Age-dependent differences in efferocytosis determine the outcome of opsonophagocytic protection from invasive pathogens.

In early life, susceptibility to invasive infection skews toward a small subset of microbes, whereas other pathogens associated with diseases later in life, including Streptococcus pneumoniae (Spn), are uncommon among neonates. To delineate mechanisms behind age-dependent susceptibility, we compared age-specific mouse models of invasive Spn infection. We show enhanced CD11b-dependent opsonophagocytosis by neonatal neutrophils improved protection against Spn during early life. The augmented function of neonatal neutrophils was mediated by higher CD11b surface expression at the population level due to dampened efferocytosis, which also resulted in more CD11bhi "aged" neutrophils in peripheral blood. Dampened efferocytosis during early life could be attributed to the lack of CD169+ macrophages in neonates and reduced systemic expressions of multiple efferocytic mediators, including MerTK. On experimentally impairing efferocytosis later in life, CD11bhi neutrophils increased and protection against Spn improved. Our findings reveal how age-dependent differences in efferocytosis determine infection outcome through the modulation of CD11b-driven opsonophagocytosis and immunity.

Impaired upper respiratory tract barrier function during postnatal development predisposes to invasive pneumococcal disease.

Infants are highly susceptible to invasive respiratory and gastrointestinal infections. To elucidate the age-dependent mechanism(s) that drive bacterial spread from the mucosa, we developed an infant mouse model using the prevalent pediatric respiratory pathogen, Streptococcus pneumoniae (Spn). Despite similar upper respiratory tract (URT) colonization levels, the survival rate of Spn-infected infant mice was significantly decreased compared to adults and corresponded with Spn dissemination to the bloodstream. An increased rate of pneumococcal bacteremia in early life beyond the newborn period was attributed to increased bacterial translocation across the URT barrier. Bacterial dissemination in infant mice was independent of URT monocyte or neutrophil infiltration, phagocyte-derived ROS or RNS, inflammation mediated by toll-like receptor 2 or interleukin 1 receptor signaling, or the pore-forming toxin pneumolysin. Using molecular barcoding of Spn, we found that only a minority of bacterial clones in the nasopharynx disseminated to the blood in infant mice, indicating the absence of robust URT barrier breakdown. Rather, transcriptional profiling of the URT epithelium revealed a failure of infant mice to upregulate genes involved in the tight junction pathway. Expression of many such genes was also decreased in early life in humans. Infant mice also showed increased URT barrier permeability and delayed mucociliary clearance during the first two weeks of life, which corresponded with tighter attachment of bacteria to the respiratory epithelium. Together, these results demonstrate a window of vulnerability during postnatal development when altered mucosal barrier function facilitates bacterial dissemination.

Effects of Capsular Polysaccharide amount on Pneumococcal-Host interactions.

Among the many oral streptococci, Streptococcus pneumoniae (Spn) stands out for the capacity of encapsulated strains to cause invasive infection. Spread beyond upper airways, however, is a biological dead end for the organism, raising the question of the benefits of expending energy to coat its surface in a thick layer of capsular polysaccharide (CPS). In this study, we compare mutants of two serotypes expressing different amounts of CPS and test these in murine models of colonization, invasion infection and transmission. Our analysis of the effect of CPS amount shows that Spn expresses a capsule of sufficient thickness to shield its surface from the deposition of complement and binding of antibody to underlying epitopes. While effective shielding is permissive for invasive infection, its primary contribution to the organism appears to be in the dynamics of colonization. A thicker capsule increases bacterial retention in the nasopharynx, the first event in colonization, and also impedes IL-17-dependent clearance during late colonization. Enhanced colonization is associated with increased opportunity for host-to-host transmission. Additionally, we document substantial differences in CPS amount among clinical isolates of three common serotypes. Together, our findings show that CPS amount is highly variable among Spn and could be an independent determinant affecting host interactions.

BlpC-mediated selfish program leads to rapid loss of Streptococcus pneumoniae clonal diversity during infection.

Successful colonization of a host requires bacterial adaptation through genetic and population changes that are incompletely defined. Using chromosomal barcoding and high-throughput sequencing, we investigate the population dynamics of Streptococcus pneumoniae during infant mouse colonization. Within 1 day post inoculation, diversity was reduced >35-fold with expansion of a single clonal lineage. This loss of diversity was not due to immune factors, microbiota, or exclusive genetic drift. Rather, bacteriocins induced by the BlpC-quorum sensing pheromone resulted in predation of kin cells. In this intra-strain competition, the subpopulation reaching a quorum likely eliminates others that have yet to activate the blp locus. Additionally, this reduced diversity restricts the number of unique clones that establish colonization during transmission between hosts. Genetic variation in the blp locus was also associated with altered transmissibility in a human population, further underscoring the importance of BlpC in clonal selection and its role as a selfish element.

Rewilding of laboratory mice enhances granulopoiesis and immunity through intestinal fungal colonization.

