T Cell Antifungal Immunity and the Role of C-Type Lectin Receptors
Authors: Emily A. Speakman, Ivy M. Dambuza, Fabián Salazar, Gordon D. Brown
Journal: Trends in Immunology (2019)

CLRs are essential pattern recognition receptors involved in fungal recognition and initiation of protective antifungal immunity.CLRs promote the differentiation of mammalian T helper cell subsets essential for the control of systemic (Th1) and mucosal (Th17) fungal infections.CLRs are involved in antigen presentation, the expression of co-stimulatory molecules, and cytokine secretion; therefore, they can regulate lymphocyte function and adaptive immune responses at different levels.Fungal morphological changes, such as the transition from yeast to hyphae in Candida albicans during tissue invasion, affects recognition by CLRs and impacts on adaptive immune responses.CLRs recognize the fungal component of the microbiome that can influence T cell responses during infection at intestinal and peripheral sites. Fungi can cause disease in humans, from mucocutaneous to life-threatening systemic infections. Initiation of antifungal immunity involves fungal recognition by pattern recognition receptors such as C-type lectin receptors (CLRs). These germline-encoded receptors trigger a multitude of innate responses including phagocytosis, fungal killing, and antigen presentation which can also shape the development of adaptive immunity. Recently, studies have shed light on how CLRs directly or indirectly modulate lymphocyte function. Moreover, CLR-mediated recognition of commensal fungi maintains homeostasis and prevents invasion from opportunistic commensals. We present an overview of current knowledge of antifungal T cell immune responses, with emphasis on the role of C-type lectins, and discuss how these receptors modulate these responses at different levels. Fungi can cause disease in humans, from mucocutaneous to life-threatening systemic infections. Initiation of antifungal immunity involves fungal recognition by pattern recognition receptors such as C-type lectin receptors (CLRs). These germline-encoded receptors trigger a multitude of innate responses including phagocytosis, fungal killing, and antigen presentation which can also shape the development of adaptive immunity. Recently, studies have shed light on how CLRs directly or indirectly modulate lymphocyte function. Moreover, CLR-mediated recognition of commensal fungi maintains homeostasis and prevents invasion from opportunistic commensals. We present an overview of current knowledge of antifungal T cell immune responses, with emphasis on the role of C-type lectins, and discuss how these receptors modulate these responses at different levels. Fungi are abundant in the environment and we are in constant contact with these organisms, from inhaled spores of Aspergillus spp. to commensal fungi (see Glossary) such as Candida spp. that become opportunistic pathogens under particular conditions. Of note, toxins secreted by fungi such as candidalysin can directly damage epithelial membranes and trigger a danger response signaling pathway that activates epithelial immunity [1Moyes D.L. et al.Candidalysin is a fungal peptide toxin critical for mucosal infection.Nature. 2016; 532: 64-68Crossref PubMed Scopus (252) Google Scholar]. Despite high levels of exposure, the incidence of lethal fungal infections in humans is relatively low, mainly because of a highly sophisticated immune system. This is underscored by the increased susceptibility to fungal infections that is associated with loss of immune function, as observed in individuals with HIV/AIDS who present with a range of invasive and noninvasive fungal infections such as cryptococcal meningitis and oropharyngeal candidiasis (OPC), respectively [2Feigal D.W. et al.The prevalence of oral lesions in HIV-infected homosexual and bisexual men: three San Francisco epidemiological cohorts.AIDS. 1991; 5: 519-525Crossref PubMed Google Scholar]. Systemic infections are relatively rare but have high mortality rates that often exceed 50%, depending on underlying conditions [3Montagna M.T. et al.Epidemiology of invasive fungal infections in the intensive care unit: results of a multicenter Italian survey (AURORA Project).Infection. 