Aspergillosis and stem cell transplantation: An overview of experimental pathogenesis studies

Epidemiology of Clinically Significant Aspergillus Species from a Large Tertiary Hospital in Shanghai, China, for the Period of Two Years

Background

Aspergillus species are becoming a major public health concern worldwide due to the increase in the incidence of aspergillosis and emergence of antifungal resistance. In this study, we surveyed all Aspergillus species isolated from aspergillosis patients in Zhongshan Hospital Fudan University, Shanghai, China, from 2019 to 2021.

Methods

We characterized the susceptibility profiles of these Aspergillus species to medical azoles (voriconazole, itraconazole and posaconazole) using YeastOne^TM broth microdilution system. To determine the underlying antifungal resistance mechanisms in azole-resistant A. fumigatus (ARAf) isolates, we characterized mutations in the cyp51A gene. Genotypic diversity of sampled A. fumigatus was investigated using CSP-typing.

Results

A total of 112 Aspergillus isolates (81 A. fumigatus, 17 A. flavus, 5 A. niger, 2 A. terreus, 2 A. lentulus, 2 A. oryzae, 1 A. nidulans, 1 A. versicolor and 1 A. sydowii) from 105 patients diagnosed with aspergillosis (including proven or probable invasive aspergillosis, chronic pulmonary aspergillosis, allergic bronchopulmonary aspergillosis and cutaneous aspergillosis) were obtained. Eight isolates (7 A. fumigatus and 1 A. niger) from seven patients were either azole non-susceptible or non-wild type. Azole non-susceptible or non-wild type rate was 7.1%/isolate and 6.7%/patient analysed. Four ARAf harbored TR34/L98H mutation, whereas one carried TR46/Y121F/T289A allele. The 81 A. fumigatus isolates were spread across 8 CSP types with t01 to be the predominant type (53.1%). ARAf isolates were distributed over CSP types t01, t02, t04A and t11.

Conclusion

Results from this study provided us with an understanding of the antifungal resistance and related characteristics of Aspergillus species in Eastern China. Further comparisons of our results with those in other countries reflect potential clonal expansion of A. fumigatus in our region. Further surveillance study is warranted to guide antifungal therapy and for epidemiological purposes.

A Strategy of On-Demand Immune Activation for Antifungal Treatment Using Near-Infrared Responsive Conjugated Polymer Nanoparticles

Pathogenic fungal infection is a major clinical threat because pathogenic fungi have developed resistant mechanisms to evade the innate immune response, especially interactions with macrophages. Herein, a strategy to activate immune responses of macrophages to fungi based on near-infrared (NIR) responsive conjugated polymer nanoparticles (CPNs-M) is reported for antifungal immunotherapy. Under NIR light irradiation, CPNs-M exposes β-glucan on the surface of fungal conidia by photothermal damage and drug released from CPNs-M. The exposed β-glucan elicits macrophage recognition and subsequently activates calcium-calmodulin (Ca²⁺-CaM) signaling followed by the LC3-associated phagocytosis (LAP) pathway to kill fungal conidia. Consequently, a remarkable elimination of intracellular fugal conidia and successful treatment of fungal pneumonia are achieved. This remote regulation strategy to restore pathogen-immune cell interaction on demand provides a new insight into combatting intractable intracellular infections.

Protective role of host complement system in Aspergillus fumigatus infection

Invasive aspergillosis (IA) is a life-threatening fungal infection for immunocompromised hosts. It is, therefore, necessary to understand the immune pathways that control this infection. Although the primary infection site is the lungs, aspergillosis can disseminate to other organs through unknown mechanisms. Herein we have examined the in vivo role of various complement pathways as well as the complement receptors C3aR and C5aR1 during experimental systemic infection by Aspergillus fumigatus, the main species responsible for IA. We show that C3 knockout (C3^-/-) mice are highly susceptible to systemic infection of A. fumigatus. Intriguingly, C4^-/- and factor B (FB)^-/- mice showed susceptibility similar to the wild-type mice, suggesting that either the complement pathways display functional redundancy during infection (i.e., one pathway compensates for the loss of the other), or complement is activated non-canonically by A. fumigatus protease. Our in vitro study substantiates the presence of C3 and C5 cleaving proteases in A. fumigatus. Examination of the importance of the terminal complement pathway employing C5^-/- and C5aR1^-/- mice reveals that it plays a vital role in the conidial clearance. This, in part, is due to the increased conidial uptake by phagocytes. Together, our data suggest that the complement deficiency enhances the susceptibility to systemic infection by A. fumigatus.

