Dectin-2-dependent host defense in mice infected with serotype 3 Streptococcus pneumoniae

Role of CARD9 in Cell- and Organ-Specific Immune Responses in Various Infections

The caspase recruitment domain-containing protein 9 (CARD9) is an intracellular adaptor protein that is abundantly expressed in cells of the myeloid lineage, such as neutrophils, macrophages, and dendritic cells. CARD9 plays a critical role in host immunity against infections caused by fungi, bacteria, and viruses. A CARD9 deficiency impairs the production of inflammatory cytokines and chemokines as well as migration and infiltration, thereby increasing susceptibility to infections. However, CARD9 signaling varies depending on the pathogen causing the infection. Furthermore, different studies have reported altered CARD9-mediated signaling even with the same pathogen. Therefore, this review focuses on and elucidates the current literature on varied CARD9 signaling in response to various infectious stimuli in humans and experimental mice models.

C-type lectin receptor expression is a hallmark of neutrophils infiltrating the skin in epidermolysis bullosa acquisita

Introduction

Inflammatory epidermolysis bullosa acquisita (EBA) is characterized by a neutrophilic response to anti-type VII collagen (COL7) antibodies resulting in the development of skin inflammation and blistering. The antibody transfer model of EBA closely mirrors this EBA phenotype.

Methods

To better understand the changes induced in neutrophils upon recruitment from peripheral blood into lesional skin in EBA, we performed single-cell RNA-sequencing of whole blood and skin dissociate to capture minimally perturbed neutrophils and characterize their transcriptome.

Results

Through this approach, we identified clear distinctions between circulating activated neutrophils and intradermal neutrophils. Most strikingly, the gene expression of multiple C-type lectin receptors, which have previously been reported to orchestrate host defense against fungi and select bacteria, were markedly dysregulated. After confirming the upregulation of Clec4n, Clec4d, and Clec4e in experimental EBA as well as in lesional skin from patients with inflammatory EBA, we performed functional studies in globally deficient Clec4e-/- and Clec4d-/- mice as well as in neutrophil-specific Clec4n-/- mice. Deficiency in these genes did not reduce disease in the EBA model.

Discussion

Collectively, our results suggest that while the upregulation of Clec4n, Clec4d, and Clec4e is a hallmark of activated dermal neutrophil populations, their individual contribution to the pathogenesis of EBA is dispensable.

CARD9 in host immunity to fungal, bacterial, viral, and parasitic infections: An update

Microbial infection, caused by fungi, bacteria, viruses, and parasites, significantly contributes to the global death burden and health costs. The innate and adaptive immune systems orchestrate a multifaceted signaling response to invading pathogens as the human antimicrobial system. In this process, caspase recruitment domain-containing protein 9 (CARD9) emerges as a critical intermediary adaptor molecule to participate in regulating a series of antimicrobial immune reactions. Previous publications have confirmed that CARD9 plays a crucial role in fungal, bacterial, viral, and parasitic infections. In this study, we aim to provide an update on the recent clinical and basic studies where the mechanism and function of CARD9 have been further studied and understood. In addition, we summarize the latest treatment and prevention strategies based on CARD9 and discuss the current perspectives and future direction of CARD9.

Effects of the Dectin-2/TNF-α Pathway on Ventricular Arrhythmia after Acute Myocardial Infarction in Mice

Background

Inflammatory responses are involved in ischemic injuries and cardiac repair after acute myocardial infarction (AMI). Dectin-2 is a C-type lectin receptor that induces cytokine production and promotes local inflammatory responses.

Methods

Sixty C57BL/6 mice were randomly assigned to a sham-surgery group, AMI group, or AMI + etanercept group, with 20 mice in each group. Programmed electrical stimulation (PES) was used to anesthetized mice to induce ventricular tachycardia. Real-time polymerase chain reaction (PCR) and western blot analysis were adopted to determine the expression and distribution of dectin-2 in heart tissues. The tumor necrosis factor-α (TNF-α), interferon-gamma (IFN)-γ, interleukin (IL) 4, and IL-5 levels in the serum were determined using ELISAs.