2013; 41: 645-653Crossref PubMed Scopus (93) Google Scholar]. Successful antifungal immunity relies on both the innate and adaptive immune systems. Innate immunity constitutes the first line of defense, which includes physical barriers such as skin and mucosa, antimicrobial peptides (AMPs), the complement system, and cell-mediated protection. Effector mechanisms of innate immunity are performed by phagocytic cells such as neutrophils, macrophages, and monocytes, which mediate several protective mechanisms including phagocytosis and the production of reactive oxygen species (ROS) and hydrolytic enzymes that can directly kill fungal pathogens, as well as releasing inflammatory mediators such as cytokines [4Brown G.D. Innate antifungal immunity: the key role of phagocytes.Annu. Rev. Immunol. 2011; 29: 1-21Crossref PubMed Scopus (223) Google Scholar]. Epithelial cells can also promote protection against fungi by secreting AMPs that have fungicidal and fungistatic activity through permeabilization of the cell wall and by promoting ROS production and mitochondrial dysfunction [5Swidergall M. Ernst J.F. Interplay between Candida albicans and the antimicrobial peptide armory.Eukaryot. Cell. 2014; 13: 950-957Crossref PubMed Scopus (58) Google Scholar, 6Wilson D. et al.Zinc exploitation by pathogenic fungi.PLoS Pathog. 2012; 8: e1003034Crossref PubMed Scopus (19) Google Scholar, 7Yang D. et al.Human neutrophil defensins selectively chemoattract naive T and immature dendritic cells.J. Leukoc. Biol. 2000; 68: 9-14Crossref PubMed Google Scholar, 8Puri S. Edgerton M. How does it kill? Understanding the candidacidal mechanism of salivary histatin 5.Eukaryot. Cell. 2014; 13: 958-964Crossref PubMed Scopus (0) Google Scholar] (Figure 1). Central to initiation of protective antifungal immunity are members of the CLR superfamily which include Dectin-1 (CLEC7A), Dectin-2 (CLEC4N), macrophage C-type lectin (MCL, CLEC4D), macrophage-inducible C-type lectin (Mincle, CLEC4E), mannose receptor (MR, CD206), dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN, CD209), and melanin-sensing C-type lectin (MelLec, CLEC1A) [9Salazar F. Brown G.D. Antifungal innate immunity: a perspective from the last 10 years.J. Innate Immun. 2018; 10: 373-397Crossref PubMed Scopus (17) Google Scholar]. CLRs are primarily expressed on cells of myeloid origin [10Sancho D. Reis e Sousa C. Signaling by myeloid C-type lectin receptors in immunity and homeostasis.Annu. Rev. Immunol. 2012; 30: 491-529Crossref PubMed Scopus (283) Google Scholar], but some are expressed by nonhematopoietic cells such as epithelial and endothelial cells [11Stappers M.H.T. et al.Recognition of DHN-melanin by a C-type lectin receptor is required for immunity to Aspergillus.Nature. 2018; 555: 382-386Crossref PubMed Scopus (51) Google Scholar]. CLRs contain at least one C-type lectin-like domain (CTLD) that is classically associated with the recognition of fungal carbohydrates such as β-glucans, mannan, and chitin that are present within the cell wall [10Sancho D. Reis e Sousa C. Signaling by myeloid C-type lectin receptors in immunity and homeostasis.Annu. Rev. Immunol. 2012; 30: 491-529Crossref PubMed Scopus (283) Google Scholar], but can also recognize other components such as melanin [11Stappers M.H.T. et al.Recognition of DHN-melanin by a C-type lectin receptor is required for immunity to Aspergillus.Nature. 2018; 555: 382-386Crossref PubMed Scopus (51) Google Scholar]. Following ligand recognition, most CLRs trigger intracellular pathways involving Syk kinase and caspase recruitment domain-containing protein 9 (CARD9), or Raf-1 (Figure 2, Key Figure). The signaling pathways of other CLRs (including MelLec and the MR) are not well defined. The CARD9 pathway is essential because defects in this pathway lead to severe systemic infections in both mice and humans [12Drummond R.A. et al.CARD9-dependent neutrophil recruitment protects against fungal invasion of the central nervous system.PLoS Pathog. 2015; 11: e1005293Crossref PubMed Scopus (72) Google Scholar,13Rieber N. et al.Extrapulmonary Aspergillus infection in patients with CARD9 deficiency.JCI Insight. 