Section for Respiratory Medicine and Allergology, Lund University, Skåne University Hospital, Lund, Sweden.
J Innate Immun. 2013;5(5):519-30. doi: 10.1159/000346709. Epub 2013 Feb 2.
To resist infections, robust defense mechanisms of the airways are essential. Retinoic acid promotes differentiation and maintains the phenotypic characteristics of bronchial epithelium. In addition, it induces the expression of the antibacterial growth factor midkine (MK). In the present study, we explored the expression and antibacterial activity of MK in an airway context. MK was detected in bronchial epithelial cells of large airways and type 2 pneumocytes of normal lungs by immunohistochemistry. Immunoelectron microscopy revealed a surface-associated distribution, both on the ciliated apical and basolateral sides, and MK was detected in sputum obtained from healthy individuals by ELISA. In vitro, MK killed the common respiratory pathogen Streptococcus pneumoniae at below micromolar concentrations, an activity retained in the presence of sodium chloride at physiological concentrations. The MK molecule consists of two domains with three anti-parallel β-sheets and a COOH-terminal tail. Although both the NH2- and COOH-terminal domains alone showed antibacterial activity, the COOH-terminal domain including the tail region possessed higher bactericidal activity, i.e. in the order of the holoprotein. Retinoic acid-induced differentiation of primary bronchial epithelial cells, using an air-liquid interface system, revealed bactericidal activity in the apical airway surface liquid, an activity that was reduced after immunoprecipitation of MK. This study shows that airway epithelial cells of large airways and alveoli have a constitutive production of MK that is part of the bactericidal activity present in the air surface liquid, at least in vitro, and may thus be an important part of this arm of airway host defense.
为了抵抗感染，呼吸道需要强大的防御机制。视黄酸促进支气管上皮细胞的分化和维持表型特征。此外，它还诱导表达具有抗菌作用的生长因子中期因子（MK）。在本研究中，我们探讨了气道中 MK 的表达和抗菌活性。免疫组织化学法检测到正常肺大气道的支气管上皮细胞和 II 型肺泡细胞中存在 MK。免疫电镜显示 MK 在上皮细胞的纤毛顶端和基底外侧均有表面相关分布，ELISA 检测到健康个体的痰液中存在 MK。体外实验表明，MK 在低于微摩尔浓度时即可杀死常见呼吸道病原体肺炎链球菌，该活性在生理浓度的氯化钠存在下仍能保留。MK 分子由两个结构域组成，包含三个反平行的β-折叠和一个羧基末端尾部。尽管 NH2-和 COOH-末端结构域本身都具有抗菌活性，但包含尾部的 COOH-末端结构域具有更高的杀菌活性，即整个蛋白的活性。应用气液界面系统对原代支气管上皮细胞进行视黄酸诱导分化，发现气道表面液中具有杀菌活性，而 MK 免疫沉淀后该活性降低。本研究表明，大气道和肺泡的气道上皮细胞持续产生 MK，这是气道表面液中杀菌活性的一部分，至少在体外是如此，因此可能是气道宿主防御的重要组成部分。
