Myosin II Synergizes with F-Actin to Promote DNGR-1-Dependent Cross-Presentation of Dead Cell-Associated Antigens

Hydatid fluid from Echinococcus granulosus induces autophagy in dendritic cells and promotes polyfunctional T-cell responses

Parasites possess remarkable abilities to evade and manipulate the immune response of their hosts. Echinococcus granulosus is a parasitic tapeworm that causes cystic echinococcosis in animals and humans. The hydatid fluid released by the parasite is known to contain various immunomodulatory components that manipulate host´s defense mechanism. In this study, we focused on understanding the effect of hydatid fluid on dendritic cells and its impact on autophagy induction and subsequent T cell responses. Initially, we observed a marked downregulation of two C-type lectin receptors in the cell membrane, CLEC9A and CD205 and an increase in lysosomal activity, suggesting an active cellular response to hydatid fluid. Subsequently, we visualized ultrastructural changes in stimulated dendritic cells, revealing the presence of macroautophagy, characterized by the formation of autophagosomes, phagophores, and phagolysosomes in the cell cytoplasm. To further elucidate the underlying molecular mechanisms involved in hydatid fluid-induced autophagy, we analyzed the expression of autophagy-related genes in stimulated dendritic cells. Our results demonstrated a significant upregulation of beclin-1, atg16l1 and atg12, indicating the induction of autophagy machinery in response to hydatid fluid exposure. Additionally, using confocal microscopy, we observed an accumulation of LC3 in dendritic cell autophagosomes, confirming the activation of this catabolic pathway associated with antigen presentation. Finally, to evaluate the functional consequences of hydatid fluid-induced autophagy in DCs, we evaluated cytokine transcription in the splenocytes. Remarkably, a robust polyfunctional T cell response, with inhibition of Th2 profile, is characterized by an increase in the expression of il-6, il-10, il-12, tnf-α, ifn-γ and tgf-β genes. These findings suggest that hydatid fluid-induced autophagy in dendritic cells plays a crucial role in shaping the subsequent T cell responses, which is important for a better understanding of host-parasite interactions in cystic echinococcosis.

SYK ubiquitination by CBL E3 ligases restrains cross-presentation of dead cell-associated antigens by type 1 dendritic cells

Cross-presentation of dead cell-associated antigens by conventional dendritic cells type 1 (cDC1s) is critical for CD8+ T cells response against many tumors and viral infections. It is facilitated by DNGR-1 (CLEC9A), an SYK-coupled cDC1 receptor that detects dead cell debris. Here, we report that DNGR-1 engagement leads to rapid activation of CBL and CBL-B E3 ligases to cause K63-linked ubiquitination of SYK and terminate signaling. Genetic deletion of CBL E3 ligases or charge-conserved mutation of target lysines within SYK abolishes SYK ubiquitination and results in enhanced DNGR-1-dependent antigen cross-presentation. We also find that cDC1 deficient in CBL E3 ligases are more efficient at cross-priming CD8+ T cells to dead cell-associated antigens and promoting host resistance to tumors. Our findings reveal a role for CBL-dependent ubiquitination in limiting cross-presentation of dead cell-associated antigens and highlight an axis of negative regulation of cDC1 activity that could be exploited to increase anti-tumor immunity.

Identification of MYH9 as a key regulator for synoviocyte migration and invasion through secretome profiling

OBJECTIVES

'Invasive pannus' is a pathological hallmark of rheumatoid arthritis (RA). This study aimed to investigate secretome profile of synovial fibroblasts of patients with RA (RA-FLSs), a major cell type comprising the invasive pannus.

METHODS

Secreted proteins from RA-FLSs were first identified using liquid chromatography-tandem mass spectrometry analysis. Ultrasonography was performed for affected joints to define synovitis severity at the time of arthrocentesis. Expression levels of myosin heavy chain 9 (MYH9) in RA-FLSs and synovial tissues were determined by ELISA, western blot analysis and immunostaining. A humanised synovitis model was induced in immuno-deficient mice.