Therapeutic Potential of Green Synthesized Gold Nanoparticles Using Extract of Leptadenia hastata against Invasive Pulmonary Aspergillosis

Gold nanoparticles are widely used in the biomedical field for the treatment of several diseases, including cancer, inflammatory diseases, and immune system disorders, due to their distinctive physicochemical characteristics. In this study, we investigated the therapeutic potential of green synthesized gold nanoparticles using ethanolic leaf extract of Leptadenia hastata (LH-AuNPs) against invasive pulmonary aspergillosis (IPA) in mice. UV/visible spectroscopy, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), and zeta potential were used to characterize the biofabricated LH-AuNPs. Antifungal activity of LH-AuNPs was determined by MTT assay, (3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide), time-kill assay, and radial growth inhibition. TEM and SEM were used to examine the mode of the antifungal action of LH-AuNPs. The in vivo activity of LH-AuNPs against IPA was studied using a well-established IPA mouse model. LH-AuNPs excreted antifungal activity against Aspergillus fumigatus with MIC 64 µg/mL and inhibited the radial growth of A. fumigatus by 30% compared to the control. LH-AuNPs caused distortion and collapse of fungal hyphae and deterioration of cell walls. Interestingly, LH-AuNPs did not display any cytotoxicity on cultured primary bone marrow stem cells (BMSCs) or A549 human lung cell line in vitro at MIC concentration. IPA mice treated with LH-AuNPs displayed significant lung tissue repair without any in vivo cytotoxicity. LH-AuNPs administration showed significant suppression of fungal burden and gliotoxin production in the lung. In addition, LH-AuNPs inhibited IPA-induced pro-inflammatory cytokines production, including interleukin-1 (IL-1), interleukin-17 (IL-17), and tumor necrosis factor-alpha (TNF-α), and reduced oxidative stress in lung. In conclusion, our data provide LH-AuNPs as a novel nanoparticle therapy for IPA.

Cytokine and Chemokine Responses in Invasive Aspergillosis Following Hematopoietic Stem Cell Transplantation: Past Evidence for Future Therapy of Aspergillosis

Invasive pulmonary aspergillosis is a frequent complication in immunocompromised individuals, and it continues to be an important cause of mortality in patients undergoing hematopoietic stem cell transplantation. In addition to antifungal therapy used for mycoses, immune-modulatory molecules such as cytokines and chemokines can modify the host immune response and exhibit a promising form of antimicrobial therapeutics to combat invasive fungal diseases. Cytokine and chemokine profiles may also be applied as biomarkers during fungal infections and clinical research has demonstrated different activation patterns of cytokines in invasive mycoses such as aspergillosis. In this review, we summarize different aspects of cytokines that have been described to date and provide possible future directions in research on invasive pulmonary aspergillosis following hematopoietic stem cell transplantation. These findings suggest that cytokines and chemokines may serve as useful biomarkers to improve diagnosis and monitoring of infection.

Experimental Models of Infectious Pulmonary Complications Following Hematopoietic Cell Transplantation

Pulmonary infections remain a major cause of morbidity and mortality in hematopoietic cell transplantation (HCT) recipients. The prevalence and type of infection changes over time and is influenced by the course of immune reconstitution post-transplant. The interaction between pathogens and host immune responses is complex in HCT settings, since the conditioning regimens create periods of neutropenia and immunosuppressive drugs are often needed to prevent graft rejection and limit graft-versus-host disease (GVHD). Experimental murine models of transplantation are valuable tools for dissecting the procedure-related alterations to innate and adaptive immunity. Here we review mouse models of post-HCT infectious pulmonary complications, primarily focused on three groups of pathogens that frequently infect HCT recipients: bacteria (often P. aeruginosa), fungus (primarily Aspergillus fumigatus), and viruses (primarily herpesviruses). These mouse models have advanced our knowledge regarding how the conditioning and HCT process negatively impacts innate immunity and have provided new potential strategies of managing the infections. Studies using mouse models have also validated clinical observations suggesting that prior or occult infections are a potential etiology of noninfectious pulmonary complications post-HCT as well.

