Acidic mammalian chitinase is not a critical target for allergic airway disease

Fungal chitin is not an independent mediator of allergic fungal asthma severity

Chitin, a polysaccharide found in the fungal cell wall and the exoskeletons of house dust mites and cockroaches, has garnered attention as a potential immunoreactive allergen. Mammals have evolved to express chitin-degrading chitinases (acidic mammalian chitinase/AMCase and chitotriosidase) that may modulate immune responses to chitin. We have previously reported that mice deficient in AMCase (Chia-/-) demonstrated better lung function during allergic fungal asthma. As expected, we show that mice overexpressing AMCase (SPAM mice) had worse airway hyperreactivity (AHR) during allergic fungal asthma. We further demonstrate that chitin-positive Aspergillus fumigatus conidia are detectable in the allergic lung during chronic exposure. Lung function in Chia-/- and SPAM mice is directly correlated with the level of chitinase activity during chronic fungal exposure (Chia-/- mice, negligible chitinase activity, lower AHR; SPAM mice, heightened chitinase activity, higher AHR), suggesting that the breakdown of chitin promoted AHR. However, chronic exposure of normal mice to purified A. fumigatus chitin resulted in only moderate inflammatory changes in the lung that were not sufficient to induce AHR. Moreover, despite having dramatic differences in chitinase activity, chronic exposure of Chia-/- and SPAM mice to purified A. fumigatus chitin likewise did not modulate AHR. Collectively, these results indicate that chronic exposure to fungal chitin alone is incapable of driving AHR. Furthermore, our data suggest that the chitinase-mediated degradation of chitin associated with A. fumigatus conidia may facilitate unmasking and/or liberation of other fungal cell wall components that drive inflammatory responses that contribute to AHR.NEW & NOTEWORTHY Humans with asthma sensitized to fungi often have more severe asthma than those who are not fungal-sensitized. Chitin makes up a significant portion of the cell wall of fungi and has been implicated as a pathogenic factor in allergic asthma. Ellis et al. demonstrate that chronic exposure to fungal chitin alone is unable to modulate lung function, even in the presence of differential lung chitinase activity.

Evolutionary activation of acidic chitinase in herbivores through the H128R mutation in ruminant livestock

Placental mammals' ancestors were insectivores, suggesting that modern mammals may have inherited the ability to digest insects. Acidic chitinase (Chia) is a crucial enzyme hydrolyzing significant component of insects' exoskeleton in many species. On the other hand, herbivorous animal groups, such as cattle, have extremely low chitinase activity compared to omnivorous species, e.g., mice. The low activity of cattle Chia has been attributed to R128H mutation. The presence of either of these amino acids correlates with the feeding behavior of different bovid species with R and H determining the high and low enzymatic activity, respectively. Evolutionary analysis indicated that selective constraints were relaxed in 67 herbivorous Chia in Cetartiodactyla. Despite searching for another Chia paralog that could compensate for the reduced chitinase activity, no active paralogs were found in this order. Herbivorous animals' Chia underwent genetic alterations and evolved into a molecule with low activity due to the chitin-free diet.

Discovery and Optimization of a Novel Macrocyclic Amidinourea Series Active as Acidic Mammalian Chitinase Inhibitors

Our research group has been involved for a long time in the development of macrocyclic amidinoureas (MCAs) as antifungal agents. The mechanistic investigation drove us to perform an in silico target fishing study, which allowed the identification of chitinases as one of their putative targets, with 1a showing a submicromolar inhibition of Trichoderma viride chitinase. In this work, we investigated the possibility to further inhibit the corresponding human enzymes, acidic mammalian chitinase (AMCase) and chitotriosidase (CHIT1), involved in several chronic inflammatory lung diseases. Thus, we first validated the inhibitory activity of 1a against AMCase and CHIT1 and then designed and synthesized new derivatives aimed at improving the potency and selectivity against AMCase. Among them, compound 3f emerged for its activity profile along with its promising in vitro ADME properties. We also gained a good understanding of the key interactions with the target enzyme through in silico studies.

Chitinases and chitinase-like proteins in asthma

Despite the lack of endogenous chitin synthesis, mammalian genomes encode two enzymatically active true chitinases (chitotriosidase and acidic mammalian chitinase) and a variable number of chitinase-like proteins (CLPs) that have no enzyme activity but bind chitin. Chitinases and CLPs are prominent components of type-2 immune response-mediated respiratory diseases. However, despite extensive research into their role in allergic airway disease, there is still no agreement on whether they are mere biomarkers of disease or actual disease drivers. Functions ascribed to chitinases and CLPs include, but are not limited to host defense against chitin-containing pathogens, directly promoting inflammation, and modulating tissue remodeling and fibrosis. Here, we discuss in detail the chitin-dependent and -independent roles of chitinases and CLPs in the context of allergic airway disease, and recent advances and emerging concepts in the field that might identify opportunities for new therapies.