RESULTS

We first identified 843 proteins secreted from RA-FLSs; 48.5% of the secretome was associated with pannus-driven pathologies. Parallel reaction monitoring analysis of the secretome facilitated discovery of 16 key proteins related to 'invasive pannus', including MYH9, in the synovial fluids, which represented synovial pathology based on ultrasonography and inflammatory activity in the joints. Particularly, MYH9, a key protein in actin-based cell motility, showed a strong correlation with fibroblastic activity in the transcriptome profile of RA synovia. Moreover, MYH9 expression was elevated in cultured RA-FLSs and RA synovium, and its secretion was induced by interleukin-1β, tumour necrosis factor α, toll-like receptor ligation and endoplasmic reticulum stimuli. Functional experiments demonstrated that MYH9 promoted migration and invasion of RA-FLSs in vitro and in a humanised synovitis model, which was substantially inhibited by blebbistatin, a specific MYH9 inhibitor.

CONCLUSIONS

This study provides a comprehensive resource of the RA-FLS-derived secretome and suggests that MYH9 represents a promising target for retarding abnormal migration and invasion of RA-FLSs.

High Bacillary Burden and the ESX-1 Type VII Secretion System Promote MHC Class I Presentation by Mycobacterium tuberculosis-Infected Macrophages to CD8 T Cells

We used a mouse model to study how Mycobacterium tuberculosis subverts host defenses to persist in macrophages despite immune pressure. CD4 T cells can recognize macrophages infected with a single bacillus in vitro. Under identical conditions, CD8 T cells inefficiently recognize infected macrophages and fail to restrict M. tuberculosis growth, although they can inhibit M. tuberculosis growth during high-burden intracellular infection. We show that high intracellular M. tuberculosis numbers cause macrophage death, leading other macrophages to scavenge cellular debris and cross-present the TB10.4 Ag to CD8 T cells. Presentation by infected macrophages requires M. tuberculosis to have a functional ESX-1 type VII secretion system. These data indicate that phagosomal membrane damage and cell death promote MHC class I presentation of the immunodominant Ag TB10.4 by macrophages. Although this mode of Ag presentation stimulates cytokine production that we presume would be host beneficial, killing of uninfected cells could worsen immunopathology. We suggest that shifting the focus of CD8 T cell recognition to uninfected macrophages would limit the interaction of CD8 T cells with infected macrophages and impair CD8 T cell-mediated resolution of tuberculosis.

The evolving biology of cross-presentation

Cross-priming was first recognized in the context of in vivo cytotoxic T lymphocyte (CTL) responses generated against minor histocompatibility antigens induced by immunization with lymphoid cells. Even though the basis for T cell antigen recognition was still largely unclear at that time, these early studies recognized the implication that such minor histocompatibility antigens were derived from the immunizing cells and were obtained exogenously by the host's antigen presenting cells (APCs) that directly prime the CTL response. As antigen recognition by the T cell receptor became understood to involve peptides derived from antigens processed by the APCs and presented by major histocompatibility molecules, the "cross-priming" phenomenon was subsequently recast as "cross-presentation" and the scope considered for examining this process gradually broadened to include many different forms of antigens, including soluble proteins, and different types of APCs that may not be involved in in vivo CTL priming. Many studies of cross-presentation have relied on in vitro cell models that were recently found to differ from in vivo APCs in particular mechanistic details. A recent trend has focused on the APCs and pathways of cross-presentation used in vivo, especially the type 1 dendritic cells. Current efforts are also being directed towards validating the in vivo role of various putative pathways and gene candidates in cross-presentation garnered from various in vitro studies and to determine the relative contributions they make to CTL responses across various forms of antigens and immunologic settings. Thus, cross-presentation appears to be carried by different pathways in various types of cells for different forms under different physiologic settings, which remain to be evaluated in an in vivo physiologic setting.

DNGR-1-mediated cross-presentation of dead cell-associated antigens

Conventional dendritic cells type 1 (cDC1) are critical for inducing protective CD8⁺ T cell responses to tumour and viral antigens. In many instances, cDC1 access those antigens in the form of material internalised from dying tumour or virally-infected cells. How cDC1 extract dead cell-associated antigens and cross-present them in the form of peptides bound to MHC class I molecules to CD8⁺ T cells remains unclear. Here we review the biology of dendritic cell natural killer group receptor-1 (DNGR-1; also known as CLEC9A), a C-type lectin receptor highly expressed on cDC1 that plays a key role in this process. We highlight recent advances that support a function for DNGR-1 signalling in promoting inducible rupture of phagocytic or endocytic compartments containing dead cell debris, thereby making dead cell-associated antigens accessible to the endogenous MHC class I processing and presentation machinery of cDC1. We further review how DNGR-1 detects dead cells, as well as the functions of the receptor in anti-viral and anti-tumour immunity. Finally, we highlight how the study of DNGR-1 has opened new perspectives into cross-presentation, some of which may have applications in immunotherapy of cancer and vaccination against viral diseases.