2016; 1: e89890Crossref PubMed Google Scholar]. In addition, CARD9 plays an essential role in the maintenance of gut homeostasis with commensal organisms and prevents tissue invasion by opportunistic fungi (see below). Recognition of fungi by CLRs activates innate host defense mechanisms which promote fungal killing but also couple with activation of the adaptive immune system. This is carried out by dendritic cells (DCs) which, upon activation, upregulate antigen-presentation molecules including major histocompatibility complex (MHC-I/II) molecules, enhance the expression of co-stimulatory molecules, and release cytokines and chemokines. This review highlights the influence of CLRs in promoting antifungal T cell immunity, with specific focus on recent advances in CD4+ T cell immunity. The emerging role of the mycobiome in antifungal T cell immunity will also be discussed. CD4+ T cells are generally considered to play a role in both the resolution and worsening of superficial or invasive fungal infections. For example, data from mouse models have indicated that T helper (Th)1 and Th17 cells are usually important for controlling fungal infections, whereas regulatory T cells (Tregs) are nonprotective. Other subsets such as Th2 and Th9 cells, as well as CD8+ T cells, have also been implicated in antifungal immunity (Box 1 for details). Of note, recent in vitro data using human leukocytes suggested that double-positive CD4hiCD8lo T cells were involved in the control of Candida albicans during infection, whereas persistent infection was associated with increased CD4loCD8hi T cells [14Misme-Aucouturier B. et al.Double positive CD4+CD8+ T cells are part of the adaptive immune response against Candida albicans.Hum. Immunol. 2019; 80: 955-1074Crossref PubMed Scopus (0) Google Scholar]. The generation of different T cell subsets relies on activities of antigen-presenting cells (APCs) which recognize fungi through CLRs. Most of what is known about the role of CLRs in adaptive T cell immunity is derived from in vitro and in vivo studies that investigate the cytokine milieu secreted by DCs or macrophages following receptor engagement with fungi in wild-type (WT) or gene-deficient mice, CLR crosslinking with antibodies, or CLR interaction with purified ligand agonists/antagonists. For instance, numerous in vitro and in vivo studies have shown that recognition of Candida spp. by CLRs including Dectin-1 results in interleukin (IL)-12 production by DCs and macrophages (which promotes Th1 cell immunity that is required for protection against systemic fungal infections), and in secretion of IL-1β, IL-6, and IL-23 (which promote Th17 immunity that is crucial for restraining oropharyngeal and mucocutaneous fungal infections) [15Martinez-Sanchez M.E. et al.Role of cytokine combinations on CD4+ T cell differentiation, partial polarization, and plasticity: continuous network modeling approach.Front. Physiol. 2018; 9: 877Crossref PubMed Scopus (4) Google Scholar] (Figure 1). Of note, CD103+CD11b+RALDH+ DCs have been found to regulate gut mycobiota by promoting Th17 immunity, Foxp3+ Treg induction, and IgA production in mice [16Zhang Z. et al.Peripheral lymphoid volume expansion and maintenance are controlled by gut microbiota via RALDH+ dendritic cells.Immunity. 2016; 44: 330-342Abstract Full Text Full Text PDF PubMed Scopus (42) Google Scholar]. By contrast, CD103+CD11b− murine DCs can support Th1 immunity via IL-12 production [17Liang J. et al.Inflammatory Th1 and Th17 in the intestine are each driven by functionally specialized dendritic cells with distinct requirements for MyD88.Cell Rep. 2016; 17: 1330-1343Abstract Full Text Full Text PDF PubMed Scopus (21) Google Scholar, 18Chudnovskiy A. et al.Host–protozoan interactions protect from mucosal infections through activation of the inflammasome.Cell. 2016; 167: 444-456Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar, 19Luda K.M. et al.IRF8 transcription-factor-dependent classical dendritic cells are essential for intestinal T cell homeostasis.Immunity. 