FAM21 is critical for TLR2/CLEC4E-mediated dendritic cell function against Candida albicans

FAM21 (family with sequence similarity 21) is a component of the Wiskott-Aldrich syndrome protein and SCAR homologue (WASH) protein complex that mediates actin polymerization at endosomal membranes to facilitate sorting of cargo-containing vesicles out of endosomes. To study the function of FAM21 in vivo, we generated conditional knockout (cKO) mice in the C57BL/6 background in which FAM21 was specifically knocked out of CD11c-positive dendritic cells. BMDCs from those mice displayed enlarged early endosomes, and altered cell migration and morphology relative to WT cells. FAM21-cKO cells were less competent in phagocytosis and protein antigen presentation in vitro, though peptide antigen presentation was not affected. More importantly, we identified the TLR2/CLEC4E signaling pathway as being down-regulated in FAM21-cKO BMDCs when challenged with its specific ligand Candida albicans Moreover, FAM21-cKO mice were more susceptible to C. albicans infection than WT mice. Reconstitution of WT BMDCs in FAM21-cKO mice rescued them from lethal C. albicans infection. Thus, our study highlights the importance of FAM21 in a host immune response against a significant pathogen.

TLR Signaling Rescues Fungicidal Activity in Syk-Deficient Neutrophils

An impaired neutrophil response to pathogenic fungi puts patients at risk for fungal infections with a high risk of morbidity and mortality. Acquired neutrophil dysfunction in the setting of iatrogenic immune modulators can include the inhibition of critical kinases such as spleen tyrosine kinase (Syk). In this study, we used an established system of conditionally immortalized mouse neutrophil progenitors to investigate the ability to augment Syk-deficient neutrophil function against Candida albicans with TLR agonist signaling. LPS, a known immunomodulatory molecule derived from Gram-negative bacteria, was capable of rescuing effector functions of Syk-deficient neutrophils, which are known to have poor fungicidal activity against Candida species. LPS priming of Syk-deficient mouse neutrophils demonstrates partial rescue of fungicidal activity, including phagocytosis, degranulation, and neutrophil swarming, but not reactive oxygen species production against C. albicans, in part due to c-Fos activation. Similarly, LPS priming of human neutrophils rescues fungicidal activity in the presence of pharmacologic inhibition of Syk and Bruton's tyrosine kinase (Btk), both critical kinases in the innate immune response to fungi. In vivo, neutropenic mice were reconstituted with wild-type or Syk-deficient neutrophils and challenged i.p. with C. albicans. In this model, LPS improved wild-type neutrophil homing to the fungal challenge, although Syk-deficient neutrophils did not persist in vivo, speaking to its crucial role on in vivo persistence. Taken together, we identify TLR signaling as an alternate activation pathway capable of partially restoring neutrophil effector function against Candida in a Syk-independent manner.

Control of β-glucan exposure by the endo-1,3-glucanase Eng1 in Candida albicans modulates virulence

Candida albicans is a major opportunistic pathogen of humans. It can grow as morphologically distinct yeast, pseudohyphae and hyphae, and the ability to switch reversibly among different forms is critical for its virulence. The relationship between morphogenesis and innate immune recognition is not quite clear. Dectin-1 is a major C-type lectin receptor that recognizes β-glucan in the fungal cell wall. C. albicans β-glucan is usually masked by the outer mannan layer of the cell wall. Whether and how β-glucan masking is differentially regulated during hyphal morphogenesis is not fully understood. Here we show that the endo-1,3-glucanase Eng1 is differentially expressed in yeast, and together with Yeast Wall Protein 1 (Ywp1), regulates β-glucan exposure and Dectin-1-dependent immune activation of macrophage by yeast cells. ENG1 deletion results in enhanced Dectin-1 binding at the septa of yeast cells; while eng1 ywp1 yeast cells show strong overall Dectin-1 binding similar to hyphae of wild-type and eng1 mutants. Correlatively, hyphae of wild-type and eng1 induced similar levels of cytokines in macrophage. ENG1 expression and Eng1-mediated β-glucan trimming are also regulated by antifungal drugs, lactate and N-acetylglucosamine. Deletion of ENG1 modulates virulence in the mouse model of hematogenously disseminated candidiasis in a Dectin-1-dependent manner. The eng1 mutant exhibited attenuated lethality in male mice, but enhanced lethality in female mice, which was associated with a stronger renal immune response and lower fungal burden. Thus, Eng1-regulated β-glucan exposure in yeast cells modulates the balance between immune protection and immunopathogenesis during disseminated candidiasis.

Measurements of Treg Cell Induction by Candida albicans DNA Using Flow Cytometry

Microbiota and their metabolites in the human gut regulate a variety of immune cells including regulatory T cells (Treg cells) and cytokine production. The T helper 17 (Th17)/Treg ratio biomarker (Th17/Treg), for example, has been linked to the development and progression of certain inflammatory diseases, insulin resistance, and systemic lupus erythematosus (SLE). Candida albicans is an opportunistic fungal pathogen that also colonizes the gut. T cell reactivity through T helper cells play critical roles in fungal clearance by the host through the secretion of proinflammatory cytokines. While these cytokines are mainly produced by the Th1 and Th17 subsets of T cells, another subset of T cells, the Treg cells, are also induced by antigenic ligands from pathogens that inhibit the responses of other effector T cells during the inflammation. The antigenic ligands for Treg induction have been found to include microbial cell wall polysaccharides (PSA), metabolites like short chain fatty acids (SCFA), or even microbial DNA.

Inflammasome-mediated GSDMD activation facilitates escape of Candida albicans from macrophages

Candida albicans is the most common cause of fungal sepsis. Inhibition of inflammasome activity confers resistance to polymicrobial and LPS-induced sepsis; however, inflammasome signaling appears to protect against C. albicans infection, so inflammasome inhibitors are not clinically useful for candidiasis. Here we show disruption of GSDMD, a known inflammasome target and key pyroptotic cell death mediator, paradoxically alleviates candidiasis, improving outcomes and survival of Candida-infected mice. Mechanistically, C. albicans hijacked the canonical inflammasome-GSDMD axis-mediated pyroptosis to promote their escape from macrophages, deploying hyphae and candidalysin, a pore-forming toxin expressed by hyphae. GSDMD inhibition alleviated candidiasis by preventing C. albicans escape from macrophages while maintaining inflammasome-dependent but GSDMD-independent IL-1β production for anti-fungal host defenses. This study demonstrates key functions for GSDMD in Candida's escape from host immunity in vitro and in vivo and suggests that GSDMD may be a potential therapeutic target in C. albicans-induced sepsis.

B Cell Recognition of Candida albicans Hyphae via TLR 2 Promotes IgG1 and IL-6 Secretion for TH17 Differentiation

Candida albicans is usually a benign member of the human gut microbiota, but can become pathogenic under certain circumstances, for example in an immunocompromised host. The innate immune system, in particular neutrophils and macrophages, constitutes a crucial first line of defense against fungal invasion, however adaptive immunity may provide long term protection and thus allow vaccination of at risk patients. While TH1 and TH17 cells are important for antifungal responses, the role of B cells and antibodies in protection from C. albicans infection is less well defined. In this study, we show that C. albicans hyphae but not yeast, as well as fungal cell wall components, directly activate B cells via MyD88 signaling triggered by Toll- like receptor 2, leading to increased IgG1 production. While Dectin-1 signals and specific recognition by the B cell receptor are dispensable for B cell activation in this system, TLR2/MyD88 signals cooperate with CD40 signals in promoting B cell activation. Importantly, recognition of C. albicans via MyD88 signaling is also essential for induction of IL-6 secretion by B cells, which promotes TH17 polarization in T-B cell coculture experiments. B cells may thus be activated directly by C. albicans in its invasive form, leading to production of antibodies and T cell help for fungal clearance.

Distinct innate immune responses between sublethal and lethal models of disseminated candidiasis in newborn BALB/c mice

Invasive candidiasis is associated with a high incidence and mortality rates in infants, especially in preterm newborns. The immunopathogenesis of the mycosis during the neonatal period is poorly understood. Although several in vivo models exist to study invasive candidiasis, the majority of studies employ distinct routes of infection and use 2 to 6 day-old mice that could be less comparable in studying candidiasis in preterm infants. In this study, by using 0-days-old mice we developed a new neonatal murine model of intravenous Candida albicans infection. Using different inoculums of Candida albicans we evaluated survival, dissemination of the fungus, frequency of CD45+ cells, and cytokine production in the liver, brain, and kidneys of newborn and adult BALB/c mice. Unexpectedly, the newborn mice infected with a low inoculum (1×105 cfu per mouse) of Candida albicans survive to the infection. Compared to adult mice, the liver and brain of newborn animals had the greatest fungal burden, fungal invasion and leukocyte infiltrate. A moderate production of TNFα, IL-1β, IL-6 and IFNγ was detected in tissues of newborn mice infected with a non-lethal inoculum of Candida albicans. In contrast, overproduction of TNFα, IL-1β, IL-6 and IL-10 was determined when injecting with a lethal inoculum. In agreement, flow cytometry of brain and liver showed an inoculum-dependent CD45+ leukocyte infiltration in newborn mice infected with Candida albicans. Overall, our data shows that Candida albicans infection in newborn mice affects mainly the brain and liver and a 2-fold increase of the inoculum rapidly becomes lethal probably due to massive fungal invasion and exacerbated CD45+ leukocyte infiltrate and cytokine production. This study is the first analysis of innate immune responses in different tissues during early neonatal disseminated candidiasis.

