Christopher M. Evans

Streptococcus pneumoniae is a leading human respiratory pathogen, yet its success depends not on disease but on its ability to persist as an asymptomatic colonizer of the nasopharynx. The nasopharyngeal mucosa is lined with mucus, whose heavily O-glycosylated mucin proteins are the most abundant macromolecules in the pneumococcal natural niche. While mucus is canonically viewed as a host defense barrier, we found that mucin actively reprograms pneumococcal physiology to promote colonization through two distinct mechanisms. First, mucin suppresses pneumococcal autolysis by specifically inhibiting the enzymatic activity of the major autolysin LytA, prolonging bacterial survival and restricting release of the cholesterol-dependent cytotoxin pneumolysin, thereby reducing host cell damage. Second, mucin induces de novo biosynthesis and extracellular release of tryptophan, an essential amino acid that humans cannot synthesize and that serves as a substrate for host immunomodulatory pathways. Tryptophan biosynthesis was critical for pneumococcal survival against macrophage killing, and its disruption abolished both colonization and invasive disease. Transcriptomic analysis revealed that tryptophan biosynthesis enables the pneumococcus to survive a bactericidal macrophage state while preventing macrophage transition to proliferative tissue repair. Together, these findings reveal that mucin is not merely a barrier but an active signal that reprograms the pneumococcus toward a persistent, host-tolerant state, identifying a distinctive repertoire of colonization mechanisms that allow a virulent respiratory pathogen to sustain long-term asymptomatic carriage in its human reservoir.

Streptococcus pneumoniae is a leading human respiratory pathogen, yet its success depends not on disease but on its ability to persist as an asymptomatic colonizer of the nasopharynx. The nasopharyngeal mucosa is lined with mucus, whose heavily O-glycosylated mucin proteins are the most abundant macromolecules in the pneumococcal natural niche. While mucus is canonically viewed as a host defense barrier, we found that mucin actively reprograms pneumococcal physiology to promote colonization through two distinct mechanisms. First, mucin suppresses pneumococcal autolysis by specifically inhibiting the enzymatic activity of the major autolysin LytA, prolonging bacterial survival and restricting release of the cholesterol-dependent cytotoxin pneumolysin, thereby reducing host cell damage. Second, mucin induces de novo biosynthesis and extracellular release of tryptophan, an essential amino acid that humans cannot synthesize and that serves as a substrate for host immunomodulatory pathways. Tryptophan biosynthesis was critical for pneumococcal survival against macrophage killing, and its disruption abolished both colonization and invasive disease. Transcriptomic analysis revealed that tryptophan biosynthesis enables the pneumococcus to survive a bactericidal macrophage state while preventing macrophage transition to proliferative tissue repair. Together, these findings reveal that mucin is not merely a barrier but an active signal that reprograms the pneumococcus toward a persistent, host-tolerant state, identifying a distinctive repertoire of colonization mechanisms that allow a virulent respiratory pathogen to sustain long-term asymptomatic carriage in its human reservoir.

T-helper (Th)-17 lymphocytes are central mediators of adaptive type 17 immunity. Decreased type-17 signaling increases severity of infections in humans and mice. However, detrimental effects of excessive type 17 responses in autoimmune and other inflammatory diseases highlight a need for type-17 immune calibration to support beneficial host defense requirements. Mechanisms of type 17 calibration are poorly understood. A gut-lung axis has been proposed to coordinate homeostatic protection and acute host defense. Factors that acutely alter the gut microbiome are heterogeneous and include acute intestinal infections, non-infectious colitis, and medical treatments such as antibiotics. How changes in the gut microbiome affect lung immune tone during homeostasis and acute pulmonary infections are also poorly understood. Prior studies have shown that antibiotics reduce expression of IL-17-mediated host defense in the gut. Since gut microbial homeostasis influences Th17 cell numbers in both the intestine and remote tissues, we postulated that antibiotic treatment would result in gut dysbiosis and weakened type-17 host defense in the lungs. We found that amoxicillin induces significant dysbiosis that is long-lasting and that there is a long-term decrease in type-17 tone in the lungs. We also found that in mice lacking the gut mucin, Muc2, Th17 cells increased in the lungs following inflammatory challenge. These findings suggest that antibiotic-induced dysbiosis can decrease lung immune defenses for long periods of time after cessation of antibiotic treatment.

Rationale: Finely tuned production and secretion of the polymeric mucins MUC5AC and MUCB are required for lung health, but knowledge of many details between their translation and their packaging into secretory granules is lacking. Objective: To analyze the structure and function of the Golgi apparatus, a key site of mucin glycosylation, folding, polymerization and packaging, in airway epithelial secretory cells. Methods: Lung tissue was obtained from mice stimulated or not with IL-13 to upregulate mucin production, and from normal human lungs. Golgi elements in mouse and human tissue were imaged by high-resolution immunofluorescence microscopy and electron microscopy. Tissue from mice with deletion of both polymeric mucins was also examined. Measurements and Main Results: By immunofluorescence microscopy, both mouse and human airway secretory cells contained ∼100 dispersed puncta labeled by markers of medial and trans Golgi cisternae and the trans-Golgi network (TGN), but only a few perinuclear puncta were labeled by markers of cis-Golgi cisternae. By electron microscopy, secretory cells contained both a perinuclear Golgi ribbon and numerous dispersed Golgi stacks, termed satellites. In mucous metaplastic cells, satellites were concentrated among immature mucin granules. Increasing mucin production by cytokine stimulation did not increase the number of TGN puncta, nor did preventing polymeric mucin production by gene deletion reduce TGN puncta. Conclusions: Mucin-producing airway secretory cells express an unusual Golgi structure consisting of a conventional perinuclear ribbon as well as dispersed satellites. While the Golgi satellites are likely an adaptation for mucin production and packaging, their presence is specified developmentally, independent of mucin production.

