New advances in invasive aspergillosis immunobiology leading the way towards personalized therapeutic approaches

Detailed characterisation of invasive aspergillosis in a murine model of X-linked chronic granulomatous disease shows new insights in infections caused by Aspergillus fumigatus versus Aspergillus nidulans

Introduction

Invasive aspergillosis (IA) is the most prevalent infectious complication in patients with chronic granulomatous disease (CGD). Yet, understanding of fungal pathogenesis in the CGD host remains limited, particularly with regards to A. nidulans infection.

Methods

We have used a murine model of X-linked CGD to investigate how the pathogenesis of IA varies between A. fumigatus and A. nidulans, comparing infection in both X-linked CGD (gp91-/-) mice and their parent C57BL/6 (WT) mice. A 14-colour flow cytometry panel was used to assess the cell dynamics over the course of infection, with parallel assessment of pulmonary cytokine production and lung histology.

Results

We observed a lack of association between pulmonary pathology and infection outcome in gp91-/- mice, with no significant mortality in A. nidulans infected mice. An overwhelming and persistent neutrophil recruitment and IL-1 release in gp91-/- mice following both A. fumigatus and A. nidulans infection was observed, with divergent macrophage, dendritic cell and eosinophil responses and distinct cytokine profiles between the two infections.

Conclusion

We have provided an in-depth characterisation of the immune response to pulmonary aspergillosis in an X-linked CGD murine model. This provides the first description of distinct pulmonary inflammatory environments in A. fumigatus and A. nidulans infection in X-linked CGD and identifies several new avenues for further research.

Human Airway Epithelium Responses to Invasive Fungal Infections: A Critical Partner in Innate Immunity

The lung epithelial lining serves as the primary barrier to inhaled environmental toxins, allergens, and invading pathogens. Pulmonary fungal infections are devastating and carry high mortality rates, particularly in those with compromised immune systems. While opportunistic fungi infect primarily immunocompromised individuals, endemic fungi cause disease in immune competent and compromised individuals. Unfortunately, in the case of inhaled fungal pathogens, the airway epithelial host response is vastly understudied. Furthering our lack of understanding, very few studies utilize primary human models displaying pseudostratified layers of various epithelial cell types at air-liquid interface. In this review, we focus on the diversity of the human airway epithelium and discuss the advantages and disadvantages of oncological cell lines, immortalized epithelial cells, and primary epithelial cell models. Additionally, the responses by human respiratory epithelial cells to invading fungal pathogens will be explored. Future investigations leveraging current human in vitro model systems will enable identification of the critical pathways that will inform the development of novel vaccines and therapeutics for pulmonary fungal infections.

NADPH Oxidase Limits Collaborative Pattern-Recognition Receptor Signaling to Regulate Neutrophil Cytokine Production in Response to Fungal Pathogen-Associated Molecular Patterns

Chronic granulomatous disease (CGD) is a primary immunodeficiency caused by genetic defects in leukocyte NADPH oxidase, which has both microbicidal and immunomodulatory roles. Hence, CGD is characterized by recurrent bacterial and fungal infections as well as aberrant inflammation. Fungal cell walls induce neutrophilic inflammation in CGD; yet, underlying mechanisms are incompletely understood. This study investigated the receptors and signaling pathways driving aberrant proinflammatory cytokine production in CGD neutrophils activated by fungal cell walls. Although cytokine responses to β-glucan particles were similar in NADPH oxidase-competent and NADPH oxidase-deficient mouse and human neutrophils, stimulation with zymosan, a more complex fungal particle, induced elevated cytokine production in NADPH oxidase-deficient neutrophils. The dectin-1 C-type lectin receptor, which recognizes β-glucans (1-3), and TLRs mediated cytokine responses by wild-type murine neutrophils. In the absence of NADPH oxidase, fungal pathogen-associated molecular patterns engaged additional collaborative signaling with Mac-1 and TLRs to markedly increase cytokine production. Mechanistically, this cytokine overproduction is mediated by enhanced proximal activation of tyrosine phosphatase SHP2-Syk and downstream Card9-dependent NF-κB and Card9-independent JNK-c-Jun. This activation and amplified cytokine production were significantly decreased by exogenous H₂O₂ treatment, enzymatic generation of exogenous H₂O₂, or Mac-1 blockade. Similar to zymosan, Aspergillus fumigatus conidia induced increased signaling in CGD mouse neutrophils for activation of proinflammatory cytokine production, which also used Mac-1 and was Card9 dependent. This study, to our knowledge, provides new insights into how NADPH oxidase deficiency deregulates neutrophil cytokine production in response to fungal cell walls.

Fungal Pathogens: Shape-Shifting Invaders

Fungal infections are on the rise due to new medical procedures that have increased the number of immune compromised patients, antibacterial antibiotics that disrupt the microbiome, and increased use of indwelling medical devices that provide sites for biofilm formation. Key to understanding the mechanisms of pathogenesis is to determine how fungal morphology impacts virulence strategies. For example, small budding cells use very different strategies to disseminate compared with long hyphal filaments. Furthermore, cell morphology must be monitored in the host, as many fungal pathogens change their shape to disseminate into new areas, acquire nutrients, and avoid attack by the immune system. This review describes the shape-shifting alterations in morphogenesis of human fungal pathogens and how they influence virulence strategies.