Signaling through Syk or CARD9 Mediates Species-Specific Anti-Candida Protection in Bone Marrow Chimeric Mice

The spleen tyrosine kinase (Syk) and the downstream adaptor protein CARD9 are crucial signaling molecules in antimicrobial immunity. Candida parapsilosis is an emerging fungal pathogen with a high incidence in neonates, while Candida albicans is the most common agent of candidiasis. While signaling through Syk/CARD9 promotes protective host mechanisms in response to C. albicans, its function in immunity against C. parapsilosis remains unclear. Here, we generated Syk-/- and CARD9-/- bone marrow chimeric mice to study the role of Syk/CARD9 signaling in immune responses to C. parapsilosis compared to C. albicans. We demonstrate various functions of this pathway (e.g., phagocytosis, phagosome acidification, and killing) in Candida-challenged, bone marrow-derived macrophages with differential involvement of Syk and CARD9 along with species-specific differences in cytokine production. We report that Syk-/- or CARD9-/- chimeras rapidly display high susceptibility to C. albicans, while C. parapsilosis infection exacerbates over a prolonged period in these animals. Thus, our results establish that Syk and CARD9 contribute to systemic resistance to C. parapsilosis and C. albicans differently. Additionally, we confirm prior studies but also detail new insights into the fundamental roles of both proteins in immunity against C. albicans. Our data further suggest that Syk has a more prominent influence on anti-Candida immunity than CARD9. Therefore, this study reinforces the Syk/CARD9 pathway as a potential target for anti-Candida immune therapy. IMPORTANCE While C. albicans remains the most clinically significant Candida species, C. parapsilosis is an emerging pathogen with increased affinity to neonates. Syk/CARD9 signaling is crucial in immunity to C. albicans, but its role in in vivo responses to other pathogenic Candida species is largely unexplored. We used mice with hematopoietic systems deficient in Syk or CARD9 to comparatively study the function of these proteins in anti-Candida immunity. We demonstrate that Syk/CARD9 signaling has a protective role against C. parapsilosis differently than against C. albicans. Thus, this study is the first to reveal that Syk can exert immune responses during systemic Candida infections species specifically. Additionally, Syk-dependent immunity to a nonalbicans Candida species in an in vivo murine model has not been reported previously. We highlight that the contribution of Syk and CARD9 to fungal infections are not identical and underline this pathway as a promising immune-therapeutic target to fight Candida infections.

Histatin 5 Metallopeptides and Their Potential against Candida albicans Pathogenicity and Drug Resistance

Usually caused by Candida albicans, buccal candidiasis begins with the morphological transition between yeast and hyphal cells. Over time and without the correct treatment, it can be disseminated through the bloodstream becoming a systemic infection with high mortality rates. C. albicans already shows resistance against antifungals commonly used in treatments. Therefore, the search for new drugs capable of overcoming antifungal resistance is essential. Histatin 5 (Hst5) is an antimicrobial peptide of the Histatin family, that can be found naturally in human saliva. This peptide presents high antifungal activity against C. albicans. However, Hst5 action can be decreased for interaction with enzymes and metal ions present in the oral cavity. The current work aims to bring a brief review of relevant aspects of the pathogenesis and resistance mechanisms already reported for C. albicans. In addition, are also reported here the main immune responses of the human body and the most common antifungal drugs. Finally, the most important aspects regarding Histatin 5 and the benefits of its interaction with metals are highlighted. The intention of this review is to show the promising use of Hst5 metallopeptides in the development of effective drugs.

Activation of Cph1 causes ß(1,3)-glucan unmasking in Candida albicans and attenuates virulence in mice in a neutrophil-dependent manner

Masking the immunogenic cell wall epitope ß(1,3)-glucan under an outer layer of mannosylated glycoproteins is an important virulence factor deployed by Candida albicans during infection. Consequently, increased ß(1,3)-glucan exposure (unmasking) reveals C. albicans to the host's immune system and attenuates its virulence. We have previously shown that activation of the Cek1 MAPK pathway via expression of a hyperactive allele of an upstream kinase (STE11ΔN467) induced unmasking. It also increased survival of mice in a murine disseminated candidiasis model and attenuated kidney fungal burden by ≥33 fold. In this communication, we utilized cyclophosphamide-induced immunosuppression to test if the clearance of the unmasked STE11ΔN467 mutant was dependent on the host immune system. Suppression of the immune response by cyclophosphamide reduced the attenuation in fungal burden caused by the STE11ΔN467 allele. Moreover, specific depletion of neutrophils via 1A8 antibody treatment also reduced STE11ΔN467-dependent fungal burden attenuation, but to a lesser extent than cyclophosphamide, demonstrating an important role for neutrophils in mediating fungal clearance of unmasked STE11ΔN467 cells. In an effort to understand the mechanism by which Ste11ΔN467 causes unmasking, transcriptomics were used to reveal that several components in the Cek1 MAPK pathway were upregulated, including the transcription factor CPH1 and the cell wall sensor DFI1. In this report we show that a cph1ΔΔ mutation restored ß(1,3)-glucan exposure to wild-type levels in the STE11ΔN467 strain, confirming that Cph1 is the transcription factor mediating Ste11ΔN467-induced unmasking. Furthermore, Cph1 is shown to induce a positive feedback loop that increases Cek1 activation. In addition, full unmasking by STE11ΔN467 is dependent on the upstream cell wall sensor DFI1. However, while deletion of DFI1 significantly reduced Ste11ΔN467-induced unmasking, it did not impact activation of the downstream kinase Cek1. Thus, it appears that once stimulated by Ste11ΔN467, Dfi1 activates a parallel signaling pathway that is involved in Ste11ΔN467-induced unmasking.

The impact of the Fungus-Host-Microbiota interplay upon Candida albicans infections: current knowledge and new perspectives

Candida albicans is a major fungal pathogen of humans. It exists as a commensal in the oral cavity, gut or genital tract of most individuals, constrained by the local microbiota, epithelial barriers and immune defences. Their perturbation can lead to fungal outgrowth and the development of mucosal infections such as oropharyngeal or vulvovaginal candidiasis, and patients with compromised immunity are susceptible to life-threatening systemic infections. The importance of the interplay between fungus, host and microbiota in driving the transition from C. albicans commensalism to pathogenicity is widely appreciated. However, the complexity of these interactions, and the significant impact of fungal, host and microbiota variability upon disease severity and outcome, are less well understood. Therefore, we summarise the features of the fungus that promote infection, and how genetic variation between clinical isolates influences pathogenicity. We discuss antifungal immunity, how this differs between mucosae, and how individual variation influences a person's susceptibility to infection. Also, we describe factors that influence the composition of gut, oral and vaginal microbiotas, and how these affect fungal colonisation and antifungal immunity. We argue that a detailed understanding of these variables, which underlie fungal-host-microbiota interactions, will present opportunities for directed antifungal therapies that benefit vulnerable patients.

Renal CD169++ resident macrophages are crucial for protection against acute systemic candidiasis

Disseminated candidiasis remains as the most common hospital-acquired bloodstream fungal infection with up to 40% mortality rate despite the advancement of medical and hygienic practices. While it is well established that this infection heavily relies on the innate immune response for host survival, much less is known for the protective role elicited by the tissue-resident macrophage (TRM) subsets in the kidney, the prime organ for Candida persistence. Here, we describe a unique CD169++ TRM subset that controls Candida growth and inflammation during acute systemic candidiasis. Their absence causes severe fungal-mediated renal pathology. CD169++ TRMs, without being actively involved in direct fungal clearance, increase host resistance by promoting IFN-γ release and neutrophil ROS activity.

The anti-inflammatory cytokine interleukin-37 is an inhibitor of trained immunity

Trained immunity (TI) is a de facto innate immune memory program induced in monocytes/macrophages by exposure to pathogens or vaccines, which evolved as protection against infections. TI is characterized by immunometabolic changes and histone post-translational modifications, which enhance production of pro-inflammatory cytokines. As aberrant activation of TI is implicated in inflammatory diseases, tight regulation is critical; however, the mechanisms responsible for this modulation remain elusive. Interleukin-37 (IL-37) is an anti-inflammatory cytokine that curbs inflammation and modulates metabolic pathways. In this study, we show that administration of recombinant IL-37 abrogates the protective effects of TI in vivo, as revealed by reduced host pro-inflammatory responses and survival to disseminated candidiasis. Mechanistically, IL-37 reverses the immunometabolic changes and histone post-translational modifications characteristic of TI in monocytes, thus suppressing cytokine production in response to infection. IL-37 thereby emerges as an inhibitor of TI and as a potential therapeutic target in immune-mediated pathologies.