Inhibition of CHIT1 as a novel therapeutic approach in idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive and eventually fatal lung disease with a complex etiology. Approved drugs, nintedanib and pirfenidone, modify disease progression, but IPF remains incurable and there is an urgent need for new therapies. We identified chitotriosidase (CHIT1) as new driver of fibrosis in IPF and a novel therapeutic target. We demonstrate that CHIT1 activity and expression are significantly increased in serum (3-fold) and induced sputum (4-fold) from IPF patients. In the lungs CHIT1 is expressed in a distinct subpopulation of profibrotic, disease-specific macrophages, which are only present in patients with ILDs and CHIT1 is one of the defining markers of this fibrosis-associated gene cluster. To define CHIT1 role in fibrosis, we used the therapeutic protocol of the bleomycin-induced pulmonary fibrosis mouse model. We demonstrate that in the context of chitinase induction and the macrophage-specific expression of CHIT1, this model recapitulates lung fibrosis in ILDs. Genetic inactivation of Chit1 attenuated bleomycin-induced fibrosis (decreasing the Ashcroft scoring by 28%) and decreased expression of profibrotic factors in lung tissues. Pharmacological inhibition of chitinases by OATD-01 reduced fibrosis and soluble collagen concentration. OATD-01 exhibited anti-fibrotic activity comparable to pirfenidone resulting in the reduction of the Ashcroft score by 32% and 31%, respectively. These studies provide a preclinical proof-of-concept for the antifibrotic effects of OATD-01 and establish CHIT1 as a potential new therapeutic target for IPF.

Occupational exposure to gasoline in gasoline station male attendants promotes M1 polarization in macrophages

Several studies have reported the toxicological implications of exposure to petroleum hydrocarbon fumes in animal models. There is little documentation on the effect of such exposure on oxidative stress levels and immune response. To our knowledge, no documentation of M1 polarization in macrophages in gasoline station male attendants. Therefore, this study aimed to evaluate the harmful effects of gasoline vapors in 62 male attendants (16-70 years) compared to 29 age- and sex-matched-unexposed controls. The attendants were recruited from Damietta governorate gasoline stations. Gasoline exposure induced a significant increase in tumor necrosis factor-α (TNF-α) level (p < 0.05) as well as a slight but non-significant increase in the activity of acidic mammalian chitinase (AMCase) (p > 0.05). Further TNF-α/AMCase ratio was significantly increased (p < 0.01) in sera of the attendants when compared to those of the healthy controls. Also, the total leucocytic and lymphocytic counts were significantly increased (p < 0.01 and p < 0.001, respectively). On contrary, neutrophils to lymphocytes ratio (NLR) and platelets to lymphocytes ratio (PLR) were significantly decreased (p < 0.05 and p < 0.001, respectively). In addition, significant reduction in hemoglobin (Hb) concentration, plasma glutathione reduced form (GSH), and catalase, as well as superoxide dismutase (SOD) activities in red blood cells were observed in the exposed attendants. As a result, malondialdehyde (MDA), nitric oxide (NO) levels, and NO/AMCase ratio were significantly increased (p < 0.05). In conclusion, this study inferred that prolonged gasoline exposure can mediate immune activation, especially M1 macrophages polarization, possibly via oxidative stress-mediated mechanism.

Mouse Acidic Chitinase Effectively Degrades Random-Type Chitosan to Chitooligosaccharides of Variable Lengths under Stomach and Lung Tissue pH Conditions

Chitooligosaccharides exhibit several biomedical activities, such as inflammation and tumorigenesis reduction in mammals. The mechanism of the chitooligosaccharides’ formation in vivo has been, however, poorly understood. Here we report that mouse acidic chitinase (Chia), which is widely expressed in mouse tissues, can produce chitooligosaccharides from deacetylated chitin (chitosan) at pH levels corresponding to stomach and lung tissues. Chia degraded chitin to produce N-acetyl-d-glucosamine (GlcNAc) dimers. The block-type chitosan (heterogenous deacetylation) is soluble at pH 2.0 (optimal condition for mouse Chia) and was degraded into chitooligosaccharides with various sizes ranging from di- to nonamers. The random-type chitosan (homogenous deacetylation) is soluble in water that enables us to examine its degradation at pH 2.0, 5.0, and 7.0. Incubation of these substrates with Chia resulted in the more efficient production of chitooligosaccharides with more variable sizes was from random-type chitosan than from the block-type form of the molecule. The data presented here indicate that Chia digests chitosan acquired by homogenous deacetylation of chitin in vitro and in vivo. The degradation products may then influence different physiological or pathological processes. Our results also suggest that bioactive chitooligosaccharides can be obtained conveniently using homogenously deacetylated chitosan and Chia for various biomedical applications.