The Roles of γδ T Cells in Hematopoietic Stem Cell Transplantation

The αβ T-cell-depleted hematopoietic stem cell transplantation (HSCT) leads to lower relapse and better outcome, and may correlate strongly with expansion of donor-derived γδ T cells. γδ T cells play an important role in immune reconstitution and can exert a graft-versus-leukemia effect after HSCT. This review showed the recent literature on immune functions of γδ T cells after HSCT. The discrepancies between studies of γδ T cells in graft-versus-host disease may cause by its heterogeneous and various distinct subsets. And reconstitution of γδ T cells may play a potential immunoregulatory role in the infections after HSCT.

How Long Do We Need to Treat an Invasive Mold Disease in Hematology Patients? Factors Influencing Duration of Therapy and Future Questions

Treatment duration for invasive mold disease (IMD) in patients with hematological malignancy is not standardized and is a challenging subject in antifungal stewardship. Concerns for IMD relapse during subsequent reinduction or consolidation chemotherapy or graft versus host disease treatment in hematopoietic stem cell transplant recipients often results in prolonged or indefinite antifungal treatment. There are no validated criteria that predict when it is safe to stop antifungals. Decisions are individualized and depend on the offending fungus, site and extent of IMD, comorbidities, hematologic disease prognosis, and future plans for chemotherapy or transplantation. Recent studies suggest that FDG-PET/CT could help discriminate between active and residual fungal lesions to support decisions for safely stopping antifungals. Validation of noninvasive biomarkers for monitoring treatment response, tests for quantifying the "net state of immunosuppression," and genetic polymorphisms associated with poor fungal immunity could lead to a personalized assessment for the continued need for antifungal therapy.

Effects of Hematopoietic Cell Transplantation on the Pulmonary Immune Response to Infection

Pulmonary infections are common in hematopoietic cell transplant (HCT) patients of all ages and are associated with high levels of morbidity and mortality. Bacterial, viral, fungal, and parasitic pathogens are all represented as causes of infection. The lung mounts a complex immune response to infection and this response is significantly affected by the pre-HCT conditioning regimen, graft characteristics, and ongoing immunomodulatory therapy. We review the published literature, including animal models as well as human data, to describe what is known about the pulmonary immune response to infection in HCT recipients. Studies have focused on the pulmonary immune response to Aspergillus fumigatus, gram-positive and gram-negative bacteria, and viruses, and show a range of defects associated with both the innate and adaptive immune responses after HCT. There are still many open areas for research, to delineate novel therapeutic targets for pulmonary infections as well as to explore linkages to non-infectious inflammatory lung conditions.

In vitro and in vivo characterization of two nonsporulating Aspergillus fumigatus clinical isolates from immunocompetent patients

Aspergillus fumigatus is a pathogenic fungus responsible for invasive aspergillosis (IA). Typically, it can produce abundant conidia to survive and spread. The infection by A. fumigatus usually occurs in immunocompromised patients due to failed clearance of inhaled conidia. However, the incidence of aspergillosis in immunocompetent hosts has been increasing, the pathogenesis of which is still unknown. Our team previously obtained two clinical nonsporulating A. fumigatus isolates from non-immunocompromised patients, which only have the form of hyphae. This present study demonstrated the in vitro and in vivo characteristics of the two nonsporulating A. fumigatus isolates and verified that their conidiation defects are associated to abolished expression of the sporulation-related central regulatory pathway brlA gene. In addition, we confirmed the mutation site of brlA gene (c.657_660delTCCT) contributes to the nonsporulating phenotype in one clinical isolate. Plate assay showed that the two nonsporulating isolates have a similar resistance to antifungal drugs, cell wall disturbing substances, and oxidative stress compared with the wild-type reference Af293. Most important of all, we employed an immunocompetent mouse model to mimic the pathogenesis of pulmonary aspergillosis in non-immunocompromised patients. It revealed that the hyphae of two nonsporulating isolates and Af293 have similar virulence in immunocompetent hosts. Interestingly, the hyphae fragments of Af293 but not conidia are able to induce invasive aspergillosis in immunocompetent mice. In conclusion, our study indicate that the form of hyphae may play a dominant causative role in pulmonary aspergillosis of immunocompetent hosts rather than conidia.

