Rhizopus oryzae hyphae are damaged by human natural killer (NK) cells, but suppress NK cell mediated immunity

COVID-19 associated mucormycosis surge: A review on multi-pathway mechanisms

Mucormycosis is a fungal infection caused by moulds from the Mucorales order. Concerns have been mounting due to the alarming increase in severe morbidity and mortality associated with mucormycosis during the COVID-19 pandemic. This condition, known as COVID-19-associated mucormycosis (CAM), has been linked to various environmental, host-related, and medical factors on a global scale. We have categorized the most significant potential risk factors for developing mucormycosis in individuals with a previous history of coronavirus infection into 10 major categories. These categories include acute hyperglycemia, the impact of cytokine release, immune response deficiencies in COVID-19 patients, microvasculopathy and dysfunction of endothelial cells, imbalances in iron metabolism, metabolic acidosis, organ damage resulting from COVID-19, underlying health conditions (such as diabetes), environmental factors, and medical treatments that can be iatrogenic in nature (such as inappropriate glucocorticoid use). Many of these factors can lead to potentially life-threatening infections that can complicate the treatment of COVID-19. Physicians should be vigilant about these factors because early detection of mucormycosis is crucial for effective management of this condition.

Mucormycosis in 2023: an update on pathogenesis and management

Mucormycosis (MCR) is an emerging and frequently lethal fungal infection caused by the Mucorales family, with Rhizopus, Mucor, and Lichtheimia, accounting for > 90% of all cases. MCR is seen in patients with severe immunosuppression such as those with hematologic malignancy or transplantation, Diabetes Mellitus (DM) and diabetic ketoacidosis (DKA) and immunocompetent patients with severe wounds. The recent SARS COV2 epidemy in India has resulted in a tremendous increase in MCR cases, typically seen in the setting of uncontrolled DM and corticosteroid use. In addition to the diversity of affected hosts, MCR has pleiotropic clinical presentations, with rhino-orbital/rhino-cerebral, sino-pulmonary and necrotizing cutaneous forms being the predominant manifestations. Major insights in MCR pathogenesis have brought into focus the host receptors (GRP78) and signaling pathways (EGFR activation cascade) as well as the adhesins used by Mucorales for invasion. Furthermore, studies have expanded on the importance of iron availability and the complex regulation of iron homeostasis, as well as the pivotal role of mycotoxins as key factors for tissue invasion. The molecular toolbox to study Mucorales pathogenesis remains underdeveloped, but promise is brought by RNAi and CRISPR/Cas9 approaches. Important recent advancements have been made in early, culture-independent molecular diagnosis of MCR. However, development of new potent antifungals against Mucorales remains an unmet need. Therapy of MCR is multidisciplinary and requires a high index of suspicion for initiation of early Mucorales-active antifungals. Reversal of underlying immunosuppression, if feasible, rapid DKA correction and in selected patients, surgical debulking are crucial for improved outcomes.

Higher proinflammatory responses possibly contributing to suppressed cytotoxicity in patients with COVID-19 associated mucormycosis

INTRODUCTION

COVID-19 Associated Mucormycosis (CAM), an opportunistic fungal infection, surged during the second wave of SARS Cov-2 pandemic. Since immune responses play an important role in controlling this infection in immunocompetent hosts, it is required to understand immune perturbations associated with this condition for devising immunotherapeutic strategies for its control. We conducted a study to determine different immune parameters altered in CAM cases as compared to COVID-19 patients without CAM.

METHODOLOGY

Cytokine levels in serum samples of CAM cases (n = 29) and COVID-19 patients without CAM (n = 20) were determined using luminex assay. Flow cytometric assays were carried out in 20 CAM cases and 10 controls for determination of frequency of NK cells, DCs, phagocytes, T cells and their functionalities. The cytokine levels were analyzed for their association with each other as well as with T cell functionality. The immune parameters were also analyzed with respect to the known risk factors such as diabetes mellitus and steroid treatment.

RESULTS

Significant reduction in frequencies of total and CD56 + CD16 + NK cells (cytotoxic subset) was noted in CAM cases. Degranulation responses indicative of cytotoxicity of T cell were significantly hampered in CAM cases as compared to the controls. Conversely, phagocytic functions showed no difference in CAM cases versus their controls except for migratory potential which was found to be enhanced in CAM cases. Levels of proinflammatory cytokines such as IFN-γ, IL-2, TNF-α, IL-17, IL-1β, IL-18 and MCP-1 were significantly elevated in cases as compared to the control with IFN-γ and IL-18 levels correlating negatively with CD4 T cell cytotoxicity. Steroid administration was associated with higher frequency of CD56 + CD16- NK cells (cytokine producing subset) and higher MCP-1 levels. Whereas diabetic participants had higher phagocytic and chemotactic potential and had higher levels of IL-6, IL-17 and MCP-1.

CONCLUSION

CAM cases differed from the controls in terms of higher titers of proinflammatory cytokines, reduced frequency of total and cytotoxic CD56 + CD16 + NK cell. They also had reduced T cell cytotoxicity correlating inversely with IFN-γ and IL-18 levels, possibly indicating induction of negative feedback mechanisms while diabetes mellitus or steroid administration did not affect the responses negatively.

Surge of mucormycosis during the COVID-19 pandemic

Patients with respiratory viral infections are more likely to develop co-infections leading to increased fatality. Mucormycosis is an epidemic amidst the COVID-19 pandemic that conveys a 'double threat' to the global health fraternity. Mucormycosis is caused by the Mucorales group of fungi and exhibits acute angioinvasion generally in immunocompromised patients. The most familiar foci of infections are sinuses (39%), lungs (24%), and skin tissues (19%) where the overall dissemination occurs in 23% of cases. The mortality rate in the case of disseminated mucormycosis is found to be 96%. Symptoms are mostly nonspecific and often resemble other common bacterial or fungal infections. Currently, COVID-19-associated mucormycosis (CAM) is being reported from a number of countries such as the USA, Turkey, France, Mexico, Iran, Austria, UK, Brazil, and Italy, while India is the hotspot for this deadly co-infection, accounting for approximately 28,252 cases up to June 8, 2021. It strikes patients within 12-18 days after COVID-19 recovery, and nearly 80% require surgery. Nevertheless, the mortality rate can reach 94% if the diagnosis is delayed or remains untreated. Sometimes COVID-19 is the sole predisposing factor for CAM. Therefore, this study may provide a comprehensive resource for clinicians and researchers dealing with fungal infections, intending to link the potential translational knowledge and prospective therapeutic challenges to counter this opportunistic pathogen.

Manipulating NK cellular therapy from cancer to invasive fungal infection: promises and challenges

The ideal strategy to fight an infection involves both (i) weakening the invading pathogen through conventional antimicrobial therapy, and (ii) strengthening defense through the augmentation of host immunity. This is even more pertinent in the context of invasive fungal infections whereby the majority of patients have altered immunity and are unable to mount an appropriate host response against the pathogen. Natural killer (NK) cells fit the requirement of an efficient, innate executioner of both tumour cells and pathogens – their unique, targeted cell killing mechanism, combined with other arms of the immune system, make them potent effectors. These characteristics, together with their ready availability (given the various sources of extrinsic NK cells available for harvesting), make NK cells an attractive choice as adoptive cellular therapy against fungi in invasive infections. Improved techniques in ex vivo NK cell activation with expansion, and more importantly, recent advances in genetic engineering including state-of-the-art chimeric antigen receptor platform development, have presented an opportune moment to harness this novel therapeutic as a key component of a multipronged strategy against invasive fungal infections.