Comparative assessment of immune evasion mechanisms in human whole-blood infection assays by a systems biology approach

Computer simulations of mathematical models open up the possibility of assessing hypotheses generated by experiments on pathogen immune evasion in human whole-blood infection assays. We apply an interdisciplinary systems biology approach in which virtual infection models implemented for the dissection of specific immune mechanisms are combined with experimental studies to validate or falsify the respective hypotheses. Focusing on the assessment of mechanisms that enable pathogens to evade the immune response in the early time course of a whole-blood infection, the least-square error (LSE) as a measure for the quantitative agreement between the theoretical and experimental kinetics is combined with the Akaike information criterion (AIC) as a measure for the model quality depending on its complexity. In particular, we compare mathematical models with three different types of pathogen immune evasion as well as all their combinations: (i) spontaneous immune evasion, (ii) evasion mediated by immune cells, and (iii) pre-existence of an immune-evasive pathogen subpopulation. For example, by testing theoretical predictions in subsequent imaging experiments, we demonstrate that the simple hypothesis of having a subpopulation of pre-existing immune-evasive pathogens can be ruled out. Furthermore, in this study we extend our previous whole-blood infection assays for the two fungal pathogens Candida albicans and C. glabrata by the bacterial pathogen Staphylococcus aureus and calibrated the model predictions to the time-resolved experimental data for each pathogen. Our quantitative assessment generally reveals that models with a lower number of parameters are not only scored with better AIC values, but also exhibit lower values for the LSE. Furthermore, we describe in detail model-specific and pathogen-specific patterns in the kinetics of cell populations that may be measured in future experiments to distinguish and pinpoint the underlying immune mechanisms.

Cell wall N-glycan of Candida albicans ameliorates early hyper- and late hypo-immunoreactivity in sepsis

Severe infection often causes a septic cytokine storm followed by immune exhaustion/paralysis. Not surprisingly, many pathogens are equipped with various anti-inflammatory mechanisms. Such mechanisms might be leveraged clinically to control septic cytokine storms. Here we show that N-glycan from pathogenic C. albicans ameliorates mouse sepsis through immunosuppressive cytokine IL-10. In a sepsis model using lipopolysaccharide (LPS), injection of the N-glycan upregulated serum IL-10, and suppressed pro-inflammatory IL-1β, TNF-α and IFN-γ. The N-glycan also improved the survival of mice challenged by LPS. Analyses of structurally defined N-glycans from several yeast strains revealed that the mannose core is key to the upregulation of IL-10. Knocking out the C-type lectin Dectin-2 abrogated the N-glycan-mediated IL-10 augmentation. Furthermore, C. albicans N-glycan ameliorated immune exhaustion/immune paralysis after acute inflammation. Our results suggest a strategy where the immunosuppressive mechanism of one pathogen can be applied to attenuate a severe inflammation/cytokine storm caused by another pathogen.

Tissue-resident memory Th17 cells maintain stable fungal commensalism in the oral mucosa

Keeping a stable equilibrium between the host and commensal microbes to which we are constantly exposed, poses a major challenge for the immune system. The host mechanisms that regulate homeostasis of the microbiota to prevent infection and inflammatory disorders are not fully understood. Here, we provide evidence that CD4+ tissue-resident memory T (TRM) cells act as central players in this process. Using a murine model of C. albicans commensalism we show that IL-17 producing CD69+CD103+CD4+ memory T cells persist in the colonized tissue long-term and independently of circulatory supplies. Consistent with the requirement of Th17 cells for limiting fungal growth, IL-17-producing TRM cells in the mucosa were sufficient to maintain prolonged colonization, while circulatory T cells were dispensable. Although TRM cells were first proposed to protect from pathogens causing recurrent acute infections, our results support a central function of TRM cells in the maintenance of commensalism.

Human gut mycobiota tune immunity via CARD9-dependent induction of anti-fungal IgG antibodies

Antibodies mediate natural and vaccine-induced immunity against viral and bacterial pathogens, whereas fungi represent a widespread kingdom of pathogenic species for which neither vaccine nor neutralizing antibody therapies are clinically available. Here, using a multi-kingdom antibody profiling (multiKAP) approach, we explore the human antibody repertoires against gut commensal fungi (mycobiota). We identify species preferentially targeted by systemic antibodies in humans, with Candida albicans being the major inducer of antifungal immunoglobulin G (IgG). Fungal colonization of the gut induces germinal center (GC)-dependent B cell expansion in extraintestinal lymphoid tissues and generates systemic antibodies that confer protection against disseminated C. albicans or C. auris infection. Antifungal IgG production depends on the innate immunity regulator CARD9 and CARD9+CX3CR1+ macrophages. In individuals with invasive candidiasis, loss-of-function mutations in CARD9 are associated with impaired antifungal IgG responses. These results reveal an important role of gut commensal fungi in shaping the human antibody repertoire through CARD9-dependent induction of host-protective antifungal IgG.

Interplay between Humoral and CLA+ T Cell Response against Candida albicans in Psoriasis

Candida albicans (CA) infections have been associated with psoriasis onset or disease flares. However, the integrated immune response against this fungus is still poorly characterized in psoriasis. We studied specific immunoglobulins in plasma and the CA response in cocultures of circulating memory CD45RA^- cutaneous lymphocyte antigen (CLA)^+/- T cell with autologous epidermal cells from plaque and guttate psoriasis patients (cohort 1, n = 52), and also healthy individuals (n = 17). A complete proteomic profile was also evaluated in plaque psoriasis patients (cohort 2, n = 114) regarding their anti-CA IgA levels. Increased anti-CA IgA and IgG levels are present in the plasma from plaque but not guttate psoriasis compared to healthy controls. CA cellular response is confined to CLA⁺ T cells and is primarily Th17. The levels of anti-CA IgA are directly associated with CLA⁺ Th17 response in plaque psoriasis. Proteomic analysis revealed distinct profiles in psoriasis patients with high anti-CA IgA. C-C motif chemokine ligand 18, chitinase-3-like protein 1 and azurocidin were significantly elevated in the plasma from plaque psoriasis patients with high anti-CA levels and severe disease. Our results indicate a mechanism by which Candida albicans exposure can trigger a clinically relevant IL-17 response in psoriasis. Assessing anti-CA IgA levels may be useful in order to evaluate chronic psoriasis patients.

Significant Differences in Host-Pathogen Interactions Between Murine and Human Whole Blood

Murine infection models are widely used to study systemic candidiasis caused by C. albicans. Whole-blood models can help to elucidate host-pathogens interactions and have been used for several Candida species in human blood. We adapted the human whole-blood model to murine blood. Unlike human blood, murine blood was unable to reduce fungal burden and more substantial filamentation of C. albicans was observed. This coincided with less fungal association with leukocytes, especially neutrophils. The lower neutrophil number in murine blood only partially explains insufficient infection and filamentation control, as spiking with murine neutrophils had only limited effects on fungal killing. Furthermore, increased fungal survival is not mediated by enhanced filamentation, as a filament-deficient mutant was likewise not eliminated. We also observed host-dependent differences for interaction of platelets with C. albicans, showing enhanced platelet aggregation, adhesion and activation in murine blood. For human blood, opsonization was shown to decrease platelet interaction suggesting that complement factors interfere with fungus-to-platelet binding. Our results reveal substantial differences between murine and human whole-blood models infected with C. albicans and thereby demonstrate limitations in the translatability of this ex vivo model between hosts.

The Inhibitory Effect of Human Beta-defensin-3 on Candida Glabrata Isolated from Patients with Candidiasis

Candida glabrata is a common non-albicans Candida species found in patients with candidiasis and it sometimes develops antifungal resistance. Human beta-defensin-3 (hBD-3) is an antimicrobial peptide of immune system active against various types of microbes including Candida spp. This study investigated antifungal activity of hBD-3 and its synergistic effect with a first-line antifungal agent on C. glabrata clinical isolates. Candida spp. were characterised in patients with candidiasis. The antifungal activities of hBD-3 and fluconazole against C. glabrata were evaluated using Broth microdilution assay. The synergistic activity of these two agents was determined by checkerboard microdilution and time-killing assays. The cytotoxicity of hBD-3 was evaluated using LDH-cytotoxicity colorimetric assay. Of 307 episodes from 254 patients diagnosed with candidiasis, C. glabrata was found in 21 clinical isolates. Antifungal susceptibility tests of C. glabrata were performed, fluconazole demonstrated an inhibitory effect at concentrations of 0.25-8 μg/ml, but one antifungal resistant strain was identified (>64 μg/ml). hBD-3 showed an inhibitory effect against all selected strains at concentrations of 50-75 μg/ml and exhibited a synergistic effect with fluconazole at the fractional inhibitory concentration index (FICI) of 0.25-0.50. A concentration of 25 μg/ml of hBD-3 alone showed no cytotoxicity but synergistic activity was seen with fluconazole. In conclusion, hBD-3 has antifungal activity against C. glabrata and synergistic effects with fluconazole at concentrations that alone, have no cytotoxicity. hBD-3 could be used as an adjunctive therapy with first-line antifungal agents for patients with C. glabrata infection particularly those infected with fluconazole-resistant strains.

