Pulmonary and Nonpulmonary Sepsis Differentially Modulate Lung Immunity toward Secondary Bacterial Pneumonia: A Critical Role for Alveolar Macrophages

Clinical observations suggest that the source of primary infection accounts for a major determinant of further nosocomial pneumonia in critically ill patients with sepsis. Here we addressed the impact of primary nonpulmonary or pulmonary septic insults on lung immunity using relevant double-hit animal models. C57BL/6J mice were first subjected to polymicrobial peritonitis induced by cecal ligation and puncture (CLP) or bacterial pneumonia induced by intratracheal challenge with Escherichia coli. Seven days later, postseptic mice received ab intratracheal challenge with Pseudomonas aeruginosa. Compared with controls, post-CLP mice became highly susceptible to P. aeruginosa pneumonia, as demonstrated by defective lung bacterial clearance and increased mortality rate. In contrast, all postpneumonia mice survived the P. aeruginosa challenge and even exhibited improved bacterial clearance. Nonpulmonary and pulmonary sepsis differentially modulated the amounts and some important immune functions of alveolar macrophages. Additionally, we observed a Toll-like receptor 2 (TLR2)-dependent increase in regulatory T cells (Tregs) in lungs from post-CLP mice. Antibody-mediated Treg depletion restored the numbers and functions of alveolar macrophages in post-CLP mice. Furthermore, post-CLP TLR2-deficient mice were found resistant to secondary P. aeruginosa pneumonia. In conclusion, polymicrobial peritonitis and bacterial pneumonia conferred susceptibility or resistance to secondary gram-negative pulmonary infection, respectively. Immune patterns in post-CLP lungs argue for a TLR2-dependent cross-talk between Tregs and alveolar macrophages as an important regulatory mechanism in postseptic lung defense.