Nontypeable Haemophilus influenzae (NTHi) is a Gram-negative human pathogen that causes infections mainly in the upper and lower respiratory tract. The bacterium is associated with bronchitis and exacerbations in patients suffering from chronic obstructive pulmonary disease and frequently causes acute otitis media in preschool children. We have previously demonstrated that the binding of C4b binding protein (C4BP) is important for NTHi complement evasion. In this study, we identified outer membrane protein 5 (P5) of NTHi as a novel ligand of C4BP. Importantly, we observed significantly lower C4BP binding and decreased serum resistance in P5-deficient NTHi mutants. Surface expression of recombinant P5 on Escherichia coli conferred C4BP binding and consequently increased serum resistance. Moreover, P5 expression was positively correlated with C4BP binding in a series of clinical isolates. We revealed higher levels of P5 surface expression and consequently more C4BP binding in isolates from the lower respiratory tract of chronic obstructive pulmonary disease patients and tonsil specimens compared with isolates from the upper respiratory tract and the bloodstream (invasive strains). Our results highlight P5 as an important protein for protecting NTHi against complement-mediated killing.

不可分型流感嗜血杆菌(NTHi) 是一种革兰氏阴性人类病原体，主要引起上呼吸道和下呼吸道感染。该细菌与慢性阻塞性肺病患者的支气管炎和恶化有关，并经常在学龄前儿童中引起急性中耳炎。我们之前已经证明 C4b 结合蛋白 (C4BP) 的结合对于 NTHi 补体逃避很重要。在这项研究中，我们将 NTHi 的外膜蛋白 5 (P5) 鉴定为 C4BP 的新型配体。重要的是，我们观察到 P5 缺陷型 NTHi 突变体中 C4BP 结合显着降低，血清抗性降低。重组 P5 在大肠杆菌上的表面表达赋予 C4BP 结合，从而增加血清抗性。此外，在一系列临床分离株中，P5 表达与 C4BP 结合呈正相关。我们发现，与来自上呼吸道和血流（侵袭性菌株）的分离物相比，来自慢性阻塞性肺病患者的下呼吸道分离物和扁桃体标本中 P5 表面表达水平更高，因此与 C4BP 结合更多。我们的结果强调 P5 作为保护 NTHi 免受补体介导的杀伤的重要蛋白质。