Distinguishing Invasive from Chronic Pulmonary Infections: Host Pentraxin 3 and Fungal Siderophores in Bronchoalveolar Lavage Fluids

Interplay between host humoral pattern recognition molecules controls undue immune responses against Aspergillus fumigatus

Pentraxin 3 (PTX3), a long pentraxin and a humoral pattern recognition molecule (PRM), has been demonstrated to be protective against Aspergillus fumigatus, an airborne human fungal pathogen. We explored its mode of interaction with A. fumigatus, and the resulting implications in the host immune response. Here, we demonstrate that PTX3 interacts with A. fumigatus in a morphotype-dependent manner: (a) it recognizes germinating conidia through galactosaminogalactan, a surface exposed cell wall polysaccharide of A. fumigatus, (b) in dormant conidia, surface proteins serve as weak PTX3 ligands, and (c) surfactant protein D (SP-D) and the complement proteins C1q and C3b, the other humoral PRMs, enhance the interaction of PTX3 with dormant conidia. SP-D, C3b or C1q opsonized conidia stimulated human primary immune cells to release pro-inflammatory cytokines and chemokines. However, subsequent binding of PTX3 to SP-D, C1q or C3b opsonized conidia significantly decreased the production of pro-inflammatory cytokines/chemokines. PTX3 opsonized germinating conidia also significantly lowered the production of pro-inflammatory cytokines/chemokines while increasing IL-10 (an anti-inflammatory cytokine) released by immune cells when compared to the unopsonized counterpart. Overall, our study demonstrates that PTX3 recognizes A. fumigatus either directly or by interplaying with other humoral PRMs, thereby restraining detrimental inflammation. Moreover, PTX3 levels were significantly higher in the serum of patients with invasive pulmonary aspergillosis (IPA) and COVID-19-associated pulmonary aspergillosis (CAPA), supporting previous observations in IPA patients, and suggesting that it could be a potential panel-biomarker for these pathological conditions caused by A. fumigatus.

Interplay of Cytokines and Chemokines in Aspergillosis

Aspergillosis is a fungal infection caused by various species of Aspergillus, most notably A. fumigatus. This fungus causes a spectrum of diseases, including allergic bronchopulmonary aspergillosis, aspergilloma, chronic pulmonary aspergillosis, and invasive aspergillosis. The clinical manifestations and severity of aspergillosis can vary depending on individual immune status and the specific species of Aspergillus involved. The recognition of Aspergillus involves pathogen-associated molecular patterns (PAMPs) such as glucan, galactomannan, mannose, and conidial surface proteins. These are recognized by the pathogen recognition receptors present on immune cells such as Toll-like receptors (TLR-1,2,3,4, etc.) and C-type lectins (Dectin-1 and Dectin-2). We discuss the roles of cytokines and pathogen recognition in aspergillosis from both the perspective of human and experimental infection. Several cytokines and chemokines have been implicated in the immune response to Aspergillus infection, including interferon-γ (IFN-γ), tumor necrosis factor-α (TNF-α), CCR4, CCR17, and other interleukins. For example, allergic bronchopulmonary aspergillosis (ABPA) is characterized by Th2 and Th9 cell-type immunity and involves interleukin (IL)-4, IL-5, IL-13, and IL-10. In contrast, it has been observed that invasive aspergillosis involves Th1 and Th17 cell-type immunity via IFN-γ, IL-1, IL-6, and IL-17. These cytokines activate various immune cells and stimulate the production of other immune molecules, such as antimicrobial peptides and reactive oxygen species, which aid in the clearance of the fungal pathogen. Moreover, they help to initiate and coordinate the immune response, recruit immune cells to the site of infection, and promote clearance of the fungus. Insight into the host response from both human and animal studies may aid in understanding the immune response in aspergillosis, possibly leading to harnessing the power of cytokines or cytokine (receptor) antagonists and transforming them into precise immunotherapeutic strategies. This could advance personalized medicine.