2016; 44: 860-874Abstract Full Text Full Text PDF PubMed Google Scholar]. Other CLRs have been linked with the regulation of adaptive T cell immunity in the context of viral and bacterial infections (Box 2). How activation of the CLR–Syk–CARD9 pathway regulates other aspects of APC maturation such as the expression of co-stimulatory molecules that are necessary for T cell differentiation, survival, and memory development remains unclear (Figure 2).Box 1Roles of Th2, Th9, and CD8+ T cells in Antifungal ImmunityCLRs have been implicated in Th2-mediated fungal allergic responses. For instance, in mice, following repeated lung exposure to A. fumigatus, CD4+ T cells produce IL-4, IL-5, and IL-13 in a Dectin-1-dependent fashion (Clec7a−/− mice) [119Lilly L.M. et al.The beta-glucan receptor dectin-1 promotes lung immunopathology during fungal allergy via IL-22.J. Immunol. 2012; 189: 3653-3660Crossref PubMed Scopus (0) Google Scholar]. Moreover, intratracheal administration of Aspergillus versicolor spores in mice exacerbated house dust mite-induced allergic asthma through the production of IL-4 and IL-13 [120Zhang Z. et al.Beta-glucan exacerbates allergic asthma independent of fungal sensitization and promotes steroid-resistant TH2/TH17 responses.J. Allergy Clin. Immunol. 2017; 139: 54-65 e8Abstract Full Text Full Text PDF PubMed Scopus (35) Google Scholar]. Although these studies support a pathological role for Th2 immunity, there are also data demonstrating a protective role for Th2 cells during fungal infections. In mice infected with pulmonary Pneumocystis murina, IL-13-induced alternatively activated macrophages were shown to be more fungicidal than noninduced macrophages [26Verma A. et al.Adaptive immunity to fungi.Cold Spring Harb. Perspect. Med. 2014; 5: a019612PubMed Google Scholar]. Furthermore, in IL-4-deficient mice, intragastric challenge with C. albicans led to increased susceptibility to disease and failure to produce a Th1 response relative to WT mice [121Mencacci A. et al.Endogenous interleukin 4 is required for development of protective CD4+ T helper type 1 cell responses to Candida albicans.J. Exp. Med. 1998; 187: 307-317Crossref PubMed Scopus (0) Google Scholar].The role of CLRs in Th9 differentiation is poorly understood and knowledge about their function in fungal infections is still emerging. During fungal infection in mice, Th9 cells have been associated with failure to clear fungal pathogens while promoting fungal asthma. For instance, IL-9 KO mice intratracheally infected with Pneumocystis exhibit increased numbers of Th17 cells, augmented production of IL-17A and IL-23, and decreased fungal burdens in the lung relative to WT mice [122Li T. et al.IL-9 deficiency promotes pulmonary Th17 response in murine model of Pneumocystis infection.Front. Immunol. 2018; 9: 1118Crossref PubMed Scopus (3) Google Scholar]. In addition, following chronic A. fumigatus allergen exposure in mice, high numbers of Th9 cells were shown to correlate with increased severity of chronic airway hyper-reactivity relative to controls [123Kerzerho J. et al.Programmed cell death ligand 2 regulates TH9 differentiation and induction of chronic airway hyperreactivity.J. Allergy Clin. Immunol. 2013; 131: 1048-1057Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar]. Recently, in experimental leaky-gut mouse models, Candida-driven IL-9 production in the gut was reported to contribute to loss of barrier integrity, fungal dissemination, and inflammation relative to controls [124Renga G. et al.IL-9 and mast cells are key players of Candida albicans commensalism and pathogenesis in the gut.Cell Rep. 2018; 23: 1767-1778Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar]. In these same mouse models, IL-9 deficiency promoted gut dysbiosis relative to WT mice, suggesting that the functions of IL-9 might also involve the regulation of the microbiota [124Renga G. et al.IL-9 and mast cells are key players of Candida albicans commensalism and pathogenesis in the gut.Cell Rep. 2018; 23: 1767-1778Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar].The contribution of CLRs in CD8+ T cell function is inferred from in vitro experiments showing that bone marrow-derived DC activation by curdlan promotes differentiation of naïve CD8+ T cells into cytotoxic T cells in vitro [72Leibundgut-Landmann S. et al.Stimulation of dendritic cells via the dectin-1/Syk pathway allows priming of cytotoxic T-cell responses.Blood. 