Unlike for other solid-organ transplant (SOT), decreased mucociliary clearance, impaired cough reflex, and continuous exposure of the lungs to the environment are among the factors that contribute to postoperative bacterial or fungal infections in recipients of lung transplants. This study shows that the incidence of IA among LTRs remains high and that IA is associated with a poor outcome, indicating that IA infection should be given more attention. Chinese lung transplant recipients have a number of different characteristics from those from other countries, which may influence the occurrence, development, prognosis and treatment response for IA infection: 1) older age; 2) more serious lung diseases when accepting lung transplant surgery; 3) more single lung transplant recipients; 4) higher percentage of pulmonary fibrosis as the underlying lung diseases; and 5) poorer quality of donor lungs. In our center, the median age of LTRs was 62 years, and 25.5% (40/157) were older than 65 years. Although age over 65 years is a relative contraindication for lung transplant surgery in other countries [11], advanced age is indeed a prominent characteristic of LTRs in China. Increasing age is often associated with more comorbid conditions that may influence the clinical outcome. On the other hand, a recent multicenter study showed that the main risk factors for postoperative IA infection included single lung transplantation (hazard ratio [HR]: 1.8, p=0.02) and posttransplant colonization with Aspergillus spp. within 1 year of transplantation (HR: 2.11, p=0.03) [12]. In our study, 74.4% of the patients received a single lung transplant, a much higher proportion than in other countries [4]. The incidence of invasive aspergillosis infection after LT was 27.4% in our study, which is not low. According to series reports, the overall incidence of IA infection in lung transplant patients ranges from 3%~23% [5, 13-15].
In our study, the median time of IA onset was 21.0 days posttransplantation, and the vast majority of infections (81.4%) occurred within 3 months after surgery. This indicates that the first three months after transplantation is the period when IA infection is most likely to occur. Compared with our findings, other countries have reported much later times of IA onset, with a median of 7.7 months posttransplantation[12]. In some centers in Western Australia, the rate of invasive fungal infection was comparatively low, showing a cumulative incidence of 3.8% at 1 year, 7.6% at 3 years and 10.1% at 5 years posttransplantation, with a median of 583 days (IQR 182~1110 days), likely because of comprehensive early antifungal use and preemptive therapy at any time after transplantation. In that cohort, in addition to nebulized amphotericin administered to all LTRs on transplantation admission, systemic mold-active azole was given to 80/130 (61.5%) LTRs in the first 6 months posttransplant, 57/121 (47.1%) 6-12 months after transplantation, and 93/124 (75%) more than 12 months posttransplantation[16]. The above differences in prevention and treatment of fungal infections probably influenced the incidence and timing of invasive aspergillosis after lung transplantation. In addition, the incidence and timing of IA infection may be related to the quality of the donor lungs. Due to traditional concepts among Chinese people, tissue and organ donation and procurement usually occur much later than abroad, which inevitably leads to longer ICU stays and poor donor lung quality. Therefore, Chinese lung transplant recipients face more complex and severe problems of early postoperative infections. In our center, severe bacterial or fungal infections and associated complications have become the leading cause of death for LTRs.
It is well known that invasive candidiasis is the most frequently observed invasive fungal infection (IFI) among all SOT recipients, except for LTRs, where invasive aspergillosis is the most common[6]. We adopted a universal prophylaxis strategy for avoiding postoperative invasive fungal infections, which was thought to be the primary cause of the low occurrence of invasive candidiasis among LTRs. Recent studies have suggested that 16.5% of patients who underwent LTx developed IFI, while Aspergillus spp. (45%, most commonly Aspergillus fumigatus) was the most common cause of IFI, followed by Candida spp. (23%, most commonly Candida albicans) and other molds [1, 2, 4]. Our finding is different from the above reports. Among 157 recipients, the vast majority of IFIs were caused by Aspergillus spp. (27.4%, 43/157), especially Aspergillus flavus, followed by Aspergillus fumigatus and Aspergillus niger. Consistent with the literature, other invasive fungi found in our study accounted for only a small proportion of IFI patients. In Western Australia, Aspergillus species, followed by Scedosporium apiospermum and Cryptococcus species were the most common fungi that caused IFIs among lung transplant recipients [16]. This difference in epidemiology may be related to the distribution of pathogens in different countries and regions.
