Neutrophil Recruitment to the Lungs during Bacterial Pneumonia

In the respiratory system, the upper tract is colonized with commensal bacteria, whereas the lower tract is sterile. The respiratory system is continuously exposed to a variety of bacteria. To combat these intruders, the lung has developed a multifaceted system of defense. One of the most important components of the initial innate immune response in the lung against bacterial infection is the vigorous recruitment of neutrophils. However, several life-threatening bacterial lung diseases are caused by excessive neutrophil-mediated inflammation. The mechanisms underlying neutrophil accumulation during lower respiratory tract bacterial infection is learned from experimental animal models, particularly with mice ( 61). A better understanding of the mechanisms underlying the regulation of neutrophil influx is crucial to designing improved therapies to augment host defense and attenuate detrimental lung inflammation. The aim of this review is to highlight some of the most important recent advances in neutrophil infiltration, particularly in the roles of Toll-like receptors (TLRs), nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs), transcription factors, chemokines, and adhesion molecules in acute lower respiratory tract bacterial infections. This minireview includes important gram-positive and gram-negative pathogens, including Streptococcus pneumoniae, Klebsiella pneumoniae, Legionella pneumophila, Haemophilus influenzae, and Staphylococcus aureus.