2008; 112: 4971-4980Crossref PubMed Scopus (0) Google Scholar]. Moreover, in this study, the crosspresentation pathway involved in MHC-I loading of fungal antigens derived from direct ingestion or uptake of apoptotic cell-associated fungal antigens was reported to be mediated by Syk/CARD9 signaling [72Leibundgut-Landmann S. et al.Stimulation of dendritic cells via the dectin-1/Syk pathway allows priming of cytotoxic T-cell responses.Blood. 2008; 112: 4971-4980Crossref PubMed Scopus (0) Google Scholar]. Studies in mice lacking CD4+ T cells have supported the role of CD8+ T cells in antifungal immunity. For instance, following oral challenge with C. albicans in mice, IL-17 from CD8+ T cells mediated protection against fungal infection in the absence of T and B cells (Rag1−/− mice) [50Hernandez-Santos N. et al.Th17 cells confer long-term adaptive immunity to oral mucosal Candida albicans infections.Mucosal Immunol. 2013; 6: 900-910Crossref PubMed Scopus (105) Google Scholar]. In addition, following intratracheal inoculation with C. neoformans, depletion of CD8+ T cells in mice lacking CD4+ T cells (via monoclonal antibody administration) resulted in failure to control lung fungal growth and systemic dissemination relative to control mice [125Lindell D.M. et al.Generation of antifungal effector CD8+ T cells in the absence of CD4+ T cells during Cryptococcus neoformans infection.J. Immunol. 2005; 174: 7920-7928Crossref PubMed Google Scholar]. Moreover, subcutaneous vaccination in mice with antigen-specific memory CD8+ T cells producing IL-17 has been reported to promote resistance to pulmonary Blastomyces dermatitidis and H. capsulatum in the absence of CD4+ T cells (antibody-mediated depletion) relative to control mice [126Nanjappa S.G. et al.Tc17 cells mediate vaccine immunity against lethal fungal pneumonia in immune deficient hosts lacking CD4+ T cells.PLoS Pathog. 2012; 8: e1002771Crossref PubMed Scopus (0) Google Scholar].Box 2CLRs in Adaptive Immunity during Viral and Bacterial InfectionsCLRs have also been linked to the regulation of adaptive immunity in the context of viral and bacterial infections, including dendritic cell natural killer lectin group receptor 1 (DNGR-1, Clec9a), DEC-205 (CD205), Clec5a, dendritic cell immunoreceptor (DCIR) (CLEC4a), and dendritic cell immune-activating receptor (DCAR) (Clec4b1) [68Brown G.D. et al.C-type lectins in immunity and homeostasis.Nat. Rev. Immunol. 2018; 18: 374-389Crossref PubMed Scopus (92) Google Scholar,127Hoving J.C. et al.Signalling C-type lectin receptors, microbial recognition and immunity.Cell Microbiol. 2014; 16: 185-194Crossref PubMed Scopus (123) Google Scholar]. For example, relative to controls, using Clec9a blocking antibodies in mice in vitro and in vivo, DNGR-1 was shown to promote crosspresentation (a process in which dead-cell-associated extracellular antigens are taken up, processed, and presented through MHC-I molecules to CD8+ T cells to induce adaptive immune responses) [128Sancho D. et al.Identification of a dendritic cell receptor that couples sensing of necrosis to immunity.Nature. 2009; 458: 899-903Crossref PubMed Scopus (446) Google Scholar]. In addition, in mice, upon systemic infection with vaccinia virus or herpes simplex virus-1 (HSV-1), DNGR-1+ DCs were found to be essential for animal protection because lack of DNGR1 blocked crosspresentation of these viral antigens (as evidenced by adoptive transfer experiments and the use of DNGR-1-deficient mice) [129Iborra S. et al.The DC receptor DNGR-1 mediates cross-priming of CTLs during vaccinia virus infection in mice.J. Clin. Invest. 2012; 122: 1628-1643Crossref PubMed Scopus (97) Google Scholar,130Zelenay S. et al.The dendritic cell receptor DNGR-1 controls endocytic handling of necrotic cell antigens to favor cross-priming of CTLs in virus-infected mice.J. Clin. Invest. 2012; 122: 1615-1627Crossref PubMed Scopus (0) Google Scholar]. Furthermore, following systemic HSV-1 injection in mice, DNGR-1 was shown to promote the development of TFH cells relative to controls [131Kato Y. et al.Targeting antigen to Clec9A primes follicular Th cell memory responses capable of robust recall.J. Immunol. 2015; 195: 1006-1014Crossref PubMed Scopus (18) Google Scholar]. In addition, the recognition of mycobacterial glycolipids by the FcRγ-coupled activating receptor DCAR in vitro can mediate the production of monocyte chemoattractant protein 1 (MCP-1) and promote Th1 immunity against mycobacteria in vivo in mice [132Toyonaga K. et al.C-type lectin receptor DCAR recognizes mycobacterial phosphatidyl-inositol mannosides to promote a Th1 response during infection.Immunity. 