Once upon a prime: DCs shape cancer immunity

Cytotoxic CD8+ T cells are potent killers of diseased cells, but their functional capacity is often compromised in cancer. The quality of antitumor T cell immunity is determined during T cell priming in the lymph node and further influenced by the local microenvironment of the tumor. Increasing evidence indicates that dendritic cells (DCs) have the capacity to precisely regulate the functional quality of antitumor T cell responses in both locations. In this review, we discuss recent advances in our understanding of how distinct DC-derived signals influence CD8+ T cell differentiation and antitumor functions. Insight into the mechanisms of DC-mediated regulation of antitumor immunity could inspire the development of improved approaches to prevent and reverse T cell dysfunction in cancer.

The diverse roles of C-type lectin-like receptors in immunity

Our understanding of the C-type lectin-like receptors (CTLRs) and their functions in immunity have continued to expand from their initial roles in pathogen recognition. There are now clear examples of CTLRs acting as scavenger receptors, sensors of cell death and cell transformation, and regulators of immune responses and homeostasis. This range of function reflects an extensive diversity in the expression and signaling activity between individual CTLR members of otherwise highly conserved families. Adding to this diversity is the constant discovery of new receptor binding capabilities and receptor-ligand interactions, distinct cellular expression profiles, and receptor structures and signaling mechanisms which have expanded the defining roles of CTLRs in immunity. The natural killer cell receptors exemplify this functional diversity with growing evidence of their activity in other immune populations and tissues. Here, we broadly review select families of CTLRs encoded in the natural killer cell gene complex (NKC) highlighting key receptors that demonstrate the complex multifunctional capabilities of these proteins. We focus on recent evidence from research on the NKRP1 family of CTLRs and their interaction with the related C-type lectin (CLEC) ligands which together exhibit essential immune functions beyond their defined activity in natural killer (NK) cells. The ever-expanding evidence for the requirement of CTLR in numerous biological processes emphasizes the need to better understand the functional potential of these receptor families in immune defense and pathological conditions.

Increased expression of Clec9A on cDC1s associated with cytotoxic CD8+ T cell response in COPD

Although C-type lectin domain family 9A (Clec9A) on conventional type 1 dendritic cells (cDC1s) plays a critical role in cytotoxic CD8⁺ T cell response in cancers and viral infections, its role in chronic obstructive pulmonary disease (COPD) is unknown. We measured the expression of Clec9A in sera, BALF, and PBMCs from controls and COPD patients. The percentages of Clec9A⁺ DCs and cytotoxic CD8⁺ T cells in the BALF were determined by flow cytometry between patients with COPD and non-obstructive chronic bronchitis (NOCB). Compared with healthy individuals, the serum levels of Clec9A were increased at different stages of COPD patients, and the mRNA and protein levels of Clec9A were both increased in COPD patients at GOLD stages III-IV. The percentage of Clec9A⁺ DCs was also increased in the BALF of COPD patients compared with NOCB patients. Moreover, enhanced Clec9A⁺ DCs recruitment was positively correlated with cytotoxic CD8⁺ T cell response in the BALF of COPD patients. This study suggests that Clec9A⁺ DCs participate in the CD8⁺ T cell-mediated chronic airway inflammation in COPD.

The Delivery of Extracellular “Danger” Signals to Cytosolic Sensors in Phagocytes

Phagocytes, such as macrophages and dendritic cells, possess the ability to ingest large quantities of exogenous material into membrane-bound endocytic organelles such as macropinosomes and phagosomes. Typically, the ingested material, which consists of diverse macromolecules such as proteins and nucleic acids, is delivered to lysosomes where it is digested into smaller molecules like amino acids and nucleosides. These smaller molecules can then be exported out of the lysosomes by transmembrane transporters for incorporation into the cell’s metabolic pathways or for export from the cell. There are, however, exceptional instances when undigested macromolecules escape degradation and are instead delivered across the membrane of endocytic organelles into the cytosol of the phagocyte. For example, double stranded DNA, a damage associated molecular pattern shed by necrotic tumor cells, is endocytosed by phagocytes in the tumor microenvironment and delivered to the cytosol for detection by the cytosolic “danger” sensor cGAS. Other macromolecular “danger” signals including lipopolysaccharide, intact proteins, and peptidoglycans can also be actively transferred from within endocytic organelles to the cytosol. Despite the obvious biological importance of these processes, we know relatively little of how macromolecular “danger” signals are transferred across endocytic organelle membranes for detection by cytosolic sensors. Here we review the emerging evidence for the active cytosolic transfer of diverse macromolecular “danger” signals across endocytic organelle membranes. We will highlight developing trends and discuss the potential molecular mechanisms driving this emerging phenomenon.