The TLR-NF-kB axis contributes to the monocytic inflammatory response against a virulent strain of Lichtheimia corymbifera, a causative agent of invasive mucormycosis

Invasive mucormycosis (IM) is a life-threatening infection caused by the fungal order Mucorales, its diagnosis is often delayed, and mortality rates range from 40-80% due to its rapid progression. Individuals suffering from hematological malignancies, diabetes mellitus, organ transplantations, and most recently COVID-19 are particularly susceptible to infection by Mucorales. Given the increase in the occurrence of these diseases, mucormycosis has emerged as one of the most common fungal infections in the last years. However, little is known about the host immune response to Mucorales. Therefore, we characterized the interaction among L. corymbifera—one of the most common causative agents of IM—and human monocytes, which are specialized phagocytes that play an instrumental role in the modulation of the inflammatory response against several pathogenic fungi. This study covered four relevant aspects of the host-pathogen interaction: i) The recognition of L. corymbifera by human monocytes. ii) The intracellular fate of L. corymbifera. iii) The inflammatory response by human monocytes against the most common causative agents of mucormycosis. iv) The main activated Pattern-Recognition Receptors (PRRs) inflammatory signaling cascades in response to L. corymbifera. Here, we demonstrate that L. corymbifera exhibits resistance to intracellular killing over 24 hours, does not germinate, and inflicts minimal damage to the host cell. Nonetheless, viable fungal spores of L. corymbifera induced early production of the pro-inflammatory cytokine IL-1β, and late release of TNF-α and IL-6 by human monocytes. Moreover, we revealed that IL-1β production predominantly depends on Toll-like receptors (TLRs) priming, especially via TLR4, while TNF-α is secreted via C-type lectin receptors (CTLs), and IL-6 is produced by synergistic activation of TLRs and CTLs. All these signaling pathways lead to the activation of NF-kB, a transcription factor that not only regulates the inflammatory response but also the apoptotic fate of monocytes during infection with L. corymbifera. Collectively, our findings provide new insights into the host-pathogen interactions, which may serve for future therapies to enhance the host inflammatory response to L. corymbifera.

Computer-Aided Tools and Resources for Fungal Pathogens: An Application of Reverse Vaccinology for Mucormycosis

Increasing fungal infections in immunocompromised hosts are a growing concern for global public health. Along with treatments, preventive measures are required. The emergence of reverse vaccinology has opened avenues for using genomic and proteomic data from pathogens in the design of vaccines. In this work, we present a comprehensive collection of various computational tools and databases with potential to aid in vaccine development. The ongoing pandemic has directed attention toward the increasing number of mucormycosis infections in COVID-19 patients. As a case study, we developed a computational pipeline for assisting vaccine development for mucormycosis. We obtained 6 proteins from 29,447 sequences from UniProtKB as potential vaccine candidates against mucormycosis, fulfilling multiple criteria. These criteria included potential characteristics, namely adhesin properties, surface or extracellular localization, antigenicity, no similarity to any human proteins, nonallergenicity, stability in vitro, and expression in fungal cells. These six proteins were predicted to have B cell and T cell epitopes, proinflammatory inducing peptides, and orthologs in several mucormycosis-causing species. These data could aid in vaccine development against mucormycosis for at-risk individuals.

Rhino-Orbital-Cerebral Mycosis and Extranodal Natural Killer or/and T-Cell Lymphoma, Nasal Type

Background

Extranodal natural killer/T-cell lymphoma, nasal type is a syndrome of middle face destruction with an association to Epstein-Barr virus. Fungi have been recovered from the diseased tissue now and then but were often seen as a lymphoma-associated secondary infection. However, there are ENKTL-NT cases with the recoveries of fungi and complete recovery with antifungal therapy, which are quite similar to rhino-orbital-cerebral mycosis (ROCM) that often confuses the physicians.

Methods

We searched Medline for English-language manuscripts limited to “human” and “case reports,” “letters,” “reviews,” and “clinical conferences” from 1966 to 2022. We used MeSH terms “lymphoma, extranodal nk-t-cell” [MeSH Terms] or “lethal midline granuloma” [MeSH Terms], in combination with MeSH terms “microbiology” [subheading] or “microbiology” [all fields] or “fungi” [all fields] or “fungi” [MeSH Terms] for ENKTL-NT with infections. We used MeSH terms “Mycoses” in combination with “Nose” [Mesh] OR “Orbital Diseases” [Mesh] for rhino-orbital-cerebral fungal infections.

Results

We appraised 149 included articles and extracted references related to ENKTL-NT and/or ROCM. Themes and subcategories were subsequently derived. Our findings revealed that ROCM and ENKTL-NT are characterized by progressive and destructive ulcers in the midline face or rhino-orbital structures. ROCM is mainly caused by fungi in the order of Mucorales, and ENKTL-NT is usually associated with Epstein-Barr virus and sometimes fungi. Radiologically, both are characterized by non-specific features of sinusitis, soft tissue infection, and necrosis. Pathologically, ROCM and ENKTL-NT share the same characteristics of inflammation, necrosis, and granuloma. ROCM is characterized by the detection of fungi in tissue, while ENKTL-NT is typically positive for NK/T-cell markers and cytotoxic granule-associated proteins, proliferation, and vascular damage of angioinvasion, which could be incited by Mucor irregularis and Rhizopus arrhizus in patients and mice.

Conclusion

ENKTL-NT and ROCM share many similarities in clinical presentations, radiology, and histopathology, and might have the same etiology. This may explain why the two diseases are tangled together in the reported cases, and suggests the role that the fungi may play in the development of these ENKTL-NT/ROCM diseases. The reason why ENKTL-NT and ROCM are sometimes confused is that the main pathogens of ROCM, Mucor irregularis and Rhizopus arrhizus, are the fungal causative agents of ENKTL-NT.

Mucormycosis, a post-COVID infection: possible adjunctive herbal therapeutics for the realigning of impaired immune-metabolism in diabetic subjects

Cytokine storm is believed as a major root cause for multi-organ failure and death in severely infected diabetic patients with COVID-19. This condition is treated with anti-inflammatory drugs, mainly steroids, to recover people from critical conditions. However, steroid therapy causes immune suppression and uncontrolled hyper-glycaemia in post-COVID. This altered immune-metabolism provides a fertile environment for the infection of a black fungus, Rhizopus arrhizus which causes mucormycosis in diabetic patients. It is a life-threatening infection causing death in different countries. It is treated either with anti-fungal drugs, surgical debridement, or adjunctive therapies. The available therapies for mucormycosis have been associated with several drawbacks. Thus, the present review has explored and suggested herbs-spices based adjunctive therapy for possible realignment of the impaired immune system in the post-COVID diabetic subjects. The consumption of herbal therapeutics after COVID-19 could realign the impaired immune-metabolism in the post-COVID and thereby exert prophylactic effects against mucormycosis. Furthermore, the suggested herbal sources could help in the discovery of novel therapeutics against the COVID-19 associated mucormycosis.