Results

The expression of dectin-2 and TNF-α was increased in the myocardium in AMI, and the susceptibility to ventricular arrhythmia (VA) was increased. The induction rate of VA was significantly decreased by etanercept. Compared with those in the sham-surgery group, the AMI group showed significantly higher serum TNF-α and IFN-γ levels and lower IL-4 and IL-5levels.

Conclusion

Dectin-2 intensifies the activation of the TNF-α immune reaction through the Th1 differentiation, which may increase vulnerability to VA in AMI.

Heterogeneity in extracellular vesicle secretion by single human macrophages revealed by super-resolution microscopy

The diverse origins, nanometre‐scale and invasive isolation procedures associated with extracellular vesicles (EVs) mean they are usually studied in bulk and disconnected from their parental cell. Here, we used super‐resolution microscopy to directly compare EVs secreted by individual human monocyte‐derived macrophages (MDMs). MDMs were differentiated to be M0‐, M1‐ or M2‐like, with all three secreting EVs at similar densities following activation. However, M0‐like cells secreted larger EVs than M1‐ and M2‐like macrophages. Proteomic analysis revealed variations in the contents of differently sized EVs as well as between EVs secreted by different MDM phenotypes. Super resolution microscopy of single‐cell secretions identified that the class II MHC protein, HLA‐DR, was expressed on ∼40% of EVs secreted from M1‐like MDMs, which was double the frequency observed for M0‐like and M2‐like EVs. Strikingly, human macrophages, isolated from the resected lungs of cancer patients, secreted EVs that expressed HLA‐DR at double the frequency and with greater intensity than M1‐like EVs. Quantitative analysis of single‐cell EV profiles from all four macrophage phenotypes revealed distinct secretion types, five of which were consistent across multiple sample cohorts. A sub‐population of M1‐like MDMs secreted EVs similar to lung macrophages, suggesting an expansion or recruitment of cells with a specific EV secretion profile within the lungs of cancer patients. Thus, quantitative analysis of EV heterogeneity can be used for single cell profiling and to reveal novel macrophage biology.

Bioinorganic Chemistry of Zinc in Relation to the Immune System

Zinc is well-known to have a central role in human inflammation and immunity and is itself an anti-inflammatory and antiviral agent. Despite its massively documented role in such processes, the underlying chemistry of zinc in relation to specific proteins and pathways of the immune system has not received much focus. This short review provides an overview of this topic, with emphasis on the structures of key proteins, zinc coordination chemistry, and probable mechanisms involved in zinc-based immunity, with some focus points for future chemical and biological research.

Effect of CARD9 Deficiency on Neutrophil-Mediated Host Defense against Pulmonary Infection with Streptococcus pneumoniae

Streptococcus pneumoniae is a major causative bacterium of community-acquired pneumonia. Dendritic cell-associated C-type lectin-2 (dectin-2), one of the C-type lectin receptors (CLRs), was previously reported to play a pivotal role in host defense against pneumococcal infection through regulating phagocytosis by neutrophils while not being involved in neutrophil accumulation. In the present study, to elucidate the possible contribution of other CLRs to neutrophil accumulation, we examined the role of caspase recruitment domain-containing protein 9 (CARD9), a common adaptor molecule for signal transduction triggered by CLRs, in neutrophilic inflammatory response against pneumococcal infection. Wild-type (WT), CARD9 knockout (KO), and dectin-2 KO mice were infected intratracheally with pneumococcus, and the infected lungs were histopathologically analyzed to assess neutrophil accumulation at 24 h postinfection. Bronchoalveolar lavage fluids (BALFs) were collected at the same time point to count the neutrophils and assess the production of inflammatory cytokines and chemokines. Neutrophil accumulation was significantly decreased in CARD9 KO mice, but not in dectin-2 KO mice. Tumor necrosis factor alpha (TNF-α), keratinocyte-derived chemokine (KC), and macrophage inflammatory protein-2 (MIP-2) production in BALFs were also attenuated in CARD9 KO mice, but not in dectin-2 KO mice. Production of TNF-α and KC by alveolar macrophages stimulated with pneumococcal culture supernatants was significantly attenuated in CARD9 KO mice, but not in dectin-2 KO mice, compared to that in each group's respective control mice. In addition, pneumococcus-infected CARD9 KO mice showed larger bacterial burdens in the lungs than did WT mice. These data indicate that CARD9 is required for neutrophil migration after pneumococcal infection, as well as inflammatory cytokine and chemokine production by alveolar macrophages, and suggest that a CLR distinct from dectin-2 may be involved in this response.