Organellar homeostasis and innate immune sensing

A cell is delimited by numerous borders that define specific organelles. The walls of some organelles are particularly robust, such as in mitochondria or endoplasmic reticulum, but some are more fluid such as in phase-separated stress granules. Either way, all organelles can be damaged at times, leading their contents to leak out into the surrounding environment. Therefore, an elegant way to construct an innate immune defence system is to recognize host molecules that do not normally reside within a particular compartment. Here, we provide several examples where organellar homeostasis is lost, leading to the activation of a specific innate immune sensor; these include NLRP3 activation owing to a disrupted trans-Golgi network, Pyrin activation due to cytoskeletal damage, and cGAS-STING activation following the leakage of nuclear or mitochondrial DNA. Frequently, organelle damage is observed downstream of pathogenic infection but it can also occur in sterile settings as associated with auto-inflammatory disease. Therefore, understanding organellar homeostasis is central to efforts that will identify new innate immune pathways, and therapeutics that balance organellar homeostasis, or target the breakdown pathways that trigger innate immune sensors, could be useful treatments for infection and chronic inflammatory diseases.

Fc-conjugated C-type lectin receptors: Tools for understanding host-pathogen interactions

The use of soluble fusion proteins of pattern recognition receptors (PRRs) used in the detection of exogenous and endogenous ligands has helped resolve the roles of PRRs in the innate immune response to pathogens, how they shape the adaptive immune response, and function in maintaining homeostasis. Using the immunoglobulin (Ig) crystallizable fragment (Fc) domain as a fusion partner, the PRR fusion proteins are soluble, stable, easily purified, have increased affinity due to the Fc homodimerization properties, and consequently have been used in a wide range of applications such as flow cytometry, screening of protein and glycan arrays, and immunofluorescent microscopy. This review will predominantly focus on the recognition of pathogens by the cell membrane-expressed glycan-binding proteins of the C-type lectin receptor (CLR) subgroup of PRRs. PRRs bind to conserved pathogen-associated molecular patterns (PAMPs), such as glycans, usually located within or on the outer surface of the pathogen. Significantly, many glycans structures are identical on both host and pathogen (e.g. the Lewis (Le) X glycan), allowing the use of Fc CLR fusion proteins with known endogenous and/or exogenous ligands as tools to identify pathogen structures that are able to interact with the immune system. Screens of highly purified pathogen-derived cell wall components have enabled identification of many unique PAMP structures recognized by CLRs. This review highlights studies using Fc CLR fusion proteins, with emphasis on the PAMPs found in fungi, bacteria, viruses, and parasites. The structure and unique features of the different CLR families is presented using examples from a broad range of microbes whenever possible.

Secreted gelsolin inhibits DNGR-1-dependent cross-presentation and cancer immunity

Cross-presentation of antigens from dead tumor cells by type 1 conventional dendritic cells (cDC1s) is thought to underlie priming of anti-cancer CD8+ T cells. cDC1 express high levels of DNGR-1 (a.k.a. CLEC9A), a receptor that binds to F-actin exposed by dead cell debris and promotes cross-presentation of associated antigens. Here, we show that secreted gelsolin (sGSN), an extracellular protein, decreases DNGR-1 binding to F-actin and cross-presentation of dead cell-associated antigens by cDC1s. Mice deficient in sGsn display increased DNGR-1-dependent resistance to transplantable tumors, especially ones expressing neoantigens associated with the actin cytoskeleton, and exhibit greater responsiveness to cancer immunotherapy. In human cancers, lower levels of intratumoral sGSN transcripts, as well as presence of mutations in proteins associated with the actin cytoskeleton, are associated with signatures of anti-cancer immunity and increased patient survival. Our results reveal a natural barrier to cross-presentation of cancer antigens that dampens anti-tumor CD8+ T cell responses.