Aspergillus fumigatus conidial metalloprotease Mep1p cleaves host complement proteins

Innate immunity in animals including humans encompasses the complement system, which is considered an important host defense mechanism against Aspergillus fumigatus, one of the most ubiquitous opportunistic human fungal pathogens. Previously, it has been shown that the alkaline protease Alp1p secreted from A. fumigatus mycelia degrades the complement components C3, C4, and C5. However, it remains unclear how the fungal spores (i.e. conidia) defend themselves against the activities of the complement system immediately after inhalation into the lung. Here, we show that A. fumigatus conidia contain a metalloprotease Mep1p, which is released upon conidial contact with collagen and inactivates all three complement pathways. In particular, Mep1p efficiently inactivated the major complement components C3, C4, and C5 and their activation products (C3a, C4a, and C5a) as well as the pattern-recognition molecules MBL and ficolin-1, either by directly cleaving them or by cleaving them to a form that is further broken down by other proteases of the complement system. Moreover, incubation of Mep1p with human serum significantly inhibited the complement hemolytic activity and conidial opsonization by C3b and their subsequent phagocytosis by macrophages. Together, these results indicate that Mep1p associated with and released from A. fumigatus conidia likely facilitates early immune evasion by disarming the complement defense in the human host.

Overcoming antibiotic resistance: Is siderophore Trojan horse conjugation an answer to evolving resistance in microbial pathogens?

Comparative study of siderophore biosynthesis pathway in pathogens provides potential targets for antibiotics and host drug delivery as a part of computationally feasible microbial therapy. Iron acquisition using siderophore models is an essential and well established model in all microorganisms and microbial infections a known to cause great havoc to both plant and animal. Rapid development of antibiotic resistance in bacterial as well as fungal pathogens has drawn us at a verge where one has to get rid of the traditional way of obstructing pathogen using single or multiple antibiotic/chemical inhibitors or drugs. 'Trojan horse' strategy is an answer to this imperative call where antibiotic are by far sneaked into the pathogenic cell via the siderophore receptors at cell and outer membrane. This antibiotic once gets inside, generates a 'black hole' scenario within the opportunistic pathogens via iron scarcity. For pathogens whose siderophore are not compatible to smuggle drug due to their complex conformation and stiff valence bonds, there is another approach. By means of the siderophore biosynthesis pathways, potential targets for inhibition of these siderophores in pathogenic bacteria could be achieved and thus control pathogenic virulence. Method to design artificial exogenous siderophores for pathogens that would compete and succeed the battle of intake is also covered with this review. These manipulated siderophore would enter pathogenic cell like any other siderophore but will not disperse iron due to which iron inadequacy and hence pathogens control be accomplished. The aim of this review is to offer strategies to overcome the microbial infections/pathogens using siderophore.

Linear Epitopes of Paracoccidioides brasiliensis and Other Fungal Agents of Human Systemic Mycoses As Vaccine Candidates

Dimorphic fungi are agents of systemic mycoses associated with significant morbidity and frequent lethality in the Americas. Among the pathogenic species are Paracoccidioides brasiliensis and Paracoccidioides lutzii, which predominate in South America; Histoplasma capsulatum, Coccidioides posadasii, and Coccidioides immitis, and the Sporothrix spp. complex are other important pathogens. Associated with dimorphic fungi other important infections are caused by yeast such as Candida spp. and Cryptococcus spp. or mold such as Aspergillus spp., which are also fungal agents of deadly infections. Nowadays, the actual tendency of therapy is the development of a pan-fungal vaccine. This is, however, not easy because of the complexity of eukaryotic cells and the particularities of different species and isolates. Albeit there are several experimental vaccines being studied, we will focus mainly on peptide vaccines or epitopes of T-cell receptors inducing protective fungal responses. These peptides can be carried by antibody inducing β-(1,3)-glucan oligo or polysaccharides, or be mixed with them for administration. The present review discusses the efficacy of linear peptide epitopes in the context of antifungal immunization and vaccine proposition.