Recruitment of plasminogen is an important infection strategy of the human pathogen Streptococcus pneumoniae to invade host tissues. In Streptococcus aureus, triosephosphate isomerase (TPI) has been reported to bind plasminogen. In this study, the TPI of S. pneumoniae (TpiA) was identified through proteomic analysis of bronchoalveolar lavage fluid from a murine pneumococcal pneumonia model. The binding kinetics of recombinant pneumococcal TpiA with plasminogen were characterized using surface plasmon resonance (SPR, Biacore), ligand blot analyses, and enzyme‐linked immunosorbent assay. Enhanced plasminogen activation and subsequent degradation by plasmin were also shown. Release of TpiA into the culture medium was observed to be dependent on autolysin. These findings suggest that S. pneumoniae releases TpiA via autolysis, which then binds to plasminogen and promotes its activation, thereby contributing to tissue invasion via degradation of the extracellular matrix.

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Pneumolysin is a potent intracellular toxin possessing multiple functions that augment pneumococcal virulence. For over 10 years, sub-MICs of macrolides, including clarithromycin, have been recognized to decrease pneumolysin production and release from pneumococcal cells.

Periodontitis is one of the most common oral diseases resulting in gingival inflammation and tooth loss. Growing evidence indicates that it results from dysbiosis of the oral microbiome, which interferes with the host immune system, leading to bone destruction. Immune cells activate periodontal ligament cells to express the receptor activator of nuclear factor kappa-B (NF-κB) ligand (RANKL) and promote osteoclast activity. Osteocytes have active roles in periodontitis progression in the bone matrix. Local proteins are involved in bone regeneration through functional immunological plasticity. Here, we discuss the current knowledge of cellular and molecular mechanisms in periodontitis, the roles of local proteins, and promising synthetic compounds generating a periodontal regeneration effect. It is anticipated that this may lead to a better perception of periodontitis pathophysiology.

Neutrophil elastase (NE) functions as a host defense factor; however, excessive NE activity can potentially destroy human tissues. Although NE activity is positively correlated to gingival crevicular fluid and clinical attachment loss in periodontitis, the underlying mechanisms by which NE aggravates periodontitis remain elusive. In this study, we investigated how NE induces periodontitis severity and whether NE inhibitors were efficacious in periodontitis treatment. In a ligature-induced murine model of periodontitis, neutrophil recruitment, NE activity, and periodontal bone loss were increased in the periodontal tissue. Local administration of an NE inhibitor significantly decreased NE activity in periodontal tissue and attenuated periodontal bone loss. Furthermore, the transcription of proinflammatory cytokines in the gingiva, which was significantly upregulated in the model of periodontitis, was significantly downregulated by NE inhibitor injection. An in vitro study demonstrated that NE cleaved cell adhesion molecules, such as desmoglein 1, occludin, and E-cadherin, and induced exfoliation of the epithelial keratinous layer in three-dimensional human oral epithelial tissue models. The permeability of fluorescein-5-isothiocyanate-dextran or periodontal pathogen was significantly increased by NE treatment in the human gingival epithelial monolayer. These findings suggest that NE induces the disruption of the gingival epithelial barrier and bacterial invasion in periodontal tissues, aggravating periodontitis.

Bacterial and viral respiratory infections can initiate acute lung injury and acute respiratory distress syndrome. Neutrophils and their granule enzymes, including neutrophil elastase, are key mediators of the pathophysiology of acute respiratory failure. Although intracellular neutrophil elastase functions as a host defensive factor against pathogens, its leakage into airway spaces induces degradation of host connective tissue components. This leakage disrupts host innate immune responses via proteolytic cleavage of Toll-like receptors and cytokines. Here, we investigated whether neutrophils possess proteases that cleave adaptive immune molecules. We found that expression of the human leukocyte antigen (HLA) class II molecule HLA-DP β1 was decreased in THP-1-derived macrophages treated with supernatants from dead neutrophils. This decreased HLA-DP β1 expression was counteracted by treatment with neutrophil elastase inhibitor, suggesting proteolytic cleavage of HLA-DP β1 by neutrophil elastase. SDS-PAGE showed that neutrophil elastase cleaved recombinant HLA-DP α1, -DP β1, -DQ α1, -DQ β1, -DR α, and -DR β1. Neutrophil elastase also cleaved HLA-DP β1 on extracellular vesicles isolated from macrophages without triggering morphological changes. Thus, leakage of neutrophil elastase may disrupt innate immune responses, antigen presentation, and T cell activation. Additionally, inhibition of neutrophil elastase is a potential therapeutic option for treating bacterial and viral pneumonia.

Quantification of single-species biofilms using crystal violet assayThe quantification of single-species biofilms of Streptococcus mutans (S. mutans) UA159, Actinomyces naeslundii (A. naeslundii) ATCC12104, and Streptococcus oralis (S. oralis) ATCC35037 against chlorhexidine gluconate (CHG) at sub-minimum inhibitory concentration (sub-MIC) was performed using the crystal violet (CV) assay (Figure S1).

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