Fungal respiratory infections in cystic fibrosis: a growing problem

Cystic fibrosis (CF) is the major genetic inherited disease in the European caucasian population and based on the number of patients, the third most common orphan disease. CF is due to mutations in the gene cystic fibrosis transmembrane conductance regulator (CFTR) which encodes a chloride channel involved in electrolytic exchanges through the plasma membrane of numerous types of epithelial cells. Therefore, several organs are involved in the pathogenic process, but prognosis essentially depends on the severity of the lesions of the lungs which are the main target organ of the disease. In the respiratory tract, mutations in CFTR result in a defective mucociliary clearance and a thickening of the bronchial mucus. These abnormal airway conditions facilitate the entrapment of the inhaled bacteria and fungal conidia, and provide a suitable environment for growth of microorganisms. This in turn leads to respiratory infections and inflammatory reactions which, more than the microorganisms themselves, contribute to the progressive deterioration of lung function. Bacteria, particularly Staphylococcus aureus, Pseudomonas aeruginosa, Burkholderiacepacia and Stenotrophomnas maltophilia, are considered as the major cause of such respiratory infections. Therefore, considerable attention has been paid over recent decades to their prevention and treatment, which has resulted in improvement of patient management and in a marked increase in the life expectancy of the patients. However, the respiratory tract of older patients may also be colonized by fungi. This is facilitated by the repeated use of antifungals and corticosteroids which may lead to true respiratory infections. The rate of these fungal infections increases as the life expectancy of patients improves. But in contrast to bacterial respiratory infections, relatively little progress has been made with regards to fungal respiratory infections. Aspergillusfumigatus is by far the most common causative agent with a reported prevalence rate varying from 5.9% to 58.3%, but a wide range of fungi may colonize the respiratory tract of CF patients. For example, other Aspergillus species like Aspergillus terreus, as well as Scedosporium spp. and Exophiala dermatitidis are increasingly described in the CF context, but their prevalence is certainly underestimated. Additionally, whether the colonization of the airways by fungi/micromycetes constitutes a clinically relevant problem is still a matter of debate. However, recent studies have clearly indicated an increase in their prevalence rate, and a relationship with increased numbers of hospital admissions. Moreover, due to their thermotolerance and to their propensity to disseminate in immunocompromised hosts, prior colonization of the airways by filamentous fungi constitutes a serious risk factor for invasive infections in cases of lung transplantation. The latter remains the ultimate treatment of CF patients. Numerous questions are created by fungal colonization of the airways, and basic research on the ecology of the fungi, their biochemistry, and their pathogenic mechanisms should be promoted in order to define prophylactic measures or to develop more efficient antifungal drugs. To address these questions, the International Society for Human and Animal Mycology (ISHAM) launched in October 2006, a working group on “Fungal Respiratory Infections in Cystic Fibrosis”, whose first formal meeting was held in Angers, France, on June 7–8, 2009. Altogether 65 clinicians, mycologists or scientists from 14 different countries participated to this meeting, as well as numerous PhD students to guarantee dynamic discussions. Thirty-eight presentations were provided by attendees, covering a wide variety of topics, including (a) “clinical surveillance and description of unusual fungal species” (19 presentations), (b) “physiopathology and treatment” (6 presentations), (c) “epidemiological and environmental studies” (5 presentations), and (d) “biological diagnosis” (8 presentations). Thirteen of these lectures summarizing the state of the art of fungal risk in CF patients, as well as the creative discussions that occurred during the meeting, were selected for publication in the framework of this special on-line issue. Three additional papers related to these topics were also included in this special issue. Several presentations dealt with clinical surveillance, most of which involved single-center studies which provided information on the isolation rates of the major fungal species recovered from respiratory secretions of CF patients in France, Spain, Italy, Denmark, the United Kingdom and Brazil. Prevalence rates varied considerably from one center to another because of the differences in the methods used in the recovery and identification of the fungi. Study durations ranged between 6 months and up to 12 years, but most investigations were performed over a period of two years, with some still ongoing. Fifty-four to 302 adult and paediatric patients were included in the investigations which involved the examination of 208 to 8,552 respiratory samples, mostly sputa. Nevertheless, A. fumigatus was always the most common filamentous fungus recovered with isolation rates ranging from 11.1% in Spain to 81% in Denmark. Isolation of other Aspergillus species, including Aspergillus terreus and Aspergillusflavus, was also reported. Similarly, in most studies, Scedosporium apiospermum ranked the second most common filamentous fungus