Initial Immune Response in Escherichia coli, Staphylococcus aureus, and Candida albicans Bacteremia

Sepsis remains a leading cause of mortality worldwide and is characterized by sustained inflammatory responses, reflected as changes in the expression profile of cytokines with time. The aim of the present study was to investigate the dynamic changes in complete blood count, serum chemistry, procalcitonin (PCT), tumor necrosis factor-alpha (TNF-α), and interleukin-6 (IL-6) in Escherichia coli, Staphylococcus aureus, and Candida albicans bacteremia. Study subjects were 32 healthy male Landrace-Large White pigs, aged 10-15 weeks and of average weight 19 ± 2 kg. Bacteremia was induced by continuous intravenous infusion of microbial suspensions during a period of 8 h. E. coli and S. aureus bacteremia were associated with a significant gradual decrease in white blood cells and platelets, respectively (p = 0.002 and p = 0.004), while candidemia was characterized by a significant gradual decrease in lymphocytes (p = 0.009). Serum PCT levels were either undetectable or very low, with no significant changes with time in all groups. E. coli bacteremia elicited a strong pro-inflammatory response, characterized by a significant increase in TNF-α expression from the onset of bacteremia (p = 0.042). C. albicans exhibited a different profile with an early, moderate increase in TNF-α followed by a subsequent marked increase in IL-6 levels (p = 0.03). The differential regulation of inflammatory and hematological responses depending on the pathogenic agent can reveal differences in the underlying inflammatory mechanisms, which may assist in the ongoing quest for the identification of a panel of circulating biomarkers during bacteremia.

Metabolic competition between host and pathogen dictates inflammasome responses to fungal infection

The NLRP3 inflammasome has emerged as a central immune regulator that senses virulence factors expressed by microbial pathogens for triggering inflammation. Inflammation can be harmful and therefore this response must be tightly controlled. The mechanisms by which immune cells, such as macrophages, discriminate benign from pathogenic microbes to control the NLRP3 inflammasome remain poorly defined. Here we used live cell imaging coupled with a compendium of diverse clinical isolates to define how macrophages respond and activate NLRP3 when faced with the human yeast commensal and pathogen Candida albicans. We show that metabolic competition by C. albicans, rather than virulence traits such as hyphal formation, activates NLRP3 in macrophages. Inflammasome activation is triggered by glucose starvation in macrophages, which occurs when fungal load increases sufficiently to outcompete macrophages for glucose. Consistently, reducing Candida’s ability to compete for glucose and increasing glucose availability for macrophages tames inflammatory responses. We define the mechanistic requirements for glucose starvation-dependent inflammasome activation by Candida and show that it leads to inflammatory cytokine production, but it does not trigger pyroptotic macrophage death. Pyroptosis occurs only with some Candida isolates and only under specific experimental conditions, whereas inflammasome activation by glucose starvation is broadly relevant. In conclusion, macrophages use their metabolic status, specifically glucose metabolism, to sense fungal metabolic activity and activate NLRP3 when microbial load increases. Therefore, a major consequence of Candida-induced glucose starvation in macrophages is activation of inflammatory responses, with implications for understanding how metabolism modulates inflammation in fungal infections.

Activation of the immune regulator NLRP3 inflammasome by microbial pathogens has been shown to play both protective and destructive roles in infection, underscoring the importance of tight control over NLRP3-driven inflammation to ensure host health. A key microbe recognised by NLRP3 is the human yeast commensal and pathogen Candida albicans, which is responsible for mucosal and invasive infections. We demonstrate that innate immune cells sense their metabolic status to trigger NLRP3 activation only when microbial numbers have reached dangerous levels. This regulation is a consequence of metabolic competition between C. albicans and macrophages for an essential nutrient–glucose. The NLRP3 inflammasome is activated when increased fungal load in the infection microenvironment drives down glucose levels, thereby causing glucose starvation in macrophages. Restoring glucose homeostasis in macrophages reduced NLRP3 activation and production of the proinflammatory cytokine IL-1β, suggesting that metabolism regulates NLRP3 inflammasome activity in fungal infections.

Redundant Trojan horse and endothelial-circulatory mechanisms for host-mediated spread of Candida albicans yeast

The host innate immune system has developed elegant processes for the detection and clearance of invasive fungal pathogens. These strategies may also aid in the spread of pathogens in vivo, although technical limitations have previously hindered our ability to view the host innate immune and endothelial cells to probe their roles in spreading disease. Here, we have leveraged zebrafish larvae as a model to view the interactions of these host processes with the fungal pathogen Candida albicans in vivo. We examined three potential host-mediated mechanisms of fungal spread: movement inside phagocytes in a "Trojan Horse" mechanism, inflammation-assisted spread, and endothelial barrier passage. Utilizing both chemical and genetic tools, we systematically tested the loss of neutrophils and macrophages and the loss of blood flow on yeast cell spread. Both neutrophils and macrophages respond to yeast-locked and wild type C. albicans in our model and time-lapse imaging revealed that macrophages can support yeast spread in a "Trojan Horse" mechanism. Surprisingly, loss of immune cells or inflammation does not alter dissemination dynamics. On the other hand, when blood flow is blocked, yeast can cross into blood vessels but they are limited in how far they travel. Blockade of both phagocytes and circulation reduces rates of dissemination and significantly limits the distance of fungal spread from the infection site. Together, this data suggests a redundant two-step process whereby (1) yeast cross the endothelium inside phagocytes or via direct uptake, and then (2) they utilize blood flow or phagocytes to travel to distant sites.

The Dual Function of the Fungal Toxin Candidalysin during Candida albicans-Macrophage Interaction and Virulence

The dimorphic fungus Candida albicans is both a harmless commensal organism on mucosal surfaces and an opportunistic pathogen. Under certain predisposing conditions, the fungus can overgrow the mucosal microbiome and cause both superficial and life-threatening systemic infections after gaining access to the bloodstream. As the first line of defense of the innate immune response, infecting C. albicans cells face macrophages, which mediate the clearance of invading fungi by intracellular killing. However, the fungus has evolved sophisticated strategies to counteract macrophage antimicrobial activities and thus evade immune surveillance. The cytolytic peptide toxin, candidalysin, contributes to this fungal defense machinery by damaging immune cell membranes, providing an escape route from the hostile phagosome environment. Nevertheless, candidalysin also induces NLRP3 inflammasome activation, leading to an increased host-protective pro-inflammatory response in mononuclear phagocytes. Therefore, candidalysin facilitates immune evasion by acting as a classical virulence factor but also contributes to an antifungal immune response, serving as an avirulence factor. In this review, we discuss the role of candidalysin during C. albicans infections, focusing on its implications during C. albicans-macrophage interactions.

Post-sepsis immunosuppression depends on NKT cell regulation of mTOR/IFN-γ in NK cells

As treatment of the early, inflammatory phase of sepsis improves, post-sepsis immunosuppression and secondary infection have increased in importance. How early inflammation drives immunosuppression remains unclear. Although IFN-γ typically helps microbial clearance, we found that increased plasma IFN-γ in early clinical sepsis was associated with the later development of secondary Candida infection. Consistent with this observation, we found that exogenous IFN-γ suppressed macrophage phagocytosis of zymosan in vivo, and antibody blockade of IFN-γ after endotoxemia improved survival of secondary candidemia. Transcriptomic analysis of innate lymphocytes during endotoxemia suggested that NKT cells drove IFN-γ production by NK cells via mTORC1. Activation of invariant NKT (iNKT) cells with glycolipid antigen drove immunosuppression. Deletion of iNKT cells in Cd1d-/- mice or inhibition of mTOR by rapamycin reduced immunosuppression and susceptibility to secondary Candida infection. Thus, although rapamycin is typically an immunosuppressive medication, in the context of sepsis, rapamycin has the opposite effect. These results implicated an NKT cell/mTOR/IFN-γ axis in immunosuppression following endotoxemia or sepsis. In summary, in vivo iNKT cells activated mTORC1 in NK cells to produce IFN-γ, which worsened macrophage phagocytosis, clearance of secondary Candida infection, and mortality.