Physiological and pathophysiological roles of acidic mammalian chitinase (CHIA) in multiple organs

Acidic mammalian chitinase (CHIA) belongs to the 18-glycosidase family and is expressed in epithelial cells and certain immune cells (such as neutrophils and macrophages) in various organs. Under physiological conditions, as a hydrolase, CHIA can degrade chitin-containing pathogens, participate in Type 2 helper T (Th2)-mediated inflammation, and enhance innate and adaptive immunity to pathogen invasion. Under pathological conditions, such as rhinitis, ocular conjunctivitis, asthma, chronic atrophic gastritis, type 2 diabetes, and pulmonary interstitial fibrosis, CHIA expression is significantly changed. In addition, studies have shown that CHIA has an anti-apoptotic effect, promotes epithelial cell proliferation and maintains organ integrity, and these effects are not related to chitinase degradation. CHIA can also be used as a biomolecular marker in diseases such as chronic atrophic gastritis, dry eye, and acute kidney damage caused by sepsis. Analysis of the authoritative TCGA database shows that CHIA expression in gastric adenocarcinoma, liver cancer, renal clear cell carcinoma and other tumors is significantly downregulated compared with that in normal tissues, but the specific mechanism is unclear. This review is based on all surveys conducted to date and summarizes the expression patterns and functional diversity of CHIA in various organs. Understanding the physiological and pathophysiological relevance of CHIA in multiple organs opens new possibilities for disease treatment.

Aspergillus fumigatus corneal infection is regulated by chitin synthases and by neutrophil-derived acidic mammalian chitinase

Aspergillus fumigatus is an important cause of pulmonary and systemic infections in immune compromised individuals, and of corneal ulcers and blindness in immune competent patients. To examine the role of chitin synthases in Aspergillus corneal infection, we analyzed Aspergillus mutants of chitin synthase family 1 and family 2, and found that compared with the parent strain, the quadruple mutants from both families were more readily killed by neutrophils in vitro, and that both also exhibited impaired hyphal growth in the cornea. Further, inhibition of chitin synthases using Nikkomycin Z enhanced neutrophil killing in vitro and in vivo in a murine model of A. fumigatus corneal infection. Acidic mammalian chitinase (AMCase) is mostly produced by macrophages in asthmatic lungs; however, we now demonstrate that neutrophils are a major source of AMCase, which inhibits hyphal growth. In A. fumigatus corneal infection, neutrophils are the major source of AMCase, and addition of AMCase inhibitors or adoptive transfer of neutrophils from AMCase^-/- mice resulted in impaired hyphal killing. Together, these findings identify chitin synthases as important fungal virulence factors and neutrophil-derived AMCase as an essential mediator of host defense.

High expression of acidic chitinase and chitin digestibility in the stomach of common marmoset (Callithrix jacchus), an insectivorous nonhuman primate

Chitin is a polymer of N-acetyl-D-glucosamine (GlcNAc) and a main constituent of insects' exoskeleton. Insects are rich in protein with high energy conversion efficiency. Recently, we have reported that acidic chitinases (Chia) act as digestive enzymes in mouse, pig and chicken (omnivorous) but not in dog (carnivorous) and bovine (herbivorous), indicating that feeding behavior affects Chia expression levels, and determines chitin digestibility in the particular animals. Common marmoset (Callithrix jacchus) belongs to New World monkey family and provides a potential bridge between mouse models and human diseases. Common marmoset is an insectivorous nonhuman primate with unknown expression levels and enzymatic functions of the Chia homologue, CHIA. Here, we report that common marmoset highly expresses pepsin-, trypsin- and chymotrypsin-resistant CHIA in the stomach. We show that CHIA is most active at pH 2.0 and degrades chitin and mealworm shells into GlcNAc dimers under gastrointestinal conditions. Although common marmoset and crab-eating monkey (Old World monkey) have two CHIA genes in their genomes, they primarily express one gene in the stomach. Thus, this study is the first to investigate expression levels and enzymatic functions of CHIA in a New World primate, contributing to the understanding of dietary adaptation and digestion in this taxon.

