Hexokinase dissociation from mitochondria promotes oligomerization of VDAC that facilitates NLRP3 inflammasome assembly and activation

NLRP3 inflammasome activation is a highly regulated process for controlling secretion of the potent inflammatory cytokines IL-1β and IL-18 that are essential during bacterial infection, sterile inflammation, and disease, including colitis, diabetes, Alzheimer's disease, and atherosclerosis. Diverse stimuli activate the NLRP3 inflammasome, and unifying upstream signals has been challenging to identify. Here, we report that a common upstream step in NLRP3 inflammasome activation is the dissociation of the glycolytic enzyme hexokinase 2 from the voltage-dependent anion channel (VDAC) in the outer membrane of mitochondria. Hexokinase 2 dissociation from VDAC triggers activation of inositol triphosphate receptors, leading to release of calcium from the ER, which is taken up by mitochondria. This influx of calcium into mitochondria leads to oligomerization of VDAC, which is known to form a macromolecule-sized pore in the outer membranes of mitochondria that allows proteins and mitochondrial DNA (mtDNA), often associated with apoptosis and inflammation, respectively, to exit the mitochondria. We observe that VDAC oligomers aggregate with NLRP3 during initial assembly of the multiprotein oligomeric NLRP3 inflammasome complex. We also find that mtDNA is necessary for NLRP3 association with VDAC oligomers. These data, together with other recent work, help to paint a more complete picture of the pathway leading to NLRP3 inflammasome activation.

Abstract 6409: Non-redundant mechanisms of immune resistance to radiotherapy converge on innate immunity

Multiple studies have demonstrated synergy between immune checkpoint blockade (ICB) and radiotherapy (RT) in preclinical murine models; however, randomized trials of RT/ICB have been inconsistent in patients with solid tumors. To better understand this discordance, we hypothesized that there are non-redundant inhibitory immune pathways that restrain the efficacy of RT beyond T-cell oriented immune checkpoints. To this end, we performed scRNA-seq and CITE-seq analysis of the EO771 syngeneic murine model of breast cancer to characterize the immune landscape following RT±ICB. We found that ICB reprograms the immune response to RT by shifting tumor-associated macrophages (TAMs) from a lipid-associated phenotype (APOE, FABP5) to an M1-like interferon-stimulated state (CXCL10, ISG15). However, ICB also promoted the late recruitment of intratumoral neutrophils, which drive resistance to RT in other contexts. To evaluate whether neutrophils may be limiting antitumor immunity to RT/ICB, we depleted intratumoral neutrophils using two separate antibodies, anti-Ly6G and anti-Gr-1. Compared to RT/ICB alone, both neutrophil depletion strategies enhanced tumor control and prolonged survival in advanced EO771 tumors (P<0.001). Given that indiscriminate neutrophil depletion is not a viable therapeutic strategy, we tested alternative immune targeting approaches to alter the TAM response to RT/ICB. By scRNA-seq, we found that RT strongly upregulated several innate immune checkpoints on TAMs (e.g., SIRPα, SLAMF3/7, LRP1). Accordingly, we disrupted the SIRPα-CD47 interaction with anti-CD47 antibodies and characterized the impact on response to RT/ICB. Anti-CD47 significantly improved tumor regression and survival when combined with RT/ICB (P<0.001). We then used scRNA-seq and CITE-seq to understand why disruption of SIRPα-CD47 improved antitumor responses to RT/ICB. We found that anti-CD47 depleted an entire cluster of chronically inflamed TAMs, expressing pro-inflammatory markers (IL1A, NOS2) and chemokines (CCL3, CXCL1/2/3). Furthermore, anti-CD47 reduced the recruitment of intratumoral neutrophils and depleted a cluster of pathologically activated neutrophils (PMNs), termed myeloid-derived suppressor cells (PMN-MDSCs), expressing WFDC17, PTGS2, S100A8/9. Lastly, anti-CD47 enhanced the recruitment of tumor-infiltrating lymphocytes including central memory TCF7+ T cells and CD19+ B cells. By inference and analysis of cell-cell communication (CellChat), we found that anti-CD47 strengthened the interactions between myeloid cells and T cells compared to RT/ICB alone. Collectively, our data indicate that innate immune cells, in particular neutrophils and chronically inflamed TAMs, promote resistance to RT/ICB in the EO771 model. These data suggest that inhibition of CD47-SIRPα is a promising therapeutic strategy to overcoming immune resistance through the elimination of PMN-MDSCs.

Citation Format: Anthony T. Nguyen, Tahir B. Dar, Jolene Viramontes, Satchel Stevens, Julie K. Jang, Emily Y. Ko, Diana J. Lu, Eric M. Chung, Samuel C. Zhang, Katelyn M. Atkins, Mitchell Kamrava, Howard M. Sandler, Jlenia Guarnerio, Simon Knott, Zachary S. Zumsteg, David M. Underhill, Stephen L. Shiao. Non-redundant mechanisms of immune resistance to radiotherapy converge on innate immunity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6409.

Mycobiota and diet-derived fungal xenosiderophores promote Salmonella gastrointestinal colonization

The fungal gut microbiota (mycobiota) has been implicated in diseases that disturb gut homeostasis, such as inflammatory bowel disease. However, little is known about functional relationships between bacteria and fungi in the gut during infectious colitis. Here we investigated the role of fungal metabolites during infection with the intestinal pathogen Salmonella enterica serovar Typhimurium, a major cause of gastroenteritis worldwide. We found that, in the gut lumen, both the mycobiota and fungi present in the diet can be a source of siderophores, small molecules that scavenge iron from the host. The ability to use fungal siderophores, such as ferrichrome and coprogen, conferred a competitive growth advantage to Salmonella strains expressing the fungal siderophore receptors FhuA or FhuE in vitro and in a mouse model. Our study highlights the role of inter-kingdom cross-feeding between fungi and Salmonella and elucidates an additional function of the gut mycobiota, revealing the importance of these understudied members of the gut ecosystem during bacterial infection.

Non-protective immune imprint underlies failure of Staphylococcus aureus IsdB vaccine

Humans frequently encounter Staphylococcus aureus (SA) throughout life. Animal studies have yielded SA candidate vaccines, yet all human SA vaccine trials have failed. One notable vaccine "failure" targeted IsdB, critical for host iron acquisition. We explored a fundamental difference between humans and laboratory animals-natural SA exposure. Recapitulating the failed phase III IsdB vaccine trial, mice previously infected with SA do not mount protective antibody responses to vaccination, unlike naive animals. Non-protective antibodies exhibit increased α2,3 sialylation that blunts opsonophagocytosis and preferentially targets a non-protective IsdB domain. IsdB vaccination of SA-infected mice recalls non-neutralizing humoral responses, further reducing vaccine efficacy through direct antibody competition. IsdB vaccine interference was overcome by immunization against the IsdB heme-binding domain. Purified human IsdB-specific antibodies also blunt IsdB passive immunization, and additional SA vaccines are susceptible to SA pre-exposure. Thus, failed anti-SA immunization trials could be explained by non-protective imprint from prior host-SA interaction.