Association of modulation of pro-inflammatory responses by dectin-2 with preterm delivery: An experimental model

PROBLEM

Pro-inflammatory responses of pathogen recognition receptors (PRR) are implicated in preterm delivery (PTD). Dectin-2 is one PRR recognizing unselective carbohydrate structures; its participation in PTD has never been studied before.

METHOD OF STUDY

In an experimental model, PTD was induced in female pregnant wild-type (WT) mice and mice with homologous deficiency for dectin-2 by the intraperitoneal injection of bacterial lipopolysaccharide (LPS) on day 14 of pregnancy. Time to delivery and fetal mortality were recorded. Challenged mice were killed for tissue collection and splenocyte isolation 6 hours later. Concentrations of tumour necrosis factor-alpha (TNFα), interleukin (IL)-1α, and IL-1β were measured.

RESULTS

Delivery was induced significantly earlier in WT than dectin-2^-/- mice; however, fetal mortality was higher among dectin-2^-/- mice. Candida albicans challenge could not lead to these changes. Sacrifice experiments showed that LPS challenge led to significant increase of TNFα, IL-1α, and IL-1β in maternal tissues of WT; this was further enhanced for TNFα and IL-1β in dectin-2^-/- mice. Pre-treatment with the prostaglandin inhibitor diclofenac delayed time to delivery of WT mice, but not of dectin2^-/- mice. TNFα stimulation of splenocytes of dectin2^-/- mice was enhanced with the addition of anti-TLR4 and decreased in the presence of lipid A.

CONCLUSIONS

Dectin-2 delays LPS-induced PTD by enhancing the production of pro-inflammatory cytokines.

C-type Lectins in Immunity to Lung Pathogens

The respiratory tract is tasked with responding to a constant and vast influx of foreign agents. It acts as an important first line of defense in the innate immune system and as such plays a crucial role in preventing the entry of invading pathogens. While physical barriers like the mucociliary escalator exert their effects through the clearance of these pathogens, diverse and dynamic cellular mechanisms exist for the activation of the innate immune response through the recognition of pathogen-associated molecular patterns (PAMPs). These PAMPs are recognized by pattern recognition receptors (PRRs) that are expressed on a number of myeloid cells such as dendritic cells, macrophages, and neutrophils found in the respiratory tract. C-type lectin receptors (CLRs) are PRRs that play a pivotal role in the innate immune response and its regulation to a variety of respiratory pathogens such as viruses, bacteria, and fungi. This chapter will describe the function of both activating and inhibiting myeloid CLRs in the recognition of a number of important respiratory pathogens as well as the signaling events initiated by these receptors.

Congenitally impaired pattern-recognition receptors in pathogenesis of pediatric invasive and recurrent pneumococcal infection

