Leukocyte kinetics and bacterial clearance during <i>Streptococcus pneumoniae</i> pneumonia and contributions of ICAM-1

<i>Streptococcus pneumoniae</i> 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 plays 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 <i>S. pneumoniae</i> and evaluated 1, 4, and 7 days later. Our results show that <i>Icam1</i>^-/- mice have a greater number of viable bacteria within the lung at each time point. The impaired clearance observed in <i>Icam1</i>^-/- mice was not due to an impediment in leukocyte recruitment. In fact, <i>Icam1</i>^-/- mice had a greater number of neutrophils and recruited inflammatory macrophages in the lung tissue and the alveoli/airways on <i>day 7</i>. In contrast, fewer alveolar macrophages were present in the bronchoalveolar lavage (BAL) of <i>Icam1^-/-</i> mice. The loss of body weight and the concentrations of inflammatory mediators in the BAL were also significantly greater in <i>Icam1</i>^-/- 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 no suggestion of a defect in phagocytosis or killing. Thus, ICAM-1 is necessary for the clearance of <i>S. pneumoniae</i> 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 <i>S. pneumoniae-</i>induced pneumonia.<b>NEW & NOTEWORTHY</b> <i>Streptococcus pneumoniae</i> is the leading cause of community-acquired pneumonia. Our study examined ICAM-1, an adhesion molecule that is expressed on most cell types and plays important roles in leukocyte adhesion, migration, and motility. The data demonstrate that ICAM-1 is necessary for the clearance of <i>S. pneumoniae</i> 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.