2016; 45: 1245-1257Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar]. Lastly, using DCIR-deficient mice, DCIR signaling – a CLR bearing an inhibitory ITIM signaling motif – was shown to sustain type-I IFN signaling in DCs and control Th1 differentiation during mycobacterial infection relative to WT mice [133Troegeler A. et al.C-type lectin receptor DCIR modulates immunity to tuberculosis by sustaining type I interferon signaling in dendritic cells.Proc. Natl. Acad. Sci. U. S. A. 2017; 114: E540-E549Crossref PubMed Scopus (0) Google Scholar]. Owing to their contribution to innate and adaptive immunity, CLRs have been used to increase the effectiveness of vaccines via approaches that involve antigen coupling to molecules (antibodies or ligands) targeting CLRs on DCs. Targeting Epstein–Barr virus (EBV) nuclear antigen 1 to DEC-205 on human PBMCs (antibody-mediated), for example, led to expansion of antigen-specific CD4+ and CD8+ memory T cells that suppressed virus-infected B cells in vitro [134Gurer C. et al.Targeting the nuclear antigen 1 of Epstein–Barr virus to the human endocytic receptor DEC-205 stimulates protective T-cell responses.Blood. 2008; 112: 1231-1239Crossref PubMed Scopus (0) Google Scholar]. Conversely, blocking Clec5a with monoclonal antibodies, before infection with Dengue or Japanese encephalitis viruses in mice, reduced the secretion of proinflammatory cytokines such as TNF and IL-6, as well as the activation of NOD-, LRR-, and pyrin domain-containing protein 3 (NLRP3) inflammasomes in mice; this restored homeostasis and led to resolution of infection, suggesting that blocking of Clec5a might potentially alleviate tissue damage and protect against some viral infections [135Chen S.T. et al.CLEC5A is critical for dengue-virus-induced lethal disease.Nature. 2008; 453: 672-676Crossref PubMed Scopus (250) Google Scholar,136Chen S.T. et al.CLEC5A regulates Japanese encephalitis virus-induced neuroinflammation and lethality.PLoS Pathog. 2012; 8: e1002655Crossref PubMed Scopus (67) Google Scholar]. Of note, Clec5a also modulates innate immunity in mice in response to bacterial infections in vivo, including Listeria monocytogenes and S. aureus, through the activation of macrophages, neutrophils, and γδ T cell effector functions [137Chen S.T. et al.CLEC5A is a critical receptor in innate immunity against Listeria infection.Nat. Commun. 2017; 8: 299Crossref PubMed Scopus (20) Google Scholar]. CLRs have been implicated in Th2-mediated fungal allergic responses. For instance, in mice, following repeated lung exposure to A. fumigatus, CD4+ T cells produce IL-4, IL-5, and IL-13 in a Dectin-1-dependent fashion (Clec7a−/− mice) [119Lilly L.M. et al.The beta-glucan receptor dectin-1 promotes lung immunopathology during fungal allergy via IL-22.J. Immunol. 2012; 189: 3653-3660Crossref PubMed Scopus (0) Google Scholar]. Moreover, intratracheal administration of Aspergillus versicolor spores in mice exacerbated house dust mite-induced allergic asthma through the production of IL-4 and IL-13 [120Zhang Z. et al.Beta-glucan exacerbates allergic asthma independent of fungal sensitization and promotes steroid-resistant TH2/TH17 responses.J. Allergy Clin. Immunol. 2017; 139: 54-65 e8Abstract Full Text Full Text PDF PubMed Scopus (35) Google Scholar]. Although these studies support a pathological role for Th2 immunity, there are also data demonstrating a protective role for Th2 cells during fungal infections. In mice infected with pulmonary Pneumocystis murina, IL-13-induced alternatively activated macrophages were shown to be more fungicidal than noninduced macrophages [26Verma A. et al.Adaptive immunity to fungi.Cold Spring Harb. Perspect. Med. 2014; 5: a019612PubMed Google Scholar]. Furthermore, in IL-4-deficient mice, intragastric challenge with C. albicans led to increased susceptibility to disease and failure to produce a Th1 response relative to WT mice [121Mencacci A. et al.Endogenous interleukin 4 is required for development of protective CD4+ T helper type 1 cell responses to Candida albicans.J. Exp. Med. 1998; 187: 307-317Crossref PubMed Scopus (0) Google Scholar]. The role of CLRs in Th9 differentiation is poorly understood and knowledge about their function in fungal infections is still emerging. During fungal infection in mice, Th9 cells have been associated with failure to clear fungal pathogens while promoting fungal asthma. For instance, IL-9 KO mice intratracheally infected with Pneumocystis exhibit increased numbers of Th17 cells, augmented production of IL-17A and IL-23, and decreased fungal burdens in the lung relative to WT mice [122Li T. et al.IL-9 deficiency promotes pulmonary Th17 response in murine model of Pneumocystis infection.Front. Immunol. 