Trichoderma longibrachiatum-Associated Skin Inflammation and Atypical Hyperplasia in Mouse

Background

The relationship between infection and tumors has attracted increasing attention. Trichoderma spp. are often isolated from tumors. However, their potential role remains unclear. We recently reported the isolation of Trichoderma longibrachiatum from a patient with pulmonary spindle cell carcinoma that was confirmed as primary infection by application of laser capture microdissection and polymerase chain reaction. To explore whether the strain is pathogenic and whether it can cause atypical cell proliferation and infiltration of NK cells and T cells, we designed a mouse infection experiment.

Methods

Twelve ICR mice were randomly separated into three groups. Cyclophosphamide was used to inhibit the immunity of mice. A mouse model of Trichoderma infection was successfully established by intracutaneous injection on the back skin with a suspension of strain PKUT180420015. The pathological manifestations of Trichoderma infection and the interaction between immune cells and fungi were observed by histopathology, immunohistochemistry and intensive fungal staining. Reisolation of the fungus was observed by infected tissue culture. The inoculated sites exhibited swelling 3 days after inoculation, and ulcers developed at approximately 14 days. Skin specimens were obtained and then cultured at 3, 7, and 14 days after inoculation. We selected mice 14 days after inoculation in Group 3, whose ulcers were the most typical, for histological analysis.

Results

Inflammation, angioinvasion and necrosis were observed. Immunohistochemistry showed positive markers of Ki67, CD3, CD56, GZMB, and PRF. Periodic acid-Schiff staining, periodic acid-silver methenamine staining, and Calcofluor staining showed fungal spores in the vascular lumen, vascular walls and around the blood vessels.

Conclusions

Our studies showed that a T. longibrachiatum strain (PKUT180420015) isolated from a biopsy specimen in a patient with pulmonary spindle cell carcinoma could induce atypical hyperplasia, with the expression of Ki67, CD3, CD56, GZMB, and PRF in mice. These data indicate that the fungus may be involved in inducing atypical hyperplasia or tumorigenesis.

Mucormycosis and COVID-19-Associated Mucormycosis: Insights of a Deadly but Neglected Mycosis

The ongoing COVID-19 pandemic has quickly become a health threat worldwide, with high mortality and morbidity among patients with comorbidities. This viral infection promotes the perfect setting in patients for the development of opportunistic infections, such as those caused by fungi. Mucormycosis, a rare but deadly fungal infection, has recently increased its incidence, especially in endemic areas, since the onset of the pandemic. COVID-19-associated mucormycosis is an important complication of the pandemic because it is a mycosis hard to diagnose and treat, causing concern among COVID-19-infected patients and even in the already recovered population. The risk factors for the development of mucormycosis in these patients are related to the damage caused by the SARS-CoV-2 itself, the patient’s overstimulated immune response, and the therapy used to treat COVID-19, causing alterations such as hyperglycemia, acidosis, endothelial and lung damage, and immunosuppression. In this review, the molecular aspects of mucormycosis and the main risk factors for the development of COVID-19-associated mucormycosis are explained to understand this virus–fungi–host interaction and highlight the importance of this neglected mycosis.

A bacterial endosymbiont of the fungus Rhizopus microsporus drives phagocyte evasion and opportunistic virulence

Opportunistic infections by environmental fungi are a growing clinical problem, driven by an increasing population of people with immunocompromising conditions. Spores of the Mucorales order are ubiquitous in the environment but can also cause acute invasive infections in humans through germination and evasion of the mammalian host immune system. How they achieve this and the evolutionary drivers underlying the acquisition of virulence mechanisms are poorly understood. Here, we show that a clinical isolate of Rhizopus microsporus contains a Ralstonia pickettii bacterial endosymbiont required for virulence in both zebrafish and mice and that this endosymbiosis enables the secretion of factors that potently suppress growth of the soil amoeba Dictyostelium discoideum, as well as their ability to engulf and kill other microbes. As amoebas are natural environmental predators of both bacteria and fungi, we propose that this tri-kingdom interaction contributes to establishing endosymbiosis and the acquisition of anti-phagocyte activity. Importantly, we show that this activity also protects fungal spores from phagocytosis and clearance by human macrophages, and endosymbiont removal renders the fungal spores avirulent in vivo. Together, these findings describe a new role for a bacterial endosymbiont in Rhizopus microsporus pathogenesis in animals and suggest a mechanism of virulence acquisition through environmental interactions with amoebas.

Neem and Turmeric in the management of Covid Associated Mucormycosis (CAM) derived through network pharmacology

Mucormycosis or 'Black Fungus' has been known to target immunocompromised individuals even before the emergence of COVID-19. Nevertheless, the present circumstances provide the best opening for Covid Associated Mucormycosis (CAM), as the global pandemic is engulfing a large part of human population making them immunocompromised. This drastic increase in Mucormycosis infections has to be addressed as early as possible. There is a growing tendency of relying upon herbal drugs that have minimal side effects and does not compromise our immune system. Recently, the concept of network pharmacology has grabbed the attention of modern science, especially advanced medical sciences. This is a new discipline that can use computational power to systematically catalogue the molecular interactions between botanical formulations and the human body. In this study, Neem and Turmeric was considered as the target plants and an attempt was made to reveal various aspects through which phytocompounds derived from them may effectively manage CAM menace. We have taken a step-by-step approach for identifying the target proteins and ligands associated with Mucormycosis treatment. Functional network analysis and Molecular docking approaches were applied to validate our findings. Quercetin derived from both Neem and Turmeric was found to be one of the main phytocompounds working against Mucormycosis. Along with that, Caffeic acid, Curcumin, Kaempferol, Tetrahydrocurcumin and Myricetin also play a pivotal role in fighting against Black-Fungus. A thorough analysis of our result suggested a triple-front attack on the fungal pathogens and the approaches are necrosis inhibition, iron chelation and immuno-boosting.Communicated by Ramaswamy H. Sarma.

Risks of mucormycosis in the current Covid-19 pandemic: a clinical challenge in both immunocompromised and immunocompetent patients

Mucormycosis, also called "Black Fungus", is a new cause for worry in the current Coronavirus disease 2019 (covid-19) pandemic. Mucormycosis is devasting due to its high rate of morbidity and mortality which is a great cause of concern. Mucormycosis, in general, affects immunocompromised patients including diabetic, people with malignancies, organ and stem cell transplants and people affected with pandemic diseases like covid-19. Diagnosis of Mucormycosis is often delayed either due to clinical complications or misdiagnosed as symptoms of other diseases, especially covid-19. This could delay the treatment protocol which results in the failure of treatment. Mortality rate due to secondary infections in covid-19 patients with uncontrolled diabetics and who are on steroid therapy can soon reach 100% if diagnosis and treatment doesn't happen on timely basis. Risk of Mucormycosis is not just in immunosuppressed patients, but immunocompetent people with late diagnosis are also prone to infection. In view of this, we present a comprehensive review on risks of Mucormycosis in immunocompromised and immunocompetent patients highlighting the epidemiology, forms of Mucormycosis, immune response against Mucorales, difficulties in diagnosis and challenges in treatment of Mucormycosis, with emphasis on covid-19 associated Mucormycosis. Importantly, we have discussed the precautions and care to effectively manage Mucormycosis in immunocompromised and immunocompetent patients. Thus, current review helps clinicians in understanding various risk factors in both immunocompromised (especially covid-19 patients) and immunocompetent patients which is critical in managing Mucormycosis in current covid-19 pandemic.

