The relative contribution of resident pulmonary alveolar macrophage and inflammatory polymorphonuclear neutrophils in host resistance to pulmonary infection by Candida albicans

The Dysbiosis and Inter-Kingdom Synergy Model in Oropharyngeal Candidiasis, a New Perspective in Pathogenesis

As more information emerges on oral microbiota using advanced sequencing methodologies, it is imperative to examine how organisms modulate the capacity of each other to colonize or trigger infection. Most mouse models of oral C. albicans infection have focused on interactions with single bacterial species. Thus, little is known about the microbiome-mediated interactions that control the switch of C. albicans from commensalism to infection. Evidence is accumulating that in immunosuppression where mucosal candidiasis is more prevalent, there is an altered oral bacterial microbiome with reduced diversity, but not an altered mycobiome. Oropharyngeal candidiasis in immunosuppressed humans and mice is associated with a further reduction in oral bacterial diversity and a dysbiotic shift with significant enrichment of streptococcal and enterococcal species. Our recent studies in a cancer chemotherapy mouse model supported the combined profound effect of immunosuppression and C. albicans in reducing oral bacterial diversity and provided the first direct evidence that these changes contribute to pathogenesis, representing dysbiosis. There is still a gap in understanding the relationship between Candida and the oral bacterial microbiome. We propose that certain oral commensal bacteria contribute to fungal pathogenesis and we identify gaps in our understanding of the mechanisms involved in this cooperative virulence.

Candida albicans induces mucosal bacterial dysbiosis that promotes invasive infection

Infectious complications are a common cause of morbidity and mortality in cancer patients undergoing chemotherapy due to increased risk of oral and gastrointestinal candidiasis, candidemia and septicemia. Interactions between C. albicans and endogenous mucosal bacteria are important in understanding the mechanisms of invasive infection. We published a mouse intravenous chemotherapy model that recapitulates oral and intestinal mucositis, and myelosuppression in patients receiving 5-fluorouracil. We used this model to study the influence of C. albicans on the mucosal bacterial microbiome and compared global community changes in the oral and intestinal mucosa of the same mice. We validated 16S rRNA gene sequencing data by qPCR, in situ hybridization and culture approaches. Mice receiving both 5Fu and C. albicans had an endogenous bacterial overgrowth on the oral but not the small intestinal mucosa. C. albicans infection was associated with loss of mucosal bacterial diversity in both sites with indigenous Stenotrophomonas, Alphaproteobacteria and Enterococcus species dominating the small intestinal, and Enterococcus species dominating the oral mucosa. Both immunosuppression and Candida infection contributed to changes in the oral microbiota. Enterococci isolated from mice with oropharyngeal candidiasis were implicated in degrading the epithelial junction protein E-cadherin and increasing the permeability of the oral epithelial barrier in vitro. Importantly, depletion of these organisms with antibiotics in vivo attenuated oral mucosal E-cadherin degradation and C. albicans invasion without affecting fungal burdens, indicating that bacterial community changes represent overt dysbiosis. Our studies demonstrate a complex interaction between C. albicans, the resident mucosal bacterial microbiota and the host environment in pathogenesis. We shed significant new light on the role of C. albicans in shaping resident bacterial communities and driving mucosal dysbiosis.

The Relationship of Candida albicans with the Oral Bacterial Microbiome in Health and Disease

Candida albicans is an opportunistic pathogen colonizing the oropharyngeal, esophageal, and gastrointestinal mucosa in most healthy humans. In immunocompromised hosts, this fungal organism can cause mucosal candidiasis in these sites. C. albicans also causes fungemia, a serious consequence of cancer cytotoxic chemotherapy, which is thought to develop from fungal translocation through compromised mucosal barriers. Changes in endogenous bacterial population size or composition as well as changes in the host environment can transform fungal commensals into opportunistic pathogens in the upper and lower GI tract. Pioneering studies from our group have shown that a ubiquitous oral commensal of the mitis streptococcal group (Streptococcus oralis) has a mutualistic relationship with C. albicans, with C. albicans enabling streptococcal biofilm growth at mucosal sites, and S. oralis facilitating invasion of the oral and esophageal mucosa by C. albicans. In these studies, we used a cortisone-induced immunosuppression mouse model. More recently, the development of a novel mouse chemotherapy model has allowed us to examine the interactions of C. albicans with the endogenous bacterial microbiota in the oral and small intestinal mucosa, two sites adversely affected by cytotoxic chemotherapy. In this model, oral inoculation with C. albicans causes severe dysbiosis in the mucosal bacterial composition in both sites. We also found that antibiotic treatment ameliorates invasion of the oral mucosa but aggravates dissemination through the intestinal mucosa. In this chapter, we discuss work from our laboratory and others examining the relationships of C. albicans with oral bacteria and their role in mucosal homeostasis or disease.