The paucity of blood granulocyte populations such as neutrophils in laboratory mice is a notable difference between this model organism and humans, but the cause of this species-specific difference is unclear. We previously demonstrated that laboratory mice released into a seminatural environment, referred to as rewilding, display an increase in blood granulocytes that is associated with expansion of fungi in the gut microbiota. Here, we find that tonic signals from fungal colonization induce sustained granulopoiesis through a mechanism distinct from emergency granulopoiesis, leading to a prolonged expansion of circulating neutrophils that promotes immunity. Fungal colonization after either rewilding or oral inoculation of laboratory mice with Candida albicans induced persistent expansion of myeloid progenitors in the bone marrow. This increase in granulopoiesis conferred greater long-term protection from bloodstream infection by gram-positive bacteria than by the trained immune response evoked by transient exposure to the fungal cell wall component ß-glucan. Consequently, introducing fungi into laboratory mice may restore aspects of leukocyte development and provide a better model for humans and free-living mammals that are constantly exposed to environmental fungi.

Serotype-Dependent Effects on the Dynamics of Pneumococcal Colonization and Implications for Transmission.

Capsule-switch mutants were compared to analyze how serotype affects the success of Streptococcus pneumoniae (Spn) during colonization and transmission. Strains of multiple serotypes were tested in highly susceptible infant mice, both singly and in competitive assays. Our findings demonstrated a role of serotype, apart from genetic background, in competitive success of strains, but this depended on timing postinoculation. As is the case for natural carriage, there was a hierarchy of success among serotypes using capsule-switch strains. The long-term dominance of a serotype was established within the first 4 h after acquisition, suggesting an effect independent of Spn-induced host responses. The hierarchy of serotype dominance correlated with decreased clearance rather than increased growth in vivo. Competitive assays staggering the timing of challenge showed that the first strain to dominate the niche sustained its competitive advantage, potentially explaining how increased density from delayed early clearance could result in serotype-dependent success. Effector molecules of intrastrain competition (fratricide), regulated by the competence operon in a quorum-sensing mechanism, were required for early niche dominance. This suggested a winner-takes-all scenario in which serotype is a major factor in achieving early niche dominance, such that once a strain reaches a threshold density it is able to exclude competitors through fratricide. Serotype was also an important determinant of transmission dynamics, although transit to a recipient host depended on effects of serotype different from its contribution to the dominance of colonization in the donor host. IMPORTANCE Capsule is the major virulence factor and surface antigen of the opportunistic respiratory pathogen Streptococcus pneumoniae (Spn). Strains of Spn express at least 100 structurally and immunologically distinct types (serotypes) of capsule, but for unknown reasons only a few are common. The effect of serotypes during the commensal interactions of Spn and its host, colonization and transmission, was tested. This was carried out by comparing genetically modified strains differing only in serotype in infant mouse models. Results show that serotype is an important factor in a strain's success during colonization. This was attributed to the effect of serotype on early clearance of the organism in the host. Competitive factors expressed by Spn (in a mechanism referred to as fratricide) allow the strain gaining this initial advantage to then dominate the upper respiratory tract niche. Serotype also plays an important role in a strain's success during transmission from one host to another.

Pneumococcal capsule blocks protection by immunization with conserved surface proteins.

Vaccines targeting Streptococcus pneumoniae (Spn) are limited by dependence on capsular polysaccharide and its serotype diversity. More broadly-based approaches using common protein antigens have not resulted in a licensed vaccine. Herein, we used an unbiased, genome-wide approach to find novel vaccine antigens to disrupt carriage modeled in mice. A Tn-Seq screen identified 198 genes required for colonization of which 16 are known to express conserved, immunogenic surface proteins. After testing defined mutants for impaired colonization of infant and adult mice, 5 validated candidates (StkP, PenA/Pbp2a, PgdA, HtrA, and LytD/Pce/CbpE) were used as immunogens. Despite induction of antibody recognizing the Spn cell surface, there was no protection against Spn colonization. There was, however, protection against an unencapsulated Spn mutant. This result correlated with increased antibody binding to the bacterial surface in the absence of capsule. Our findings demonstrate how the pneumococcal capsule interferes with mucosal protection by antibody to common protein targets.

Pneumococcal quorum sensing drives an asymmetric owner-intruder competitive strategy during carriage via the competence regulon.

Competition among microorganisms is a key determinant of successful host colonization and persistence. For Streptococcus pneumoniae, lower than predicted rates of co-colonizing strains suggest a competitive advantage for resident bacteria over newcomers. In light of evolutionary theory, we hypothesized that S. pneumoniae use owner-intruder asymmetries to settle contests, leading to the disproportionate success of the initial resident 'owner', regardless of the genetic identity of the 'intruder'. We investigated the determinants of within-host competitive success utilizing S. pneumoniae colonization of the upper respiratory tract of infant mice. Within 6 h, colonization by the resident inhibited colonization by an isogenic challenger. The competitive advantage of the resident was dependent on quorum sensing via the competence (Com) regulon and downstream choline binding protein D (CbpD) and on the competence-induced bacteriocins A and B (CibAB) implicated in fratricide. CbpD and CibAB are highly conserved across pneumococcal lineages, indicating evolutionary advantages for asymmetric competitive strategies within the species. Mathematical modelling supported a significant role for quorum sensing via the Com regulon in competition, even for strains with different competitive advantages. Our study suggests that asymmetric owner-intruder competitive strategies do not require complex cognition and are used by a major human pathogen to determine 'ownership' of human hosts.