Immunopathology of Aspergillus Infections in Children with Chronic Granulomatous Disease and Cystic Fibrosis

Introduction

Children with chronic granulomatous disease (CGD) and cystic fibrosis (CF) share some important clinical characteristics which are the consequence of their intrinsic susceptibility to recurrent and opportunistic infections and exaggerated inflammation. Although the clinical phenotypes of Aspergillus disease differ substantially between those two non-neutropenic patient groups, the underlying pathophysiology show remarkable commonalities with excessive inflammation being a hallmark of Aspergillus disease. The occurrence of Aspergillus infections in these two patient populations is associated with decreased quality of life and premature death.

Our knowledge of the pathophysiology of Aspergillus infections is mainly derived from infections caused by a single species - Aspergillus fumigatus - in the neutropenic host. The insights obtained in this setting do not translate well to the non-neutropenic host. Recently, more insight has been obtained in the underlying molecular mechanisms driving the excessive inflammation during Aspergillus infection and disease in those 2 patient groups and is summarized here.

Chronic granulomatous disease

Chronic granulomatous disease (CGD) is a rare primary immunodeficiency (prevalence 1:200,000) characterized by a defective NADPH-oxidase in phagocytic cells. Patients with CGD have the highest lifetime risk of invasive aspergillosis (IA; incidences 26%-45%) and, despite the availability of antifungal prophylaxis and targeted antifungal therapy, IA remains the most common infectious complication and the most frequent cause of death in CGD [1]. Aspergillus-specific mortality in CGD appears to be decreasing over time, from around 30% to below 20% in the most recent case series. Curative options to treat CGD, e.g. hematopoietic stem cell transplantation, are available, though not each patient is benefiting from this treatment. Impaired production of reactive oxygen species (ROS) underlies the clinical phenotype of CGD. Lack of the direct microbicidal effects of ROS renders children with CGD susceptible to IA, while the lack of a functional NADPH oxidase leads to dysregulated inflammatory responses explaining the pathophysiology of IA as observed in CGD. Invasive aspergillosis in CGD is characterized by a sub-acute infection, non-angio-invasive, excessive granuloma formation in the affected tissue.

There is a clear association between the NADPH-oxidase mutation, the extent to which superoxide production is impaired, and the occurrence and severity of IA in CGD. X-linked CGD patients have both a higher incidence and worse outcome of IA compared to those with autosomal recessive CGD. Invasive aspergillosis in CGD is caused predominantly by two different Aspergillus species, A. fumigatus and A. nidulans. Interestingly, A. nidulans infection is found almost exclusively in patients with X-linked CGD, and seldom other (non-CGD) patient groups, indicating a unique unsolved interaction in the clinical setting [1]. Both in vitro and murine studies have shown that infection with A. nidulans and A. fumigatus induce a hyperinflammatory response driven by an exaggerated interleukin-1 (IL-1) production [2,3].

Cystic fibrosis

Cystic fibrosis (CF) is caused by a mutation in the gene that encodes the cystic fibrosis transmembrane conductance regulator (CFTR) protein, an ion channel, expressed in epithelial cells as well as immune cells. This mutation accounts for the most common fatal genetically inherited disease in humans (prevalence 1:13,500). Median age of death is still a mere 29 years and progressive lung damage caused by inflammation and infection is the major cause of this fatal outcome. Up to 60% of patients with CF will be infected with Aspergillus and it has been suggested that persistent infection is associated with progressive lung function decline [4]. A small proportion of those patients will develop allergic responses to Aspergillus (e.g. allergic bronchopulmonary aspergillosis [ABPA] or fungal sensitization), while others will develop airway-invasive aspergillosis (e.g. Aspergillus bronchitis). Studies reporting on the relative number of A. fumigatus versus non-fumigatus Aspergillus species have shown that between 36% and 58% of colonization is caused by A. fumigatus [5]. Although infection with Aspergillus has long been considered as innocent colonization due to the entrapment of Aspergillus conidia by the sticky mucus in the CF lung, recent data suggest otherwise. Adhesion, phagocytosis and killing of Aspergillus conidia has been shown to be impaired in CF bronchial epithelial cells [6]. In the same paper, the authors demonstrated, by using an experimental murine model, impaired clearance of Aspergillus conidia and epithelial necrosis and fibrin deposition in CFTR-deficient but not wild-type mouse airways. CFTR protein has been found in cells of both the innate and adaptive immune system and has shown to play a critical role for their normal function. The observation that CF patients have a higher risk of developing invasive aspergillosis after lung transplantation than non-CF transplant patients additionally suggests impaired antifungal effector mechanisms of CF immune cells [4]. We have recently shown that although human CF neutrophils are capable of efficiently phagocytose and kill A. fumigatus, this is at a cost of excessive ROS [7]. This excessive ROS response was significantly correlated to disease severity in terms of clinical exacerbations and lung function and suggests that the hyperinflammatory response by CF neutrophils upon exposure to A. fumigatus is likely to contribute to progressive lung disease [7].