New insights on mucormycosis and its association with the COVID-19 pandemic

COVID-19 continues to cause significant fatality worldwide. Glucocorticoids prove to play essential roles in COVID-19 management; however, the extensive use of steroids together with the virus immune dysregulation may increase the danger of secondary infections with mucormycosis, an angioinvasive fungal infection. Unfortunately, a definite correlation between COVID-19 and elevated mucormycosis infection cases is now clear worldwide. In this review, we discuss the historical record and epidemiology of mucormycosis as well as pathogenesis and associated host immune response, risk factors, clinical presentation, diagnosis and treatment. Special emphasis is given to its association with the current COVID-19 pandemic, including latest updates on COVID-19-associated mucormycosis cases globally, with recommendations for efficacious management.

A definite correlation between COVID-19 and elevated mucormycosis infection cases is now clear worldwide. This article can be used as comprehensive tool to help clinicians and the healthcare team in getting a clearer look on the historical record and epidemiology of mucormycosis as well as pathogenesis and associated host immune response. Risk factors, clinical presentation, diagnosis, treatment options and also the latest updates on COVID-19-associated mucormycosis are also discussed, which will help in the understanding of the fatal mucormycosis infection.

COVID-19 associated mucormycosis: evolving technologies for early and rapid diagnosis

The post-coronavirus disease (COVID-19) mucormycosis is a deadly addition to the pandemic spectrum. Although it’s a rare, aggressive, and opportunistic disease, the associated morbidity and mortality are significant. The complex interplay of factors aggravating CAM is uncontrolled diabetes, irrational and excessive use of antibiotics, steroids, and an impaired immune system. Recently, India has been witnessing a rapid surge in the cases of coronavirus disease-associated mucormycosis (CAM), since the second wave of COVID-19. The devastating and lethal implications of CAM had now become a matter of global attention. A delayed diagnosis is often associated with a poor prognosis. Therefore, the rapid and early diagnosis of infection would be life-saving. Prevention and effective management of mucormycosis depend upon its early and accurate diagnosis followed by a multimodal therapeutic approach. The current review summarizes an array of detection methods and highlights certain evolving technologies for early and rapid diagnosis of CAM. Furthermore, several potential management strategies have also been discussed, which would aid in tackling the neglected yet fatal crisis of mucormycosis associated with COVID-19.

Dental and Oral Manifestations of COVID-19 Related Mucormycosis: Diagnoses, Management Strategies and Outcomes

It has been nearly two years since the pandemic caused by the novel coronavirus disease (COVID-19) has affected the world. Several innovations and discoveries related to COVID-19 are surfacing every day and new problems associated with the COVID-19 virus are also coming to light. A similar situation is with the emergence of deep invasive fungal infections associated with severe acute respiratory syndrome 2 (SARS-CoV-2). Recent literature reported the cases of pulmonary and rhino-cerebral fungal infections appearing in patients previously infected by COVID-19. Histopathological analysis of these cases has shown that most of such infections are diagnosed as mucormycosis or aspergillosis. Rhino-orbital-cerebral mucormycosis usually affects the maxillary sinus with involvement of maxillary teeth, orbits, and ethmoidal sinuses. Diabetes mellitus is an independent risk factor for both COVID-19 as well as mucormycosis. At this point, there is scanty data on the subject and most of the published literature comprises of either case reports or case series with no long-term data available. The aim of this review paper is to present the characteristics of COVID-19 related mucormycosis and associated clinical features, outcome, diagnostic and management strategies. A prompt diagnosis and aggressive treatment planning can surely benefit these patients.

IFN-γ Mediated Signaling Improves Fungal Clearance in Experimental Pulmonary Mucormycosis

We established three immunocompetent murine models of pulmonary mucormycosis to determine the involvement of the adaptive immune response in host resistance in pulmonary mucormycosis, a rapidly fatal disease caused mainly by Rhizopus spp. Immunocompetent inbred (C57BL/6, BALB/c) and outbred (Swiss) strains of mice were inoculated with R. oryzae via the intratracheal route. The inoculation resulted in a disseminated infection that spread to the brain, spleen, kidney, and liver. After 7 and 30 days of R. oryzae infection, BALB/c mice showed the lowest fungal load and highest production of IFN-γ and IL-2 by splenocytes. Swiss mice showed a higher fungal load 30 days p.i. and was associated with a weak development of the Th-1 profile. To confirm our findings, R. oryzae-infected IFN-γ⁻/⁻ mice were evaluated after 60 days, where the mice still showed viable fungi in the lungs. This study showed, for the first time, that pulmonary mucormycosis in three widely used mouse strains resulted in an acute fungal dissemination without immunosuppression whose outcome varies according to the genetic background of the mice. We also identified the partial role of IFN-γ in the efficient elimination of R. oryzae during pulmonary infection.

Mucormycosis: An opportunistic pathogen during COVID-19

The pandemic of coronavirus disease 2019 (COVID-19) still remains on an upsurge trend. The second wave of this disease has led to panic in many countries, including India and some parts of the world suffering from the third wave. As there are no proper treatment options or remedies available for this deadly infection, supportive care equipment's such as oxygen cylinders, ventilators and heavy use of steroids play a vital role in the management of COVID-19. In the midst of this pandemic, the COVID-19 patients are acquiring secondary infections such as mucormycosis also known as black fungus disease. Mucormycosis is a serious, but rare opportunistic fungal infection that spreads rapidly, and hence prompt diagnosis and treatment are necessary to avoid high rate of mortality and morbidity rates. Mucormycosis is caused by the inhalation of its filamentous (hyphal form) fungi especially in the patients who are immunosuppressed. Recent studies have documented alarming number of COVID-19 patients with mucormycosis infection. Most of these patients had diabetes and were administered steroids for Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection and were consequently more prone to mucormycosis. Hence, the present review emphasizes mucormycosis and its related conditions, its mechanism in normal and COVID-19 affected individuals, influencing factors and challenges to overcome this black mold infection. Early identification and further investigation of this fungus will significantly reduce the severity of the disease and mortality rate in COVID-19 affected patients.

