**7. Neutrophil-mediated monocyte recruitment. View on neutrophil-monocyte axis**

The sequence of phagocyte recruitment to the site of inflammation comprises initial extravasation of neutrophils followed by a subsequent emigration of monocytes. The experiments of Gallin (Gallin et al., 1982) pointed to the importance of ready made neutrophil granule proteins in the recruitment of monocytes. Granule proteins are stored in 4 distinct sets of granules. Primary and secondary granules discharged from emigrated neutrophils contain mainly antimicrobial polypeptides. Rapidly mobilized secretory vesicles contain mainly receptors important for adhesion and recognition of foreign particles. Tertiary granules released during transendothelial migration contain mainly proteases (Soehlein et al., 2009).

It has been shown that neutrophils that have migrated to the site of inflammation can upregulate their production of chemokines, supporting the notion that, in this way, neutrophils participate in the regulation of leukocyte accumulation (Soehlein et al., 2009). In terms of production, the principal chemokine produced by neutrophils is IL-8, which activates neutrophils in an autocrine loop. IL-8 binds to CXCR2 expressed not just on neutrophils, but also monocytes. IL-8 also mediates adhesion both human neutrophils and monocytes to the endothelium (Soehlein et al., 2009).

Adhesion of neutrophils to the endothelial cells results in rapid release of secretory vesicles: proteinase-3 and azurocidin (also known as cationic antimicrobial protein). Both azurocidin and proteinase-3 are strongly positively charged and may therefore act with negatively charged endothelial proteoglycans. Azuricidin was recently shown (Soehnlein et al., 2009) to induce monocyte extravasation, it has been demonstrated that depletion of neutrophils reduces the recruitment of inflammatory monocytes. Interestingly, this deficiency in recruitment can be almost completely rescued by the local application of the supernatant from activated human neutrophils (Soehlein et al., 2009).

Activated neutrophils are short-lived cells. Their apoptosis is a tightly regulated process involving ROI and pathogens. Once neutrophils migrate toward the site of inflammation, their life span increases because of the presence of survival signals in the inflammatory milieu. In response to pro-inflammatory signals, neutrophils not only extend their life span, but also release a web of DNA in which granule proteins are enweaved (Brinkmann et al., 2004). Exposure of granule proteins and entrapment within a net of DNA may contribute to creating a gradient of chemotactic stimuli relevant to monocyte recruitment. Apart from release of granule proteins, apoptotic neutrophils may release attraction signals leading to influx monocytes. In recent years, several apoptotic cell-derived "find-me" signals were identified. Among them is lysophosphatidylcholine (LPC), of which the latter has received much attention. LPC was identified (Kim et al., 2002) as an "eat-me" signal on the apoptotic cell surface. More recently, it has been shown that changes in membrane composition of apoptotic cells (negative surface charges) in initiate attractive signals for phagocytes including monocyte. It is feasible that apoptotic neutrophils generate such electric signals resulting in electrotaxis of monocytes (Zhao et al., 2006).

In addition, neutrophil granule proteins enhance the production of ROI by monocytes (Soehnlein et a., 2008). Thus, various experimental setups provide evidence that the axis of neutrophils and inflammatory monocytes promotes and sustained inflammation. Taken together, the multifaceted action of neutrophils in recruiting and activating monocytes may offer a powerful target for interfering with the sustained inflammatory reponse in RA (Soehnlein et al., 2009).