Gut microbiota modulates the development of murine Kawasaki disease vasculitis

Alterations of gut microbiota composition and function influence the development of cardiovascular disease, but the role of these aberrations remains poorly understood in Kawasaki Disease (KD), an acute pediatric vasculitis that targets coronary arteries. Using a murine model of KD vasculitis, we found that germ-free and antibiotic-treated mice displayed reduced inflammation and cardiovascular lesions. Development of KD vasculitis in mice was associated with alterations in the composition of the intestinal microbiota, specifically decreased abundance of Akkermansia muciniphila and Faecalibacterium prausnitzii. Supplementation with live or pasteurized A. muciniphila or F. prausnitzii attenuated the severity of KD cardiovascular inflammation. Oral administration of the short-chain fatty acids acetate or butyrate, which are produced by A. muciniphila and F. prausnitzii respectively, or treatment with Amuc_1100, a purified protein isolated from the outer membrane of A. muciniphila, ameliorated the severity of KD cardiovascular lesions. Reduced development of KD vasculitis in mice receiving either pasteurized A. muciniphila or Amuc_1100 was associated with improvements of gut barrier function. These results reveal an underappreciated gut microbiota-cardiovascular inflammation axis during murine KD vasculitis. Our findings may stimulate the development of novel diagnostic tools and therapeutic strategies that modulate the intestinal microbiota composition and function for KD patients.

Research is supported by the NIH grants R01AI072726 to M.A. and R01HL139766 to M.N.R. 

Pathogen size alters C-type lectin receptor signaling in dendritic cells to influence CD4 Th9 cell differentiation

Dectin-1 recognizes β-glucan in fungal cell walls, and activation of Dectin-1 in dendritic cells (DCs) influences immune responses against fungi. Although many studies have shown that DCs activated via Dectin-1 induce different subsets of T helper cells according to different cytokine milieus, the mechanisms underlying such differences remain unknown. By harnessing polymorphic Candida albicans and polystyrene beads of different sizes, we find that target size influences production of cytokines that control differentiation of T helper cell subsets. Hyphal C. albicans and large beads activate DCs but cannot be phagocytosed due to their sizes, which prolongs the duration of Dectin-1 signaling. Transcriptomic analysis reveals that expression of Il33 is significantly increased by larger targets, and increased IL-33 expression promotes T_H9 responses. Expression of IL-33 is regulated by the Dectin-1-SYK-PLCγ-CARD9-ERK pathway. Altogether, our study demonstrates that size of fungi can be a determining factor in how DCs induce context-appropriate adaptive immune responses.

Oh et al. show that dendritic cells exposed to C. albicans hyphae more strongly induce IL-9-producing T cells compared with cells exposed to yeast. They find that this T_H9 response is driven in large part by Dectin-1 sensing microbe size, leading to elevated production of IL-33.

Fungal microbiome in inflammatory bowel disease: a critical assessment

The gut microbiome is at the center of inflammatory bowel disease (IBD) pathogenesis and disease activity. While this has mainly been studied in the context of the bacterial microbiome, recent advances have provided tools for the study of host genetics and metagenomics of host-fungal interaction. Through these tools, strong evidence has emerged linking certain fungal taxa, such as Candida and Malassezia, with cellular and molecular pathways of IBD disease biology. Mouse models and human fecal microbial transplant also suggest that some disease-participatory bacteria and fungi may act not via the host directly, but via their fungal-bacterial ecologic interactions. We hope that these insights, and the study design and multi-omics strategies used to develop them, will facilitate the inclusion of the fungal community in basic and translational IBD research.

Unsupervised Machine Learning Approaches Reveal Distinct Phenotypes of Perceived Bladder Pain: A Pilot Study

Interstitial cystitis/bladder pain syndrome (IC/BPS) is defined as an unpleasant sensation perceived to be related to the bladder with associated urinary symptoms. Due to difficulties discriminating pelvic visceral sensation, IC/BPS likely represents multiple phenotypes with different etiologies that present with overlapping symptomatic manifestations, which complicates clinical management. We hypothesized that unique bladder pain phenotypes or "symptomatic clusters" would be identifiable using machine learning analysis (unsupervised clustering) of validated patient-reported urinary and pain measures. Patients (n = 145) with pelvic pain/discomfort perceived to originate in the bladder and lower urinary tract symptoms answered validated questionnaires [OAB Questionnaire (OAB-q), O'Leary-Sant Indices (ICSI/ICPI), female Genitourinary Pain Index (fGUPI), and Pelvic Floor Disability Index (PFDI)]. In comparison to asymptomatic controls (n = 69), machine learning revealed three bladder pain phenotypes with unique, salient features. The first group chiefly describes urinary frequency and pain with the voiding cycle, in which bladder filling causes pain relieved by bladder emptying. The second group has fluctuating pelvic discomfort and straining to void, urinary frequency and urgency without incontinence, and a sensation of incomplete emptying without urinary retention. Pain in the third group was not associated with voiding, instead being more constant and focused on the urethra and vagina. While not utilized as a feature for clustering, subjects in the second and third groups were significantly younger than subjects in the first group and controls without pain. These phenotypes defined more homogeneous patient subgroups which responded to different therapies on chart review. Current approaches to the management of heterogenous populations of bladder pain patients are often ineffective, discouraging both patients and providers. The granularity of individual phenotypes provided by unsupervised clustering approaches can be exploited to help objectively define more homogeneous patient subgroups. Better differentiation of unique phenotypes within the larger group of pelvic pain patients is needed to move toward improvements in care and a better understanding of the etiologies of these painful symptoms.

Candida-induced asthma steps up to the plate-lets

Fungal proteases are well-known allergens. In this issue of Immunity, Wu et al. (2021) observe that allergic airway responses to Candida albicans are mediated by the peptide toxin candidalysin rather than proteases. Candidalysin promotes these responses by stimulating platelets to release the Wnt antagonist Dickkopf-1.

Commensal bacteria and fungi differentially regulate tumor responses to radiation therapy

Studies suggest that the efficacy of cancer chemotherapy and immunotherapy is influenced by intestinal bacteria. However, the influence of the microbiome on radiation therapy is not as well understood, and the microbiome comprises more than bacteria. Here, we find that intestinal fungi regulate antitumor immune responses following radiation in mouse models of breast cancer and melanoma and that fungi and bacteria have opposite influences on these responses. Antibiotic-mediated depletion or gnotobiotic exclusion of fungi enhances responsiveness to radiation, whereas antibiotic-mediated depletion of bacteria reduces responsiveness and is associated with overgrowth of commensal fungi. Further, elevated intratumoral expression of Dectin-1, a primary innate sensor of fungi, is negatively associated with survival in patients with breast cancer and is required for the effects of commensal fungi in mouse models of radiation therapy.