Inflammasomes and adaptive immune responses

A fundamental concept in immunology is that the innate immune system initiates or instructs downstream adaptive immune responses. Inflammasomes are central players in innate immunity to pathogens, but how inflammasomes shape adaptive immunity is complex and relatively poorly understood. Here we highlight recent work on the interplay between inflammasomes and adaptive immunity. We address how inflammasome-dependent release of cytokines and antigen activates, shapes or even inhibits adaptive immune responses. We consider how distinct tissue or cellular contexts may alter the effects of inflammasome activation on adaptive immunity and how this contributes to beneficial or detrimental outcomes in infectious diseases, cancer and autoimmunity. We aspire to provide a framework for thinking about inflammasomes and their connection to the adaptive immune response.

Expanding the Known Repertoire of C-Type Lectin Receptors Binding to Toxoplasma gondii Oocysts Using a Modified High-Resolution Immunofluorescence Assay

The environmental stage of the apicomplexan Toxoplasma gondii oocyst is vital to its life cycle but largely understudied. Because oocysts are excreted only by infected felids, their availability for research is limited. We report the adaptation of an agarose-based method to immobilize minute amounts of oocysts to perform immunofluorescence assays. Agarose embedding allows high-resolution confocal microscopy imaging of antibodies binding to the oocyst surface as well as unprecedented imaging of intracellular sporocyst structures with Maclura pomifera agglutinin after on-slide permeabilization of the immobilized oocysts. To identify new possible molecules binding to the oocyst surface, we used this method to screen a library of C-type lectin receptor (CLR)-human IgG constant region fusion proteins from the group of related CLRs called the Dectin-1 cluster against oocysts. In addition to CLEC7A that was previously reported to decorate T. gondii oocysts, we present experimental evidence for specific binding of three additional CLRs to the surface of this stage. We discuss how these CLRs, known to be expressed on neutrophils, dendritic cells, or macrophages, could be involved in the early immune response by the host, such as oocyst antigen uptake in the intestine. In conclusion, we present a modified immunofluorescence assay technique that allows material-saving immunofluorescence microscopy with T. gondii oocysts in a higher resolution than previously published, which allowed us to describe three additional CLRs binding specifically to the oocyst surface.

IMPORTANCE Knowledge of oocyst biology of Toxoplasma gondii is limited, not the least due to its limited availability. We describe a method that permits us to process minute amounts of oocysts for immunofluorescence microscopy without compromising their structural properties. This method allowed us to visualize internal structures of sporocysts by confocal microscopy in unprecedented quality. Moreover, the method can be used as a low- to medium-throughput method to screen for molecules interacting with oocysts, such as antibodies, or compounds causing structural damage to oocysts (i.e., disinfectants). Using this method, we screened a small library of C-type lectin receptors (CLRs) present on certain immune cells and found three CLRs able to decorate the oocyst wall of T. gondii and which were not known before to bind to oocysts. These tools will allow further study into oocyst wall composition and could also provoke experiments regarding immunological recognition of oocysts.

Enhancing the immunogenicity of cancer vaccines by harnessing CLEC9A

Dendritic cell (DC) vaccines are a safe and effective means of inducing tumor immune responses, however, a better understanding of DC biology is required in order to realize their full potential. Recent advances in DC biology have identified a crucial role for cDC1 in tumor immune responses, making this DC subset an attractive vaccine target. Human cDC1 exclusively express the C-type-lectin-like receptor, CLEC9A (DNGR-1) that plays an important role in cross-presentation, the process by which effective CD8⁺ T cell responses are generated. CLEC9A antibodies deliver antigen specifically to cDC1 for the induction of humoral, CD4⁺ and CD8⁺ T cell responses and are therefore promising candidates to develop as vaccines for infectious diseases and cancer. The development of human CLEC9A antibodies now facilitates their application as vaccines for cancer immunotherapy. Here we discuss the recent advances in CLEC9A targeting antibodies as vaccines for cancer and their translation to the clinic.

The receptor DNGR-1 signals for phagosomal rupture to promote cross-presentation of dead-cell-associated antigens

Type 1 conventional dendritic (cDC1) cells are necessary for cross-presentation of many viral and tumor antigens to CD8+ T cells. cDC1 cells can be identified in mice and humans by high expression of DNGR-1 (also known as CLEC9A), a receptor that binds dead-cell debris and facilitates XP of corpse-associated antigens. Here, we show that DNGR-1 is a dedicated XP receptor that signals upon ligand engagement to promote phagosomal rupture. This allows escape of phagosomal contents into the cytosol, where they access the endogenous major histocompatibility complex class I antigen processing pathway. The activity of DNGR-1 maps to its signaling domain, which activates SYK and NADPH oxidase to cause phagosomal damage even when spliced into a heterologous receptor and expressed in heterologous cells. Our data reveal the existence of innate immune receptors that couple ligand binding to endocytic vesicle damage to permit MHC class I antigen presentation of exogenous antigens and to regulate adaptive immunity.