Cytosolic phospholipase A2 contributes to innate immune defense against Candida albicans lung infection

BACKGROUND

The lung is exposed to airborne fungal spores, and fungi that colonize the oral cavity such as Candida albicans, but does not develop disease to opportunistic fungal pathogens unless the immune system is compromised. The Group IVA cytosolic phospholipase A2 (cPLA2α) is activated in response to Candida albicans infection resulting in the release of arachidonic acid for eicosanoid production. Although eicosanoids such as prostaglandins and leukotrienes modulate inflammation and immune responses, the role of cPLA2α and eicosanoids in regulating C. albicans lung infection is not understood.

METHODS

The responses of cPLA2α(+/+) and cPLA2α(-/-) Balb/c mice to intratracheal instillation of C. albicans were compared. After challenge, we evaluated weight loss, organ fungal burden, and the recruitment of cells and the levels of cytokines and eicosanoids in bronchoalveolar lavage fluid. The ability of macrophages and neutrophils from cPLA2α(+/+) and cPLA2α(-/-) mice to recognize and kill C. albicans was also compared.

RESULTS

After C. albicans instillation, cPLA2α(+/+) mice recovered a modest weight loss by 48 h and completely cleared fungi from the lung by 12 h with no dissemination to the kidneys. In cPLA2α(-/-) mice, weight loss continued for 72 h, C. albicans was not completely cleared from the lung and disseminated to the kidneys. cPLA2α(-/-) mice exhibited greater signs of inflammation including higher neutrophil influx, and elevated levels of albumin and pro-inflammatory cytokines/chemokines (IL1α, IL1β, TNFα, IL6, CSF2, CXCL1, CCL20) in bronchoalveolar lavage fluid. The amounts of cysteinyl leukotrienes, thromboxane B2 and prostaglandin E2 were significantly lower in bronchoalveolar lavage fluid from C. albicans-infected cPLA2α(-/-) mice compared to cPLA2α(+/+) mice. Alveolar macrophages and neutrophils from uninfected cPLA2α(-/-) mice exhibited less killing of C. albicans in vitro than cells from cPLA2α(+/+) mice. In addition alveolar macrophages from cPLA2α(-/-) mice isolated 6 h after instillation of GFP-C. albicans contained fewer internalized fungi than cPLA2α(+/+) macrophages.

CONCLUSIONS

The results demonstrate that cPLA2α contributes to immune surveillance and host defense in the lung to prevent infection by the commensal fungus C. albicans and to dampen inflammation.

Candida Soluble Cell Wall β-D-Glucan Induces Lung Inflammation in Mice

Bioactivity of cell wall component(s) of fungi has not been fully elucidated, especially in vivo. We isolated Candida soluble beta-D-glucan (CSBG) from Candida albicans (C. albicans). We investigated the effects of airway exposure to CSBG on the immune systems in the airways in mice. CSBG exposure induced neutrophilic and eosinophilic inflammation in the lung, which was concomitant with the increased local expression of proinflammatory cytokines including tumor necrosis factor - α, interleukin (IL)-1 β, IL-6, macrophage inflammatory protein -1 α, macrophage chemoattractant protein -1, RANTES (regulated on activation and normal T cells expressed and secreted), and eotaxin. The lung inflammation with enhanced expression of proinflammatory proteins caused by CSBG was directly related to its structure, since structurally degraded products of CSBG by formic acid induced negligible responses in the lung. CSBG enhanced nuclear localization of phosphorylated signal transducer and activator of transcription (STAT)-6 in the lung. These results suggest that airway exposure to CSBG induces lung inflammation, at least partly, via the enhanced expression of proinflammatory cytokines and the activation of STAT-6 pathway, and can be a proper murine model for fungal lung inflammation.