The clinical manifestations of aspergillosis range from asymptomatic colonization to invasive presentations, including TBA, IPA, and disseminated extrapulmonary aspergillosis. Asymptomatic colonization with Aspergillus was found in 11 LTRs (11/157, 7.0%) postoperation. Whether these asymptomatic colonizations are a risk factor for IA infection still needs further follow-up. In the majority of IA patients, the clinical symptoms include cough, hemoptysis, pleuritic chest pain, or fever. Respiratory failure secondary to IA is uncommon[5]. It is worth mentioning that mixed infections, especially multidrug-resistant bacterial infections and invasive fungal infections, are very common at the early stage of lung transplantation, making it almost impossible to distinguish the clinical symptoms caused by bacterial or Aspergillus infections. In our center, TBA (including a special type,BAI) was the main form of invasive aspergillosis infection (76.7%), followed by IPA (32.6%). As reported in the literature, TBA occurs mainly among LTRs and may lead to airway obstruction, bronchial ulcerations and pseudomembrane formation [17]. In our study, LTRs with TBA were significantly more likely to suffer from anastomotic airway complications (stenosis, 10/33, and fistulae, 9/33) than those with IPA (0/14). Chong PP and his colleagues showed that Aspergillus fumigatus was the most common cause of invasive fungal infections, with the pulmonary parenchyma being the most common site of infection [4]. However, the tracheal bronchus (bronchial anastomosis included) of the transplanted lung was found to be the most common site of IA infection in our center. On the one hand, the incidence of IPA was much lower than that of TBA in our study, partly due to the targeted prevention or preemptive treatment for postoperative fungal infections. On the other hand, the diagnosis of IPA is more difficult than that of TBA in clinical practice due to concerns about the underlying procedure-related risks from lung tissue biopsy not only for the patients but also for the respiratory physicians.
Bronchial anastomotic infection is a special type of IA infection in lung transplant recipients that can lead to anastomotic dehiscence or stenosis, necessitating aggressive therapy [18, 19]. It has been reported that following lung transplantation, saprophytic fungal infections of the bronchial anastomosis are associated with serious airway complications. The risk of patients with infections developing an additional complication following an anastomotic infection was five times that of recipients without an infection (relative risk, 5.36; 95% CI, 1.82 to 15.79). The odds in favor of a bronchial complication in patients following infection were eight times greater than in recipients without infection (odds ratio, 8.31; 95% CI, 1.96 to 35.16) [20]. A typical case is shown in Fig. 2. This was a 67-year-old male recipient who complained of dry cough 2 months after a left single LT for hypersensitivity pneumonitis. The growth of fungal filaments can be seen at the anastomotic site under direct bronchofibroscopic observation. Positive histopathology and tissue culture both confirmed that this was a proven IA infection caused by Aspergillus flavus. Because of cultural complexities and differences between Eastern and Western countries, it is relatively harder for Chinese LTRs to agree to invasive operations, such as lung biopsy or bronchial anastomotic tissue biopsy, especially in the early postoperative period. This may explain why only a small percentage of recipients (9.3%) with IA were diagnosed with proven IA infection according to ISHLT criteria in our study.
According to the guidelines, BAL-GM is the preferred sampling method for the diagnosis of IPA among SOT recipients (strong; high quality), and a BAL-GM cutoff index value ≥1.0 ng/ml is preferred for the diagnosis of IA in lung and nonlung transplant recipients, in combination with other fungal diagnostic modalities (e.g., chest CT scan, culture) [5]. In the diagnosis of IA infection among lung transplant recipients, the sensitivity of BAL-GM ranges from 67%~100%, while the specificity ranges from 89%~93% [21]. Therefore, we also regarded BAL-GM as an important surveillance and diagnostic modality for IA infection in lung transplant recipients during follow-up. Recently, some studies have shown that because of the high sensitivity of bronchial aspirate culturing, unlike BAL (k 0.817, CI 0.664~0.840, p<0.001), it can be used successfully for most patients with ventilator-associated pneumonia [22]. BAL is not always well tolerated by LTRs, especially recipients at the early stage following lung transplantation; it may cause some minor complications, such as fever, cough, wheezing, pulmonary infiltrates and hypoxemia. Serious complications, including pneumothorax, bleeding and cardiac arrest, are rare, but the consequence is serious and easily leads to medical damage disputes [23]. Simple bronchial aspiration, on the other hand, is well tolerated and frequently performed during the routine care of LTRs in our center. We found that the median value of BA-GM from all 43 recipients with IA infection reached 6.76 ng/ml [IQR, 4.25-9.15 ng/ml], which was much higher than the recommended diagnostic cutoff value for BAL-GM. The diagnostic performance and cutoff value of BA-GM for IA infection in lung transplant recipients will be further explored in a future study.