Debaryomyces is enriched in Crohn's disease intestinal tissue and impairs healing in mice

Alterations of the mycobiota composition associated with Crohn's disease (CD) are challenging to link to defining elements of pathophysiology, such as poor injury repair. Using culture-dependent and -independent methods, we discovered that Debaryomyces hansenii preferentially localized to and was abundant within incompletely healed intestinal wounds of mice and inflamed mucosal tissues of CD human subjects. D. hansenii cultures from injured mice and inflamed CD tissues impaired colonic healing when introduced into injured conventionally raised or gnotobiotic mice. We reisolated D. hansenii from injured areas of these mice, fulfilling Koch's postulates. Mechanistically, D. hansenii impaired mucosal healing through the myeloid cell-specific type 1 interferon-CCL5 axis. Taken together, we have identified a fungus that inhabits inflamed CD tissue and can lead to dysregulated mucosal healing.

Malassezia spp. induce inflammatory cytokines and activate NLRP3 inflammasomes in phagocytes

Malassezia spp. are common eukaryotic yeasts that colonize mammalian skin. Recently, the authors and others have observed that Malassezia globosa and Malassezia restricta can be found in the intestines in the context of certain diseases, including Crohn's disease and pancreatic cancer. In order to better understand the nature of innate inflammatory responses to these yeasts, inflammatory responses induced by M. restricta and M. globosa in mouse bone marrow-derived Mϕs (BMDM) and dendritic cells (BMDC) are evaluated. While Malassezia yeasts induce proinflammatory cytokine production from both Mϕs and dendritic cells, the levels of production from BMDC were more pronounced. Both M. restricta and M. globosa activated inflammatory cytokine production from BMDC in large part through Dectin2 and CARD9 signaling, although additional receptors appear to be involved in phagocytosis and activation of reactive oxygen production in response to the yeasts. Both M. restricta and M. globosa stimulate production of pro-IL-1β as well as activation of the NLRP3 inflammasome. NLRP3 inflammasome activation by Malassezia fungi requires SYK signaling, potassium efflux and actin rearrangement. Together, the data further the understanding of the coordinated involvement of multiple innate immune receptors in recognizing Malassezia globosa and Malassezia restricta and orchestrating phagocyte inflammatory and antimicrobial responses.

Harnessing antifungal immunity in pursuit of a Staphylococcus aureus vaccine strategy

Staphylococcus aureus (S. aureus) is one of the most common bacterial infections worldwide, and antibiotic resistant strains such as Methicillin-Resistant S. aureus (MRSA) are a major threat and burden to public health. MRSA not only infects immunocompromised patients but also healthy individuals and has rapidly spread from the healthcare setting to the outside community. However, all vaccines tested in clinical trials to date have failed. Immunocompromised individuals such as patients with HIV or decreased levels of CD4+ T cells are highly susceptible to S. aureus infections, and they are also at increased risk of developing fungal infections. We therefore wondered whether stimulation of antifungal immunity might promote the type of immune responses needed for effective host defense against S. aureus. Here we show that vaccination of mice with a fungal β-glucan particle (GP) loaded with S. aureus antigens provides protective immunity to S. aureus. We generated glucan particles loaded with the four S. aureus proteins ClfA, IsdA, MntC, and SdrE, creating the 4X-SA-GP vaccine. Vaccination of mice with three doses of 4X-SA-GP promoted protection in a systemic model of S. aureus infection with a significant reduction in the bacterial burden in the spleen and kidneys. 4X-SA-GP vaccination induced antigen-specific Th1 and Th17 CD4+ T cell and antibody responses and provided long-term protection. This work suggests that the GP vaccine system has potential as a novel approach to developing vaccines for S. aureus.

Antifungals improve anti-PD1 efficacy in a murine breast cancer model

Every year over 200,000 North American women are diagnosed with breast cancer, which is the second leading cause of cancer death among women. The introduction of effective immunotherapies has revolutionized patient care in many cancers, including breast, but only a subset of patients exhibit clinical response. Factors determining a patient’s response to immunotherapy is an area of intense research. The microbiome is one potential contributing factor. The human intestinal microbiome is comprised largely of bacteria, but also harbors fungi, viruses and archaea that undoubtedly have significant biological functions. Recent reports have detailed the association of specific strains of bacteria with positive response to PD-1/PDL1 therapy in several cancers however, little is known regarding the involvement of commensal fungi. Previous work in our lab found a high diversity of fungi associated with the human gut, which interact with the immune system and influence the severity of gastrointestinal inflammation and allergic airway disease. Using a syngeneic murine model of triple negative breast cancer, we found that antifungal treatment sensitizes previously resistant tumors to anti-PD1 therapy, leading to a significant decrease in growth rate as well as increased survival compared to anti-PD-1 treatment alone. Antifungal treatment altered the microbiome composition as evidenced by 16s and ITS1 sequencing of fecal pellets, as well as altered the tumor immune compartment to promote anti-tumor activity when combined with anti-PD1. These data suggest fungi have a role in shaping the immune-tumor microenvironment and provide insight into how perturbation of the microbiome might improve response to immunotherapy.

Optimization of DNA extraction from human urinary samples for mycobiome community profiling

INTRODUCTION

Recent data suggest the urinary tract hosts a microbial community of varying composition, even in the absence of infection. Culture-independent methodologies, such as next-generation sequencing of conserved ribosomal DNA sequences, provide an expansive look at these communities, identifying both common commensals and fastidious organisms. A fundamental challenge has been the isolation of DNA representative of the entire resident microbial community, including fungi.

MATERIALS AND METHODS

We evaluated multiple modifications of commonly-used DNA extraction procedures using standardized male and female urine samples, comparing resulting overall, fungal and bacterial DNA yields by quantitative PCR. After identifying protocol modifications that increased DNA yields (lyticase/lysozyme digestion, bead beating, boil/freeze cycles, proteinase K treatment, and carrier DNA use), all modifications were combined for systematic confirmation of optimal protocol conditions. This optimized protocol was tested against commercially available methodologies to compare overall and microbial DNA yields, community representation and diversity by next-generation sequencing (NGS).

RESULTS

Overall and fungal-specific DNA yields from standardized urine samples demonstrated that microbial abundances differed significantly among the eight methods used. Methodologies that included multiple disruption steps, including enzymatic, mechanical, and thermal disruption and proteinase digestion, particularly in combination with small volume processing and pooling steps, provided more comprehensive representation of the range of bacterial and fungal species. Concentration of larger volume urine specimens at low speed centrifugation proved highly effective, increasing resulting DNA levels and providing greater microbial representation and diversity.

CONCLUSIONS

Alterations in the methodology of urine storage, preparation, and DNA processing improve microbial community profiling using culture-independent sequencing methods. Our optimized protocol for DNA extraction from urine samples provided improved fungal community representation. Use of this technique resulted in equivalent representation of the bacterial populations as well, making this a useful technique for the concurrent evaluation of bacterial and fungal populations by NGS.