Ibrutinib-associated invasive fungal diseases in patients with chronic lymphocytic leukaemia and non-Hodgkin lymphoma: An observational study

BACKGROUND

Invasive fungal diseases (IFD) are life-threatening infections most commonly diagnosed in acute leukaemia patients with prolonged neutropenia and are uncommonly diagnosed in patients with lymphoproliferative diseases.

OBJECTIVES

Following the initial report of aspergillosis diagnosed shortly after beginning ibrutinib for chronic lymphocytic leukaemia, a survey was developed to seek additional cases of IFD during ibrutinib treatment.

METHODS

Local and international physicians and groups were approached for relevant cases. Patients were included if they met the following criteria: diagnosis of chronic lymphocytic leukaemia/non-Hodgkin lymphoma; proven or probable IFD; and ibrutinib treatment on the date IFD were diagnosed. Clinical and laboratory data were captured using REDCap software.

RESULT

Thirty-five patients with IFD were reported from 22 centres in eight countries: 26 (74%) had chronic lymphocytic leukaemia. The median duration of ibrutinib treatment before the onset of IFD was 45 days (range 1-540). Aspergillus species were identified in 22 (63%) of the patients and Cryptococcus species in 9 (26%). Pulmonary involvement occurred in 69% of patients, cranial in 60% and disseminated disease in 60%. A definite diagnosis was made in 21 patients (69%), and the mortality rate was 69%. Data from Israel regarding ibrutinib treated patients were used to evaluate a prevalence of 2.4% IFD.

CONCLUSIONS

The prevalence of IFD among chronic lymphocytic leukaemia/non-Hodgkin lymphoma patients treated with ibrutinib appears to be higher than expected. These patients often present with unusual clinical features. Mortality from IFD in this study was high, indicating that additional studies are urgently needed to identify patients at risk for ibrutinib-associated IFD.

Call for Action: Invasive Fungal Infections Associated With Ibrutinib and Other Small Molecule Kinase Inhibitors Targeting Immune Signaling Pathways

Opportunistic infections caused by Pneumocystis jirovecii, Cryptococcus neoformans, and ubiquitous airborne filamentous fungi have been recently reported in patients with hematological cancers historically considered at low risk for invasive fungal infections (IFIs), after receipt of the Bruton tyrosine kinase inhibitor ibrutinib. The spectrum and severity of IFIs often observed in these patients implies the presence of a complex immunodeficiency that may not be solely attributed to mere inhibition of Bruton tyrosine kinase. In view of the surge in development of small molecule kinase inhibitors for treatment of malignant and autoimmune diseases, it is possible that there would be an emergence of IFIs associated with the effects of these molecules on the immune system. Preclinical assessment of the immunosuppressive effects of kinase inhibitors and human studies aimed at improving patient risk stratification for development of IFIs could lead to prevention, earlier diagnosis, and better outcomes in affected patients.

Host-Derived Leukotriene B4 Is Critical for Resistance against Invasive Pulmonary Aspergillosis

Aspergillus fumigatus is a mold that causes severe pulmonary infections. Our knowledge of how immune competent hosts maintain control of fungal infections while constantly being exposed to fungi is rapidly emerging. It is known that timely neutrophil recruitment to and activation in the lungs is critical to the host defense against development of invasive pulmonary aspergillosis, but the inflammatory sequelae necessary remains to be fully defined. Here, we show that 5-Lipoxygenase (5-LO) and Leukotriene B₄ (LTB₄) are critical for leukocyte recruitment and resistance to pulmonary A. fumigatus challenge in a fungal-strain-dependent manner. 5-LO activity was needed in radiosensitive cells for an optimal anti-fungal response and in vivo LTB₄ production was at least partially dependent on myeloid-derived hypoxia inducible factor-1α. Overall, this study reveals a role for host-derived leukotriene synthesis in innate immunity to A. fumigatus.

Interleukin 1α Is Critical for Resistance against Highly Virulent Aspergillus fumigatus Isolates

Heterogeneity among Aspergillus fumigatus isolates results in unique virulence potential and inflammatory responses. How these isolates drive specific immune responses and how this affects fungally induced lung damage and disease outcome are unresolved. We demonstrate that the highly virulent CEA10 strain is able to rapidly germinate within the immunocompetent lung environment, inducing greater lung damage, vascular leakage, and interleukin 1α (IL-1α) release than the low-virulence Af293 strain, which germinates with a lower frequency in this environment. Importantly, the clearance of CEA10 was consequently dependent on IL-1α, in contrast to Af293. The release of IL-1α occurred by a caspase 1/11- and P2XR7-independent mechanism but was dependent on calpain activity. Our finding that early fungal conidium germination drives greater lung damage and IL-1α-dependent inflammation is supported by three independent experimental lines. First, pregermination of Af293 prior to in vivo challenge drives greater lung damage and an IL-1α-dependent neutrophil response. Second, the more virulent EVOL20 strain, derived from Af293, is able to germinate in the airways, leading to enhanced lung damage and IL-1α-dependent inflammation and fungal clearance. Third, primary environmental A. fumigatus isolates that rapidly germinate under airway conditions follow the same trend toward IL-1α dependency. Our data support the hypothesis that A. fumigatus phenotypic variation significantly contributes to disease outcomes.