Case Report: Rhizopus arrhizus Rhino-Orbital-Cerebral Mycosis and Lethal Midline Granuloma: Another Fungal Etiological Agent

Objective: Both rhino-orbital-cerebral mycosis and lethal midline granuloma (LMG) may result in midline destruction. LMG has now been generally considered as a natural killer/T cell lymphoma, nasal type (ENKTL-NT) with an association of EBV. Fungi have been detected from the diseased tissues now and then but are often considered as lymphoma-associated infections. We previously reported an ENKTL-NT case with Mucor irregularis, which played a causal role in the disease and was involved in the overexpression of Ki67 and CD56 in the mouse experiment. The present study describes a chronic Rhizopus arrhizus infection with immunological parameters that are closely similar to LMG. We aim to explore the relationship of another Mucorales fungus, R. arrhizus, and LMG in a patient and in mice. Methods: Case study and mouse infection modules were designed for our observation. A 35-year-old man with midline face ulcers which was clinically suspected as LMG was selected. Biopsy specimens were sent for lymphoma diagnosis and microbiological detection. The isolated fungus was tested in an ICR mouse model for mycological and histological analyses. Results: Five tissue samples yielded Rhizopus arrhizus. In the pathology, characteristic inflammation, necrosis, and granulation with thin-walled hyphae are observed. Immunohistochemistry showed NK/T cell infiltration (CD3+, CD8+, TIA1+, GZMB+, PRF+, individual CD56+) with hyperplasia (Ki67+) and angioinvasion. The patient recovered completely with amphotericin B. In the murine experiment, R. arrhizus caused angioinvasion with NK/T cell infiltration (CD3+, CD56+, TIA1+, GZMB +, PRF+) with proliferation (Ki67+) and was re-isolated from the infected host. Conclusions: We here describe a mid-face destruction patient, which was diagnosed by the top pathologists in China according to the current criteria of NK/T cell lymphoma, with a negative result for EBV and positive result for R. arrhizus. With a then developed mouse experiment, the R. arrhizus in the diseased lesions was responsible for the NK/T cell infiltration (CD3+, CD8+, CD56+, TIA1+, GZMB+, PRF+), proliferation (Ki67+), and angioinvasion, suggesting another fungal etiological agent for LMG, which could be eradicated with amphotericin B. Limitations: The sample size is not sufficient for statistical analysis. However, our findings are suggestive for the role fungus plays in LMG.

Immunology of Fungal Infections

Complex processes mediate immunity to fungal infections. Responses vary depending on the organism, morphogenic state, and infection site. Innate immune effectors such as epithelia, phagocytes, and soluble molecules detect pathogens, kill fungi, release cytokines, and prime the adaptive response. Adaptive responses to mucocutaneous or invasive disease are markedly different but intersect at certain pathways (molecules required for IL-23 and IL-12 signaling). Many of these pathways have been elucidated from the study of inborn errors of immunity. This review explores the general aspects of antifungal immunity and delves into the mechanisms that mediate protection from frequently encountered fungi.

Natural Killer Cell Line NK-92-Mediated Damage of Medically Important Fungi

Invasive fungal disease (IFD) in hematopoietic stem cell transplantation is associated with high morbidity and mortality. As the antifungal host response determines risk and outcome of IFD, there is growing interest in adoptive immunotherapy using T cells or natural killer (NK) cells. Although the NK-92 cell line has been tested as anticancer therapy in clinical trials, data on the antifungal activity of NK-92 cells are lacking. Here, we show that the NK-92 cell line exhibits considerable fungal damage on all medically important fungi tested, such as different species of Aspergillus, Candida, mucormycetes, and Fusarium. The extent of fungal damage differs across various species of mucormycetes and Fusarium, whereas it is comparable across different species of Aspergillus and Candida. Interferon (IFN)-γ levels in the supernatant were lower when NK-92 cells are co-incubated with Aspergillus fumigatus, Candida albicans, or Rhizopus arrhizus compared to the levels when NK-92 cells are incubated alone. Different to primary human NK cells, no increase of perforin levels in the supernatant was observed when the fungi were added to NK-92 cells. Our in vitro data demonstrated that the NK-92 cell line could be a feasible tool for antifungal immunotherapy, but data of animal models are warranted prior to clinical trials.

Host Immune Defense upon Fungal Infections with Mucorales: Pathogen-Immune Cell Interactions as Drivers of Inflammatory Responses

During the last few decades, mucormycosis has emerged as one of the most common fungal infections, following candidiasis and aspergillosis. The fungal order responsible for causing mucormycosis is the Mucorales. The main hallmarks of this infection include the invasion of blood vessels, infarction, thrombosis, and tissue necrosis, which are exhibited at the latest stages of the infection. Therefore, the diagnosis is often delayed, and the rapid progression of the infection severely endangers the life of people suffering from diabetes mellitus, hematological malignancies, or organ transplantation. Given the fact that mortality rates for mucormycosis range from 40 to 80%, early diagnosis and novel therapeutic strategies are urgently needed to battle the infection. However, compared to other fungal infections, little is known about the host immune response against Mucorales and the influence of inflammatory processes on the resolution of the infection. Hence, in this review, we summarized our current understanding of the interplay among pro-inflammatory cytokines, chemokines, and the host-immune cells in response to mucoralean fungi, as well as their potential use for immunotherapies.

Mucorales Species and Macrophages

Mucormycosis is an emerging fungal infection caused by Mucorales with an unacceptable high mortality rate. Mucorales is a complex fungal group, including eleven different genera that can infect humans. This heterogeneity is associated with species-specific invasion pathways and responses to the host defense mechanisms. The host innate immune system plays a major role in preventing Mucorales growth and host invasion. In this system, macrophages are the main immune effector cells in controlling these fungi by rapid and efficient phagocytosis of the spores. However, Mucorales have evolved mechanisms to block phagosomal maturation and species-specific mechanisms to either survive as dormant spores inside the macrophage, as Rhizopus species, or geminate and escape, as Mucor species. Classical fungal models of mucormycosis, mostly Rhizopus, have made important contributions to elucidate key aspects of the interaction between Mucorales and macrophages, but they lack robust tools for genetic manipulation. The recent introduction of the genetically tractable Mucor circinelloides as a model of mucormycosis offers the possibility to analyze gene function. This has allowed the identification of regulatory pathways that control the fungal response to phagocytosis, including a non-canonical RNAi pathway (NCRIP) that regulates the expression of most genes regulated by phagocytosis.

Immunotherapy as an Antifungal Strategy in Immune Compromised Hosts

Purpose of Review

IFIs cause high morbidity and mortality in the immunocompromised host worldwide. Although highly effective, conventional antifungal chemotherapy faces new challenges due to late diagnosis and increasing numbers of drug-resistant fungal strains. Thus, antifungal immunotherapy represents a viable treatment option, and recent advances in the field are summarized in this review.

Recent Findings

Antifungal immunotherapies include application of immune cells as well as the administration of cytokines, growth factors, and antibodies. Novel strategies to treat IFIs in the immunocompromised host target intracellular signaling pathways using SMTs such as checkpoint inhibitors.

Summary

Studies using cytokines or chemokines exerted a potential adjuvant role to conventional antifungal therapy, but issues on toxicity for some agents have to be resolved. Cell-based immunotherapies are very labor-intense and costly, but NK cell transfer and CAR T cell therapy provide exciting strategies to combat IFIs. Antibody-mediated protection and checkpoint inhibition are additional novel immunotherapeutic approaches.