Human Chitinases: Structure, Function, and Inhibitor Discovery

Chitinases are glycosyl hydrolases that hydrolyze the β-(1-4)-linkage of N-acetyl-D-glucosamine units present in chitin polymers. Chitinases are widely distributed enzymes and are present in a wide range of organisms including insects, plants, bacteria, fungi, and mammals. These enzymes play key roles in immunity, nutrition, pathogenicity, and arthropod molting. Humans express two chitinases, chitotriosidase 1 (CHIT1) and acid mammalian chitinase (AMCase) along with several chitinase-like proteins (CLPs). Human chitinases are reported to play a protective role against chitin-containing pathogens through their capability to degrade chitin present in the cell wall of pathogens. Now, human chitinases are gaining attention as the key players in innate immune response. Although the exact mechanism of their role in immune response is not known, studies in recent years begin to relate chitin recognition and degradation with the activation of signaling pathways involved in inflammation. The roles of both CHIT1 and AMCase in the development of various diseases have been revealed and several classes of inhibitors have been developed. However, a clear understanding could not be established due to complexities in the design of the right experiment for studying the role of human chitinase in various diseases. In this chapter, we will first outline the structural features of CHIT1 and AMcase. We will then review the progress in understanding the role of human chitinases in the development of various diseases. Finally, we will summarize the inhibitor discovery efforts targeting both CHIT1 and AMCase.

L. Chitins and chitinase activity in airway diseases

Chitin, one of the most abundant biopolymers on Earth, is bound and degraded by chitinases, specialized enzymes that are similarly widespread in nature. Chitin catabolism affects global carbon and nitrogen cycles through a host of diverse biological processes, but recent work has focused attention on systems of chitin recognition and degradation conserved in mammals, connecting an ancient pathway of polysaccharide processing to human diseases influenced by persistent immune triggering. Here we review current advances in our understanding of how chitin-chitinase interactions affect mucosal immune feedback mechanisms essential to maintaining homeostasis and organ health.

Acidic Mammalian Chitinase Negatively Affects Immune Responses during Acute and Chronic Aspergillus fumigatus Exposure

Chitin is a polysaccharide that provides structure and rigidity to the cell walls of fungi and insects. Mammals possess multiple chitinases, which function to degrade chitin, thereby supporting a role for chitinases in immune defense. However, chitin degradation has been implicated in the pathogenesis of asthma. Here, we determined the impact of acidic mammalian chitinase (AMCase) (Chia) deficiency on host defense during acute exposure to the fungal pathogen Aspergillus fumigatus as well as its contribution to A. fumigatus-associated allergic asthma. We demonstrate that chitin in the fungal cell wall was detected at low levels in A. fumigatus conidia, which emerged at the highest level during hyphal transition. In response to acute A. fumigatus challenge, Chia^-/- mice unexpectedly demonstrated lower A. fumigatus lung burdens at 2 days postchallenge. The lower fungal burden correlated with decreased lung interleukin-33 (IL-33) levels yet increased IL-1β and prostaglandin E₂ (PGE₂) production, a phenotype that we reported previously to promote the induction of IL-17A and IL-22. During chronic A. fumigatus exposure, AMCase deficiency resulted in lower dynamic and airway lung resistance than in wild-type mice. Improved lung physiology correlated with attenuated levels of the proallergic chemokines CCL17 and CCL22. Surprisingly, examination of inflammatory responses during chronic exposure revealed attenuated IL-17A and IL-22 responses, but not type 2 responses, in the absence of AMCase. Collectively, these data suggest that AMCase functions as a negative regulator of immune responses during acute fungal exposure and is a contributor to fungal asthma severity, putatively via the induction of proinflammatory responses.

C-Type Lectin Receptors in Asthma

Asthma is a heterogeneous disease that affects approximately 300 million people worldwide, largely in developed countries. The etiology of the disease is poorly understood, but is likely to involve specific innate and adaptive responses to inhaled microbial components that are found in allergens. Fungal-derived allergens represent a major contributing factor in the initiation, persistence, exacerbation, and severity of allergic asthma. C-type lectin like receptors, such as dectin-1, dectin-2, DC-specific intercellular adhesion molecule 3-grabbing nonintegrin, and mannose receptor, recognize many fungal-derived allergens and other structurally similar allergens derived from house dust mites (HDM). In some cases, the fungal derived allergens have been structurally and functionally identified alongside their respective receptors in both humans and mice. In this review, we discuss recent understanding on how selected fungal and HDM derived allergens as well as their known or unknown receptors shape allergic airway diseases.

Targeting Acidic Mammalian chitinase Is Effective in Animal Model of Asthma

This article highlights our work toward the identification of a potent, selective, and efficacious acidic mammalian chitinase (AMCase) inhibitor. Rational design, guided by X-ray analysis of several inhibitors bound to human chitotriosidase (hCHIT1), led to the identification of compound 7f as a highly potent AMCase inhibitor (IC₅₀ values of 14 and 19 nM against human and mouse enzyme, respectively) and selective (>150× against mCHIT1) with very good PK properties. This compound dosed once daily at 30 mg/kg po showed significant anti-inflammatory efficacy in HDM-induced allergic airway inflammation in mice, reducing inflammatory cell influx in the BALF and total IgE concentration in plasma, which correlated with decrease of chitinolytic activity. Therapeutic efficacy of compound 7f in the clinically relevant aeroallergen-induced acute asthma model in mice provides a rationale for developing AMCase inhibitor for the treatment of asthma.