Malassezia Is Associated with Crohn's Disease and Exacerbates Colitis in Mouse Models

Inflammatory bowel disease (IBD) is characterized by alterations in the intestinal microbiota and altered immune responses to gut microbiota. Evidence is accumulating that IBD is influenced by not only commensal bacteria but also commensal fungi. We characterized fungi directly associated with the intestinal mucosa in healthy people and Crohn's disease patients and identified fungi specifically abundant in patients. One of these, the common skin resident fungus Malassezia restricta, is also linked to the presence of an IBD-associated polymorphism in the gene for CARD9, a signaling adaptor important for anti-fungal defense. M. restricta elicits innate inflammatory responses largely through CARD9 and is recognized by Crohn's disease patient anti-fungal antibodies. This yeast elicits strong inflammatory cytokine production from innate cells harboring the IBD-linked polymorphism in CARD9 and exacerbates colitis via CARD9 in mouse models of disease. Collectively, these results suggest that targeting specific commensal fungi may be a therapeutic strategy for IBD.

Expansion of commensal fungus Wallemia mellicola in the gastrointestinal mycobiota enhances the severity of allergic airway disease in mice

The gastrointestinal microbiota influences immune function throughout the body. The gut-lung axis refers to the concept that alterations of gut commensal microorganisms can have a distant effect on immune function in the lung. Overgrowth of intestinal Candida albicans has been previously observed to exacerbate allergic airways disease in mice, but whether subtler changes in intestinal fungal microbiota can affect allergic airways disease is less clear. In this study we have investigated the effects of the population expansion of commensal fungus Wallemia mellicola without overgrowth of the total fungal community. Wallemia spp. are commonly found as a minor component of the commensal gastrointestinal mycobiota in both humans and mice. Mice with an unaltered gut microbiota community resist population expansion when gavaged with W. mellicola; however, transient antibiotic depletion of gut microbiota creates a window of opportunity for expansion of W. mellicola following delivery of live spores to the gastrointestinal tract. This phenomenon is not universal as other commensal fungi (Aspergillus amstelodami, Epicoccum nigrum) do not expand when delivered to mice with antibiotic-depleted microbiota. Mice with Wallemia-expanded gut mycobiota experienced altered pulmonary immune responses to inhaled aeroallergens. Specifically, after induction of allergic airways disease with intratracheal house dust mite (HDM) antigen, mice demonstrated enhanced eosinophilic airway infiltration, airway hyperresponsiveness (AHR) to methacholine challenge, goblet cell hyperplasia, elevated bronchoalveolar lavage IL-5, and enhanced serum HDM IgG1. This phenomenon occurred with no detectable Wallemia in the lung. Targeted amplicon sequencing analysis of the gastrointestinal mycobiota revealed that expansion of W. mellicola in the gut was associated with additional alterations of bacterial and fungal commensal communities. We therefore colonized fungus-free Altered Schaedler Flora (ASF) mice with W. mellicola. ASF mice colonized with W. mellicola experienced enhanced severity of allergic airways disease compared to fungus-free control ASF mice without changes in bacterial community composition.

Commensal Fungi in Health and Disease

Fungi are increasingly being recognized as common members of the microbiomes found on nearly all mucosal surfaces, and interest is growing in understanding how these organisms may contribute to health and disease. In this review, we investigate recent developments in our understanding of the fungal microbiota or "mycobiota" including challenges faced in characterizing it, where these organisms are found, their diversity, and how they interact with host immunity. Growing evidence indicates that, like the bacterial microbiota, the fungal microbiota is often altered in disease states, and increasingly studies are being designed to probe the functional consequences of such fungal dysbiosis on health and disease.

Cryptococcal meningitis in a daily cannabis smoker without evidence of immunodeficiency

Cryptococcal meningitis is a life-threatening condition most commonly observed in immunocompromised individuals. We describe a daily cannabis smoker without evidence of immunodeficiency presenting with confirmed Cryptococcus neoformans meningitis. An investigation of cannabis samples from the patient's preferred dispensary demonstrated contamination with several varieties of Cryptococcus, including C. neoformans, and other opportunistic fungi. These findings raise concern regarding the safety of dispensary-grade cannabis, even in immunocompetent users.

Peptidoglycan recognition by the innate immune system

The innate immune system recognizes microbial products using germline-encoded receptors that initiate inflammatory responses to infection. The bacterial cell wall component peptidoglycan is a prime example of a conserved pathogen-associated molecular pattern (PAMP) for which the innate immune system has evolved sensing mechanisms. Peptidoglycan is a direct target for innate immune receptors and also regulates the accessibility of other PAMPs to additional innate immune receptors. Subtle structural modifications to peptidoglycan can influence the ability of the innate immune system to detect bacteria and can allow bacteria to evade or alter host defences. This Review focuses on the mechanisms of peptidoglycan recognition that are used by mammalian cells and discusses new insights into the role of peptidoglycan recognition in inflammation, metabolism, immune homeostasis and disease.

Host-microbe interactions: commensal fungi in the gut

Fungi are ubiquitous microbes that are common in diverse environments including as commensal organisms on the human body. In addition to its obvious role as a digestive organ, the intestines have been further appreciated as important for the development, maintenance, and instruction of the immune system. The gut harbors many types of microorganisms including bacteria, archaea, fungi, and viruses, and many studies over the past couple of decades have documented an important role for intestinal bacteria in immunological function. Recent studies are now suggesting that intestinal fungi (the gut 'mycobiome') may similarly play important roles in host immunity and inflammation. This review will discuss recent studies that will influence our growing understanding of the role(s) of intestinal fungi in health and disease.

O-Acetylation of Peptidoglycan Limits Helper T Cell Priming and Permits Staphylococcus aureus Reinfection

Humans do not usually develop effective immunity to Staphylococcus aureus reinfection. Using a murine model that mimics human infection, we show that lack of protective immunity to S. aureus systemic reinfection is associated with robust interleukin-10 (IL-10) production and impaired protective Th17 responses. In dendritic cell co-culture assays, priming with S. aureus promotes robust T cell proliferation, but limits Th cells polarization and production of IL-1β and other cytokines important for Th1 and Th17 differentiation. We show that O-acetylation of peptidoglycan, a mechanism utilized by S. aureus to block bacterial cell wall breakdown, limits the induction of pro-inflammatory signals required for optimal Th17 polarization. IL-10 deficiency in mice restores protective immunity to S. aureus infection, and adjuvancy with a staphylococcal peptidoglycan O-acetyltransferase mutant reduces IL-10, increases IL-1β, and promotes development of IL-17-dependent, Th cell-transferable protective immunity. Overall, our study suggests a mechanism whereby S. aureus modulates cytokines critical for induction of protective Th17 immunity.

