Streptococcus pneumoniae (S. pneumoniae, also known as pneumococcus) infections are major causes of death worldwide. Despite the development and use of effective antibiotics, high, early mortality due to pneumococcal infections has not been decreased for the last few decades. Recent study found a deadly hemorrhagic acute lung injury (ALI) as a major cause of death at the early stage of severe pneumococcal infections. Interleukin (IL)-1β was known to play critical roles not only for the development of ALI but also resolution of it. The role of IL-1β on the pathogenesis of pneumococcal ALI, however, has not been well understood yet. This study aims to investigate the role of IL-1β on the development of pneumococcal ALI and subsequent death. IL-1β expression was upregulated in the lungs of pneumococcal ALI in wild-type (WT) mice, but not in the plasma. Despite an increased expression of pulmonary IL-1β, no inflammatory cell infiltration into airway has been observed. Upregulation of IL-1β expression was indeed dependent on pneumococcal cytoplasmic toxin pneumolysin and its cell surface receptor Toll-like receptor 4. Deficiency of IL-1R1, a cell surface receptor of IL-1β, resulted in a markedly reduced hemorrhagic pulmonary edema and early death in pneumococcal ALI. Finally, IL-1β neutralization in WT mice protects against pulmonary hemorrhagic edema and death. These data suggest that pulmonary expression of IL-1β exacerbates pneumolysin-induced ALI and death by promoting alveolar hemorrhagic edema.

Coagulation factor 2 receptor (F2R), also well-known as a protease-activated receptor 1 (PAR1), is the first known thrombin receptor and plays a critical role in transmitting thrombin-mediated activation of intracellular signaling in many types of cells. It has been known that bacterial infections lead to activation of coagulation systems, and recent studies suggest that PAR1 may be critically involved not only in mediating bacteria-induced detrimental coagulation, but also in innate immune and inflammatory responses. Community-acquired pneumonia, which is frequently caused by Streptococcus pneumoniae (S. pneumoniae), is characterized as an intra-alveolar coagulation and an interstitial neutrophilic inflammation. Recently, the role of PAR1 in regulating pneumococcal infections has been proposed. However, the role of PAR1 in pneumococcal infections has not been clearly understood yet. In this review, recent findings on the role of PAR1 in pneumococcal infections and possible underlying molecular mechanisms by which S. pneumoniae regulates PAR1-mediated immune and inflammatory responses will be discussed.