Distinct Effects of Immunosuppressive Drugs on the Anti-Aspergillus Activity of Human Natural Killer Cells

As the prognosis of invasive aspergillosis remains unacceptably poor in patients undergoing hematopoietic stem cell transplantation (HSCT), there is a growing interest in the adoptive transfer of antifungal effector cells, such as Natural Killer (NK) cells. Because immunosuppressive agents are required in most HSCT recipients, knowledge of the impact of these compounds on the antifungal activity of NK cells is a prerequisite for clinical trials. We, therefore, assessed the effect of methylprednisolone (mPRED), cyclosporin A (CsA) and mycophenolic acid (MPA) at different concentrations on proliferation, apoptosis/necrosis, and the direct and indirect anti-Aspergillus activity of human NK cells. Methylprednisolone decreased proliferation and increased apoptosis of NK cells in a significant manner. After seven days, a reduction of viable NK cells was seen for all three immunosuppressants, which was significant for MPA only. Cyclosporin A significantly inhibited the direct hyphal damage by NK cells in a dose-dependent manner. None of the immunosuppressive compounds had a major impact on the measured levels of interferon-γ, granulocyte-macrophage colony-stimulating factor and RANTES (regulated on activation, normal T cell expressed and secreted; CCL5). Our data demonstrate that commonly used immunosuppressive compounds have distinct effects on proliferation, viability and antifungal activity of human NK cells, which should be considered in designing studies on the use of NK cells for adoptive antifungal immunotherapy.

Pathogenicity patterns of mucormycosis: epidemiology, interaction with immune cells and virulence factors

Fungi of the basal lineage order Mucorales are able to cause infections in animals and humans. Mucormycosis is a well-known, life-threatening disease especially in patients with a compromised immune system. The rate of mortality and morbidity caused by mucormycosis has increased rapidly during the last decades, especially in developing countries. The systematic, phylogenetic, and epidemiological distributions of mucoralean fungi are addressed in relation to infection in immunocompromised patients. The review highlights the current achievements in (i) diagnostics and management of mucormycosis, (ii) the study of the interaction of Mucorales with cells of the innate immune system, (iii) the assessment of the virulence of Mucorales in vertebrate and invertebrate infection models, and (iv) the determination of virulence factors that are key players in the infection process, for example, high-affinity iron permease (FTR1), spore coat protein (CotH), alkaline Rhizopus protease enzyme (ARP), ADP-ribosylation factor (ARF), dihydrolipoyl dehydrogenase, calcineurin (CaN), serine and aspartate proteases (SAPs). The present mini-review attempts to increase the awareness of these difficult-to-manage fungal infections and to encourage research in the detection of ligands and receptors as potential diagnostic parameters and drug targets.

Impact of Antifungal Compounds on Viability and Anti-Aspergillus Activity of Human Natural Killer Cells

Despite the availability of new antifungal compounds, invasive aspergillosis carries high morbidity and mortality in hematopoietic stem cell transplant recipients. In vitro studies and animal models suggest that the adoptive transfer of natural killer (NK) cells might be a promising immunotherapeutic option in this setting. As it is unclear whether the viability and function of human NK cells are affected by common antifungal agents, we analyzed the interaction of various concentrations of amphotericin B deoxycholate (AmB-D), liposomal amphotericin B, caspofungin, fluconazole, voriconazole, and posaconazole with human NK cells. When adding NK cells to therapeutic concentrations of antifungal agents, a significant increase in the antifungal effect was seen for caspofungin and voriconazole, whereas NK cells significantly decreased the hyphal damage of escalated doses of AmB-D. In contrast, therapeutic concentrations of all antifungal compounds tested did not have a negative effect on proliferation, viability, and the release of soluble immunomodulatory molecules of NK cells. These data indicate that therapeutic concentrations of the antifungal agents tested do not negatively affect the functional properties of human NK cells, which is a prerequisite for further studies evaluating NK cells as antifungal immunotherapy in immunocompromised patients suffering from invasive aspergillosis.

Immunomodulation as a Therapy for Aspergillus Infection: Current Status and Future Perspectives

Invasive aspergillosis (IA) is the most serious life-threatening infectious complication of intensive remission induction chemotherapy and allogeneic stem cell transplantation in patients with a variety of hematological malignancies. Aspergillus fumigatus is the most commonly isolated species from cases of IA. Despite the various improvements that have been made with preventative strategies and the development of antifungal drugs, there is an urgent need for new therapeutic approaches that focus on strategies to boost the host’s immune response, since immunological recovery is recognized as being the major determinant of the outcome of IA. Here, we aim to summarize current knowledge about a broad variety of immunotherapeutic approaches against IA, including therapies based on the transfer of distinct immune cell populations, and the administration of cytokines and antibodies.

First Insights in NK-DC Cross-Talk and the Importance of Soluble Factors During Infection With Aspergillus fumigatus

Invasive aspergillosis (IA) is an infectious disease caused by the fungal pathogen Aspergillus fumigatus that mainly affects immunocompromised hosts. To investigate immune cell cross-talk during infection with A. fumigatus, we co-cultured natural killer (NK) cells and dendritic cells (DC) after stimulation with whole fungal structures, components of the fungal cell wall, fungal lysate or ligands for distinct fungal receptors. Both cell types showed activation after stimulation with fungal components and were able to transfer activation signals to the counterpart not stimulated cell type. Interestingly, DCs recognized a broader spectrum of fungal components and thereby initiated NK cell activation when those did not recognize fungal structures. These experiments highlighted the supportive function of DCs in NK cell activation. Furthermore, we focused on soluble DC mediated NK cell activation and showed that DCs stimulated with the TLR2/Dectin-1 ligand zymosan could maximally stimulate the expression of CD69 on NK cells. Thus, we investigated the influence of both receptors for zymosan, Dectin-1 and TLR2, which are highly expressed on DCs but show only minimal expression on NK cells. Specific focus was laid on the question whether Dectin-1 or TLR2 signaling in DCs is important for the secretion of soluble factors leading to NK cell activation. Our results show that Dectin-1 and TLR2 are negligible for NK cell activation. We conclude that besides Dectin-1 and TLR2 other receptors on DCs are able to compensate for the missing signal.

Immunomodulation as Therapy for Fungal Infection: Are We Closer?

Invasive fungal disease (IFD) causes significant morbidity in immunocompromised patients due to their weakened immune system. Immunomodulatory therapy, in synergy with existing antifungal therapy, is an attractive option to enhance their immune system and aid clearance of these opportunistic pathogens. From a scientific and clinical perspective, we explore the immunotherapeutic options to augment standard antifungal drugs for patients with an IFD. We discuss the range of immunomodulatory therapies being considered in IFD - from cytokines, including G-CSF, GM-CSF, M-CSF, IFN-γ, and cytokine agonists, to cellular therapies, consisting of granulocyte transfusion, adoptive T-cell, CAR T-cell, natural killer cell therapies, and monoclonal antibodies. Adjunct pharmaceutical agents which augment the immunity are also being considered. Lastly, we explore the likelihood of the use of probiotics and manipulation of the microbiome/mycobiome to enhance IFD treatment outcomes.

Natural killer cells as a therapeutic tool for infectious diseases - current status and future perspectives

Natural Killer (NK) cells are involved in the host immune response against infections due to viral, bacterial and fungal pathogens, all of which are a significant cause of morbidity and mortality in immunocompromised patients. Since the recovery of the immune system has a major impact on the outcome of an infectious complication, there is major interest in strengthening the host response in immunocompromised patients, either by using cytokines or growth factors or by adoptive cellular therapies transfusing immune cells such as granulocytes or pathogen-specific T-cells. To date, relatively little is known about the potential of adoptively transferring NK cells in immunocompromised patients with infectious complications, although the anti-cancer property of NK cells is already being investigated in the clinical setting. This review will focus on the antimicrobial properties of NK cells and the current standing and future perspectives of generating and using NK cells as immunotherapy in patients with infectious complications, an approach which is promising and might have an important clinical impact in the future.