Immunopathology

Aspergillus infection triggers an aberrant host immune response characterized by hyperinflammation in patients with CGD and CF, resulting in insufficient fungal control and tissue damage. Over the last few years a number of studies have shown commonalities in the molecular drivers of the observed immunopathology in both patients groups.

Autophagy is the process of effective microbial killing and clearance following phagocytosis and phagolysosome maturation, and controls inflammasome activation. Defective autophagy has been reported in both CGD and CF immune cells [8,9]. The link between autophagy and ROS remains poorly understood, although it is known that reactive oxygen species are essential for effective autophagy. On the other hand, excessive ROS production inhibits the process of autophagy. Therefore, the observations that the process of autophagy is defective in both the CF (excessive reactive oxygen species) as well as the CGD (deficient in reactive oxygen species) host suggest a common element to the immunopathology.

Rapamycin, a macrolide compound, was initially developed as an antifungal agent until its potent immunosuppressive properties were discovered. Rapamycin is a mTOR (mammalian target of rapamycin) inhibitor and is mainly been used in clinical care to prevent rejections of organ transplants and uncontrolled lymphoproliferation. A first study showing its potential use in patients with CGD demonstrated that rapamycin induced autophagy induction thereby inhibiting inflammasome activation in human CGD phagocytes [10]. Inhibition of pro-inflammatory cytokines including TNF-α and IL-17, independently of caspase-1 inhibition, was observed as well. Anakinra, an IL-1-receptor antagonist, enhanced the inhibitory effect of rapamycin on IL-1β secretion by phagocytes in the in vitro model used, suggesting that this combination could have extended benefits for dampening the immunopathology in CGD. Comparable studies are not performed yet using human CF phagocytes.

Another component in the vicious perpetual cycle of fungal induced inflammation is the activation of the inflammasome. Increased activation of the inflammasome has been shown under conditions in which either too much or too little reactive oxygen species is induced. This pathway has been shown to be upregulated in both CGD and CF immune cells upon fungal stimulation and leads to increased release of IL-1 [3,11]. IL-1 inhibitors like IL-1-receptor antagonists, have shown to reduce inflammation in human immune cells and experimental murine models of CGD and CF [3,11]. We have shown previously that chloroquine, an antimalarial drug exhibiting anti-inflammatory properties, inhibits IL-1 release by Aspergillus infected human CGD phagocytes [12]. The mechanisms of the anti-inflammatory activity of chloroquine though, is poorly understood. Our recent observations show that prophylactic hydroxychloroquine can attenuate mortality, weight loss and pulmonary inflammation in gp91-/- mice infected with A. nidulans. This beneficial effect is not observed in A. fumigatus infections. Chloroquine and hydroxychloroquine have similar pharmacological properties, but hydroxychloroquine has a more favorable side effect profile, increasing its appeal for long-term clinical use.

Studies in murine models of pulmonary aspergillosis showed that a defective NADPH-oxidase complex and CFTR dysfunction leads to defective indoleamine 2,3-dioxygenase (IDO) activity [13,14]. Indoleamine 2,3-dioxygenase (IDO) is a rate-limiting enzyme for tryptophan degradation in the kynurenine pathway. Impaired tryptophan catabolism resulted in exaggerated inflammation and abnormalities in T cell polarization with exaggerated Th17 responses. Therapeutic modulation of this pathway to enhance IDO activity enabled resolution of excessive inflammation in those experimental models. However, subsequent studies have found preserved tryptophan/kynurenine metabolism and Th17 differentiation in CGD patients [15], suggesting this effect is likely limited to the p47-/- murine model. Studies assessing its relevance in patients with CF are lacking.

Immunotherapy

Corticosteroids are used to dampen the allergic inflammation in ABPA in CF patients and the inflammatory complications post-infection in CGD patients [1,2]. Due to the well-known severe side-effects and its unspecific and broad mode of action, new and more targeted immunotherapies are needed. Interferon-γ has been carefully studied as an antifungal adjuvant immunotherapy. Interferon-γ enhances antifungal activity of macrophages and polymorphonuclear neutrophils, and its first successful clinical application was in CGD patients in whom substantial protection was seen against IA [1]. Currently, only sparse preclinical data is available to support the potential beneficial effect of novel additional immunomodulatory agents in the management of Aspergillus infection in children with CGD or CF.

The CFTR modulators are the first causative treatment options for CF patients and have achieved significant improvement in lung function and quality of life. The results of a recent clinical study looking at the effect of Ivacaftor on specific airway microbial colonization showed a 53% reduction in Aspergillus colonization [16]. This effect may be related to a reduction in inflammation, an improved immune function of the epithelial cells or a direct effect on immune cells present in the lung environment, or a combination of both.

Summary

Patients suffering from CGD or CF are two non-neutropenic patient populations characterized by an increased susceptibility to Aspergillus infections. To improve the outcome of Aspergillus infections in these particular patient groups, a better understanding of the host-fungus interaction is urgently required in order to develop targeted and individualized management strategies. Targeting either the primary immune defect, or the defective autophagy, or dampening the activity of the inflammasome has the potential to reduce the immunopathology caused by Aspergillus species and consequently to improve patient outcome. Yet, the translation of the exciting preclinical data into careful designed clinical trials is awaited.