated alveolar macrophages (Soukup and Becker, 2001). It remains unclear whether particle-related reduction of respiratory burst is attributed to cytotoxic events. Past studies have shown that exposures of macrophage cells to air pollution particles (Imrich et al., 1999 and Soukup et al., 2000), metals (Benson et al., 1988 and Riley et al., 2005) or minerals (Costantini et al., 2011 and Fubini and Hubbard, 2003) may result in cytotoxicity often leading to apoptotic or necrotic cell death. In contrast, extracts of airborne particulates from industrialized Rhine-Ruhr area have been shown to inhibit phagocytosis in human peripheral blood macrophages without apparent effect on cell viability (Hadnagy and Seemayer, 1994).