Here we review currently available data showing that innate immune signs predisposing to recurrent and invasive pneumococcal infections were identified in children. Streptococcus pneumoniae (pneumococcus) belongs to Grampositive bacteria being the major cause of morbidity and mortality in infants, especially in developing countries and in communities with low socioeconomic status. Due to the lack of anti-pneumococcal vaccination, the significant proportion of pneumococcus carriers develop non-invasive (pneumonia, otitis media, sinusitis) and severe invasive (bacteremia/septicemia, meningitis) pneumococcal infection. A great deal of diverse factors related to pneumococcus biological features (virulence factors) as well individualized host-specific immunity are implicated in efficient bacterial penetration across the mucous membranes. The TLR signaling system plays a crucial role in the human nonspecific defense upon the first encounter with the pathogen. Various TLRs comprise the first pattern recognition receptor fami ly ever described which sense ligands derived from the outer bacterial wall. The complement system is the ancient innate immunity component mainly involved in intravascular elimination of bacterial agents. In addition, the complement proteins serve as a bridge between innate and adaptive immunity, ensuring optimal conditions for B- and T-cell maturation and differentiation. Because pneumococcus secretes the IgA protease, a local protective effects related to IgA antibodies might not be so prominent. Therefore, B-cell immunodeficiency and impaired complement system hold a lead place among congenital causes resulting in severe and recurrent pneumococcal infections in children. Thus, based on available data, we concluded that impaired B-cell function, the complement components deficiency as well as receptor-recognition receptors (TLR-2, -9, -4, MYD88 adapter protein, TLR cascade enzymes: IRAK4, NEMO, NOD-like receptors: NOD2, NLRP3; C-type lectins: MBL, Dextin-2, and, possibly, ficoline) play the most important role among congenital immunodeficiencies predisposing to invasive and recurrent pneumococcal infections play the most important role among congenital immunodeficiencies predisposing to invasive and recurrent pneumococcal infections, and should be used as a rationale for immunological surveillance and organizing immunogenetics screening in these patients.  

Nasal vaccination with pneumococcal surface protein A in combination with cationic liposomes consisting of DOTAP and DC-chol confers antigen-mediated protective immunity against Streptococcus pneumoniae infections in mice

Infectious diseases are the second leading cause of death worldwide, suggesting that there is still a need for the development of new and improved strategies for combating pathogens effectively. Streptococcus pneumoniae is the most virulent bacteria causing pneumonia with high mortality, especially in children and the elderly. Because of the emergence of antibiotic resistance in S. pneumoniae, employing a serotype-independent mucosal vaccine would be the best approach to prevent and treat the diseases caused by S. pneumoniae. In this study, we have developed a pneumococcal nasal vaccine, consisting of pneumococcal surface protein A (PspA) and cationic liposomes composed of 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) and cholesteryl 3β-N-(dimethylaminoethyl)-carbamate (DC-chol) (DOTAP/DC-chol liposome). The efficiency of this cationic liposome-based PspA nasal vaccine was examined in a murine model of S. pneumoniae infection. Intranasal vaccination with PspA and DOTAP/DC-chol liposomes conferred protective immunity against lethal inhalation of S. pneumoniae, improving the survival rate of infected mice. Moreover, intranasal immunization with PspA and DOTAP/DC-chol liposomes not only induced the production of PspA-specific IgA and IgG by both mucosal and systemic compartments but also elicited PspA-specific Th17 responses, which play a pivotal role in controlling S. pneumoniae infection by host innate immune response. We further demonstrated that DOTAP/DC-chol liposomes enhanced PspA uptake by nasal dendritic cells (DCs), which might be a mechanism for the induction of protective immune responses to S. pneumoniae infection. These results show that DOTAP/DC-chol liposome would be an efficient mucosal vaccine system for a serotype-independent universal nasal vaccine against pneumococcal infection.

C-type lectin receptors in anti-fungal immunity

Host immune systems are constantly engaged with fungal pathogens which are common in environments as well as in healthy human skin and mucosa. C-type lectin receptors (CLRs) are expressed in myeloid cells and play central roles in host defenses against fungal infections by coordinating innate and adaptive immune systems. Upon ligand binding, CLRs stimulate cellular responses by inducing the production of cytokines and reactive oxygen species via the Syk/CARD9 signaling pathway, leading to fungal elimination. Due to identification and characterization of the CLRs, the underlying mechanisms of the anti-fungal immunity are being unveiled in the present decade. In this review, we focus on the anti-fungal activities of CLRs and summarize of current knowledge of the related expression profiles, modes of ligand recognition, and signaling cascades.

