Human alkaline phosphatase dephosphorylates microbial products and is elevated in preterm neonates with a history of late-onset sepsis

BackgroundA host defense function for Alkaline phosphatases (ALPs) is suggested by the contribution of intestinal ALP to detoxifying bacterial lipopolysaccharide (endotoxin) in animal models in vivo and the elevation of ALP activity following treatment of human cells with inflammatory stimuli in vitro. However the activity of ALP in human plasma (primarily tissue-nonspecific ALP; TNAP) on lipopolysaccharide and other microbial products has not been assessed, nor has its expression been studied in preterm newborns, a vulnerable population at high risk of sepsis. In this context, the aim of our study was to characterize the activity of TNAP on Toll-like receptor (TLR) agonists and assess the concentrations of plasma ALP during late-onset sepsis in preterm newborns.MethodsRecombinant human TNAP was incubated with microbial products and phosphate release was measured by malachite green assay. Plasma ALP activity was measured serially in a cohort of preterm (N = 129) infants at high risk of late-onset sepsis (LOS).ResultsTNAP dephosphorylates poly-inosine:cytosine (Toll-like receptor (TLR) 3 agonist) and LPS from Klebsiella pneumoniae and Salmonella minnesota (TLR4 agonists). Plasma ALP significantly increased postnatally over the first 4 weeks of life in preterm and term newborns. Bacteremic LOS in preterm infants (gestational age ≤ 30 weeks) was associated with significantly elevated plasma ALP at 4 weeks postnatal age.ConclusionsTNAP, the main circulating isozyme of ALP, de-phosphorylates TLR agonists, demonstrates a post-natal age dependent increase in preterm and term plasma across the first 4 weeks of life, and is elevated in association with preterm LOS.

研究背景：碱性磷酸酶（Alkaline phosphatases, ALPs）的宿主防御功能已有提示：体内动物实验显示，肠道碱性磷酸酶可通过降解细菌脂多糖（内毒素）发挥解毒作用；体外实验中，炎症刺激处理人细胞后可使ALP活性升高。然而，目前尚未明确人血浆中主要的组织非特异性碱性磷酸酶（tissue-nonspecific ALP, TNAP）对脂多糖及其他微生物产物的活性，也未在脓毒症高风险的易感人群——早产儿中对其表达进行研究。鉴于此，本研究旨在明确TNAP对Toll样受体（Toll-like receptor, TLR）激动剂的活性，并检测早产儿晚发性脓毒症（late-onset sepsis, LOS）期间的血浆ALP浓度。
研究方法：将重组人TNAP与微生物产物共孵育，采用孔雀石绿法检测磷酸根释放量。本研究纳入129名晚发性脓毒症高风险早产儿队列，对其血浆ALP活性进行连续检测。
研究结果：TNAP可使聚肌苷酸：胞苷酸（TLR3激动剂）以及肺炎克雷伯菌、明尼苏达沙门菌来源的脂多糖（lipopolysaccharide, LPS，TLR4激动剂）发生去磷酸化。早产儿与足月儿的血浆ALP活性在出生后前4周内显著升高。胎龄≤30周的早产儿发生菌血症性晚发性脓毒症时，其出生后4周的血浆ALP活性显著升高。
研究结论：作为ALP的主要循环同工酶，TNAP可使TLR激动剂去磷酸化；其血浆活性在早产儿与足月儿出生后前4周内呈出生后年龄依赖性升高，且与早产儿晚发性脓毒症存在显著关联。