Soluble cell wall beta-glucan of Candida induces/enhances apoptosis and oxidative stress in murine lung

The bioactivity of cell wall component(s) of fungi has not been fully investigated, especially in vivo. We have successfully extracted a soluble cell wall beta-glucan from C. albicans (CSBG), and shown that pulmonary exposure to CSBG induces lung inflammation in mice. CSBG-induced lung inflammation was concomitant with the nuclear translocation of signal transducer and activator of transcription (STAT)6 and enhanced lung expression of various cytokines and chemokines. However, the effects of CSBG on the murine respiratory system and their mechanisms have not been fully investigated. In this study, we further explored the effects of pulmonary exposure to CSBG on lung pathophysiology in terms of the induction of apoptosis and enhancement of oxidative stress. ICR mice were intratracheally instilled with vehicle, CSBG, or structurally degraded products of CSBG by formic acid (DEG-CSBG), and 24 h later, the lungs were isolated and evaluated for apoptosis by the TUNEL assay and oxidative stress by immunohistochemistry of 8-hydroxy-2'-deoxyguanosine (8-OHdG), a proper marker of the oxidative DNA damage. In another experiment, the mice were sacrificed and lungs were isolated 4 h after the instillation with vehicle or CSBG for evaluation of transcriptional level of heme oxygenase (HO)-1, a stress responsive protein. Pulmonary exposure to CSBG significantly induced apoptosis and enhanced the formation of 8-OHdG in the lung parenchyma as compared to vehicle or DEG-CSBG exposure. CSBG significantly induced HO-1 in the lung. Taken together, CSBG induces/enhances apoptosis and oxidative stress in the lung in association with lung inflammation/injury.

Cytosolic phospholipase a2 activation by Candida albicans in alveolar macrophages: role of dectin-1

Candida albicans is an increasingly important pulmonary fungal pathogen. Resident alveolar macrophages are important in host defense against opportunistic fungal infections. Activation of Group IVA cytosolic phospholipase A(2)alpha (cPLA(2)alpha) in macrophages initiates arachidonic acid (AA) release for production of eicosanoids, which regulate inflammation and immune responses. We investigated the ability of C. albicans to activate cPLA(2)alpha in unprimed alveolar macrophages and after priming with granulocyte macrophage colony-stimulating factor (GM-CSF), which regulates alveolar macrophage maturation. AA was released within minutes by GM-CSF-primed but not unprimed alveolar macrophages in response to C. albicans, and was blocked by soluble glucan phosphate (S-GP). The expression of the beta-glucan receptor dectin-1 was increased in GM-CSF-primed macrophages, and AA release from GM-CSF-primed dectin-1(-/-) alveolar macrophages was reduced to basal levels. The enhanced activation of extracellular signal-regulated kinases and phosphorylation of cPLA(2)alpha on Ser-505 that occurred in GM-CSF-primed macrophages were reduced by MEK1 and Syk inhibitors, which also suppressed AA release. At later times after C. albicans infection (6 h), unprimed and GM-CSF-primed macrophages released similar levels of AA. The expression of cyclooxygenase 2 and prostanoid production at 6 hours was higher in GM-CSF-primed macrophages, but the responses were not dependent on dectin-1. However, dectin-1 contributed to the C. albicans-stimulated increase in TNF-alpha production that occurred in GM-CSF-primed macrophages. The results demonstrate that dectin-1 mediates the acute activation of cPLA(2)alpha in GM-CSF-primed alveolar macrophages, but not in the more delayed phase of AA release and GM-CSF-dependent prostanoid production.

Role of chemokines in fungal infections

The production of chemokines at the site of a fungal infection is critical for effective recruitment of leukocytes to that site. Over 40 chemokines and 20 chemokine receptors have been identified. The most intriguing biological property of chemokines is that they often play non-redundant roles in vivo even though they are highly related, have multiple activities and bind multiple chemokine receptors. Almost all of the chemokine studies to date have concentrated on responses to Cryptococcus, Candida, Aspergillus or Pneumocystis. The role of chemokines in infections caused by fungi such as Histoplasma, Blastomyces, Coccidioides and Paracoccidioides remains to be explored. In this review we have summarized what is currently known about the role of chemokines during fungal infection, including the influence of these signaling proteins on effector cell recruitment and development of cell-mediated immunity.