Contact, Collaboration, and Conflict: Signal Integration of Syk-Coupled C-Type Lectin Receptors

Several spleen tyrosine kinase-coupled C-type lectin receptors (CLRs) have emerged as important pattern recognition receptors for infectious danger. Because encounter with microbial pathogens leads to the simultaneous ligation of several CLRs and TLRs, the signals emanating from different pattern recognition receptors have to be integrated to achieve appropriate biological responses. In this review, we briefly summarize current knowledge about ligand recognition and core signaling by Syk-coupled CLRs. We then address mechanisms of synergistic and antagonistic crosstalk between different CLRs and with TLRs. Emerging evidence suggests that signal integration occurs through 1) direct interaction between receptors, 2) regulation of expression levels and localization, and 3) collaborative or conflicting signaling interference. Accordingly, we aim to provide a conceptual framework for the complex and sometimes unexpected outcome of CLR ligation in bacterial and fungal infection.

Dectin-2 Deficiency Modulates Th1 Differentiation and Improves Wound Healing After Myocardial Infarction

RATIONALE

Macrophages are involved in wound healing after myocardial infarction (MI). The role of Dectin-2, a pattern recognition receptor mainly expressed on myeloid cells, in the infarct healing remains unknown.

OBJECTIVE

The aim of this study is to determine whether Dectin-2 signaling is involved in the healing process and cardiac remodeling after MI and to elucidate the underlying molecular mechanisms.

METHODS AND RESULTS

In a mouse model of permanent coronary ligation, Dectin-2, mainly expressed in macrophages, was shown to be increased in the early phase after MI. Dectin-2 knockout mice showed an improvement in the infarct healing and cardiac remodeling, compared with wild-type mice, which was demonstrated by significantly lower mortality because of cardiac rupture, increased wall thickness, and better cardiac function. Increased expression of α-smooth muscle actin and collagen I/III was observed, whereas the levels of matrix metalloproteinase-2 and matrix metalloproteinase-9 were decreased in the hearts of Dectin-2 knockout mice after MI. Dectin-2 deficiency inhibited the rate of apoptotic and necrotic cell death. However, Dectin-2 did not affect immune cell infiltration and macrophage polarization, but it led to a stronger activation of the Th1/interferon-γ immune reaction, through the enhancement of interleukin-12 production in the heart. Interferon-γ was shown to downregulate transforming growth factor-β-induced expression of α-smooth muscle actin and collagen I/III in isolated cardiac fibroblasts, leading to a decrease in migration and myofibroblast differentiation. Finally, Dectin-2 knockout improved myocardial ischemia-reperfusion injury and infarct healing.

CONCLUSIONS

Dectin-2 leads to an increase in cardiac rupture, impairs wound healing, and aggravates cardiac remodeling after MI through the modulation of Th1 differentiation.

Mechanistic Insights into the Role of C-Type Lectin Receptor/CARD9 Signaling in Human Antifungal Immunity

Human CARD9 deficiency is an autosomal recessive primary immunodeficiency disorder caused by biallelic mutations in the gene CARD9, which encodes a signaling protein that is found downstream of many C-type lectin receptors (CLRs). CLRs encompass a large family of innate recognition receptors, expressed predominantly by myeloid and epithelial cells, which bind fungal carbohydrates and initiate antifungal immune responses. Accordingly, human CARD9 deficiency is associated with the spontaneous development of persistent and severe fungal infections that primarily localize to the skin and subcutaneous tissue, mucosal surfaces and/or central nervous system (CNS). In the last 3 years, more than 15 missense and nonsense CARD9 mutations have been reported which associate with the development of a wide spectrum of fungal infections caused by a variety of fungal organisms. The mechanisms by which CARD9 provides organ-specific protection against these fungal infections are now emerging. In this review, we summarize recent immunological and clinical advances that have provided significant mechanistic insights into the pathogenesis of human CARD9 deficiency. We also discuss how genetic mutations in CARD9-coupled receptors (Dectin-1, Dectin-2) and CARD9-binding partners (MALT1, BCL10) affect human antifungal immunity relative to CARD9 deficiency, and we highlight major understudied research questions which merit future investigation.