2018; 9: 1118Crossref PubMed Scopus (3) Google Scholar]. In addition, following chronic A. fumigatus allergen exposure in mice, high numbers of Th9 cells were shown to correlate with increased severity of chronic airway hyper-reactivity relative to controls [123Kerzerho J. et al.Programmed cell death ligand 2 regulates TH9 differentiation and induction of chronic airway hyperreactivity.J. Allergy Clin. Immunol. 2013; 131: 1048-1057Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar]. Recently, in experimental leaky-gut mouse models, Candida-driven IL-9 production in the gut was reported to contribute to loss of barrier integrity, fungal dissemination, and inflammation relative to controls [124Renga G. et al.IL-9 and mast cells are key players of Candida albicans commensalism and pathogenesis in the gut.Cell Rep. 2018; 23: 1767-1778Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar]. In these same mouse models, IL-9 deficiency promoted gut dysbiosis relative to WT mice, suggesting that the functions of IL-9 might also involve the regulation of the microbiota [124Renga G. et al.IL-9 and mast cells are key players of Candida albicans commensalism and pathogenesis in the gut.Cell Rep. 2018; 23: 1767-1778Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar]. The contribution of CLRs in CD8+ T cell function is inferred from in vitro experiments showing that bone marrow-derived DC activation by curdlan promotes differentiation of naïve CD8+ T cells into cytotoxic T cells in vitro [72Leibundgut-Landmann S. et al.Stimulation of dendritic cells via the dectin-1/Syk pathway allows priming of cytotoxic T-cell responses.Blood. 2008; 112: 4971-4980Crossref PubMed Scopus (0) Google Scholar]. Moreover, in this study, the crosspresentation pathway involved in MHC-I loading of fungal antigens derived from direct ingestion or uptake of apoptotic cell-associated fungal antigens was reported to be mediated by Syk/CARD9 signaling [72Leibundgut-Landmann S. et al.Stimulation of dendritic cells via the dectin-1/Syk pathway allows priming of cytotoxic T-cell responses.Blood. 2008; 112: 4971-4980Crossref PubMed Scopus (0) Google Scholar]. Studies in mice lacking CD4+ T cells have supported the role of CD8+ T cells in antifungal immunity. For instance, following oral challenge with C. albicans in mice, IL-17 from CD8+ T cells mediated protection against fungal infection in the absence of T and B cells (Rag1−/− mice) [50Hernandez-Santos N. et al.Th17 cells confer long-term adaptive immunity to oral mucosal Candida albicans infections.Mucosal Immunol. 2013; 6: 900-910Crossref PubMed Scopus (105) Google Scholar]. In addition, following intratracheal inoculation with C. neoformans, depletion of CD8+ T cells in mice lacking CD4+ T cells (via monoclonal antibody administration) resulted in failure to control lung fungal growth and systemic dissemination relative to control mice [125Lindell D.M. et al.Generation of antifungal effector CD8+ T cells in the absence of CD4+ T cells during Cryptococcus neoformans infection.J. Immunol. 2005; 174: 7920-7928Crossref PubMed Google Scholar]. Moreover, subcutaneous vaccination in mice with antigen-specific memory CD8+ T cells producing IL-17 has been reported to promote resistance to pulmonary Blastomyces dermatitidis and H. capsulatum in the absence of CD4+ T cells (antibody-mediated depletion) relative to control mice [126Nanjappa S.G. et al.Tc17 cells mediate vaccine immunity against lethal fungal pneumonia in immune deficient hosts lacking CD4+ T cells.PLoS Pathog. 2012; 8: e1002771Crossref PubMed Scopus (0) Google Scholar]. CLRs have also been linked to the regulation of adaptiv