RNF41 regulates the damage recognition receptor Clec9A and antigen cross-presentation in mouse dendritic cells

The dendritic cell receptor Clec9A facilitates processing of dead cell-derived antigens for cross-presentation and the induction of effective CD8⁺ T cell immune responses. Here, we show that this process is regulated by E3 ubiquitin ligase RNF41 and define a new ubiquitin-mediated mechanism for regulation of Clec9A, reflecting the unique properties of Clec9A as a receptor specialized for delivery of antigens for cross-presentation. We reveal RNF41 is a negative regulator of Clec9A and the cross-presentation of dead cell-derived antigens by mouse dendritic cells. Intriguingly, RNF41 regulates the downstream fate of Clec9A by directly binding and ubiquitinating the extracellular domains of Clec9A. At steady-state, RNF41 ubiquitination of Clec9A facilitates interactions with ER-associated proteins and degradation machinery to control Clec9A levels. However, Clec9A interactions are altered following dead cell uptake to favor antigen presentation. These findings provide important insights into antigen cross-presentation and have implications for development of approaches to modulate immune responses.

Sensing Tissue Damage by Myeloid C-Type Lectin Receptors

After both sterile and infectious insults, damage is inflicted on tissues leading to accidental or programmed cell death. In addition, events of programmed cell death also take place under homeostatic conditions, such as in embryo development or in the turnover of hematopoietic cells. Mammalian tissues are seeded with myeloid immune cells, which harbor a plethora of receptors that allow the detection of cell death, modulating immune responses. The myeloid C-type lectin receptors (CLRs) are one of the most prominent families of receptors involved in tailoring immunity after sensing dead cells. In this chapter, we will cover a diversity of signals arising from different forms of cell death and how they are recognized by myeloid CLRs. We will also explore how myeloid cells develop their sentinel function, exploring how some of these CLRs identify cell death and the type of responses triggered thereof. In particular, we will focus on DNGR-1 (CLEC9A), Mincle (CLEC4E), CLL-1 (CLEC12A), LOX-1 (OLR1), CD301 (CLEC10A) and DEC-205 (LY75) as paradigmatic death-sensing CLRs expressed by myeloid cells. The molecular processes triggered after cell death recognition by myeloid CLRs contribute to the regulation of immune responses in pathologies associated with tissue damage, such as infection, autoimmunity and cancer. A better understanding of these processes may help to improve the current approaches for therapeutic intervention.

Cracking the Cell Death Code

Cell death is an invariant feature throughout our life span, starting with extensive scheduled cell death during morphogenesis and continuing with death under homeostasis in adult tissues. Additionally, cells become victims of accidental, unscheduled death following injury and infection. Cell death in each of these occasions triggers specific and specialized responses in the living cells that surround them or are attracted to the dying/dead cells. These responses sculpt tissues during morphogenesis, replenish lost cells in homeostasis to maintain tissue/system function, and repair damaged tissues after injury. Wherein lies the information that sets in motion the cascade of effector responses culminating in remodeling, renewal, or repair? Here, we attempt to provide a framework for thinking about cell death in terms of the specific effector responses that accompanies various modalities of cell death. We also propose an integrated threefold "cell death code" consisting of information intrinsic to the dying/dead cell, the surroundings of the dying cell, and the identity of the responder.