Recent Advances in the Pathogenesis of Mucormycoses

PURPOSE

The purposes of this review are to describe the pathogenesis of mucormycosis and to address recent research advances in understanding the mechanisms of fungal invasion and dissemination.

METHODS

Studies and reviews published in the PubMed and ClinicalTrials.gov databases until December 2017 that explored or reported recent advances in the understanding of the pathogenesis of mucormycosis were reviewed.

FINDINGS

To cause disease, fungal spores need to evade the innate immune system and germinate, leading to angioinvasion and tissue destruction. Recent studies have found that Mucorales are able to downregulate several host defense mechanisms and have identified the specific receptors through which Mucorales attach to the endothelium, facilitating their endocytosis and subsequent angioinvasion. In addition, certain conditions found to act through various mechanisms and pathways in experimental and animal studies, such as hyperglycemia, elevated iron concentrations, and acidosis (particularly diabetic ketoacidosis), increase the virulence of the fungi and enhance their attachment to the endothelium, rendering patients with uncontrolled diabetes and patients with iron overload susceptible to mucormycosis. The role and various antifungal functions of platelets and natural killer cells are highlighted, and the potential contribution of alternative therapies, such as manipulating the innate immune host defenses with granulocyte transfusions or administration of growth factors and using the antifungal effects of calcineurin inhibitors, are presented. Finally, directions and possible implications for future research are provided.

IMPLICATIONS

This article provides a comprehensive overview of research advances in the pathogenesis of infections caused by Mucorales and helps future studies develop effective treatment strategies and improve patient outcomes.

Aspergillus fumigatus responds to natural killer (NK) cells with upregulation of stress related genes and inhibits the immunoregulatory function of NK cells

Natural Killer (NK) cells are active against Aspergillus fumigatus, which in turn is able to impair the host defense. Unfortunately, little is known on the mutual interaction of NK cells and A. fumigatus. We coincubated human NK cells with A. fumigatus hyphae and assessed the gene expression and protein concentration of selected molecules. We found that A. fumigatus up-regulates the gene expression of pro-inflammatory molecules in NK cells, but inhibited the release of these molecules resulting in intracellular accumulation and limited extracellular availability. A. fumigatus down-regulatedmRNA levels of perforin in NK cells, but increased its intra- and extracellular protein concentration. The gene expression of stress related molecules of A. fumigatus such as heat shock protein hsp90 was up-regulated by human NK cells. Our data characterize for the first time the immunosuppressive effect of A. fumigatus on NK cells and may help to develop new therapeutic antifungal strategies.

The mucormycete-host interface

Mucormycosis is a fungal infection with fulminant angioinvasion leading to high morbidity and mortality in susceptible individuals. The major predisposing conditions are uncontrolled diabetes, neutropenia, malignancies, receipt of a transplant and traumatic injury [1]. Over the past decade, mucormycosis has become an emerging fungal infection due to the increase in patient groups presenting with these pre-disposing conditions and our medical advances in diagnosing the infection [2-4]. Yet, we currently lack clinical interventions to treat mucormycosis effectively. This in turn is due to a lack of understanding of mucormycosis pathogenesis. Here, we discuss our current understanding of selected aspects of interactions at the mucormycete-host interface. We will highlight open questions that might guide future research directions for investigations into the pathogenesis of mucormycosis and potential innovative therapeutic approaches.

Natural Killer Cells in Antifungal Immunity

Invasive fungal infections are still an important cause of morbidity and mortality in immunocompromised patients such as patients suffering from hematological malignancies or patients undergoing hematopoietic stem cell transplantion. In addition, other populations such as human immunodeficiency virus-patients are at higher risk for invasive fungal infection. Despite the availability of new antifungal compounds and better supportive care measures, the fatality rate of invasive fungal infection remained unacceptably high. It is therefore of major interest to improve our understanding of the host-pathogen interaction to develop new therapeutic approaches such as adoptive immunotherapy. As experimental methodologies have improved and we now better understand the complex network of the immune system, the insight in the interaction of the host with the fungus has significantly increased. It has become clear that host resistance to fungal infections is not only associated with strong innate immunity but that adaptive immunity (e.g., T cells) also plays an important role. The antifungal activity of natural killer (NK) cells has been underestimated for a long time. In vitro studies demonstrated that NK cells from murine and human origin are able to attack fungi of different genera and species. NK cells exhibit not only a direct antifungal activity via cytotoxic molecules but also an indirect antifungal activity via cytokines. However, it has been show that fungi exert immunosuppressive effects on NK cells. Whereas clinical data are scarce, animal models have clearly demonstrated that NK cells play an important role in the host response against invasive fungal infections. In this review, we summarize clinical data as well as results from in vitro and animal studies on the impact of NK cells on fungal pathogens.

The influence of the mating type on virulence of Mucor irregularis

Mucor irregularis is an emerging fungal pathogen that cause cutaneous infection and could cause death. However, little is known about its mechanism of pathogenesis. There is evidence suggesting virulence vary with mating types in fungi, including the Mucorales. Here, we characterized the mating type locus of M. irregularis and the mating type ratio of 17 clinical isolates in China. Genomic data indicated M. irregularis is heterothallic having two mating types – bearing either SexP or SexM allele. Also, we employed a mice model to study the inflammation and pathological effects of different mating types. The comparison of the inflammatory response, cytokine profiles and Th-1, Th-2 and Th-17 cells numbers in each mating type treated mice showed that the severity and disease progress were enhanced in (+) mating type treated mice. One (+/0) mutant strain, with multiple mutations at the mating locus, had defects in sexual mating ability but appeared to be more virulent than the (−) mating type. Although (+) mating type appeared to be more virulent, most of our clinical isolates presented belonged to (−) mating type. Our findings support the involvement of MAT genes in sexual fertility, and the influence of mating type on the severity of cutaneous infection.

Innate and Adaptive Immunity to Mucorales

Mucormycosis is an invasive fungal infection characterised by rapid filamentous growth, which leads to angioinvasion, thrombosis, and tissue necrosis. The high mortality rates (50-100%) associated with mucormycosis are reflective of not only the aggressive nature of the infection and the poor therapeutics currently employed, but also the failure of the human immune system to successfully clear the infection. Immune effector interaction with Mucorales is influenced by the developmental stage of the mucormycete spore. In a healthy immune environment, resting spores are resistant to phagocytic killing. Contrarily, swollen spores and hyphae are susceptible to damage and degradation by macrophages and neutrophils. Under the effects of immune suppression, the recruitment and efficacy of macrophage and neutrophil activity against mucormycetes is considerably reduced. Following penetration of the endothelial lining, Mucorales encounter platelets. Platelets adhere to both mucormycete spores and hyphae, and exhibit germination suppression and hyphal damage capacity in vitro. Dendritic cells are activated in response to Mucorales hyphae only, and induce adaptive immunity. It is crucial to further knowledge regarding our immune system's failure to eradicate resting spores under intact immunity and inhibit fungal growth under immunocompromised conditions, in order to understand mucormycosis pathogenicity and enhance therapeutic strategies for mucormycosis.

