Monoclonal antibodies specific for immunorecessive epitopes of glucuronoxylomannan, the major capsular polysaccharide of Cryptococcus neoformans, reduce serotype bias in an immunoassay for cryptococcal antigen

Diagnostic Performances of an in-House Immunochromatography Test Based on the Monoclonal Antibody 18B7 to Glucuronoxylomannan for Clinical Suspected Cryptococcosis: a Large-Scale Prototype Evaluation in Northern Thailand

OBJECTIVE

Cryptococcosis predominantly presents as a meningoencephalitis in Thailand. Early and expeditious diagnosis is essential for reducing both mortality and morbidity associated with cryptococcal meningitis. We aim to define and establish the diagnostic performances between the benchmark commercially available diagnostic kit (CrAg® LFA) and the large-scale prototype of an inexpensive in-house immunochromatographic test (ICT) based on monoclonal antibody (MAb) 18B7.

METHODS

We have developed the large-scale prototype for the rapid detection of cryptococcal polysaccharide antigens by utilizing a single antibody sandwich ICT format employing MAb 18B7, which is highly specific to Cryptococcus neoformans glucuronoxylomannan (GXM) antigens. An in-house MAb18B7 ICT was manufactured in accordance with industry standards under the control of the International Organization for Standardization (ISO) 13485.

RESULTS

The diagnostic sensitivity, specificity, and accuracy for the in-house MAb 18B7 ICT were 99.10%, 97.61%, and 97.83%, respectively. The agreement kappa (κ) coefficient was 0.968 based on the retrospective evaluation of 580 specimens from patients living in northern Thailand with clinically suspected cryptococcosis.

CONCLUSION

The data suggest that this in-house MAb 18B7 ICT will be highly beneficial for addressing the issue of cryptococcal infection in Thailand. Moreover, it is anticipated that this inexpensive ICT can play a pivotal role in various global strategies aimed at eradicating cryptococcal meningitis among individuals living with HIV by 2030.

Cryptococcal meningitis

Cryptococcus neoformans and Cryptococcus gattii species complexes cause meningoencephalitis with high fatality rates and considerable morbidity, particularly in persons with deficient T cell-mediated immunity, most commonly affecting people living with HIV. Whereas the global incidence of HIV-associated cryptococcal meningitis (HIV-CM) has decreased over the past decade, cryptococcosis still accounts for one in five AIDS-related deaths globally due to the persistent burden of advanced HIV disease. Moreover, mortality remains high (~50%) in low-resource settings. The armamentarium to decrease cryptococcosis-associated mortality is expanding: cryptococcal antigen screening in the serum and pre-emptive azole therapy for cryptococcal antigenaemia are well established, whereas enhanced pre-emptive combination treatment regimens to improve survival of persons with cryptococcal antigenaemia are in clinical trials. Short courses (≤7 days) of amphotericin-based therapy combined with flucytosine are currently the preferred options for induction therapy of cryptococcal meningitis. Whether short-course induction regimens improve long-term morbidity such as depression, reduced neurocognitive performance and physical disability among survivors is the subject of further study. Here, we discuss underlying immunology, changing epidemiology, and updates on the management of cryptococcal meningitis with emphasis on HIV-associated disease.

Diagnostic performance of the IMMY cryptococcal antigen lateral flow assay on serum and cerebrospinal fluid for diagnosis of cryptococcosis in HIV-negative patients: a systematic review

BACKGROUND

The incidence of cryptococcosis amongst HIV-negative persons is increasing. Whilst the excellent performance of the CrAg testing in people living with HIV is well described, the diagnostic performance of the CrAg LFA has not been systematically evaluated in HIV-negative cohorts on serum or cerebrospinal fluid.

METHODS

We performed a systematic review to characterise the diagnostic performance of IMMY CrAg® LFA in HIV-negative populations on serum and cerebrospinal fluid. A systematic electronic search was performed using Medline, Embase, Global Health, CENTRAL, WoS Science Citation Index, SCOPUS, Africa-Wide Information, LILACS and WHO Global Health Library. Studies were screened and data extracted from eligible studies by two independent reviewers. A fixed effect meta-analysis was used to estimate the diagnostic sensitivity and specificity.

