Leukocyte kinetics and bacterial clearance during S. pneumoniae pneumonia and contributions of ICAM-1

Streptococcus pneumoniae is a leading cause of community-acquired pneumonia. Intercellular adhesion molecule-1 (ICAM-1) is an adhesion molecule that is highly expressed on the pulmonary capillary endothelium, alveolar epithelium and other cell types within the lung. ICAM-1 is known to play important roles in leukocyte adhesion, migration, and motility. To determine the contributions of ICAM-1 to bacterial clearance and leukocyte kinetics during pneumonia, mice were inoculated with S. pneumoniae and evaluated 1, 4 and 7 days later. Our results show that Icam1-/- mice have a greater number of viable bacteria within the lung at each time point. The impaired clearance observed in Icam1-/- mice was not due to an impediment in leukocyte recruitment. In fact, Icam1-/- mice had a greater number of neutrophils and recruited inflammatory macrophages in the lung tissue and the alveoli/airways on day 7. In contrast, fewer alveolar macrophages were present in the BAL of Icam1-/- mice. The loss of body weight and the concentrations of inflammatory mediators in the BAL were also significantly greater in Icam1-/- mice. Mechanistic studies to understand the defect in clearance show that neutrophils and macrophage subpopulations had no defect in phagocytosis or acidification of phagosomes. RNA sequencing reveals many differences in gene expression, but does not suggest° a defect in phagocytosis. Thus, ICAM-1 is necessary for the clearance of S. pneumoniae and for the resolution of pneumonia, but is not required for the recruitment of neutrophils or inflammatory macrophages into the pneumonic lung parenchyma or the alveoli/airways during S. pneumoniae-induced pneumonia. Overall design: Gene expression profiling analysis was performed on 1) naïve primary alveolar macrophages isolated from ICAM-1 null or wild type mice or 2) primary alveolar macrophages isolated from ICAM-1 null or wild type mice following exposure to Staphylococcus aureus pHrodo-labeled bioparticles for 1 hour.

肺炎链球菌（Streptococcus pneumoniae）是社区获得性肺炎的首要致病菌。细胞间黏附分子-1（Intercellular Adhesion Molecule-1, ICAM-1）是一种黏附分子，在肺毛细血管内皮细胞、肺泡上皮细胞及肺内其他多种细胞中呈高表达。已知ICAM-1在白细胞黏附、迁移及运动过程中发挥关键作用。为明确ICAM-1在肺炎病程中对细菌清除及白细胞动力学的贡献，研究者向小鼠接种肺炎链球菌，并于接种后1、4、7天开展检测评估。结果显示，Icam1基因敲除（Icam1-/-）小鼠在各个检测时间点的肺组织内活菌数量均显著更高。Icam1-/-小鼠的细菌清除能力受损并非源于白细胞募集障碍。事实上，接种后第7天，Icam1-/-小鼠的肺组织及肺泡/气道内的中性粒细胞和募集的炎性巨噬细胞数量反而更多。与之相反，Icam1-/-小鼠的支气管肺泡灌洗液（BAL）中肺泡巨噬细胞数量更少。此外，Icam1-/-小鼠的体重丢失量及支气管肺泡灌洗液中炎症介质浓度均显著升高。为阐明清除障碍的潜在机制，研究者开展了机制性研究，结果发现中性粒细胞及巨噬细胞亚群的吞噬功能或吞噬体酸化过程均无异常。RNA测序结果显示二者存在大量基因表达差异，但并未提示吞噬功能存在缺陷。综上，ICAM-1对于肺炎链球菌的清除及肺炎的转归是必需的，但在肺炎链球菌诱导的肺炎病程中，并不参与中性粒细胞或炎性巨噬细胞向肺炎肺实质及肺泡/气道的募集过程。总体实验设计：本研究对两类样本进行基因表达谱分析：1）从ICAM-1敲除或野生型小鼠体内分离的未激活原代肺泡巨噬细胞；2）从ICAM-1敲除或野生型小鼠体内分离的原代肺泡巨噬细胞，经pHrodo标记的金黄色葡萄球菌（Staphylococcus aureus）生物颗粒刺激1小时后获取的样本。