Direct Antimicrobial Activity of IFN-β

Type I IFNs are a cytokine family essential for antiviral defense. More recently, type I IFNs were shown to be important during bacterial infections. In this article, we show that, in addition to known cytokine functions, IFN-β is antimicrobial. Parts of the IFN-β molecular surface (especially helix 4) are cationic and amphipathic, both classic characteristics of antimicrobial peptides, and we observed that IFN-β can directly kill Staphylococcus aureus Further, a mutant S. aureus that is more sensitive to antimicrobial peptides was killed more efficiently by IFN-β than was the wild-type S. aureus, and immunoblotting showed that IFN-β interacts with the bacterial cell surface. To determine whether specific parts of IFN-β are antimicrobial, we synthesized IFN-β helix 4 and found that it is sufficient to permeate model prokaryotic membranes using synchrotron x-ray diffraction and that it is sufficient to kill S. aureus These results suggest that, in addition to its well-known signaling activity, IFN-β may be directly antimicrobial and be part of a growing family of cytokines and chemokines, called kinocidins, that also have antimicrobial properties.

Myeloid ATG16L1 Facilitates Host-Bacteria Interactions in Maintaining Intestinal Homeostasis

Intact ATG16L1 plays an essential role in Paneth cell function and intestinal homeostasis. However, the functional consequences of ATG16L1 deficiency in myeloid cells, particularly macrophages, are not fully characterized. We generated mice with Atg16l1 deficiency in myeloid and dendritic cells and showed that mice with myeloid Atg16l1 deficiency had exacerbated colitis in two acute and one chronic model of colitis with increased proinflammatory to anti-inflammatory macrophage ratios, production of proinflammatory cytokines, and numbers of IgA-coated intestinal microbes. Mechanistic analyses using primary murine macrophages showed that Atg16l1 deficiency led to increased reactive oxygen species production, impaired mitophagy, reduced microbial killing, impaired processing of MHC class II Ags, and altered intracellular trafficking to the lysosomal compartments. Increased production of reactive oxygen species and reduced microbial killing may be general features of the myeloid compartment, as they were also observed in Atg16l1-deficient primary murine neutrophils. A missense polymorphism (Thr³⁰⁰Ala) in the essential autophagy gene ATG16L1 is associated with Crohn disease (CD). Previous studies showed that this polymorphism leads to enhanced cleavage of ATG16L1 T300A protein and thus reduced autophagy. Similar findings were shown in primary human macrophages from controls and a population of CD patients carrying the Atg16l1 T300A risk variant and who were controlled for NOD2 CD-associated variants. This study revealed that ATG16L1 deficiency led to alterations in macrophage function that contribute to the severity of CD.

The mycobiome of the human urinary tract: potential roles for fungi in urology

The mycobiome, defined as the fungal microbiota within a host environment, is an important but understudied component of the human microbial ecosystem. New culture-independent approaches to determine microbial diversity, such as next-generation sequencing methods, have discovered specific, characteristic, commensal fungal populations present in different body sites. These studies have also identified diverse patterns in fungal communities associated with various diseases. While alterations in urinary bacterial communities have been noted in disease states, a comprehensive description of the urinary mycobiome has been lacking. Early evidence suggests the urinary mycobiome is a diverse community with high intraindividual variability. In other disease systems, the mycobiome is thought to interact with other biomes and the host to play a role in organ homeostasis and pathology; further study will be needed to elucidate the role fungi play in bladder health and disease.

Immunity to Commensal Fungi: Detente and Disease

Fungi are ubiquitous in our environment, and a healthy immune system is essential to maintain adequate protection from fungal infections. When this protection breaks down, superficial and invasive fungal infections cause diseases that range from irritating to life-threatening. Millions of people worldwide develop invasive infections during their lives, and mortality for these infections often exceeds 50%. Nevertheless, we are normally colonized with many of the same disease-causing fungi (e.g., on the skin or in the gut). Recent research is dramatically expanding our understanding of the mechanisms by which our immune systems interact with these organisms in health and disease. In this review, we discuss what is currently known about where and how the immune system interacts with common fungi.

Autocrine Type I IFN Signaling in Dendritic Cells Stimulated with Fungal β-Glucans or Lipopolysaccharide Promotes CD8 T Cell Activation

Type I IFNs are key mediators of immune defense against viruses and bacteria. Type I IFNs were also previously implicated in protection against fungal infection, but their roles in antifungal immunity have not been thoroughly investigated. A recent study demonstrated that bacterial and fungal β-glucans stimulate IFN-β production by dendritic cells (DCs) following detection by the Dectin-1 receptor, but the effects of β-glucan-induced type I IFNs have not been defined. We investigated whether type I IFNs regulate CD8 T cell activation by fungal β-glucan particle-stimulated DCs. We demonstrate that β-glucan-stimulated DCs induce CD8 T cell proliferation, activation marker (CD44 and CD69) expression, and production of IFN-γ, IL-2, and granzyme B. Moreover, we show that type I IFNs support robust CD8 T cell activation (proliferation and IFN-γ and granzyme B production) by β-glucan-stimulated DCs in vitro and in vivo due to autocrine effects on the DCs. Specifically, type I IFNs promote Ag presentation on MHC I molecules, CD86 and CD40 expression, and the production of IL-12 p70, IL-2, IL-6, and TNF-α by β-glucan-stimulated DCs. We also demonstrate a role for autocrine type I IFN signaling in bacterial LPS-induced DC maturation, although, in the context of LPS stimulation, this mechanism is not so critical for CD8 T cell activation (promotes IFN-γ production but not proliferation or granzyme B production). This study provides insight into the mechanisms underlying CD8 T cell activation during infection, which may be useful in the rational design of vaccines directed against pathogens and tumors.

Group B Streptococcus Evades Host Immunity by Degrading Hyaluronan

In response to tissue injury, hyaluronan (HA) polymers are cleaved by host hyaluronidases, generating small fragments that ligate Toll-like receptors (TLRs) to elicit inflammatory responses. Pathogenic bacteria such as group B Streptococcus (GBS) express and secrete hyaluronidases as a mechanism for tissue invasion, but it is not known how this activity relates to immune detection of HA. We found that bacterial hyaluronidases secreted by GBS and other Gram-positive pathogens degrade pro-inflammatory HA fragments to their component disaccharides. In addition, HA disaccharides block TLR2/4 signaling elicited by both host-derived HA fragments and other TLR2/4 ligands, including lipopolysaccharide. Application of GBS hyaluronidase or HA disaccharides reduced pulmonary pathology and pro-inflammatory cytokine levels in an acute lung injury model. We conclude that breakdown of host-generated pro-inflammatory HA fragments to disaccharides allows bacterial pathogens to evade immune detection and could be exploited as a strategy to treat inflammatory diseases.