Protease resistance of porcine acidic mammalian chitinase under gastrointestinal conditions implies that chitin-containing organisms can be sustainable dietary resources

Chitin, a polymer of N-acetyl-D-glucosamine (GlcNAc), is a major structural component in chitin-containing organism including crustaceans, insects and fungi. Mammals express two chitinases, chitotriosidase (Chit1) and acidic mammalian chitinase (AMCase). Here, we report that pig AMCase is stable in the presence of other digestive proteases and functions as chitinolytic enzyme under the gastrointestinal conditions. Quantification of chitinases expression in pig tissues using quantitative real-time PCR showed that Chit1 mRNA was highly expressed in eyes, whereas the AMCase mRNA was predominantly expressed in stomach at even higher levels than the housekeeping genes. AMCase purified from pig stomach has highest activity at pH of around 2–4 and remains active at up to pH 7.0. It was resistant to robust proteolytic activities of pepsin at pH 2.0 and trypsin and chymotrypsin at pH 7.6. AMCase degraded polymeric chitin substrates including mealworm shells to GlcNAc dimers. Furthermore, we visualized chitin digestion of fly wings by endogenous AMCase and pepsin in stomach extract. Thus, pig AMCase can function as a protease resistant chitin digestive enzyme at broad pH range present in stomach as well as in the intestine. These results indicate that chitin-containing organisms may be a sustainable feed ingredient in pig diet.

Type 2 immunity in tissue repair and fibrosis

Type 2 immunity is characterized by the production of IL-4, IL-5, IL-9 and IL-13, and this immune response is commonly observed in tissues during allergic inflammation or infection with helminth parasites. However, many of the key cell types associated with type 2 immune responses - including T helper 2 cells, eosinophils, mast cells, basophils, type 2 innate lymphoid cells and IL-4- and IL-13-activated macrophages - also regulate tissue repair following injury. Indeed, these cell populations engage in crucial protective activity by reducing tissue inflammation and activating important tissue-regenerative mechanisms. Nevertheless, when type 2 cytokine-mediated repair processes become chronic, over-exuberant or dysregulated, they can also contribute to the development of pathological fibrosis in many different organ systems. In this Review, we discuss the mechanisms by which type 2 immunity contributes to tissue regeneration and fibrosis following injury.

Gastric and intestinal proteases resistance of chicken acidic chitinase nominates chitin-containing organisms for alternative whole edible diets for poultry

Chitin, a polymer of N-acetyl-D-glucosamine (GlcNAc), functions as a major structural component in crustaceans, insects and fungi and is the second most abundant polysaccharide in the nature. Although these chitin-containing organisms have been suggested as novel animal feed resources, chitin has long been considered as indigestible fibers in the animal body. Recently, we reported that acidic chitinase (Chia) is a protease-resistant major glycosidase in mouse gastrointestinal tract (GIT) and that it digests chitin in the mouse stomach. However, the physiological role of Chia in other animals including poultry remains unknown. Here, we report that Chia can function as a digestive enzyme that breaks down chitin-containing organisms in chicken GIT. Chia mRNA is predominantly expressed in the glandular stomach tissue in normal chicken. We also show that chicken Chia has a robust chitinolytic activity at pH 2.0 and is highly resistant to proteolysis by pepsin and trypsin/chymotrypsin under conditions mimicking GIT. Chia degraded shells of mealworm larvae in the presence of digestive proteases and produced (GlcNAc)₂. Thus, functional similarity of chicken Chia with the mouse enzyme suggests that chitin-containing organisms can be used for alternative poultry diets not only as whole edible resources but also as enhancers of their nutritional value.

Spontaneous Chitin Accumulation in Airways and Age-Related Fibrotic Lung Disease

The environmentally widespread polysaccharide chitin is degraded and recycled by ubiquitous bacterial and fungal chitinases. Although vertebrates express active chitinases from evolutionarily conserved loci, their role in mammalian physiology is unclear. We show that distinct lung epithelial cells secrete acidic mammalian chitinase (AMCase), which is required for airway chitinase activity. AMCase-deficient mice exhibit premature morbidity and mortality, concomitant with accumulation of environmentally derived chitin polymers in the airways and expression of pro-fibrotic cytokines. Over time, these mice develop spontaneous pulmonary fibrosis, which is ameliorated by restoration of lung chitinase activity by genetic or therapeutic approaches. AMCase-deficient epithelial cells express fibrosis-associated gene sets linked with cell stress pathways. Mice with lung fibrosis due to telomere dysfunction and humans with interstitial lung disease also accumulate excess chitin polymers in their airways. These data suggest that altered chitin clearance could exacerbate fibrogenic pathways in the setting of lung diseases characterized by epithelial cell dysfunction.

