The Role of Endotoxin-neutralizing and Antimicrobial Proteins in Innate Immunity

CAP18(阳离子抗菌蛋白，18 kDa)是一种142个氨基酸的蛋白质，最初是通过凝集包被内毒素的红细胞的方法从兔粒细胞中分离出来的。CAP18由一个功能未知的N-末端结构域(CAP18_&lt；1-105&gt；)和一个C-末端的内毒素结合结构域(CAP18_&lt；106-142&gt；)组成。人工合成的CAP18 106-142和由CAP18-106-142-gt；C端5个氨基酸组成的32个氨基酸组成的CAP18-106-137&gt；可抑制脂多糖诱导的巨噬细胞组织因子的产生、一氧化氮的产生和肿瘤坏死因子的释放。经CAP18或CAP18 106-137处理的小鼠显著免受内毒素致死。虽然CAP18_lt；106-142&gt；和CAP18_&lt；106-137&gt；具有高活性，但CAP18_&lt；106-142&gt；的其他片段，包括N端截短的CAP18_&lt；110-142&gt；没有表现出与内毒素结合和中和内毒素的活性。这两种多肽对革兰氏阴性菌如大肠埃希菌、鼠伤寒沙门氏菌、肺炎克雷伯氏菌、铜绿假单胞菌(IC_&lt；50&gt；；40-100 nm)和革兰氏阳性菌(如金黄色葡萄球菌(甲氧西林敏感株和耐药株)和肺炎链球菌(IC_&lt；50&gt；100-200 nm)均具有广泛的抗菌活性。该克隆的基因编码140个氨基酸残基。人CAP18(CAP18)与兔CAP18基因高度同源。由32个氨基酸组成的C末端片段(CAP18&lt；104-135&gt；)能与内毒素结合，抑制小鼠巨噬细胞产生组织因子，保护小鼠免受内毒素致死。该多肽对革兰氏阴性菌和革兰氏阳性菌均显示出抗菌活性。我们推测，CAP18及其衍生的多肽与内毒素结合，改变了内毒素启动弥散性血管内凝血的能力，CAP可能作为宿主防御蛋白对抗感染性疾病，对脓毒症和内毒素休克具有治疗潜力。

CAP18 (cationic antimicrobial protein, 18kDa) is a 142 amino acid protein originally isolated from rabbit granulocytes using agglutination of LPS-coated erythrocytes as an assay. CAP18 is composed of an N-terminal domain of unknown function (CAP18_<1-105>) and a C-terminal LPS-binding domain (CAP18_<106-142>). Synthetic CAP18_<106-142> and CAP18_<106-137>, a 32-amino acid peptide resulting from the truncation of 5 amino acids from the C-terminus of CAP18_<106-142>, inhibited LPS-induced tissue factor generation, nitric oxide production and TNF release by macrophages. Mice treated with CAP18_<106-142> or CAP18_<106-137> were significantly protected from LPS lethality. Although CAP18_<106-142> and CAP18_<106-137> were highly active, other fragments of CAP18_<106-142>, including CAP18_<110-142> with a truncated N-terminus, did not exhibit LPS-binding and LPS-neutralizing activities. Both peptides had broad antimicrobial activity against both gram-negative bacteria such as Escherichia coli, Salmonella typhimurium, Klebsiella pneumoniae, Pseudomonas aeruginosa (IC_<50> ; 40-100 nM) and gram-positive bacteria such as Staphylococcus aureus (Methicillin sensitive and resistant strains) and Streptococcus pneumoniae (IC_<50> ; 100-200nM).We cloned a CAP-18 family protein from human granulocytes. The cloned cDNA encoded 140 amino acid residues. Human CAP18 (CAP18_<1-140>) was highly homologous to that of rabbit. A32-amino-acid C-terminal fragment (CAP18_<104-135>) was shown to bind LPS,inhibit LPS-induced tissue factor generation by murine macrophages, and protect mice from LPS lethality. This peptide exhibited antimicrobial activity against both gram-negative and gram-positive bacteria. We hypothesize that CAP18 and the derived peptides bind to LPS and alter the capacity of LPS to initiate disseminated intravascular coagulation.In this regard, CAP may act as host defense protein against infectious diseases, and have therapeutic potential for sepsis and endotoxin shock.