Poorly Cross-Linked Peptidoglycan in MRSA Due to mecA Induction Activates the Inflammasome and Exacerbates Immunopathology

Methicillin-resistant S. aureus (MRSA) is a leading health problem. Compared to methicillin-sensitive S. aureus, MRSA infections are associated with greater morbidity and mortality, but the mechanisms underlying MRSA pathogenicity are unclear. Here we show that the protein conferring β-lactam antibiotic resistance, penicillin-binding protein 2A (encoded by the mecA gene), directly contributes to pathogenicity during MRSA infection. MecA induction leads to a reduction in peptidoglycan cross-linking that allows for enhanced degradation and detection by phagocytes, resulting in robust IL-1β production. Peptidoglycan isolated from β-lactam-challenged MRSA strongly induces the NLRP3 inflammasome in macrophages, but these effects are lost upon peptidoglycan solubilization. Mutant MRSA bacteria with naturally occurring reduced peptidoglycan cross-links induce high IL-1β levels in vitro and cause increased pathology in vivo. β-lactam treatment of MRSA skin infection exacerbates immunopathology, which is IL-1 dependent. Thus, antibiotic-induced expression of mecA during MRSA skin infection contributes to immunopathology by altering peptidoglycan structure.

Macrophage recognition of zymosan particles

Zymosan particles have served as a model for recognition of microbes by the innate immune system for over 50 years. Zymosan induces inflammatory signals in macrophages through Toll-like receptors TLR2 and TLR6. In addition, phagocytic receptors on macrophages bind zymosan and stimulate particle engulfment. We have further examined the requirements for induction of inflammatory responses such as TNF-α production and NF-κ B activation by zymosan in mouse macrophages. We have observed that direct particle contact is required (excluding a role for soluble components of zymosan preparations) and that contact with a single particle is sufficient to trigger cytokine production. Further, ablation of the Toll-like receptor-stimulating activity of zymosan does not affect the ability of phagocytic receptors to internalise the particle.

Inflammatory properties of antibiotic-treated bacteria

Antibiotics have proven to be enormously effective tools in combating infectious diseases. A common roadblock to the effective use of antibiotics is the development of antibiotic resistance. We have recently observed that the very mechanism by which methicillin-resistant Staphylococcus aureus (MRSA) becomes antibiotic resistant causes the organism to be more inflammatory to innate immune cells. In this review, we offer some thoughts on the ways in which antibiotics have been observed to influence immune responses to bacteria.

Hexokinase Is an Innate Immune Receptor for the Detection of Bacterial Peptidoglycan

Degradation of Gram-positive bacterial cell wall peptidoglycan in macrophage and dendritic cell phagosomes leads to activation of the NLRP3 inflammasome, a cytosolic complex that regulates processing and secretion of interleukin (IL)-1β and IL-18. While many inflammatory responses to peptidoglycan are mediated by detection of its muramyl dipeptide component in the cytosol by NOD2, we report here that NLRP3 inflammasome activation is caused by release of N-acetylglucosamine that is detected in the cytosol by the glycolytic enzyme hexokinase. Inhibition of hexokinase by N-acetylglucosamine causes its dissociation from mitochondria outer membranes, and we found that this is sufficient to activate the NLRP3 inflammasome. In addition, we observed that glycolytic inhibitors and metabolic conditions affecting hexokinase function and localization induce inflammasome activation. While previous studies have demonstrated that signaling by pattern recognition receptors can regulate metabolic processes, this study shows that a metabolic enzyme can act as a pattern recognition receptor. PAPERCLIP.

Immunological Consequences of Intestinal Fungal Dysbiosis

Compared to bacteria, the role of fungi within the intestinal microbiota is poorly understood. In this study we investigated whether the presence of a "healthy" fungal community in the gut is important for modulating immune function. Prolonged oral treatment of mice with antifungal drugs resulted in increased disease severity in acute and chronic models of colitis, and also exacerbated the development of allergic airway disease. Microbiota profiling revealed restructuring of fungal and bacterial communities. Specifically, representation of Candida spp. was reduced, while Aspergillus, Wallemia, and Epicoccum spp. were increased. Oral supplementation with a mixture of three fungi found to expand during antifungal treatment (Aspergillus amstelodami, Epicoccum nigrum, and Wallemia sebi) was sufficient to recapitulate the exacerbating effects of antifungal drugs on allergic airway disease. Taken together, these results indicate that disruption of commensal fungal populations can influence local and peripheral immune responses and enhance relevant disease states.

C9orf72 is required for proper macrophage and microglial function in mice

Expansions of a hexanucleotide repeat (GGGGCC) in the noncoding region of the C9orf72 gene are the most common genetic cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia. Decreased expression of C9orf72 is seen in expansion carriers, suggesting that loss of function may play a role in disease. We found that two independent mouse lines lacking the C9orf72 ortholog (3110043O21Rik) in all tissues developed normally and aged without motor neuron disease. Instead, C9orf72 null mice developed progressive splenomegaly and lymphadenopathy with accumulation of engorged macrophage-like cells. C9orf72 expression was highest in myeloid cells, and the loss of C9orf72 led to lysosomal accumulation and altered immune responses in macrophages and microglia, with age-related neuroinflammation similar to C9orf72 ALS but not sporadic ALS human patient tissue. Thus, C9orf72 is required for the normal function of myeloid cells, and altered microglial function may contribute to neurodegeneration in C9orf72 expansion carriers.

Immune Interactions with Pathogenic and Commensal Fungi: A Two-Way Street

We are exposed to a wide spectrum of fungi including innocuous environmental organisms, opportunistic pathogens, commensal organisms, and fungi that can actively and explicitly cause disease. Much less is understood about effective host immunity to fungi than is generally known about immunity to bacterial and viral pathogens. Innate and adaptive arms of the immune system are required for effective host defense against Candida, Aspergillus, Cryptococcus, and others, with specific elements of the host response regulating specific types of fungal infections (e.g., mucocutaneous versus systemic). Here we will review themes and controversies that are currently shaping investigation of antifungal immunity (primarily to Candida and Aspergillus) and will also examine the emerging field of the role of fungi in the gut microbiome.

Mycobiome: Approaches to analysis of intestinal fungi

Massively parallel sequencing (MPSS) of bacterial 16S rDNA has been widely used to characterize the microbial makeup of the human and mouse gastrointestinal tract. However, techniques for fungal microbiota (mycobiota) profiling remain relatively under-developed. Compared to 16S profiling, the size and sequence context of the fungal Internal Transcribed Spacer 1 (ITS1), the most common target for mycobiota profiling, are highly variable. Using representative gastrointestinal tract fungi to build a known "mock" library, we examine how this sequence variability affects data quality derived from Illumina Miseq and Ion Torrent PGM sequencing pipelines. Also, while analysis of bacterial 16S profiles is facilitated by the presence of high-quality well-accepted databases of bacterial 16S sequences, such an accepted database has not yet emerged to facilitate fungal ITS sequence characterization, and we observe that redundant and inconsistent ITS1 sequence representation in publically available fungal reference databases affect quantitation and annotation of species in the gut. To address this problem, we have constructed a manually curated reference database optimized for annotation of gastrointestinal fungi. This targeted host-associated fungi (THF) database contains 1817 ITS1 sequences representing sequence diversity in genera previously identified in human and mouse gut. We observe that this database consistently outperforms three common ITS database alternatives on comprehensiveness, taxonomy assignment accuracy and computational efficiency in analyzing sequencing data from the mouse gastrointestinal tract.

