Sensing of Interleukin-1 Cytokines during Streptococcus pneumoniae Colonization Contributes to Macrophage Recruitment and Bacterial Clearance

Abstract

Streptococcus pneumoniae (the pneumococcus), a leading cause of bacterial disease, is most commonly carried in the human nasopharynx. Colonization induces inflammation that promotes the organism's growth and transmission. This inflammatory response is dependent on intracellular sensing of bacterial components that access the cytosolic compartment via the pneumococcal pore-forming toxin pneumolysin. In vitro, cytosolic access results in cell death that includes release of the proinflammatory cytokine interleukin-1␤ (IL-1␤). IL-1 family cytokines, including IL-1␤, are secreted upon activation of inflammasomes, although the role of this activation in the host immune response to pneumococcal carriage is unknown. Using a murine model of pneumococcal nasopharyngeal colonization, we show that mice deficient in the interleukin-1 receptor type 1 (Il1r1 ؊/؊ ) have reduced numbers of neutrophils early after infection, fewer macrophages later in carriage, and prolonged bacterial colonization. Moreover, intranasal administration of Il-1␤ promoted clearance. Macrophages are the effectors of clearance, and characterization of macrophage chemokines in colonized mice revealed that Il1r1 ؊/؊ mice have lower expression of the C-C motif chemokine ligand 6 (CCL6), correlating with reduced macrophage recruitment to the nasopharynx. IL-1 family cytokines are known to promote adaptive immunity; however, we observed no difference in the development of humoral or cellular immunity to pneumococcal colonization between wild-type and Il1r1 ؊/؊ mice. Our findings show that sensing of IL-1 cytokines during colonization promotes inflammation without immunity, which may ultimately benefit the pneumococcus. Streptococcus pneumoniae (the pneumococcus) is an opportunistic bacterial pathogen that is responsible for over 1 million deaths annually, mostly in children under the age of 5 years (1). The pneumococcus serially colonizes the mucosal surfaces of the human upper respiratory tract, and carriage of the organism provides the reservoir for all pneumococcal disease (2). Colonization induces airway inflammation that is characterized by a suppurative rhinitis and increased mucus secretion. These secretions promote bacterial growth (3), and inflammation is important for bacterial transmission in a viral coinfection model (4). Human studies have demonstrated that higher bacterial burdens are correlated with a more profound rhinitis (5); however, as a result of this inflammatory response, colonization is normally cleared by the host's immune system within several weeks (6).A well-defined murine model of pneumococcal colonization (7) has elucidated bacterial and host factors that are critical to immune recognition of the pneumococcus, which drives the eventual resolution of the carrier state. Although early colonization triggers the recruitment of neutrophils, these are ineffective at resolving the infection. Clearance of pneumococci from the upper airway over a period of weeks requires a sustained presence of macrophages in the nasopharynx (8). Althou...

Host-to-Host Transmission of Streptococcus pneumoniae Is Driven by Its Inflammatory Toxin, Pneumolysin

“…Ply has been shown to be a potent inducer of CXC-motif chemokines involved in neutrophil recruitment in human dendritic cells (Bernatoniene et al, 2008). IL-1β , another proinflammatory mediator detected in our study, is secreted through activation of the inflammasome, in conjunction with a TLR2-dependent increase in its transcription, during colonization in a Ply-dependent manner (Lemon et al, 2015). …”

“…Total RNA and cDNA generation was carried out as previously described (Lemon et al, 2015). qRT-PCR reactions were performed with Power SYBR Green Master Mix (Applied Biosystems) using 10 ng cDNA and 0.5 μM primers per reaction.…”

“…RNA expression was quantified using the ΔΔCT method. Primers used in this study were previously described (Lemon et al, 2015; Siegel et al, 2015). …”

“…Purification and activity of recombinant pneumolysin (Ply) and its toxoid (PdB) containing the W433F point mutation were carried out as previously described (Gelber et al, 2008; Lemon et al, 2015). Proteins were stored at −20° C with 50% (w/v) glycerol to maintain biological activity.…”

Purified Streptococcus pneumoniae Endopeptidase O (PepO) Enhances Particle Uptake by Macrophages in a Toll-Like Receptor 2- and miR-155-Dependent Manner

“…Alveolar macrophages are important players in the pulmonary immune system (3)(4)(5)(6)(7)(8). During pneumococcal infection, clearance of pneumococci is initiated by macrophages, not neutrophils.…”