INTRODUCTION
Alkaline phosphatase (AP) has multiple tissue subtypes such as tissue non-specific AP (TNAP), intestinal AP (IAP), placenta AP (PLAP) and is widely distributed in various tissues of the human body, including cardiac blood vasculature, brain, liver and placenta and intestinal epithelial brush border (Schultz-Hector, Balz, Böhm, Ikehara, Rieke, 1993; Bell, Scarrow, 1984; Hirschmugl et al., 2018; Bell, Williams, 1979; HAGerstrand, Lindholm, Lindroth, 1976). Currently, the physiological function of IAP has not been fully elucidated. The function of alkaline phosphatase was originally thought to be involved in bone calcification (Millan, Whyte, 2016). In the early 1990s, Hui et al. used gene transfer technology to induce AP gene expression in various cell types and found that AP is likely related to pathological vascular calcification (Hui, Li, Holmyard, Cheng 1997; Hui, Tenenbaum, 1998). But the mystery surrounding the main physiological function of such a widely distributed membrane-bound enzyme continued to prevail until an original study by Dutch scientists Poelstra and Meijer first reported that IAP is capable of inactivation of endotoxin, suggesting that IAP could inhibit endotoxin-induced inflammation and might have a potential for treatment lipopolysaccharide (LPS)-related diseases (Poelstra, Bakker, Klok, Hardonk, Meijer, 1997). By knocking out the mouse IAP respective gene, Bhan and Sonoko found that animals deficient in IAP develop type 2 diabetes (Kaliannan, 2013) and hyperlipidemia (Narisawa, 2013). Clinical studies by Peters and Lukas sponsored by Dutch biopharmaceutical company AM-Pharma have already validated the use of recombinant human IAP in providing a novel approach targeting inflammation in endotoxin-associated sepsis kidney injury and colitis (Peters, 2016a; Lukas, 2010).
Leukocytes contain 55-70% neutrophils, 20-40% lymphocytes and 3-10% monocytes. Neutrophil AP activity is increased in pathological sites. A characteristic response of neutrophils to bacterial infections is the pronounced increase in AP activity. IAP is thought to be involved in gut mucosal defense, mainly through the detoxification of LPS. However, the migration of neutrophils into inflammatory tissues acts like a double-edged sword. As a primary cell type that removes invasive microorganisms, neutrophils also contribute to the pathogenesis of human inflammatory diseases by releasing excess proinflammatory cytokines, such as TNF-α and IL-6 (Peiseler, Kubes, 2019; Suzuki, 2017; Powell, Huttenlocher, 2016; Jasper, McIver, Sapey, Walton, 2019; Kovtun, Messerer, Scharffetter-Kochanek, Huber-Lang, Ignatius 2018; Williams, Chambers, 2016; Wright, Moots, Bucknall, Edwards, 2010; Suzuki, 2018; Zhang, 2019; Mortaz, Alipoor, Adcock, Mumby, Koenderman, 2018). TNF-α is a mammalian-secreted protein capable of inducing a wide variety of effects on many cell types. TNF-α is essential to the initiation course of inflammatory reactions in the body and is involved in the triggering and/or amplification of local inflammatory responses related to apoptotic cell death. Dysregulated continual synthesis of IL-6 plays a pathological effect on chronic inflammation and autoimmunity. Currently, it has been unclear if AP has a direct mechanism of inhibiting leukocytic TNF-α and IL-6 upregulation in inflammatory human diseases. This study was designed to investigate the effects of IAP on TNF-α and IL-6 secretion using our cell-based model comprised of freshly extracted human leukocytes in the presence and absence of endotoxin LPS.