Streptococcus pneumoniae (the pneumococcus) is a human respiratory tract pathogen and a major cause of morbidity and mortality globally. Although the pneumococcus is a commensal bacterium that colonizes the nasopharynx, it also causes lethal diseases such as meningitis, sepsis, and pneumonia, especially in immunocompromised patients, in the elderly, and in young children. Due to the acquisition of antibiotic resistance and the emergence of nonvaccine serotypes, the pneumococcus has been classified as one of the priority pathogens for which new antibacterials are urgently required by the World Health Organization, 2017. Understanding molecular mechanisms behind the pathogenesis of pneumococcal infections and bacterial interactions within the host is crucial to developing novel therapeutics. Previously considered to be an extracellular pathogen, it is becoming evident that pneumococci may also occasionally establish intracellular niches within the body to escape immune surveillance and spread within the host. Intracellular survival within host cells also enables pneumococci to resist many antibiotics. Within the host cell, the bacteria exist in unique vacuoles, thereby avoiding degradation by the acidic lysosomes, and modulate the expression of its virulence genes to adapt to the intracellular environment. To invade and survive intracellularly, the pneumococcus utilizes a combination of virulence factors such as pneumolysin (PLY), pneumococcal surface protein A (PspA), pneumococcal adhesion and virulence protein B (PavB), the pilus-1 adhesin RrgA, pyruvate oxidase (SpxB), and metalloprotease (ZmpB). In this review, we discuss recent findings showing the intracellular persistence of Streptococcus pneumoniae and its underlying mechanisms.

Streptococcus pneumoniae is a major human pathogen and a leading cause of pneumonia, septicemia, and meningitis worldwide. Despite clinical studies linking vitamin D deficiency and pneumonia, molecular mechanisms behind these observations remain unclear. In particular, the effects of vitamin D on neutrophil responses remain unknown. Using pneumococcal strains, primary neutrophils isolated from human blood, and sera from patients with frequent respiratory tract infections (RTIs), we investigated the effects of vitamin D on neutrophil bactericidal and inflammatory responses, including pattern recognition receptors, antimicrobial peptides, and cytokine regulation. We found that vitamin D upregulated pattern recognition receptors, TLR2, and NOD2, and induced the antimicrobial human neutrophil peptides (HNP1-3) and LL-37, resulting in increased killing of pneumococci in a vitamin D receptor-dependent manner. Antibodies targeting HNP1-3 inhibited bacterial killing. Vitamin D supplementation of serum from patients with bacterial RTIs enhanced neutrophil killing. Moreover, vitamin D lowered inflammatory cytokine production by infected neutrophils via IL-4 production and the induction of suppressor of cytokine signaling (SOCS) proteins SOCS-1 and SOCS-3, leading to the suppression of NF-κB signaling. Thus, vitamin D enhances neutrophil killing of S. pneumoniae while dampening excessive inflammatory responses and apoptosis, suggesting that vitamin D could be used alongside antibiotics when treating pneumococcal infections.

Lysis of RBCs during numerous clinical settings such as severe hemolytic anemia, infection, tissue injury, or blood transfusion releases the endogenous damage-associated molecular pattern, hemoglobin (Hb), into the plasma. The redox-reactive Hb generates cytotoxic reactive oxygen species, disrupting the redox balance and impairing the immune-responsive blood cells. Therefore, it is crucial to understand how the immune system defends against the cytotoxic Hb. We identified a shortcut “capture and quench” mechanism of detoxification of Hb by the monocyte scavenger receptor CD163, independent of the well-known dominant antioxidant, haptoglobin. Our findings support a highly efficient two-pass mechanism of detoxification and clearance of Hb: 1) a direct suppression of Hb-pseudoperoxidase activity by CD163, involving an autocrine loop of CD163 shedding, sequestration of Hb, recycling, and homeostasis of CD163 in human monocytes and 2) paracrine transactivation of endothelial cells by the shedded soluble CD163 (sCD163), which further detoxifies and clears residual Hb. We showed that sCD163 and IgG interact with free Hb in the plasma and subsequently the sCD163-Hb-IgG complex is endocytosed into monocytes via FcγR. The endocytosed sCD163 is recycled to restore the homeostasis of CD163 on the monocyte membrane in an autocrine cycle, whereas the internalized Hb is catabolized. Using ex vivo coculture experiments, we demonstrated that the monocyte-derived sCD163 and IgG shuttle residual plasma Hb into the proximal endothelial cells. These findings suggest that CD163 and IgG collaborate to engage monocytes and endothelial cells in a two-pass detoxification mechanism to mount a systemic defense against Hb-induced oxidative stress.

S. pseudopneumoniae is an overlooked pathogen emerging as the causative agent of lower-respiratory-tract infections and associated with chronic obstructive pulmonary disease (COPD) and exacerbation of COPD. However, much remains unknown on its clinical importance and epidemiology, mainly due to the lack of specific markers to distinguish it from S. pneumoniae . Here, we provide a new molecular marker entirely specific for S. pseudopneumoniae and offer a comprehensive view of the virulence and colonization genes found in this species. Finally, our results pave the way for further studies aiming at understanding the pathogenesis and epidemiology of S. pseudopneumoniae .

Cholesterol‐dependent cytolysins (CDCs) are essential virulence factors for many human pathogens like Streptococcus pneumoniae (pneumolysin, PLY), Streptococcus pyogenes (streptolysin O, SLO), and Listeria monocytogenes (Listeriolysin, LLO) and induce cytolysis and inflammation. Recently, we identified that pneumococcal PLY interacts with the mannose receptor (MRC‐1) on specific immune cells thereby evoking an anti‐inflammatory response at sublytic doses. Here, we identified the interaction sites between MRC‐1 and CDCs using computational docking. We designed peptides from the CTLD4 domain of MRC‐1 that binds to PLY, SLO, and LLO, respectively. In vitro , the peptides blocked CDC‐induced cytolysis and inflammatory cytokine production by human macrophages. Also, they reduced PLY‐induced damage of the epithelial barrier integrity as well as blocked bacterial invasion into the epithelium in a 3D lung tissue model. Pre‐treatment of human DCs with peptides blocked bacterial uptake via MRC‐1 and reduced intracellular bacterial survival by targeting bacteria to autophagosomes. In order to use the peptides for treatment in vivo , we developed calcium phosphate nanoparticles (CaP NPs) as peptide nanocarriers for intranasal delivery of peptides and enhanced bioactivity. Co‐administration of peptide‐loaded CaP NPs during infection improved survival and bacterial clearance in both zebrafish and mice models of pneumococcal infection. We suggest that MRC‐1 peptides can be employed as adjunctive therapeutics with antibiotics to treat bacterial infections by countering the action of CDCs.