Characterisation of Candida albicans infections of haematogenous and mucosal origin in mice lacking the interferon gamma receptor protein

Mice harbouring a null deletion mutation in the IFNgamma receptor gene were used to study the role of IFNgamma responsiveness during experimental systemic candidiasis of mucosal or haematogenous origin. After intravenous (i.v.) or intranasal (i.n.) challenge with Candida albicans the progression of infection and concomitant cellular and antibody anti-C. albicans immune responses were analysed. During the week following i.v. challenge, the rate of C. albicans multiplication in kidneys, liver and spleen was faster in IFNgammaR (-/-) than IFNgammaR (+/+) mice. As a result, IFNgammaR (-/-) mice perished earlier than IFNgammaR (+/+) mice when challenged with equal numbers of live yeast cells. However, the overall susceptibility of the two mouse strains, in terms of survival against different C. albicans challenge doses over a 60-day period, was similar. No differences were found in the cellular anti-C. albicans response generated by i.v. challenge in both mouse strains. In contrast the kinetics and strength of the serum anti-C. albicans antibody responses were markedly different. Significantly stronger, predominantly IgG2a antibody responses accompanied the eventual control of C. albicans infection in IFNgammaR (-/-) mice. Following intranasal challenge, there was no difference in the rate of C. albicans clearance from the lungs of IFNgammaR (-/-) and IFNgammaR (+/+) mice. However, 48 h after challenge, large, conspicuous abscesses appeared in the lungs, liver, kidneys and spleen of IFNgammaR (-/-) mice. These abscesses were characterised by the presence of C. albicans and abundant neutrophilic infiltrates, but very few macrophages. No such abscesses developed in i.n. challenged IFNgammaR (+/+) mice. In both mouse strains, i.n. challenge induced strong systemic anti-C. albicans cellular responses, but relatively low titre systemic antibody responses. Mucosal anti-C. albicans antibody responses were detected in IFNgammaR (+/+), but not IFNgammaR (-/-) mice. Splenic adherent macrophages obtained from IFNgammaR (-/-) mice exhibited a significantly lower candidacidal activity than those of IFNgammaR (+/+) mice, and as expected, were not responsive to IFNgamma. In summary, these data suggest that IFNgamma has a role in limiting C. albicans multiplication during the early stages of infection, as well as in preventing the development of C. albicans-associated abscesses. Activation of macrophages by IFNgamma might be pivotal in mediating this role.

Macrophages in resistance to candidiasis

Candida albicans, an increasingly common opportunistic pathogenic fungus, frequently causes disease in immunodeficient but not immunocompetent hosts. Clarifying the role of the phagocytic cells that participate in resistance to candidiasis not only is basic to understanding how the host copes with this dimorphic pathogen but also will expedite the development of innovative prophylactic and therapeutic approaches for treating the multiple clinical presentations that candidiasis encompasses. In this review, we present evidence that a diverse population of mononuclear phagocytes, in different states of activation and differentiation and from a variety of host species, can phagocytize C. albicans blastoconidia via an array of opsonic and nonopsonic mechanisms and can kill C. albicans blastoconidia and hyphae by means of oxygen-dependent and -independent mechanisms. Reactive nitrogen intermediates should now be added to the well-established candidacidal reactive oxygen intermediates of macrophages. Furthermore, what were thought to be two independent pathways, i.e., nitric oxide and superoxide anion, have now been shown to combine to form a potent macrophage candidacidal molecule, peroxynitrite. In contrast to monocytes and neutrophils, which are important in resistance to early stages of C. albicans infections, more differentiated macrophages activated by cytokines such as gamma interferon participate in the acquired resistance of hosts with C. albicans-specific, cell-mediated immunity. Evidence presented in this review demonstrates that mononuclear phagocytes, in some instances in the absence of other professional phagocytes such as neutrophils, play an import role in resistance to systemic and mucosal candidiasis.