Characterization of Bacterial and Fungal Microbiome in Children with Hirschsprung Disease with and without a History of Enterocolitis: A Multicenter Study

Development of potentially life-threatening enterocolitis is the most frequent complication in children with Hirschsprung disease (HSCR), even after definitive corrective surgery. Intestinal microbiota likely contribute to the etiology of enterocolitis, so the aim of this study was to compare the fecal bacterial and fungal communities of children who developed Hirschsprung-associated enterocolitis (HAEC) with HSCR patients who had never had enterocolitis. Eighteen Hirschsprung patients who had completed definitive surgery were enrolled: 9 had a history of HAEC and 9 did not. Fecal DNA was isolated and 16S and ITS-1 regions sequenced using Next Generation Sequencing and data analysis for species identification. The HAEC group bacterial composition showed a modest reduction in Firmicutes and Verrucomicrobia with increased Bacteroidetes and Proteobacteria compared with the HSCR group. In contrast, the fecal fungi composition of the HAEC group showed marked reduction in diversity with increased Candida sp., and reduced Malassezia and Saccharomyces sp. compared with the HSCR group. The most striking finding within the HAEC group is that the Candida genus segregated into “high burden” patients with 97.8% C. albicans and 2.2% C. tropicalis compared with “low burden” patients 26.8% C. albicans and 73% C. tropicalis. Interestingly even the low burden HAEC group had altered Candida community structure with just two species compared to more diverse Candida populations in the HSCR patients. This is the first study to identify Candida sp. as potentially playing a role in HAEC either as expanded commensal species as a consequence of enterocolitis (or treatment), or possibly as pathobioants contributing to the pathogenesis of HAEC. These findings suggest a dysbiosis in the gut microbial ecosystem of HAEC patients, such that there may be dominance of fungi and bacteria predisposing patients to development of HAEC.

Cutting edge: FYCO1 recruitment to dectin-1 phagosomes is accelerated by light chain 3 protein and regulates phagosome maturation and reactive oxygen production

L chain 3 (LC3)-associated phagocytosis is a process in which LC3, a protein canonically involved in engulfing intracellular materials (autophagy), is recruited to traditional phagosomes during internalization of extracellular payloads. LC3's association with phagosomes has been implicated in regulating microbial killing, Ag processing, and phagosome maturation; however, the mechanism by which LC3 influences these processes has not been clear. In this study, we report that FYVE and coiled-coil domain containing 1 (FYCO1), a protein previously implicated in autophagosome trafficking, is recruited directly by LC3 to Dectin-1 phagosomes. During LC3-associated phagocytosis, FYCO1 recruitment facilitates maturation of early p40phox(+) phagosomes into late LAMP1(+) phagosomes. When FYCO1 is lacking, phagosomes stay p40phox(+) longer and produce more reactive oxygen.

Detection of a TLR2 agonist by hematopoietic stem and progenitor cells impacts the function of the macrophages they produce

Several groups have shown that detection of microbial components by TLRs on hematopoietic stem and progenitor cells (HSPCs) instructs myeloid cell generation, raising interest in the possibility of targeting TLRs on HSPCs to boost myelopoiesis. However, although "TLR-derived" cells exhibit myeloid cell characteristics (phagocytosis, cytokine production, antigen presentation), it is not clear whether they are functionally equivalent to macrophages derived in the absence of TLR activation. Our in vitro and in vivo studies show that macrophages derived from mouse and human HSPC subsets (including stem cells) exposed to a TLR2 agonist prior to or during macrophage differentiation produce lower levels of inflammatory cytokines (TNF-α, IL-6, and IL-1β) and reactive oxygen species. This is in contrast to prior exposure of differentiated macrophages to the TLR2 agonist ("tolerance"), which suppresses inflammatory cytokine production, but elevates reactive oxygen species. Soluble factors produced following exposure of HSPCs to a TLR2 agonist can also act in a paracrine manner to influence the function of macrophages derived from unexposed HSPCs. Our data demonstrate that macrophage function can be influenced by TLR signaling in the HSPCs from which they are derived, and that this may impact the clinical utility of targeting TLRs on HSPCs to boost myelopoiesis.

β-Glucan signaling connects phagocytosis to autophagy

A growing list of innate immune receptors is being defined that recognize polysaccharides of microbial cell walls. Fungal β-glucan recognition by the receptor Dectin-1 triggers inflammatory immune responses in macrophages and dendritic cells that are appropriate for defense against fungal pathogens. Among these responses is the specific recruitment of the autophagy-related protein light chain 3 (LC3) to phagosomes containing fungi. Studies documenting LC3's recruitment to phagosomes containing β-glucan and other nonsugar particles suggest that LC3 plays a role in regulating phagocytosis and its related immunological responses.

Striking a balance: fungal commensalism versus pathogenesis

The environment is suffused with nearly countless types of fungi, and our immune systems must be tuned to cope with constant exposure to them. In addition, it is becoming increasingly clear that many surfaces of our bodies are colonized with complex populations of fungi (the mycobiome) in the same way that they are colonized with complex populations of bacteria. The immune system must tolerate colonization with commensal fungi but defend against fungal invasion. Truly life-threatening fungal infections are common only when this balance is disrupted through, for example, profound immunosuppression or genetic mutation. Recent studies have begun to shed light on how this balance is established and maintained, and suggest future studies on the role of fungi in homeostatic conditions.

Inhibitory receptor paired Ig-like receptor B is exploited by Staphylococcus aureus for virulence

The innate immune system has developed to acquire a wide variety of pattern-recognition receptors (PRRs) to identify potential pathogens, whereas pathogens have also developed to escape host innate immune responses. ITIM-bearing receptors are attractive targets for pathogens to attenuate immune responses against them; however, the in vivo role of the inhibitory PRRs in host-bacteria interactions remains unknown. We demonstrate in this article that Staphylococcus aureus, a major Gram-positive bacteria, exploits inhibitory PRR paired Ig-like receptor (PIR)-B on macrophages to suppress ERK1/2 and inflammasome activation, and subsequent IL-6 and IL-1β secretion. Consequently, Pirb(-/-) mice infected with S. aureus showed enhanced inflammation and more effective bacterial clearance, resulting in resistance to the sepsis. Screening of S. aureus mutants identified lipoteichoic acid (LTA) as an essential bacterial cell wall component required for binding to PIR-B and modulating inflammatory responses. In vivo, however, an LTA-deficient S. aureus mutant was highly virulent and poorly recognized by macrophages in both wild-type and Pirb(-/-) mice, demonstrating that LTA recognition by PRRs other than PIR-B mediates effective bacterial elimination. These results provide direct evidence that bacteria exploit the inhibitory receptor for virulence, and host immune system counterbalances the infection.