Acidic chitinase primes the protective immune response to gastrointestinal nematodes

Acidic mammalian chitinase (AMCase) is known to be induced by allergens and helminths, yet its role in immunity is unclear. Using AMCase-deficient mice, we show that AMCase deficiency reduced the number of group 2 innate lymphoid cells during allergen challenge but was not required for establishment of type 2 inflammation in the lung in response to allergens or helminths. In contrast, AMCase-deficient mice showed a profound defect in type 2 immunity following infection with the chitin-containing gastrointestinal nematodes Nippostrongylus brasiliensis and Heligmosomoides polygyrus bakeri. The impaired immunity was associated with reduced mucus production and decreased intestinal expression of the signature type 2 response genes Il13, Chil3, Retnlb, and Clca1. CD103(+) dendritic cells, which regulate T cell homing, were also reduced in mesenteric lymph nodes of infected AMCase-deficient mice. Thus, AMCase functions as a critical initiator of protective type 2 responses to intestinal nematodes but is largely dispensable for allergic responses in the lung.

AMCase is a crucial regulator of type 2 immune responses to inhaled house dust mites

Chitinases are enzymes that cleave chitin, a component of the exoskeleton of many organisms including the house dust mite (HDM). Here we show that knockin mice expressing an enzymatically inactive acidic mammalian chitinase (AMCase), the dominant true chitinase in mouse lung, showed enhanced type 2 immune responses to inhaled HDM. We found that uncleaved chitin promoted the release of IL-33, whereas cleaved chitin could be phagocytosed and could induce the activation of caspase-1 and subsequent activation of caspase-7; this results in the resolution of type 2 immune responses, probably by promoting the inactivation of IL-33. These data suggest that AMCase is a crucial regulator of type 2 immune responses to inhaled chitin-containing aeroallergens.

Chitin recognition via chitotriosidase promotes pathologic type-2 helper T cell responses to cryptococcal infection

Pulmonary mycoses are often associated with type-2 helper T (Th2) cell responses. However, mechanisms of Th2 cell accumulation are multifactorial and incompletely known. To investigate Th2 cell responses to pulmonary fungal infection, we developed a peptide-MHCII tetramer to track antigen-specific CD4+ T cells produced in response to infection with the fungal pathogen Cryptococcus neoformans. We noted massive accruement of pathologic cryptococcal antigen-specific Th2 cells in the lungs following infection that was coordinated by lung-resident CD11b+ IRF4-dependent conventional dendritic cells. Other researchers have demonstrated that this dendritic cell subset is also capable of priming protective Th17 cell responses to another pulmonary fungal infection, Aspergillus fumigatus. Thus, higher order detection of specific features of fungal infection by these dendritic cells must direct Th2 cell lineage commitment. Since chitin-containing parasites commonly elicit Th2 responses, we hypothesized that recognition of fungal chitin is an important determinant of Th2 cell-mediated mycosis. Using C. neoformans mutants or purified chitin, we found that chitin abundance impacted Th2 cell accumulation and disease. Importantly, we determined Th2 cell induction depended on cleavage of chitin via the mammalian chitinase, chitotriosidase, an enzyme that was also prevalent in humans experiencing overt cryptococcosis. The data presented herein offers a new perspective on fungal disease susceptibility, whereby chitin recognition via chitotriosidase leads to the initiation of harmful Th2 cell differentiation by CD11b+ conventional dendritic cells in response to pulmonary fungal infection.

Humans often inhale potentially pathogenic fungi in the environment. While CD4+ helper T (Th) cells are required for protection against invasive disease, a subset of Th cells, called Th2 cells, are associated with increased mortality and allergy/asthma morbidity. Our study aimed to unravel the cellular and molecular basis of pulmonary Th2 cell induction in response to lethal infection with Cryptococcus neoformans. Antigen-presenting cells coordinate naïve Th cell priming and differentiation, but the precise leukocyte responsible for Th2 cell expansion to pulmonary cryptococcal infection has not been determined. Using an experimental mouse model of pulmonary cryptococcosis, we show that a subset of lung-resident dendritic cells is uniquely required for Th2 cell induction. We additionally sought to identify the molecular signal received by the host that allows dendritic cells to selectively induce Th2 cells. Since parasites and fungi elicit Th2 cell responses and both produce chitin, a molecule not found in vertebrates, we hypothesized that recognition of fungal chitin is a determinant of fungal disease. Here, we demonstrate that C. neoformans chitin and the host-derived chitinase, chitotriosidase, promote Th2 cell accumulation and disease. These findings highlight a promising target of next generation therapies aimed at limiting immunopathology caused by pulmonary fungal infection.