associated with CF, whereas E. dermatitidis seemed to be highly prevalent in some countries, e.g., Germany, the Netherlands and Belgium. In addition, new fungal species in the context of CF were discussed, including some species within the section Fumigati. For example, A. lentulus, was recovered transiently from one CF patient, and N. pseudofischeri, which was responsible for chronic colonization of the airways in three patients in France and Germany. Additional lectures focused on the description of other unusual fungal species associated with CF, Scedosporium aurantiacum in Australia and Pneumocystis jirovecii in Spain. Several presentations on physiopathology were given during the meeting, providing new insights into this poorly investigated area. For example, the first report of an independent association between long-term azithromycin therapy and Aspergillus colonization was presented suggesting that the antibiotic had an inhibitory affect on both the recruitment and the activation of neutrophils. The latter represent the first line defence against A. fumigatus. A retrospective single-center study also suggested a close association between colonization by Scedosporium apiospermum and development of allergic bronchopulmonary aspergillosis (ABPA) like signs or symptoms. Finally, IgE blockade using omalizumab was suggested as adjuvant therapy to control ABPA in corticosteroid-dependent CF patients, but to get more concrete data, a randomized, placebo controlled, double-blinded therapeutic study has been initiated in several American or European CF centers. Greater attention should be given to the interactions within the microbial communities colonizing the airways of CF patients and biofilm formation. Aspergillus fumigatus clinical isolates grown under biofilm producing conditions were found to be less susceptible to azole antifungals. For improvement of patient management and design of prophylactic measures, it is important to enhance our knowledge of the epidemiology and ecology of the fungi colonizing the airway of CF patients. In this regard, new genotyping methods were presented, such as the analysis of variable number of short tandem repeats and multiloci-sequence typing. These procedures have indicated that some A. fumigatus genotypes can chronically colonize airways. Conversely, other genotypes were only found transiently, underlying the importance of genotype selection. Two other techniques, solid-phase cytometry and sandwich ELISA may also be useful to detect azole resistant A. fumigatus in the environment and to clarify the ecology of the Pseudallescheriaboydii/Scedosporium apiospermum complex. The occurrence of Scedosporium species in the environment of France, Australia, Germany, Thailand, Israel and Italy was estimated in some studies through the use of conventional methods. Besides the predominant species P. boydii and S. apiospermum, Scedosporium aurantiacum, Scedosporium dehoogii, Pseudallescheriaminutispora and Scedosporium prolificans were also found in some environmental samples, with a high prevalence of S. aurantiacum in urban areas in Australia. Significant improvements in the biological diagnosis of the fungi associated with CF were presented in several lectures. Two prospective studies dealing with serological tests demonstrated the utility of serum IgE specific to recombinant A. fumigatus antigens in detecting sensitization of patients to this fungus. Such tests were also useful in the diagnosis of the ABPA, especially during its early phase when there are limited clinical symptoms. More precisely, the presence of serum IgE specific to rASp f1 was shown to be a good predictor of the sensitization of the patients to A. fumigatus. In contrast, serum IgE specific to rAsp f4 was found to be less sensitive, but more specific than the presence of serum specific IgG for diagnosis of ABPA, and more sensitive and specific than the total serum IgE titer. Presentations during the meeting confirmed important differences in the culture methods used by CF centers (including volume of the samples, nature and number of the culture media, duration and temperature of incubation) which, at least in part, may account for the variations noted in the prevalence rate of different fungal species. Further studies are needed to optimize routine procedures and to propose guidelines for the mycological examination of respiratory secretions from CF patients. Non-culture methods, such as the detection of some fungal metabolites like siderophores, may also constitute valuable alternatives in the diagnosis of fungal colonization of the airways. Despite significant improvements in the biological diagnosis of fungal colonization of the airways and/or fungal respiratory infections in CF patients, as well as new insights into the pathogenesis of airway colonization, the diversity of responsible fungi and the epidemiology and ecology of these moulds, a great deal still needs to be done. Some of these investigations will be performed through collaborations initiated during the meeting. Furthermore, another significant result obtained through the meeting was the plans for more cooperation between national patient organizations and cystic fibrosis societies, as well as between ISHAM and the European Cystic Fibrosis Society. Hopefully, these efforts will result in a better understanding of the clinical relevance of airway colonization and improved patient management eventually contributing to an improved quality of life of CF patients. Conflict of interest: None to declare.