Failure to induce IFN-β production during Staphylococcus aureus infection contributes to pathogenicity

The importance of type I IFNs in the host response to viral infection is well established; however, their role in bacterial infection is not fully understood. Several bacteria (both Gram-positive and -negative) have been shown to induce IFN-β production in myeloid cells, but this IFN-β is not always beneficial to the host. We examined whether Staphylococcus aureus induces IFN-β from myeloid phagocytes, and if so, whether it is helpful or harmful to the host to do so. We found that S. aureus poorly induces IFN-β production compared with other bacteria. S. aureus is highly resistant to degradation in the phagosome because it is resistant to lysozyme. Using a mutant that is more sensitive to lysozyme, we show that phagosomal degradation and release of intracellular ligands is essential for induction of IFN-β and inflammatory chemokines downstream of IFN-β. Further, we found that adding exogenous IFN-β during S. aureus infection (in vitro and in vivo) was protective. Together, the data demonstrate that failure to induce IFN-β production during S. aureus infection contributes to pathogenicity.

Dectin-1-triggered recruitment of light chain 3 protein to phagosomes facilitates major histocompatibility complex class II presentation of fungal-derived antigens

Dectin-1 is a pattern recognition receptor that is important for innate immune responses against fungi in humans and mice. Dectin-1 binds to β-glucans in fungal cell walls and triggers phagocytosis, production of reactive oxygen by the NADPH oxidase, and inflammatory cytokine production which all contribute to host immune responses against fungi. Although the autophagy pathway was originally characterized for its role in the formation of double-membrane compartments engulfing cytosolic organelles and debris, recent studies have suggested that components of the autophagy pathway may also participate in traditional phagocytosis. In this study, we show that Dectin-1 signaling in macrophages and bone marrow-derived dendritic cells triggers formation of LC3II, a major component of the autophagy machinery. Further, Dectin-1 directs the recruitment of LC3II to phagosomes, and this requires Syk, activation of reactive oxygen production by the NADPH oxidase, and ATG5. Using LC3-deficient dendritic cells we show that whereas LC3 recruitment to phagosomes is not important for triggering phagocytosis, killing or Dectin-1-mediated inflammatory cytokine production, it facilitates recruitment of MHC class II molecules to phagosomes and promotes presentation of fungal-derived antigens to CD4 T cells.

Mechanisms of Fc receptor and dectin-1 activation for phagocytosis

Phagocytosis is a key cellular process, both during homeostasis and upon infection or tissue damage. Receptors on the surface of professional phagocytic cells bind to target particles either directly or through opsonizing ligands, and trigger actin-mediated ingestion of the particles. The process must be carefully controlled to ensure that phagocytosis is triggered efficiently and specifically, and that the antimicrobial cytotoxic responses that often accompany it are initiated only when required. In this review, we will describe and compare the molecular mechanisms that regulate phagocytosis triggered by Fcγ receptors, which mediate the uptake of immunoglobulin G-opsonized targets, and Dectin-1, which is responsible for internalization of fungi with exposed cell wall β-glucan. We will examine how these receptors detect their ligands, how signal transduction is initiated and regulated, and how internalization is instructed to achieve rapid and yet controlled uptake of their targets.

Information processing during phagocytosis

Phagocytosis - the process by which macrophages, dendritic cells and other myeloid phagocytes internalize diverse particulate targets - is a key mechanism of innate immunity. The molecular and cellular events that underlie the binding of targets to a phagocyte and their engulfment into phagosomes have been extensively studied. More recent data suggest that the process of phagocytosis itself provides information to myeloid phagocytes about the nature of the targets they are engulfing and that this helps to tailor inflammatory responses. In this Review, we discuss how such information is acquired during phagocytosis and how it is processed to coordinate an immune response.

Interactions between commensal fungi and the C-type lectin receptor Dectin-1 influence colitis

The intestinal microflora, typically equated with bacteria, influences diseases such as obesity and inflammatory bowel disease. Here, we show that the mammalian gut contains a rich fungal community that interacts with the immune system through the innate immune receptor Dectin-1. Mice lacking Dectin-1 exhibited increased susceptibility to chemically induced colitis, which was the result of altered responses to indigenous fungi. In humans, we identified a polymorphism in the gene for Dectin-1 (CLEC7A) that is strongly linked to a severe form of ulcerative colitis. Together, our findings reveal a eukaryotic fungal community in the gut (the "mycobiome") that coexists with bacteria and substantially expands the repertoire of organisms interacting with the intestinal immune system to influence health and disease.

Phagosomal degradation increases TLR access to bacterial ligands and enhances macrophage sensitivity to bacteria

Signaling by innate immune receptors initiates and orchestrates the overall immune responses to infection. Macrophage receptors recognizing pathogens can be broadly grouped into surface receptors and receptors restricted to intracellular compartments, such as phagosomes and the cytoplasm. There is an expectation that ingestion and degradation of microorganisms by phagocytes contributes to activation of intracellular innate receptors, although direct demonstrations of this are rare, and many model ligands are studied in soluble form, outside of their microbial context. By comparing a wild-type strain of Staphylococcus aureus and a lysozyme-sensitive mutant, we have been able directly to address the role of degradation of live bacteria by mouse macrophages in determining the overall innate cellular inflammatory response. Our investigations revealed a biphasic response to S. aureus that consisted of an initial signal resulting from the engagement of surface TLR2, followed by a later, second wave on inflammatory gene induction. This second wave of inflammatory signaling was dependent on and correlated with the timing of bacterial degradation in phagosomes. We found that TLR2 signaling followed by TLR2/TLR9 signaling enhanced sensitivity to small numbers of bacteria. We further found that treating wild-type bacteria with the peptidoglycan synthesis-inhibiting antibiotic vancomycin made S. aureus more susceptible to degradation and resulted in increased inflammatory responses, similar to those observed for mutant degradation-sensitive bacteria.

Adaptor protein Lnk inhibits c-Fms-mediated macrophage function

The M-CSFR (c-Fms) participates in proliferation, differentiation, and survival of macrophages and is involved in the regulation of distinct macrophage functions. Interaction with the ligand M-CSF results in phosphorylation of tyrosine residues on c-Fms, thereby creating binding sites for molecules containing SH2 domains. Lnk is a SH2 domain adaptor protein that negatively regulates hematopoietic cytokine receptors. Here, we show that Lnk binds to c-Fms. Biological and functional effects of this interaction were examined in macrophages from Lnk-deficient (KO) and WT mice. Clonogenic assays demonstrated an elevated number of M-CFUs in the bone marrow of Lnk KO mice. Furthermore, the M-CSF-induced phosphorylation of Akt in Lnk KO macrophages was increased and prolonged, whereas phosphorylation of Erk was diminished. Zymosan-stimulated production of ROS was increased dramatically in a M-CSF-dependent manner in Lnk KO macrophages. Lastly, Lnk inhibited M-CSF-induced migration of macrophages. In summary, we show that Lnk binds to c-Fms and can blunt M-CSF stimulation. Modulation of levels of Lnk in macrophages may provide a unique therapeutic approach to increase innate host defenses.