C-Type Lectin-Like Receptors: Head or Tail in Cell Death Immunity

C-type lectin-like receptors (CLRs) represent a family of transmembrane pattern recognition receptors, expressed primarily by myeloid cells. They recognize not only pathogen moieties for host defense, but also modified self-antigens such as damage-associated molecular patterns released from dead cells. Upon ligation, CLR signaling leads to the production of inflammatory mediators to shape amplitude, duration and outcome of the immune response. Thus, following excessive injury, dysregulation of these receptors leads to the development of inflammatory diseases. Herein, we will focus on four CLRs of the "Dectin family," shown to decode the immunogenicity of cell death. CLEC9A on dendritic cells links F-actin exposed by dying cells to favor cross-presentation of dead-cell associated antigens to CD8⁺ T cells. Nevertheless, CLEC9A exerts also feedback mechanisms to temper neutrophil recruitment and prevent additional tissue damage. MINCLE expressed by macrophages binds nuclear SAP130 released by necrotic cells to potentiate pro-inflammatory responses. However, the consequent inflammation can exacerbate pathogenesis of inflammatory diseases. Moreover, in a tumor microenvironment, MINCLE induces macrophage-induced immune suppression and cancer progression. Similarly, triggering of LOX-1 by oxidized LDL, amplifies pro-inflammatory response but promotes tumor immune escape and metastasis. Finally, CLEC12A that recognizes monosodium urate crystals formed during cell death, inhibits activating signals to prevent detrimental inflammation. Interestingly, CLEC12A also sustains type-I IFN response to finely tune immune responses in case of viral-induced collateral damage. Therefore, CLRs acting in concert as sensors of injury, could be used in a targeted way to treat numerous diseases such as allergies, obesity, tumors, and autoimmunity.

DNGR-1, a Dendritic Cell-Specific Sensor of Tissue Damage That Dually Modulates Immunity and Inflammation

DNGR-1 (encoded by CLEC9A) is a C-type lectin receptor (CLR) with an expression profile that is mainly restricted to type 1 conventional dendritic cells (cDC1s) both in mice and humans. This delimited expression pattern makes it appropriate for defining a cDC1 signature and for therapeutic targeting of this population, promoting immunity in mouse models. Functionally, DNGR-1 binds F-actin, which is confined within the intracellular space in healthy cells, but is exposed when plasma membrane integrity is compromised, as happens in necrosis. Upon F-actin binding, DNGR-1 signals through SYK and mediates cross-presentation of dead cell-associated antigens. Cross-presentation to CD8⁺ T cells promoted by DNGR-1 during viral infections is key for cross-priming tissue-resident memory precursors in the lymph node. However, in contrast to other closely related CLRs such as Dectin-1, DNGR-1 does not activate NFκB. Instead, recent findings show that DNGR-1 can activate SHP-1 to limit inflammation triggered by heterologous receptors, which results in reduced production of inflammatory chemokines and neutrophil recruitment into damaged tissues in both sterile and infectious processes. Hence, DNGR-1 reduces immunopathology associated with tissue damage, promoting disease tolerance to safeguard tissue integrity. How DNGR-1 signals are conditioned by the microenvironment and the detailed molecular mechanisms underlying DNGR-1 function have not been elucidated. Here, we review the expression pattern and structural features of DNGR-1, and the biological relevance of the detection of tissue damage through this CLR.

Phagocytosis of Necrotic Debris at Sites of Injury and Inflammation

Clearance of cellular debris is required to maintain the homeostasis of multicellular organisms. It is intrinsic to processes such as tissue growth and remodeling, regeneration and resolution of injury and inflammation. Most of the removal of effete and damaged cells is performed by macrophages and neutrophils through phagocytosis, a complex phenomenon involving ingestion and degradation of the disposable particles. The study of the clearance of cellular debris has been strongly biased toward the removal of apoptotic bodies; as a result, the mechanisms underlying the removal of necrotic cells have remained relatively unexplored. Here, we will review the incipient but growing knowledge of the phagocytosis of necrotic debris, from their recognition and engagement to their internalization and disposal. Critical insights into these events were gained recently through the development of new in vitro and in vivo models, along with advances in live-cell and intravital microscopy. This review addresses the classes of "find-me" and "eat-me" signals presented by necrotic cells and their cognate receptors in phagocytes, which in most cases differ from the extensively characterized counterparts in apoptotic cell engulfment. The roles of damage-associated molecular patterns, chemokines, lipid mediators, and complement components in recruiting and activating phagocytes are reviewed. Lastly, the physiological importance of necrotic cell removal is emphasized, highlighting the key role of impaired debris clearance in autoimmunity.