The in vitro effects of interferon-gamma, alone or in combination with amphotericin B, tested against the pathogenic fungi Candida albicans and Aspergillus fumigatus

Objective

Recent studies into the antifungal activity of NK-cells against the Aspergillus fumigatus have presented differing accounts on their mode of antifungal activity. One of these mechanisms proposed that NK-cells may kill the fungus via the direct effects of exposure to Interferon gamma (IFN-γ).

Results

In this study we investigated the direct antifungal effects of recombinant human IFN-γ against a range of pathogenic fungi by measuring cellular damage using an XTT-based assay and cell viability through plate counts. It was found that 32 pg/ml of IFN-γ exhibited a significant but small antifungal effect on A. fumigatus (p = 0.02), Aspergillus flavus (p = 0.04) and Saccharomyces cerevisiae (p = 0.03), inhibiting growth by 6, 11 and 17% respectively. No significant inhibitory effects were observed in Candida species (p > 0.05 for all species tested) or Cryptococus neoformans (p = 0.98). Short term exposure (3 h) to a combination of amphotericin B (1 µg/ml) and IFN-γ (32 pg/ml) increased the effectiveness of amphotericin B against A. fumigatus and S. cerevisiae but not Candida albicans. These data suggest that IFN-γ does not possess strong antifungal activity but can enhance the effect of amphotericin B under some testing conditions against Aspergillus species.

Immunotherapeutic approaches to treatment of fungal diseases

Fungal infections cause morbidity worldwide and are associated with an unacceptably high mortality despite the availability of antifungal drugs. The incidence of mycoses is rising because of the HIV pandemic and because immunomodulatory drugs are increasingly used to treat autoimmune diseases and cancer. New classes of antifungal drugs have only been partly successful in improving the prognosis for patients with fungal infection. Adjunctive host-directed therapy is therefore believed to be the only option to further improve patient outcomes. Recent advances in the understanding of complex interactions between fungi and host have led to the design and exploration of novel therapeutic strategies in cytokine therapy, vaccines, and cellular immunotherapy, each of which might become viable adjuncts to existing antifungal regimens. In this report, we discuss immunotherapeutic approaches-the rationale behind their design, the challenges in their use, and the progress that is so urgently needed to overcome the devastating effect of fungal diseases.

CD56 Is a Pathogen Recognition Receptor on Human Natural Killer Cells

Aspergillus (A.) fumigatus is an opportunistic fungal mold inducing invasive aspergillosis (IA) in immunocompromised patients. Although antifungal activity of human natural killer (NK) cells was shown in previous studies, the underlying cellular mechanisms and pathogen recognition receptors (PRRs) are still unknown. Using flow cytometry we were able to show that the fluorescence positivity of the surface receptor CD56 significantly decreased upon fungal contact. To visualize the interaction site of NK cells and A. fumigatus we used SEM, CLSM and dSTORM techniques, which clearly demonstrated that NK cells directly interact with A. fumigatus via CD56 and that CD56 is re-organized and accumulated at this interaction site time-dependently. The inhibition of the cytoskeleton showed that the receptor re-organization was an active process dependent on actin re-arrangements. Furthermore, we could show that CD56 plays a role in the fungus mediated NK cell activation, since blocking of CD56 surface receptor reduced fungal mediated NK cell activation and reduced cytokine secretion. These results confirmed the direct interaction of NK cells and A. fumigatus, leading to the conclusion that CD56 is a pathogen recognition receptor. These findings give new insights into the functional role of CD56 in the pathogen recognition during the innate immune response.

NK Cells and Their Role in Invasive Mold Infection

There is growing evidence that Natural Killer (NK) cells exhibit in vitro activity against both Aspergillus and non-Aspergillus molds. Cytotoxic molecules such as NK cell-derived perforin seem to play an important role in the antifungal activity. In addition, NK cells release a number of cytokines upon stimulation by fungi, which modulate both innate and adaptive host immune responses. Whereas the in vitro data of the antifungal activity of NK cells are supported by animal studies, clinical data are scarce to date.

Mechanisms by Which Interleukin-12 Corrects Defective NK Cell Anticryptococcal Activity in HIV-Infected Patients

Cryptococcus neoformans is a pathogenic yeast and a leading cause of life-threatening meningitis in AIDS patients. Natural killer (NK) cells are important immune effector cells that directly recognize and kill C. neoformans via a perforin-dependent cytotoxic mechanism. We previously showed that NK cells from HIV-infected patients have aberrant anticryptococcal killing and that interleukin-12 (IL-12) restores the activity at least partially through restoration of NKp30. However, the mechanisms causing this defect or how IL-12 restores the function was unknown. By examining the sequential steps in NK cell killing of Cryptococcus, we found that NK cells from HIV-infected patients had defective binding of NK cells to C. neoformans. Moreover, those NK cells that bound to C. neoformans failed to polarize perforin-containing granules to the microbial synapse compared to healthy controls, suggesting that binding was insufficient to restore a defect in perforin polarization. We also identified lower expression of intracellular perforin and defective perforin release from NK cells of HIV-infected patients in response to C. neoformans. Importantly, treatment of NK cells from HIV-infected patients with IL-12 reversed the multiple defects in binding, granule polarization, perforin content, and perforin release and restored anticryptococcal activity. Thus, there are multiple defects in the cytolytic machinery of NK cells from HIV-infected patients, which cumulatively result in defective NK cell anticryptococcal activity, and each of these defects can be reversed with IL-12.

The mechanisms by which NK cells bind directly to pathogens and deploy their deadly cytolytic machinery during microbial host defense are only beginning to be elucidated. With the goal of understanding this process, we used NK cells from HIV-infected patients, which were known to have a defect in killing of Cryptococcus neoformans. Taking advantage of previous studies that had shown that IL-12 restored killing, we used the cytokine as a gain-of-function approach to define the relevance of multiple steps in the recognition and cytolytic pathway. We demonstrated that NK cells from HIV-infected patients failed to kill Cryptococcus due to defects in perforin expression, granule polarization, and release of perforin. Additionally, IL-12 restored recognition of C. neoformans through binding of the NK-activating receptor NKp30. These observations identify important mechanisms used by NK cells to kill microbes and determine that defects in NK cells from HIV-infected patients are reversible.

Innate immune cell response upon Candida albicans infection

Candida albicans is a polymorphic fungus which is the predominant cause of superficial and deep tissue fungal infections. This microorganism has developed efficient strategies to invade the host and evade host defense systems. However, the host immune system will be prepared for defense against the microbe by recognition of receptors, activation of signal transduction pathways and cooperation of immune cells. As a consequence, C. albicans could either be eliminated by immune cells rapidly or disseminate hematogenously, leading to life-threatening systemic infections. The interplay between Candida albicans and the host is complex, requiring recognition of the invaded pathogens, activation of intricate pathways and collaboration of various immune cells. In this review, we will focus on the effects of innate immunity that emphasize the first line protection of host defense against invaded C. albicans including the basis of receptor-mediated recognition and the mechanisms of cell-mediated immunity.