Targeting AMCase reduces esophageal eosinophilic inflammation and remodeling in a mouse model of egg induced eosinophilic esophagitis

Studies of AMCase inhibition in mouse models of lung eosinophilic inflammation have produced conflicting results with some studies demonstrating inhibition of eosinophilic inflammation and others not. No studies have investigated the role of AMCase inhibition in eosinophilic esophagitis (EoE). We have used a mouse model of egg (OVA) induced EoE to determine whether pharmacologic inhibition of AMCase with allosamidin reduced eosinophilic inflammation and remodeling in the esophagus in EoE. Administration of intra-esophageal OVA for 6weeks to BALB/c mice induced increased levels of esophageal eosinophils, mast cells, and features of esophageal remodeling (fibrosis, basal zone hyperplasia, deposition of the extracellular matrix protein fibronectin). Administration of intraperitoneal (ip) allosamidin to BALB/c mice significantly inhibited AMCase enzymatic activity in the esophagus. Pharmacologic inhibition of AMCase with ip allosamidin inhibited both OVA induced increases in esophageal eosinophilic inflammation and OVA induced esophageal remodeling (fibrosis, epithelial basal zone hyperplasia, extracellular matrix deposition of fibronectin). This inhibition of eosinophilic inflammation in the esophagus by ip allosamidin was associated with reduced eotaxin-1 expression in the esophagus. Oral allosamidin inhibited eosinophilic inflammation in the epithelium but did not inhibit esophageal remodeling. These studies suggest that pharmacologic inhibition of AMCase results in inhibition of eosinophilic inflammation and remodeling in the esophagus in a mouse model of egg induced EoE partially through effects in the esophagus on reducing chemokines (i.e. eotaxin-1) implicated in the pathogenesis of EoE.

From bacteria to human: a journey into the world of chitinases

Chitinases, the enzymes responsible for the biological degradation of chitin, are found in a wide range of organisms from bacteria to higher plants and animals. They participate in numerous physiological processes such as nutrition, parasitism, morphogenesis and immunity. Many organisms, in addition to chitinases, produce inactive chitinase-like lectins that despite lacking enzymatic activity are involved in several regulatory functions. Most known chitinases belong to families 18 and 19 of glycosyl hydrolases, however a few chitinases that belong to families 23 and 48 have also been identified in recent years. In this review, different aspects of chitinases and chi-lectins from bacteria, fungi, insects, plants and mammals are discussed.

An IL-4/IL-13 dual antagonist reduces lung inflammation, airway hyperresponsiveness, and IgE production in mice

IL-4 and IL-13 comprise promising targets for therapeutic interventions in asthma and other Th2-associated diseases, but agents targeting either IL-4 or IL-13 alone have shown limited efficacy in human clinical studies. Because these cytokines may involve redundant function, dual targeting holds promise for achieving greater efficacy. We describe a bifunctional therapeutic targeting IL-4 and IL-13, developed by a combination of specific binding domains. IL-4-targeted and IL-13-targeted single chain variable fragments were joined in an optimal configuration, using appropriate linker regions on a novel protein scaffold. The bifunctional IL-4/IL-13 antagonist displayed high affinity for both cytokines. It was a potent and efficient neutralizer of both murine IL-4 and murine IL-13 bioactivity in cytokine-responsive Ba/F3 cells, and exhibited a half-life of approximately 4.7 days in mice. In a murine model of ovalbumin-induced ear swelling, the bifunctional molecule blocked both the IL-4/IL-13-dependent early-phase response and the IL-4-dependent late-phase response. In the ovalbumin-induced lung inflammation model, the bifunctional IL-4/IL-13 antagonist reduced the IL-4-dependent rise in serum IgE titers, and reduced IL-13-dependent airway hyperresponsiveness, lung inflammation, mucin gene expression, and serum chitinase responses. Taken together, these findings demonstrate the effective dual blockade of IL-4 and IL-13 with a single agent, which resulted in the modulation of a more extensive range of endpoints than could be achieved by targeting either cytokine alone.