In the aging HIV (human immunodeficiency virus)-infected population, comorbid diseases are important determinants of morbidity and mortality. People living with HIV (PLWH) demonstrate increased lung inflammation and incidence of chronic obstructive pulmonary disease (COPD), even after adjusting for smoking status. Disruption of the lung molecular clock has been implicated in the increased lung inflammation observed in COPD and smokers. We hypothesize that the expression of HIV TAT protein in the lungs of PLWHpromotes lung inflammation and features of emphysema due to dysregulation of lung circadian rhythm, determined by the lung molecular clock genes. We demonstrate that HIV TAT upregulates miR-126-3p in primary bronchial epithelial cells and suppresses Sirtuin 1 (SIRT1), resulting in downstream effects on core circadian genes such as BMAL1 and PER2, leading to dysregulation of the lung molecular clock. This study identifies TAT/miR-126-3p/SIRT1 axis as an important mediator of HIV-induced lung inflammation in primary human bronchial epithelial cells, SPC-TAT transgenic mice with lung-specific TAT expression, and lungs from HIV-positive donors. Using single-cell RNA sequencing of lungs from 4-month-old SP-C TAT mice, we further show that these mice already exhibit significant alterations in clock gene expression and elevated expression of proinflammatory markers in their young adult stage. Understanding the pathophysiological mechanisms by which HIV disrupts the lung molecular clock and promotes inflammation may help identify therapeutic strategies to mitigate HIV-associated COPD.

Respiratory surfaces are exposed daily to billions of inhaled particles that could cause injury to alveolar tissues where gas exchange occurs. Accordingly, robust defense is essential but must involve minimal physiologic disruption. Airspace macrophages (AMs) protect lung surfaces while also maintaining tissue integrity through non-inflammatory homeostatic responses in health. However, in chronic obstructive pulmonary disease (COPD), AMs contribute to emphysematous alveolar destruction through mechanisms that are poorly understood. We hypothesized that damaging effects of AMs in emphysema result from loss of mechanisms that normally restrain homeostatic AMs. We found that Siglec-F, a common marker of AMs, exerts suppressive effects on tissue resident AMs (RAMs), thereby supporting alveolar integrity in mouse lungs during health and in a model of elastase-induced alveolar destruction. Siglec-F-deficient mice exhibited decreased alveolar numbers at baseline, and they had worsened alveolar damage after elastase challenge that was unexpectedly RAM-mediated. Transcriptomic profiling revealed dysregulation of key pathways involved in tissue remodeling and repair, including extracellular matrix degradation, TGF-β signaling, and phagocytosis. These findings uncover previously unrecognized roles for RAMs and Siglec-F in preserving alveolar integrity. Our findings implicate RAMs and Siglecs as potential therapeutic targets for preserving alveolar integrity in health and for limiting alveolar damage in COPD.

Respiratory tract infections pose considerable global health challenges. Upper airway colonization is pivotal to these infections, including those caused by Bordetella species. We identified an oligosaccharide, bordetellae colonization oligosaccharide (b-Cool), crucial for early nasal colonization of Bordetella bronchiseptica . We characterized the structure of b-Cool by LC-MS and NMR and found that it is prevalent across a diverse range of bordetellae, including Bordetella pertussis , which causes whooping cough in humans. A B. bronchiseptica mutant lacking b-Cool (Δ b-Cool ) showed significantly delayed and decreased colonization in mouse nasopharynx and nasal epithelia, resulting in decreased transmission. The colonization defect of Δ b-Cool was rescued in mucin deficient mice, suggesting that b-Cool may facilitate colonization in the presence of airway mucins.

Mucus is a crucial component of airway host defense. For optimal protection, its chief components—the mucins MUC5AC and MUC5B—need to be tightly regulated. Their expression localizes to specific secretory epithelial cell types capable of producing and secreting massive glycopolymers. In asthma, abnormal mucus is an important clinical problem that is effectively treated with therapies that directly target mucins. This review summarizes what is known about how mucin gene regulation, protein synthesis, and secretion are regulated in healthy and asthmatic lungs. Ultimately, a better understanding of these processes could help identify novel ways of preventing or reversing airway mucus dysfunction.

Mucus hypersecretion is an important pathological problem in respiratory diseases. Mucus accumulates in the airways of people with asthma and contributes to airflow limitation by forming plugs that occlude airways. Current treatments have minimal effects on mucus or its chief components, the polymeric mucin glycoproteins MUC5AC and MUC5B. This treatment gap reflects a poor molecular understanding of mucins that could be used to determine how they contribute to airway obstruction. Because of the prominence of glycosylation as a defining characteristic of mucins, we investigated characteristics of mucin glycans in asthma and in a mouse model of allergic asthma. Mucin fucosylation was observed in asthma, and in healthy mice it was induced as part of a mucous metaplastic response to allergic inflammation. In allergically inflamed mouse airways, mucin fucosylation was dependent on the enzyme fucosyltransferase 2. Fut2 gene–deficient mice were protected from asthma-like airway hyperreactivity and mucus plugging. These findings provide mechanistic and translational links between observations in human asthma and a mouse model that may help improve therapeutic targeting of airway mucus.