Spleen Tyrosine Kinase Is a Critical Regulator of Neutrophil Responses to Candida Species

Invasive fungal infections constitute a lethal threat, with patient mortality as high as 90%. The incidence of invasive fungal infections is increasing, especially in the setting of patients receiving immunomodulatory agents, chemotherapy, or immunosuppressive medications following solid-organ or bone marrow transplantation. In addition, inhibitors of spleen tyrosine kinase (Syk) have been recently developed for the treatment of patients with refractory autoimmune and hematologic indications. Neutrophils are the initial innate cellular responders to many types of pathogens, including invasive fungi. A central process governing neutrophil recognition of fungi is through lectin binding receptors, many of which rely on Syk for cellular activation. We previously demonstrated that Syk activation is essential for cellular activation, phagosomal maturation, and elimination of phagocytosed fungal pathogens in macrophages. Here, we used combined genetic and chemical inhibitor approaches to evaluate the importance of Syk in the response of neutrophils to Candida species. We took advantage of a Cas9-expressing neutrophil progenitor cell line to generate isogenic wild-type and Syk-deficient neutrophils. Syk-deficient neutrophils are unable to control the human pathogens Candida albicans, Candida glabrata, and Candida auris Neutrophil responses to Candida species, including the production of reactive oxygen species and of cytokines such as tumor necrosis factor alpha (TNF-α), the formation of neutrophil extracellular traps (NETs), phagocytosis, and neutrophil swarming, appear to be critically dependent on Syk. These results demonstrate an essential role for Syk in neutrophil responses to Candida species and raise concern for increased fungal infections with the development of Syk-modulating therapeutics.IMPORTANCE Neutrophils are recognized to represent significant immune cell mediators for the clearance and elimination of the human-pathogenic fungal pathogen Candida The sensing of fungi by innate cells is performed, in part, through lectin receptor recognition of cell wall components and downstream cellular activation by signaling components, including spleen tyrosine kinase (Syk). While the essential role of Syk in macrophages and dendritic cells is clear, there remains uncertainty with respect to its contribution in neutrophils. In this study, we demonstrated that Syk is critical for multiple cellular functions in neutrophils responding to major human-pathogenic Candida species. These data not only demonstrate the vital nature of Syk with respect to the control of fungi by neutrophils but also warn of the potential infectious complications arising from the recent clinical development of novel Syk inhibitors for hematologic and autoimmune disorders.

Adapting to survive: How Candida overcomes host-imposed constraints during human colonization

Successful human colonizers such as Candida pathogens have evolved distinct strategies to survive and proliferate within the human host. These include sophisticated mechanisms to evade immune surveillance and adapt to constantly changing host microenvironments where nutrient limitation, pH fluctuations, oxygen deprivation, changes in temperature, or exposure to oxidative, nitrosative, and cationic stresses may occur. Here, we review the current knowledge and recent findings highlighting the remarkable ability of medically important Candida species to overcome a broad range of host-imposed constraints and how this directly affects their physiology and pathogenicity. We also consider the impact of these adaptation mechanisms on immune recognition, biofilm formation, and antifungal drug resistance, as these pathogens often exploit specific host constraints to establish a successful infection. Recent studies of adaptive responses to physiological niches have improved our understanding of the mechanisms established by fungal pathogens to evade the immune system and colonize the host, which may facilitate the design of innovative diagnostic tests and therapeutic approaches for Candida infections.

Integrating GWAS with bulk and single-cell RNA-sequencing reveals a role for LY86 in the anti-Candida host response

Candida bloodstream infection, i.e. candidemia, is the most frequently encountered life-threatening fungal infection worldwide, with mortality rates up to almost 50%. In the majority of candidemia cases, Candida albicans is responsible. Worryingly, a global increase in the number of patients who are susceptible to infection (e.g. immunocompromised patients), has led to a rise in the incidence of candidemia in the last few decades. Therefore, a better understanding of the anti-Candida host response is essential to overcome this poor prognosis and to lower disease incidence. Here, we integrated genome-wide association studies with bulk and single-cell transcriptomic analyses of immune cells stimulated with Candida albicans to further our understanding of the anti-Candida host response. We show that differential expression analysis upon Candida stimulation in single-cell expression data can reveal the important cell types involved in the host response against Candida. This confirmed the known major role of monocytes, but more interestingly, also uncovered an important role for NK cells. Moreover, combining the power of bulk RNA-seq with the high resolution of single-cell RNA-seq data led to the identification of 27 Candida-response QTLs and revealed the cell types potentially involved herein. Integration of these response QTLs with a GWAS on candidemia susceptibility uncovered a potential new role for LY86 in candidemia susceptibility. Finally, experimental follow-up confirmed that LY86 knockdown results in reduced monocyte migration towards the chemokine MCP-1, thereby implying that this reduced migration may underlie the increased susceptibility to candidemia. Altogether, our integrative systems genetics approach identifies previously unknown mechanisms underlying the immune response to Candida infection.

Candida albicans is a fungus that can cause a life-threatening infection in individuals with an impaired immune system. To improve the prognosis and treatment of patients with such an infection, a better understanding of an individual’s immune response against Candida is required. However, small patient group sizes have limited our ability to gain such understanding. Here we show that integrating many different data layers can improve the sensitivity to detect the effects of genetics on the response to Candida infection and the roles different immune cell types have herein. Using this approach, we were able to prioritize genes that are associated with an increased risk of developing systemic Candida infections. We expand on the gene with the strongest risk association, LY86, and describe a potential mechanism through which this gene affects the immune response against Candida infection. Through experimental follow-up, we provided additional insights into how this gene is associated with an increased risk to develop a Candida infection. We expect that our approach can be generalized to other infectious diseases for which small patient group sizes have restricted our ability to unravel the disease mechanism in more detail. This will provide new opportunities to identify treatment targets in the future.

In silico Design of a Multivalent Vaccine Against Candida albicans

Invasive candidiasis (IC) is the most common nosocomial infection and a leading cause of mycoses-related deaths. High-systemic toxicity and emergence of antifungal-resistant species warrant the development of newer preventive approaches against IC. Here, we have adopted an immunotherapeutic peptide vaccine-based approach, to enhance the body's immune response against invasive candida infections. Using computational tools, we screened the entire candida proteome (6030 proteins) and identified the most immunodominant HLA class I, HLA class II and B- cell epitopes. By further immunoinformatic analyses for enhanced vaccine efficacy, we selected the 18- most promising epitopes, which were joined together using molecular linkers to create a multivalent recombinant protein against Candida albicans (mvPC). To increase mvPC's immunogenicity, we added a synthetic adjuvant (RS09) to the mvPC design. The selected mvPC epitopes are homologous against all currently available annotated reference sequences of 22 C. albicans strains, thus offering a higher coverage and greater protective response. A major advantage of the current vaccine approach is mvPC's multivalent nature (recognizing multiple-epitopes), which is likely to provide enhanced protection against complex candida antigens. Here, we describe the computational analyses leading to mvPC design.

IL-9 and Mast Cells Are Key Players of Candida albicans Commensalism and Pathogenesis in the Gut

Candida albicans is implicated in intestinal diseases. Identifying host signatures that discriminate between the pathogenic versus commensal nature of this human commensal is clinically relevant. In the present study, we identify IL-9 and mast cells (MCs) as key players of Candida commensalism and pathogenicity. By inducing TGF-β in stromal MCs, IL-9 pivotally contributes to mucosal immune tolerance via the indoleamine 2,3-dioxygenase enzyme. However, Candida-driven IL-9 and mucosal MCs also contribute to barrier function loss, dissemination, and inflammation in experimental leaky gut models and are upregulated in patients with celiac disease. Inflammatory dysbiosis occurs with IL-9 and MC deficiency, indicating that the activity of IL-9 and MCs may go beyond host immunity to include regulation of the microbiota. Thus, the output of the IL-9/MC axis is highly contextual during Candida colonization and reveals how host immunity and the microbiota finely tune Candida behavior in the gut.

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IL-9/IL-9R signaling affects MC function in mucosal candidiasis

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IL-9 and mucosal MCs contribute to barrier function loss in leaky gut models

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IL-9 and stromal MCs induce local protective tolerance in infection via IDO1

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IL-9 and mucosal MCs expand and IDO1 decreases in human celiac disease

T-cell-specific mTOR deletion in mice ameliorated CD4+ T-cell survival in lethal sepsis induced by severe invasive candidiasis

The mammalian target of rapamycin (mTOR) pathway can mediate T-cell survival; however, the role of this pathway in T-cell survival during fungal sepsis is unclear. Here, we investigated the role of the mTOR pathway in CD4+ T-cell survival in a mouse model of rapidly progressive lethal sepsis induced by severe invasive candidiasis and explored the possible mechanism. The decrease in CD4+ T-cell survival following fungal sepsis was ameliorated in mice with a T-cell-specific mTOR deletion, whereas it was exacerbated in mice with a T-cell-specific tuberous sclerosis complex (TSC)1 deletion. To explore the mechanism further, we measured expression of autophagy proteins light chain 3B and p62/sequestosome 1 in CD4+ T cells. Both proteins were increased in T-cell-specific mTOR knockout mice but lower in T-cell-specific TSC1 knockout mice. Transmission electron microscopy revealed that T-cell-specific mTOR knockout mice had more autophagosomes than wild-type mice following fungal sepsis. CD4+ T-cell mTOR knockout decreased CD4+ T-cell apoptosis in fungal sepsis. Most notably, the T-cell-specific mTOR deletion mice had an increased survival rate after fungal sepsis. These results suggest that the mTOR pathway plays a vital role in CD4+ T-cell survival during fungal sepsis, partly through the autophagy-apoptosis pathway.