PD-L2 modulates asthma severity by directly decreasing dendritic cell IL-12 production

Studies examining the role of programmed death 1 (PD-1) ligand 2 (PD-L2)/PD-1 in asthma have yielded conflicting results. To clarify its role, we examined the PD-L2 expression in biopsies from human asthmatics and the lungs of aeroallergen-treated mice. PD-L2 expression in bronchial biopsies correlated with the severity of asthma. In mice, allergen exposure increased PD-L2 expression on pulmonary myeloid dendritic cells (DCs), and PD-L2 blockade diminished allergen-induced airway hyperresponsiveness (AHR). By contrast, PD-1 blockade had no impact, suggesting that PD-L2 promotes AHR in a PD-1-independent manner. Decreased AHR was associated with enhanced serum interleukin (IL)-12 p40, and in vitro stimulation of DCs with allergen and PD-L2-Fc reduced IL-12 p70 production, suggesting that PD-L2 inhibits allergen-driven IL-12 production. In our model, IL-12 did not diminish T helper type 2 responses but rather directly antagonized IL-13-inducible gene expression, highlighting a novel role for IL-12 in regulation of IL-13 signaling. Thus, allergen-driven enhancement of PD-L2 signaling through a PD-1-independent mechanism limits IL-12 secretion, exacerbating AHR.

Innate immune responses in house dust mite allergy

Sensitizations to house dust mites (HDM) trigger strong exacerbated allergen-induced inflammation of the skin and airways mucosa from atopic subjects resulting in atopic dermatitis as well as allergic rhinitis and asthma. Initially, the Th2-biased HDM allergic response was considered to be mediated only by allergen B- and T-cell epitopes to promote allergen-specific IgE production as well as IL-4, IL-5, and IL-13 to recruit inflammatory cells. But this general molecular model of HDM allergenicity must be revisited as a growing literature suggests that stimulations of innate immune activation pathways by HDM allergens offer new answers to the following question: what makes an HDM allergen an allergen? Indeed, HDM is a carrier not only for allergenic proteins but also microbial adjuvant compounds, both of which are able to stimulate innate signaling pathways leading to allergy. This paper will describe the multiple ways used by HDM allergens together with microbial compounds to control the initiation of the allergic response through engagement of innate immunity.

Chitinase dependent control of protozoan cyst burden in the brain

Chronic infections represent a continuous battle between the host's immune system and pathogen replication. Many protozoan parasites have evolved a cyst lifecycle stage that provides it with increased protection from environmental degradation as well as endogenous host mechanisms of attack. In the case of Toxoplasma gondii, these cysts are predominantly found in the immune protected brain making clearance of the parasite more difficult and resulting in a lifelong infection. Currently, little is known about the nature of the immune response stimulated by the presence of these cysts or how they are able to propagate. Here we establish a novel chitinase-dependent mechanism of cyst control in the infected brain. Despite a dominant Th1 immune response during Toxoplasma infection there exists a population of alternatively activated macrophages (AAMØ) in the infected CNS. These cells are capable of cyst lysis via the production of AMCase as revealed by live imaging, and this chitinase is necessary for protective immunity within the CNS. These data demonstrate chitinase activity in the brain in response to a protozoan pathogen and provide a novel mechanism to facilitate cyst clearance during chronic infections.

Described here is a novel mechanism of protozoan cyst clearance in the CNS during chronic infection. These data show the presence of a population of alternatively activated macrophages in the brain that secrete the active chitinase, AMCase, in response to chitin in the cyst wall. Using both chemical and genetic inhibition in vitro, it is revealed that this enzyme is required for efficient degradation and destruction of the cyst. The necessity for AMCase is demonstrated in vivo, as the absence of the enzyme resulted in a significant increase in cyst burden and decrease in survival during chronic infection. Together, these data identify an important mechanism of parasite control and cyst clearance in the CNS. Currently, no therapies exist that lead to the total clearance of this parasite from the brain. Therefore, developing an understanding of the natural mechanisms of cyst clearance has the potential to lead to new and effective therapies for this and other chronic infections.

Developing inhibitors of glycan processing enzymes as tools for enabling glycobiology

Glycoconjugates are ubiquitous biomolecules found in all kingdoms of life. These diverse structures are metabolically responsive and occur in a cell line- and protein-specific manner, conferring tissue type-specific properties. Glycans have essential roles in diverse processes, including, for example, intercellular signaling, inflammation, protein quality control, glucohomeostasis and cellular adhesion as well as cell differentiation and proliferation. Many mysteries remain in the field, however, and uncovering the physiological roles of various glycans remains a key pursuit. Realizing this aim necessitates the ability to subtly and selectively manipulate the series of different glycoconjugates both in cells and in vivo. Selective small-molecule inhibitors of glycan processing enzymes hold great potential for such manipulation as well as for determining the function of 'orphan' carbohydrate-processing enzymes. In this review, we discuss recent advances and existing inhibitors, the prospects for small-molecule inhibitors and the challenges associated with generating high-quality chemical probes for these families of enzymes. The coordinated efforts of chemists, biochemists and biologists will be crucial for creating and characterizing inhibitors that are useful tools both for advancing a basic understanding of glycobiology in mammals as well as for validating new potential therapeutic targets within this burgeoning field.