Immune Sensing of Cell Death through Recognition of Histone Sequences by C-Type Lectin-Receptor-2d Causes Inflammation and Tissue Injury

The immune system monitors the health of cells and is stimulated by necrosis. Here we examined the receptors and ligands driving this response. In a targeted screen of C-type lectin receptors, a Clec2d reporter responded to lysates from necrotic cells. Biochemical purification identified histones, both free and bound to nucleosomes or neutrophil extracellular traps, as Clec2d ligands. Clec2d recognized poly-basic sequences in histone tails and this recognition was sensitive to post-translational modifications of these sequences. As compared with WT mice, Clec2d^-/- mice exhibited reduced proinflammatory responses to injected histones, and less tissue damage and improved survival in a hepatotoxic injury model. In macrophages, Clec2d localized to the plasma membrane and endosomes. Histone binding to Clec2d did not stimulate kinase activation or cytokine production. Rather, histone-bound DNA stimulated endosomal Tlr9-dependent responses in a Clec2d-dependent manner. Thus, Clec2d binds to histones released upon necrotic cell death, with functional consequences to inflammation and tissue damage.

C-type lectin receptors of the Dectin-1 cluster: Physiological roles and involvement in disease

C-type lectin receptors (CLRs) are essential for multicellular existence, having diverse functions ranging from embryonic development to immune function. One subgroup of CLRs is the Dectin-1 cluster, comprising of seven receptors including MICL, CLEC-2, CLEC-12B, CLEC-9A, MelLec, Dectin-1, and LOX-1. Reflecting the larger CLR family, the Dectin-1 cluster of receptors has a broad range of ligands and functions, but importantly, is involved in numerous pathophysiological processes that regulate health and disease. Indeed, these receptors have been implicated in development, infection, regulation of inflammation, allergy, transplantation tolerance, cancer, cardiovascular disease, arthritis, and other autoimmune diseases. In this mini-review, we discuss the latest advancements in elucidating the function(s) of each of the Dectin-1 cluster CLRs, focussing on their physiological roles and involvement in disease.

Cytoskeletal Exposure in the Regulation of Immunity and Initiation of Tissue Repair

This article reviews and discusses emerging evidence suggesting an evolutionarily-conserved connection between injury-associated exposure of cytoskeletal proteins and the induction of tolerance to infection, repair of tissue damage and restoration of homeostasis. While differences exist between vertebrates and invertebrates with respect to the receptor(s), cell types, and effector mechanisms involved, the response to exposed cytoskeletal proteins appears to be protective and to rely on a conserved signaling cassette involving Src family kinases, the nonreceptor tyrosine kinase Syk, and tyrosine phosphatases. A case is made for research programs that integrate different model organisms in order to increase the understanding of this putative response to tissue damage.

Use of Dendritic Cell Receptors as Targets for Enhancing Anti-Cancer Immune Responses

A successful anti-cancer vaccine construct depends on its ability to induce humoral and cellular immunity against a specific antigen. Targeting receptors of dendritic cells to promote the loading of cancer antigen through an antibody-mediated antigen uptake mechanism is a promising strategy in cancer immunotherapy. Researchers have been targeting different dendritic cell receptors such as Fc receptors (FcR), various C-type lectin-like receptors such as dendritic and thymic epithelial cell-205 (DEC-205), dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN), and Dectin-1 to enhance the uptake process and subsequent presentation of antigen to T cells through major histocompatibility complex (MHC) molecules. In this review, we compare different subtypes of dendritic cells, current knowledge on some important receptors of dendritic cells, and recent articles on targeting those receptors for anti-cancer immune responses in mouse models.

DNGR-1 in dendritic cells limits tissue damage by dampening neutrophil recruitment

Host injury triggers feedback mechanisms that limit tissue damage. Conventional type 1 dendritic cells (cDC1s) express dendritic cell natural killer lectin group receptor-1 (DNGR-1), encoded by the gene Clec9a, which senses tissue damage and favors cross-presentation of dead-cell material to CD8⁺ T cells. Here we find that DNGR-1 additionally reduces host-damaging inflammatory responses induced by sterile and infectious tissue injury in mice. DNGR-1 deficiency leads to exacerbated caerulein-induced necrotizing pancreatitis and increased pathology during systemic Candida albicans infection without affecting fungal burden. This effect is B and T cell-independent and attributable to increased neutrophilia in DNGR-1-deficient settings. Mechanistically, DNGR-1 engagement activates SHP-1 and inhibits MIP-2 (encoded by Cxcl2) production by cDC1s during Candida infection. This consequently restrains neutrophil recruitment and promotes disease tolerance. Thus, DNGR-1-mediated sensing of injury by cDC1s serves as a rheostat for the control of tissue damage, innate immunity, and immunopathology.