Rhesus Theta Defensin 1 Promotes Long Term Survival in Systemic Candidiasis by Host Directed Mechanisms

Invasive candidiasis is an increasingly frequent cause of serious and often fatal infections in hospitalized and immunosuppressed patients. Mortality rates associated with these infections have risen sharply due to the emergence of multidrug resistant (MDR) strains of C. albicans and other Candida spp., highlighting the urgent need of new antifungal therapies. Rhesus theta (θ) defensin-1 (RTD-1), a natural macrocyclic antimicrobial peptide, was recently shown to be rapidly fungicidal against clinical isolates of MDR C. albicans in vitro. Here we found that RTD-1 was rapidly fungicidal against blastospores of fluconazole/caspofungin resistant C. albicans strains, and was active against established C. albicans biofilms in vitro. In vivo, systemic administration of RTD-1, initiated at the time of infection or 24 h post-infection, promoted long term survival in candidemic mice whether infected with drug-sensitive or MDR strains of C. albicans. RTD-1 induced an early (4 h post treatment) increase in neutrophils in naive and infected mice. In vivo efficacy was associated with fungal clearance, restoration of dysregulated inflammatory cytokines including TNF-α, IL-1β, IL-6, IL-10, and IL-17, and homeostatic reduction in numbers of circulating neutrophils and monocytes. Because these effects occurred using peptide doses that produced maximal plasma concentrations (Cmax) of less than 1% of RTD-1 levels required for in vitro antifungal activity in 50% mouse serum, while inducing a transient neutrophilia, we suggest that RTD-1 mediates its antifungal effects in vivo by host directed mechanisms rather than direct fungicidal activity. Results of this study suggest that θ-defensins represent a new class of host-directed compounds for treatment of disseminated candidiasis.

Candida albicans triggers NADPH oxidase-independent neutrophil extracellular traps through dectin-2

Candida albicans is one of the top leading causes of healthcare-associated bloodstream infection. Neutrophil extracellular traps (NET) are known to capture and kill pathogens. It is reported that opsonized C. albicans-triggered NETosis is NADPH oxidase-dependent. We discovered a NADPH oxidase-independent NETosis pathway in neutrophil response to unopsonized C. albicans. While CR3 engagement with opsonized C. albicans triggered NET, dectin-2 recognized unopsonized C. albicans and mediated NET formation. Engagement of dectin-2 activated the downstream Syk-Ca²⁺-PKCδ-protein arginine deiminase 4 (PAD4) signaling pathway which modulated nuclear translocation of neutrophil elastase (NE), histone citrullination and NETosis. In a C. albicans peritonitis model we observed Ki67⁺Ly6G⁺ NETotic cells in the peritoneal exudate and mesenteric tissues within 3 h of infection. Treatment with PAD4 inhibitor GSK484 or dectin-2 deficiency reduced % Ki67⁺Ly6G⁺ cells and the intensity of Ki67 in peritoneal neutrophils. Employing DNA digestion enzyme micrococcal nuclease, GSK484 as well as dectin-2-deficient mice, we further showed that dectin-2-mediated PAD4-dependent NET formation in vivo restrained the spread of C. albicans from the peritoneal cavity to kidney. Taken together, this study reveals that unopsonized C. albicans evokes NADPH oxidase-independent NETosis through dectin-2 and its downstream signaling pathway and dectin-2-mediated NET helps restrain fungal dissemination.

Candida albicans as a dimorphic fungal pathogen is one of the top leading causes of overall healthcare-associated bloodstream infection worldwide. Invasive candidiasis affects more than 250,000 people each year and leads to more than 50,000 deaths. Upon stimulation, neutrophils release nuclear DNA that forms a web-like structure named neutrophil extracellular traps (NET). NET is known to capture pathogens and restrain the spread of infection in the host. It has been reported that opsonized C. albicans induces NET through NADPH oxidase. Here we show a NADPH oxidase-independent NETosis in response to unopsonized C. albicans. Signaling pathway leading to NETosis involves dectin-2 downstream Syk-Ca²⁺-PKCδ-PAD4/NE. In a C. albicans peritonitis model, NETotic cells are found in the peritoneal exudates and they adhere to mesenteric tissue. Treatment with PAD4 inhibitor or dectin-2 deficiency dampens the ability of neutrophil to undergo NETosis and facilitates the spread of fungus from the peritoneal cavity to kidney. Our work defines the molecular mechanism involved in NADPH oxidase-independent NET formation and sheds light on the role of dectin-2 in neutrophil anti-C. albicans function.

Candida albicans-Cell Interactions Activate Innate Immune Defense in Human Palate Epithelial Primary Cells via Nitric Oxide (NO) and β-Defensin 2 (hBD-2)

The presence of Candida albicans in the biofilm underlying the dental prosthesis is related to denture stomatitis (DS), an inflammatory reaction of the oral mucosa. The oral epithelium, a component of the innate immune response, has the ability to react to fungal invasion. In this study, we evaluated the in vitro effect of viable C. albicans on the apoptosis, nitric oxide (NO) production, and β-defensin 2 (hBD-2) expression and production of human palate epithelial cells (HPECs). We further determined whether or not these effects were correlated with fungal invasion of epithelial cells. Interaction between HPEC primary culture and C. albicans was obtained through either direct or indirect cell–cell contact with a supernatant from a hyphal fungus. We found that the hyphae supernatants were sufficient to induce slight HPEC apoptosis, which occurred prior to the activation of the specific mechanisms of epithelial defense. The epithelial defense responses were found to occur via NO and antimicrobial peptide hBD-2 production only during direct contact between C. albicans and HPECs and coincided with the fungus’s intraepithelial invasion. However, although the hBD-2 levels remained constant in the HPEC supernatants over time, the NO release and hBD-2 gene expression were reduced at a later time (10 h), indicating that the epithelial defense capacity against the fungal invasion was not maintained in later phases. This aspect of the immune response was associated with increased epithelial invasion and apoptosis maintenance.

Variations in the Clinical Course of Patients with Herpes Simplex Virus Esophagitis Based on Immunocompetence and Presence of Underlying Esophageal Disease

BACKGROUND AND AIMS

Herpes simplex esophagitis (HSE) is the second most common cause of infectious esophagitis and occurs in both immunocompetent and immunocompromised patients. The aim of this study was to reappraise the clinical course of HSE in different patient populations based on degree of immunocompetence and the presence or absence of underlying esophageal disease.

METHODS

Patients with histopathologically confirmed HSE identified from the Mayo Clinic pathology database from 2006 to 2016 were included in this study. Relevant demographic, clinical, and endoscopic data were retrospectively reviewed and compared between two cohorts: (a) immunocompromised and immunocompetent patients and (b) patients with and without underlying esophageal disorders.

RESULTS

Forty-six patients were included in the study. The most common presenting symptoms were odynophagia (34.8%) and dysphagia (30.4%). Thirty-three (71.7%) patients were immunocompromised, and these patients who experienced longer duration of symptoms (25.5 ± 23.4 days vs. 7.0 ± 5.5 days, p = 0.04) were more likely to require an extension of treatment course (38.1% vs. 8.3%, p = 0.05) compared to their immunocompetent counterparts. Seventeen (37%) patients had underlying esophageal disease, and these patients were more likely to have concomitant esophageal candidiasis (41.2% vs. 10.3%, respectively; p = 0.01).

CONCLUSION

Herpes simplex virus causes esophagitis in both immunocompetent and immunocompromised patients. While the disease course appears to be self-limited for all patient populations, clinical and endoscopic differences in the disease presentation and clinical course based on immune status and the presence or absence of underlying esophageal disease exist.

Transcriptional profiling unveils type I and II interferon networks in blood and tissues across diseases

Understanding how immune challenges elicit different responses is critical for diagnosing and deciphering immune regulation. Using a modular strategy to interpret the complex transcriptional host response in mouse models of infection and inflammation, we show a breadth of immune responses in the lung. Lung immune signatures are dominated by either IFN-γ and IFN-inducible, IL-17-induced neutrophil- or allergy-associated gene expression. Type I IFN and IFN-γ-inducible, but not IL-17- or allergy-associated signatures, are preserved in the blood. While IL-17-associated genes identified in lung are detected in blood, the allergy signature is only detectable in blood CD4+ effector cells. Type I IFN-inducible genes are abrogated in the absence of IFN-γ signaling and decrease in the absence of IFNAR signaling, both independently contributing to the regulation of granulocyte responses and pathology during Toxoplasma gondii infection. Our framework provides an ideal tool for comparative analyses of transcriptional signatures contributing to protection or pathogenesis in disease.

