Pneumolysin: stimulating protection

Lung epithelium and myeloid cells cooperate to clear acute pneumococcal infection

The Gram-positive bacterium Streptococcus pneumoniae causes life-threatening infections, especially among immunocompromised patients. The host's immune system senses S. pneumoniae via different families of pattern recognition receptors, in particular the Toll-like receptor (TLR) family that promotes immune cell activation. Yet, while single TLRs are dispensable for initiating inflammatory responses against S. pneumoniae, the central TLR adapter protein myeloid differentiation factor 88 (MyD88) is of vital importance, as MyD88-deficient mice succumb rapidly to infection. Since MyD88 is ubiquitously expressed in hematopoietic and non-hematopoietic cells, the extent to which MyD88 signaling is required in different cell types to control S. pneumoniae is unknown. Therefore, we used novel conditional knockin mice to investigate the necessity of MyD88 signaling in distinct lung-resident myeloid and epithelial cells for the initiation of a protective immune response against S. pneumoniae. Here, we show that MyD88 signaling in lysozyme M (LysM)- and CD11c-expressing myeloid cells, as well as in pulmonary epithelial cells, is critical to restore inflammatory cytokine and antimicrobial peptide production, leading to efficient neutrophil recruitment and enhanced bacterial clearance. Overall, we show a novel synergistic requirement of compartment-specific MyD88 signaling in S. pneumoniae immunity.

Streptococcus pneumoniae induces autophagy through the inhibition of the PI3K-I/Akt/mTOR pathway and ROS hypergeneration in A549 cells

The present study focused on the action mechanism of S. pneumoniae (Sp) in inducing autophagy in human alveolar epithelial cells. Sp, a gram-positive extracellular bacterium, activates autophagy with considerably increased microtuble-associated protein light chain 3 (LC3) punctation in A549 cells. The accumulation of typical autophagosomes and conjugation of LC3 to phosphatidylethanolamine were observed in Sp-infected cells as an indication of autophagy. Using the pneumolysin (PLY) mutant, we successfully demonstrated that PLY is involved in initiating autophagy without affecting the expression levels of PI3K-III and Beclin1. PLY-mediated autophagy depends on the inhibition of the phosphoinositide 3-kinase/Akt/mammalian target of rapamycin (PI3K/Akt/mTOR) pathway. Furthermore, Sp could also lead to the reactive oxygen species (ROS) hypergeneration in A549 cells. Taken together, Sp infection-induced autophagy is PLY-mediated through ROS hypergeneration and mTOR inhibition. PI3K-I and rapamycin (autophagy inducers) enhanced bacterial clearance, whereas wortmannin (autophagy inhibitor) and acetylcysteine (ROS inhibitor) reduced intracellular bacteria clearance. Thus, Sp-induced autophagy represents a host-protective mechanism, providing new insight into the pathogenesis of respiratory tract Sp infection.

Physical characterization and formulation development of a recombinant pneumolysoid protein-based pneumococcal vaccine

Streptococcus pneumoniae is a major cause of death in children worldwide. There are more than 90 known pneumococcus serotypes that vary by geographical location. Pneumolysin is a protein toxin produced by virtually all invasive strains of S. pneumoniae and is considered an important virulence factor. Pneumolysin is immunogenic and has the potential to be a new vaccine antigen offering broad serotype-independent coverage. To develop a stable vaccine formulation, the conformational stability of a recombinant pneumolysin mutant (pneumolysoid L460D) was characterized by various techniques. Three data visualization diagrams were constructed to summarize the biophysical data of the L460D pneumolysoid; the protein is most stable in solution at pH 6-7, and loses conformational integrity above 48°C. Excipient screening assays were performed and sugars such as trehalose and sucrose stabilized the pneumolysin mutant with respect to improving thermal transition temperatures and minimizing aggregation. In addition, the protein antigen showed efficient binding to aluminum hydroxide adjuvant. The conformational stability of the L460D pneumolysoid on the surface of alhydrogel adjuvant was little affected by adsorption, either with or without excipients. These studies provide important preformulation characterization information useful for the development of a stable pneumolysin mutant-based vaccine.