An optimal antifungal prophylactic strategy has yet to be defined for lung transplant recipients. The antifungal agent employed and the duration of prophylaxis remain controversial among different lung transplantation centers. Given the findings of previous studies that tracheobronchial or anastomotic aspergillus infections were the most frequently occurring infections within 3 months[24], antifungal prophylaxis with aerosolized amphotericin B deoxycholate (AmBd) was typically employed for 3 months in LTRs in our study. Amphotericin B, via inhaled administration, can provide local protection to the airways and bronchial anastomoses of transplanted lungs at risk of infection and is also associated with a low rate of invasive pulmonary fungal infection in the early posttransplantation period [25]. However, the optimal dosage, formulation (deoxycholate or lipid formulations) and duration of prophylaxis with inhaled amphotericin B are highly variable among lung transplant centers [1]. In previous studies, aerosolized AmBd was discontinued due to intolerance in 12.2% of LTRs following postoperative administration at a dose of 25 mg once every day [25]. But in our study, all recipients ultimately tolerated inhaled AmBd well at a dose of 12.5 mg twice daily, although a few complained of irritable cough or bitter taste.
In addition to local administration of antifungal agents, systemic therapies are also needed because inhaled amphotericin B is not systemically absorbed and does not provide protection against fungal infections in locations beyond the airways and lung parenchyma1. There is no consensus on the choice of systemic antifungal prophylactic agent, rout of administration, or duration of prophylaxis. Considering the greater intolerance to the side effects of triazoles and greater interaction with anti-rejection drugs, intravenous echinocandins (e.g., caspofungin 50 mg daily) for three weeks and in combination with inhaled AmBd for three months are the preferred first-line agents in our center of lung transplantation. Almost no recipient suffered from invasive candidiasis due to the above antifungal prophylaxis, except for one patient who suffered from candidemia mixed with invasive aspergillosis.
Voriconazole remains the most common drug of choice for treating invasive aspergillosis infections, followed by isavuconazole and lipid formulations of amphotericin B [5, 26]. However, whether voriconazole monotherapy or combination antifungals should be used as the primary IA therapy remains controversial. In our study, the vast majority of IA recipients (36/43,83.7%) were treated with voriconazole monotherapy at the discretion of the attending physicians, showing the mortality rate of 30.6%(11/36). The remaining recipients (7/43,16.3%) received combined antifungal therapy because of a fungal breakthrough infection or mixed infections with other invasive molds, such as Rhizopus microsporum, Scopulariopsis/Microascus or candidemia, showing a higher mortality rate of 57.1%(4/7) , although not significantly so (p=0.359) .Consistent with the recommendations from both the Infectious Diseases Society of America (IDSA) [18] and the European Society for Clinical Microbiology and Infectious Diseases [27], we agree that triazoles are preferred agents in the treatment of IA for most recipients. However, more attention should be paid to those patients with fungal breakthrough infection or mixed infections, due to the poor outcome for voriconazole monotherapy or combination therapy. Therapeutic drug monitoring (TDM) for all patients receiving azole antifungals should be carried out to ensure the effectiveness and safety of antifungal therapy and especially to minimize drug interactions with antirejection agents such as tacrolimus or cyclosporine.
The mortality rate of IA among LTRs varies according to the clinical presentation, ranging from 23%~29% for patients with tracheobronchitis to as high as 67%~82% for patients with IPA [28]. In recent studies, the 1-year survival for patients who developed IFI following transplant has increased to 77%~78% [4, 29]. In our study, the all-cause mortality 1 month and 6 months postoperatively was 4.7% and 18.6%, respectively, and 34.9% of the 43 LTRs with IA infections had died at the conclusion of this study. This shows that the mortality rate in our center was also high and is basically consistent with that reported in the above literature. More attention should be paid to the 40% mortality rate among patients with early-onset IA, as this may be higher than that of patients with late-onset IA (12.5%), although not significantly so(p=0.289). This may be partly related to our small sample size. However, another study reported the opposite result; patients with late-onset Aspergillus infections had significantly higher mortality than those with early-onset infections (57% vs 28%, p=0.045) [24]. Thus, the data from different lung transplant centers can sometimes vary greatly. In addition, recipients with mixed fungal infections resulting polymicrobial invasion seemed to have the worst outcome, with mortality as high as 50% in this study. It has been reported that invasive aspergillosis is associated with early and high mortality in lung transplant patients, and colonization with Aspergillus is also associated with a significant increase in mortality after 5 years [14]; however, whether mixed fungal infection is an independent risk factor for higher mortality remains unknown, and prospective studies with larger samples are needed to verify this hypothesis in the future.
The study had some limitations. First, the study has a single-center, cross-sectional study design with a small number of patients and a lack of a control group, which may limit the generalizability of our findings to other practice settings in China. Second, the follow-up time was limited, and we did not explore the risk factors for IA development. Finally, due to the constant adjustment of transplantation practices due to the infective state of recipients, such as the regimen and intensity of immunosuppressive therapy, we could not capture relevant data to analyze the association between the occurrence of posttransplantation IA and immunosuppressive treatment.