The protective effect of inflammatory monocytes during systemic C. albicans infection is dependent on collaboration between C-type lectin-like receptors

Invasive candidiasis, mainly caused by Candida albicans, is a serious healthcare problem with high mortality rates, particularly in immunocompromised patients. Innate immune cells express pathogen recognition receptors (PRRs) including C-type lectin-like receptors (CLRs) that bind C. albicans to initiate an immune response. Multiple CLRs including Dectin-1, Dectin-2 and Mincle have been proposed individually to contribute to the immune response to C. albicans. However how these receptors collaborate to clear a fungal infection is unknown. Herein, we used novel multi-CLR knockout (KO) mice to decipher the individual, collaborative and collective roles of Dectin-1, Dectin-2 and Mincle during systemic C. albicans infection. These studies revealed an unappreciated and profound role for CLR co-operation in anti-fungal immunity. The protective effect of multiple CLRs was markedly greater than any single receptor, and was mediated through inflammatory monocytes via recognition and phagocytosis of C. albicans, and production of C. albicans-induced cytokines and chemokines. These CLRs were dispensable for mediating similar responses from neutrophils, likely due to lower expression of these CLRs on neutrophils compared to inflammatory monocytes. Concurrent deletion of Dectin-1 and Dectin-2, or all three CLRs, resulted in dramatically increased susceptibility to systemic C. albicans infection compared to mice lacking a single CLR. Multi-CLR KO mice were unable to control fungal growth due to an inadequate early inflammatory monocyte-mediated response. In response to excessive fungal growth, the multi-CLR KO mice mounted a hyper-inflammatory response, likely leading to multiple organ failure. Thus, these data reveal a critical role for CLR co-operation in the effective control of C. albicans and maintenance of organ function during infection.

Fungal infections including invasive candidiasis are a serious healthcare problem particularly for immunocompromised patients. Mortality rates for invasive candidiasis are very high and complex anti-fungal immune responses are poorly understood, hindering the development of novel immunotherapies. Dectin-1, Dectin-2 and Mincle are three cell surface receptors that are proposed to be involved in the immune response to fungal pathogens. However, if or how these receptors work together during infection is currently unknown. Here we demonstrate that these receptors, in particular Dectin-1 and Dectin-2, work together to promote fungal clearance by a group of innate immune cells called inflammatory monocytes. Furthermore, we found that mice lacking these three receptors are dramatically susceptible to systemic Candida albicans infection due to defective early innate immune responses. These mice develop hyper-inflammation to try to control excessive fungal growth likely resulting in multi-organ failure. Our work helps explain how these receptors work together to clear/control invasive candidiasis. Our improved knowledge of the interactions between these receptors could be used to help design novel anti-fungal immunotherapies.

Novel mouse monoclonal antibodies specifically recognizing β-(1→3)-D-glucan antigen

β-(1→3)-D-Glucan is an essential component of the fungal cell wall. Mouse monoclonal antibodies (mAbs) against synthetic nona-β-(1→3)-D-glucoside conjugated with bovine serum albumin (BSA) were generated using hybridoma technology. The affinity constants of two selected mAbs, 3G11 and 5H5, measured by a surface plasmon resonance biosensor assay using biotinylated nona-β-(1→3)-D-glucan as the ligand, were approximately 11 nM and 1.9 nM, respectively. The glycoarray, which included a series of synthetic oligosaccharide derivatives representing β-glucans with different lengths of oligo-β-(1→3)-D-glucoside chains, demonstrated that linear tri-, penta- and nonaglucoside, as well as a β-(1→6)-branched octasaccharide, were recognized by mAb 5H5. By contrast, only linear oligo-β-(1→3)-D-glucoside chains that were not shorter than pentaglucosides (but not the branched octaglucoside) were ligands for mAb 3G11. Immunolabelling indicated that 3G11 and 5H5 interact with both yeasts and filamentous fungi, including species from Aspergillus, Candida, Penicillium genera and Saccharomyces cerevisiae, but not bacteria. Both mAbs could inhibit the germination of Aspergillus fumigatus conidia during the initial hours and demonstrated synergy with the antifungal fluconazole in killing C. albicans in vitro. In addition, mAbs 3G11 and 5H5 demonstrated protective activity in in vivo experiments, suggesting that these β-glucan-specific mAbs could be useful in combinatorial antifungal therapy.

The planarian Schmidtea mediterranea is a new model to study host-pathogen interactions during fungal infections

Candida albicans is one of the most common fungal pathogens of humans. Currently, there are limitations in the evaluation of C. albicans infection in existing animal models, especially in terms of understanding the influence of specific infectious stages of the fungal pathogen on the host. We show that C. albicans infects, grows and invades tissues in the planarian flatworm Schmidtea mediterranea, and that the planarian responds to infection by activating components of the host innate immune system to clear and repair host tissues. We study different stages of C. albicans infection and demonstrate that planarian stem cells increase division in response to fungal infection, a process that is likely evolutionarily conserved in metazoans. Our results implicate MORN2 and TAK1/p38 signaling pathways as possible mediators of the host innate immune response to fungal infection. We propose the use of planarians as a model system to investigate host-pathogen interactions during fungal infections.

Glyoxylate cycle gene ICL1 is essential for the metabolic flexibility and virulence of Candida glabrata

The human fungal pathogen Candida glabrata appears to utilise unique stealth, evasion and persistence strategies in subverting the onslaught of host immune response during systemic infection. However, macrophages actively deprive the intracellular fungal pathogen of glucose, and therefore alternative carbon sources probably support the growth and survival of engulfed C. glabrata. The present study aimed to investigate the role of the glyoxylate cycle gene ICL1 in alternative carbon utilisation and its importance for the virulence of C. glabrata. The data showed that disruption of ICL1 rendered C. glabrata unable to utilise acetate, ethanol or oleic acid. In addition, C. glabrata icl1∆ cells displayed significantly reduced biofilm growth in the presence of several alternative carbon sources. It was also found that ICL1 is crucial for the survival of C. glabrata in response to macrophage engulfment. Disruption of ICL1 also conferred a severe attenuation in the virulence of C. glabrata in the mouse model of invasive candidiasis. In conclusion, a functional glyoxylate cycle is essential for C. glabrata to utilise certain alternative carbon sources in vitro and to display full virulence in vivo. This reinforces the view that antifungal drugs that target fungal Icl1 have potential for future therapeutic intervention.

Endothelial nitric oxide synthase limits host immunity to control disseminated Candida albicans infections in mice

Three isoforms of nitric oxide synthase (NOS) occur in mammals. High levels of NO produced by NOS2/iNOS can protect against bacterial and parasitic infections, but the role of NOS in fungal innate immunity is less clear. Compared to wild type mice, Nos3-/- mice showed significantly higher survival of candidemia caused by Candida albicans SC5314. NOS3/eNOS is expressed by endothelial cells in the kidney, and colonization of this organ was decreased during the sub-acute stage of disseminated candidiasis. Nos3-/- mice more rapidly eliminated Candida from the renal cortex and exhibited more balanced local inflammatory reactions, with similar macrophage but less neutrophil infiltration than in infected wild type. Levels of the serum cytokines IL-9, IL-12, IL-17 and chemokines GM-CSF, MIP1α, and MIP1β were significantly elevated, and IL-15 was significantly lower in infected Nos3-/- mice. Spleens of infected Nos3-/- mice had significantly more Th2 and Th9 but not other CD4+ T cells compared with wild type. Inflammatory genes associated with leukocyte chemotaxis, IL-1 signaling, TLR signaling and Th1 and Th2 cell differentiation pathways were significantly overexpressed in infected Nos3-/- kidneys, with Nos2 being the most strongly induced. Conversely, the general NOS inhibitor NG-nitro-L-arginine methyl ester increased virulence in the mouse candidemia model, suggesting that iNOS contributes to the protective mechanism in infected Nos3-/- mice. By moderating neutrophil infiltration, the absence of eNOS may reduce the collateral damage to kidney cortex, and Th-9 CD4+ cells may enhance clearance of the infection. These data suggest that selective eNOS inhibition could mitigate candidemia by a combination of systemic and local responses that promote a more effective host immune response.

Impact of the Environment upon the Candida albicans Cell Wall and Resultant Effects upon Immune Surveillance

The fungal cell wall is an essential organelle that maintains cellular morphology and protects the fungus from environmental insults. For fungal pathogens such as Candida albicans, it provides a degree of protection against attack by host immune defences. However, the cell wall also presents key epitopes that trigger host immunity and attractive targets for antifungal drugs. Rather than being a rigid shield, it has become clear that the fungal cell wall is an elastic organelle that permits rapid changes in cell volume and the transit of large liposomal particles such as extracellular vesicles. The fungal cell wall is also flexible in that it adapts to local environmental inputs, thereby enhancing the fitness of the fungus in these microenvironments. Recent evidence indicates that this cell wall adaptation affects host-fungus interactions by altering the exposure of major cell wall epitopes that are recognised by innate immune cells. Therefore, we discuss the impact of environmental adaptation upon fungal cell wall structure, and how this affects immune recognition, focussing on C. albicans and drawing parallels with other fungal pathogens.