Microbiological Diagnosis of Pulmonary Aspergillus Infections

As microbiological tests play an important role in our diagnostic algorithms and clinical approach towards patients at-risk for pulmonary aspergillosis, a good knowledge of the diagnostic possibilities and especially their limitations is extremely important. In this review, we aim to reflect critically on the available microbiological diagnostic modalities for diagnosis of pulmonary aspergillosis and formulate some future prospects. Timely start of adequate antifungal treatment leads to a better patient outcome, but overuse of antifungals should be avoided. Current diagnostic possibilities are expanding, and are mainly driven by enzyme immunoassays and lateral flow device tests for the detection of Aspergillus antigens. Most of these tests are directed towards similar antigens, but new antibodies towards different targets are under development. For chronic forms of pulmonary aspergillosis, anti-Aspergillus IgG antibodies and precipitins remain the cornerstone. More studies on the possibilities and limitations of molecular testing including targeting resistance markers are ongoing. Also, metagenomic next-generation sequencing is expanding our future possibilities. It remains important to combine different test results and interpret them in the appropriate clinical context to improve performance. Test performances may differ according to the patient population and test results may be influenced by timing, the tested matrix, and prophylactic and empiric antifungal therapy. Despite the increasing armamentarium, a simple blood or urine test for the diagnosis of aspergillosis in all patient populations at-risk is still lacking. Research on diagnostic tools is broadening from a pathogen focus on biomarkers related to the patient and its immune system.

Opportunities and challenges of microbial siderophores in the medical field

Siderophores are low-molecular-weight secondary metabolites that function as iron chelators. Under iron-deficiency conditions, they are produced by a wide variety of microbes, allowing them to increase their iron uptake. The primary function of these compounds is the environmental iron scavenging and its transport into the cytosol. Iron is then reduced to its ferrous form to operate as an enzymatic cofactor for various functions, including respiration, nitrogen fixation, photosynthesis, methanogenesis, and amino acid synthesis. Depending on their functional group, siderophores are classified into hydroxamate, catecholate, phenolate, carboxylate, and mixed types. They have achieved great importance in recent years due to their medical applications as antimicrobial, antimalarial, or anticancer drugs, vaccines, and drug-delivery agents. This review integrates current advances in specific healthcare applications of microbial siderophores, delineating new opportunities and challenges as viable therapies to fight against diseases that represent crucial public health problems in the medical field.Key points• Siderophores are low-molecular-weight secondary metabolites functioning as iron chelators.• The siderophore's properties offer viable options to face diverse clinical problems.• Siderophores are alternatives for the enhancement of antibiotic activities.

Humoral Immunity Against Aspergillus fumigatus

Aspergillus fumigatus is one the most ubiquitous airborne opportunistic human fungal pathogens. Understanding its interaction with host immune system, composed of cellular and humoral arm, is essential to explain the pathobiology of aspergillosis disease spectrum. While cellular immunity has been well studied, humoral immunity has been poorly acknowledge, although it plays a crucial role in bridging the fungus and immune cells. In this review, we have summarized available data on major players of humoral immunity against A. fumigatus and discussed how they may help to identify at-risk individuals, be used as diagnostic tools or promote alternative therapeutic strategies. Remaining challenges are highlighted and leads are given to guide future research to better grasp the complexity of humoral immune interaction with A. fumigatus.

Current and Future Pathways in Aspergillus Diagnosis

Aspergillus fumigatus has been designated by the World Health Organization as a critical priority fungal pathogen. Some commercially available diagnostics for many forms of aspergillosis rely on fungal metabolites. These encompass intracellular molecules, cell wall components, and extracellular secretomes. This review summarizes the shortcomings of antibody tests compared to tests of fungal products in body fluids and highlights the application of β-d-glucan, galactomannan, and pentraxin 3 in bronchoalveolar lavage fluids. We also discuss the detection of nucleic acids and next-generation sequencing, along with newer studies on Aspergillus metallophores.

Diagnosis of Aspergillosis in Horses

Invasive pulmonary aspergillosis (IPA) may be a rare cause of granulomatous pneumonia in horses. The mortality of IPA is almost 100%; direct diagnostic tools in horses are needed. Bronchoalveolar lavage fluid (BALF) and serum samples were collected from 18 horses, including individuals suffering from IPA (n = 1), equine asthma (EA, n = 12), and 5 healthy controls. Serum samples were collected from another 6 healthy controls. Samples of BALF (n = 18) were analyzed for Aspergillus spp. DNA, fungal galactomannan (GM), ferricrocin (Fc), triacetylfusarinin C (TafC), and gliotoxin (Gtx). Analysis of 24 serum samples for (1,3)-β-D-glucan (BDG) and GM was performed. Median serum BDG levels were 131 pg/mL in controls and 1142 pg/mL in IPA. Similar trends were observed in BALF samples for GM (Area under the Curve (AUC) = 0.941) and DNA (AUC = 0.941). The fungal secondary metabolite Gtx was detected in IPA BALF and lung tissue samples (86 ng/mL and 2.17 ng/mg, AUC = 1).