Production and function of cytokines in natural and acquired immunity to Candida albicans infection

Host resistance against infections caused by the yeast Candida albicans is mediated predominantly by polymorphonuclear leukocytes and macrophages. Antigens of Candida stimulate lymphocyte proliferation and cytokine synthesis, and in both humans and mice, these cytokines enhance the candidacidal functions of the phagocytic cells. In systemic candidiasis in mice, cytokine production has been found to be a function of the CD4+ T helper (Th) cells. The Th1 subset of these cells, characterized by the production of gamma interferon and interleukin-2, is associated with macrophage activation and enhanced resistance against reinfection, whereas the Th2 subset, which produces interleukins-4, -6, and -10, is linked to the development of chronic disease. However, other models have generated divergent data. Mucosal infection generally elicits Th1-type cytokine responses and protection from systemic challenge, and identification of cytokine mRNA present in infected tissues of mice that develop mild or severe lesions does not show pure Th1- or Th2-type responses. Furthermore, antigens of C. albicans, mannan in particular, can induce suppressor cells that modulate both specific and nonspecific cellular and humoral immune responses, and there is an emerging body of evidence that molecular mimicry may affect the efficiency of anti-Candida responses within defined genetic contexts.

Suppression of anti-Candida activity of murine and human neutrophils by glucocorticoids

Effects of glucocorticoid (GC) compounds on inhibitory activity of neutrophils to mycelial growth of Candida albicans were examined by in vitro crystal violet staining method with 14 hr coculture. Both GC hormones (hydrocortisone > or = 6 x 10(-7) M and corticosterone > or = 10(-6)M) and anti-inflammatory GC agents (prednisolone > or = 10(-7) M and dexamethasone > or = 10(-8) M) significantly suppressed anti-Candida activity of murine casein-induced neutrophils. Anti-Candida activity of human neutrophils prepared from peripheral blood was also suppressed by hydrocortisone (> or = 6 x 10(-7) M). These GC compounds did not affect the Candida growth in the absence of neutrophils. Steroidal compounds without anti-inflammatory activity, cholesterol, cholic acid, aldosterone did not suppress neutrophil activity. These results suggest that GCs at their physiological or clinical concentration may suppress anti-Candida activity of neutrophils in vivo.

Effect of lectins on hepatic clearance and killing of Candida albicans by the isolated perfused mouse liver

The isolated perfused mouse liver model was used to study the effects of various lectins on hepatic trapping and killing of Candida albicans. After mouse livers were washed with 20 to 30 ml of perfusion buffer, 10(6) C. albicans CFU were infused into the livers. At the time of recovery, 63% +/- 2% (mean +/- standard error of the mean) of the infused C. albicans CFU were recovered from the liver and 14% +/- 1% were recovered from the effluent for a total recovery of 77% +/- 2%. This indicated that 86% +/- 9% of the original inoculum was trapped by the liver and that 23% +/- 2% was killed within the liver. When included in both preperfusion and postperfusion buffers (0.2 mg of lectin per ml), Ulex europeaus lectin (binding specificity for fucose) decreased hepatic trapping of C. albicans by 37% and eluted trapped C. albicans from the liver only when included in postperfusion buffer. By comparison, treatment of C. albicans with U. europeaus lectin before infusion had no effect on the trapping or killing of yeast cells. When Lens culinaris lectin (binding specificity for mannose) was included in the perfusion buffers, hepatic killing of C. albicans increased by 16% with no significant effect on hepatic killing when yeast cells were treated with L. culinaris lectin before infusion. Forty to 55% of the infused C. albicans were killed when concanavalin A (binding specificities for mannose and glucose), Glycine max (binding specificity for N-acetylgalactosamine), or Arachis hypogea (binding specificity for galactose) lectin was included in the perfusion buffer or when yeast cells were treated with these lectins before their infusion. When C. albicans was treated with concanavalin A at a concentration of less than 0.02 mg/ml, hepatic killing of yeast cells was not significantly increased. The data suggest that a fucose-containing receptor on the surface of either sinusoidal endothelial cells or Kupffer cells is involved in the trapping of C. albicans by the perfused mouse liver. Moreover, lectins with binding specificities for mannose, N-acetylgalactosamine, and galactose increased hepatic killing of C. albicans.