Antifungal activities of surfactant protein D in an environment closely mimicking the lung lining

At the lung lining innate defenses protect our lungs against inhaled fungal cells that could pose a threat to our health. These defenses are comprised of mucociliary clearance, soluble effector molecules and roaming phagocytic cells, such as macrophages and neutrophils. How important each of these defenses is during fungal clearance depends on the specific fungal pathogen in question and on the stage of infection. In this study the localization and antifungal activity of the lung surfactant protein D (SP-D) was studied in an environment mimicking the lung lining. To this end Calu-3 cells were grown on an air-liquid interface allowing them to polarize and to produce mucus at their apical surface. Additionally, neutrophils were added to study their role in fungal clearance. Two fungal pathogens were used for these experiments: Candida albicans and Aspergillus fumigatus, both of clinical relevance. During fungal infection SP-D localized strongly to both fungal surfaces and stayed bound through the different stages of infection. Furthermore, SP-D decreased fungal adhesion to the epithelium and increased fungal clearance by neutrophils from the epithelial surface. These findings suggest that SP-D plays an important role at the different stages of pulmonary defense against fungal intruders.

Mast Cells Respond to Candida albicans Infections and Modulate Macrophages Phagocytosis of the Fungus

Mast cells (MCs) are long-lived immune cells widely distributed at mucosal surfaces and are among the first immune cell type that can get in contact with the external environment. This study aims to unravel the mechanisms of reciprocal influence between mucosal MCs and Candida albicans as commensal/opportunistic pathogen species in humans. Stimulation of bone marrow-derived mast cells (BMMCs) with live forms of C. albicans induced the release of TNF-α, IL-6, IL-13, and IL-4. Quite interestingly, BMMCs were able to engulf C. albicans hyphae, rearranging their α-tubulin cytoskeleton and accumulating LAMP1+ vesicles at the phagocytic synapse with the fungus. Candida-infected MCs increased macrophage crawling ability and promoted their chemotaxis against the infection. On the other side, resting MCs inhibited macrophage phagocytosis of C. albicans in a contact-dependent manner. Taken together, these results indicate that MCs play a key role in the maintenance of the equilibrium between the host and the commensal fungus C. albicans, limiting pathological fungal growth and modulating the response of resident macrophages during infections.

Candida: Platelet Interaction and Platelet Activity in vitro

Over the last 2 decades, platelets have been recognized as versatile players of innate immunity. The interaction of platelets with fungal pathogens and subsequent processes may critically influence the clinical outcome of invasive mycoses. Since the role of platelets in Candida infections is poorly characterized and controversially discussed, we studied interactions of human platelets with yeast cells, (pseudo-)hyphae, biofilms and secretory products of human pathogenic Candida species applying platelet rich plasma and a whole blood model. Incubation of Candida with platelets resulted in moderate mutual interaction with some variation between different species. The rate of platelets binding to -Candida (pseudo-) hyphae and candidal biofilm was comparably low as that to the yeast form. Candida-derived secretory products did not affect platelet activity - neither stimulatory nor inhibitory. The small subset of platelets that bound to Candida morphotypes was consequently activated. However, this did not result in reduced growth or viability of the different Candida species. A whole blood model simulating in vivo conditions confirmed platelet activation in the subpopulation of Candida-bound platelets. Thus, the inability of platelets to efficiently react on Candida presence might favor fungal survival in the blood and contribute to high morbidity of Candida sepsis.

IL-36 and IL-1/IL-17 Drive Immunity to Oral Candidiasis via Parallel Mechanisms

Protection against microbial infection by the induction of inflammation is a key function of the IL-1 superfamily, including both classical IL-1 and the new IL-36 cytokine families. Candida albicans is a frequent human fungal pathogen causing mucosal infections. Although the initiators and effectors important in protective host responses to C. albicans are well described, the key players in driving these responses remain poorly defined. Recent work has identified a central role played by IL-1 in inducing innate Type-17 immune responses to clear C. albicans infections. Despite this, lack of IL-1 signaling does not result in complete loss of immunity, indicating that there are other factors involved in mediating protection to this fungus. In this study, we identify IL-36 cytokines as a new player in these responses. We show that C. albicans infection of the oral mucosa induces the production of IL-36. As with IL-1α/β, induction of epithelial IL-36 depends on the hypha-associated peptide toxin Candidalysin. Epithelial IL-36 gene expression requires p38-MAPK/c-Fos, NF-κB, and PI3K signaling and is regulated by the MAPK phosphatase MKP1. Oral candidiasis in IL-36R-/- mice shows increased fungal burdens and reduced IL-23 gene expression, indicating a key role played by IL-36 and IL-23 in innate protective responses to this fungus. Strikingly, we observed no impact on gene expression of IL-17 or IL-17-dependent genes, indicating that this protection occurs via an alternative pathway to IL-1-driven immunity. Thus, IL-1 and IL-36 represent parallel epithelial cell-driven protective pathways in immunity to oral C. albicans infection.

Development and characterization of a 3D oral mucosa model as a tool for host-pathogen interactions

The aim of this study was to (i) design, develop and validate a practical and physiologically relevant reconstituted in vitro oral mucosa tissue model and (ii) to assess its applicability in in vitro host-pathogen interactions with C. albicans and S. aureus. Co-culture organotypic constructions were created by incorporating specific numbers of keratinocytes (NOK-si) onto cellularised, collagen gel scaffolds containing human gingival fibroblasts incubated in KGM media and cultured for 14 days. The detection of the appropriate oral mucosa/epithelial structure was evaluated by histology (hematoxylin and eosin (HE), periodic acid-Schiff (P.A.S.) and Picrosirius red), and immunocytochemistry (cytokeratin 13, cytokeratin 14, Ki-67 and collagen IV) compared to a normal human gingiva. The morphology of the reconstituted tissue was analyzed by Transmission Electron Microscopy. To further quantitate tissue damage, lactate dehydrogenase (LDH) was measured in the tissue supernatant. NOK-si grown upon a gingival scaffold provided an organotypic model in an in vitro setting and exhibited structural characteristics typically associated with normal oral mucosa. Immunocytochemistry revealed the detection of epithelial cytokeratins 13 and 14, Col IV and Ki-67 in the reconstituted oral mucosa model. Infection was detected after 8 h and 16 h. This study presents an in vitro cellularised, organotypic model of reconstituted oral mucosa, which enables close control and characterization of its structure and differentiation over a mid-length period of time in culture.

Short-Form Bomanins Mediate Humoral Immunity in Drosophila

The Bomanins (Boms) are a family of a dozen secreted peptides that mediate the innate immune response governed by the Drosophila Toll receptor. We recently showed that deleting a cluster of 10 Bom genes blocks Toll-mediated defenses against a range of fungi and gram-positive bacteria. Here, we characterize the activity of individual Bom family members. We provide evidence that the Boms overlap in function and that a single Bom gene encoding a mature peptide of just 16 amino acids can act largely or entirely independent of other family members to provide phenotypic rescue in vivo. We further demonstrate that the Boms function in Drosophila humoral immunity, mediating the killing of the fungal pathogen Candida glabrata in an in vitro assay of cell-free hemolymph. In addition, we find that the level of antifungal activity both in vivo and in vitro is linked to the level of Bom gene expression. Although Toll dictates expression of the antimicrobial peptides (AMPs) drosomycin and metchnikowin, we find no evidence that Boms act by modifying the expression of the mature forms of these antifungal AMPs.

Interleukin-2 and other cytokines in candidiasis: expression, clinical significance, and future therapeutic targets

Susceptibility to Candida spp. infection is largely determined by the status of host immunity, whether immunocompromised/immunodeficient or immunocompetent. Interleukin-2 (IL-2), a potent lymphoid cell growth factor, is a four-α-helix bundle cytokine induced by activated T cells with two important roles: the activation and maintenance of immune responses, and lymphocyte production and differentiation. We reviewed the roles of cytokines as immune stimulators and suppressors of Candida spp. infections as an update on this continuously evolving field. We performed a comprehensive search of the Cochrane Central Register of Controlled Trials, Medline (PubMed), and Embase databases for articles published from March 2010 to March 2016 using the following search terms: interleukins, interleukin-2, Candida spp., and immunosuppression. Data from our own studies were also reviewed. Here, we provide an overview focusing on the ability of IL-2 to induce a large panel of trafficking receptors in skin inflammation and control T helper (Th)2 cytokine production in response to contact with Candida spp. Immunocompromised patients have reduced capacity to secrete Th1-related cytokines such as IL-2. The ability to secrete the Th1-related cytokine IL-2 is low in immunocompromised patients. This prevents an efficient Th1 immune response to Candida spp. antigens, making immunocompromised patients more susceptible to candidal infections.