RESULTS

Of 447 records assessed for eligibility, nine studies met our inclusion criteria, including 528 participants overall. Amongst eight studies that evaluated the diagnostic performance of the IMMY CrAg® LFA on serum, the pooled median sensitivity was 96% (95% Credible Interval (CrI) 68-100%) with a pooled specificity estimate of 96% (95%CrI 84-100%). Amongst six studies which evaluated the diagnostic performance of IMMY CrAg® LFA on CSF, the pooled median sensitivity was 99% (95%CrI 95-100%) with a pooled specificity median of 99% (95%CrI 95-100%).

CONCLUSIONS

This review demonstrates a high pooled sensitivity and specificity for the IMMY CrAg® LFA in HIV-negative populations, in keeping with findings in HIV-positive individuals. The review was limited by the small number of studies. Further studies using IMMY CrAg® LFA in HIV-negative populations would help to better determine the diagnostic value of this test.

Cryptococcus neoformans, a global threat to human health

BACKGROUND

Emerging fungal pathogens pose important threats to global public health. The World Health Organization has responded to the rising threat of traditionally neglected fungal infections by developing a Fungal Priority Pathogens List (FPPL). Taking the highest-ranked fungal pathogen in the FPPL, Cryptococcus neoformans, as a paradigm, we review progress made over the past two decades on its global burden, its clinical manifestation and management of cryptococcal infection, and its antifungal resistance. The purpose of this review is to drive research efforts to improve future diagnoses, therapies, and interventions associated with fungal infections.

METHODS

We first reviewed trends in the global burden of HIV-associated cryptococcal infection, mainly based on a series of systematic studies. We next conducted scoping reviews in accordance with the guidelines described in the Preferred Reporting Items for Systematic Reviews and Meta-analyses extension for Scoping Reviews using PubMed and ScienceDirect with the keyword Cryptococcus neoformans to identify case reports of cryptococcal infections published since 2000. We then reviewed recent updates on the diagnosis and antifungal treatment of cryptococcal infections. Finally, we summarized knowledge regarding the resistance and tolerance of C. neoformans to approved antifungal drugs.

RESULTS

There has been a general reduction in the estimated global burden of HIV-associated cryptococcal meningitis since 2009, probably due to improvements in highly active antiretroviral therapies. However, cryptococcal meningitis still accounts for 19% of AIDS-related deaths annually. The incidences of CM in Europe and North America and the Latin America region have increased by approximately two-fold since 2009, while other regions showed either reduced or stable numbers of cases. Unfortunately, diagnostic and treatment options for cryptococcal infections are limited, and emerging antifungal resistance exacerbates the public health burden.

CONCLUSION

The rising threat of C. neoformans is compounded by accumulating evidence for its ability to infect immunocompetent individuals and the emergence of antifungal-resistant variants. Emphasis should be placed on further understanding the mechanisms of pathogenicity and of antifungal resistance and tolerance. The development of novel management strategies through the identification of new drug targets and the discovery and optimization of new and existing diagnostics and therapeutics are key to reducing the health burden.

Immunologic Diagnosis of Endemic Mycoses

The endemic mycoses blastomycosis, coccidioidomycosis, histoplasmosis, paracoccidioidomycosis, cryptococcosis, sporotrichosis, talaromycosis, adiaspiromycosis, and emergomycosis are mostly caused by geographically limited thermally dimorphic fungi (except for cryptococcosis), and their diagnoses can be challenging. Usual laboratory methods involved in endemic mycoses diagnosis include microscopic examination and culture of biological samples; however, serologic, histopathologic, and molecular techniques have been implemented in the last few years for the diagnosis of these mycoses since the recovery and identification of their etiologic agents is time-consuming and lacks in sensitivity. In this review, we focus on the immunologic diagnostic methods related to antibody and antigen detection since their evidence is presumptive diagnosis, and in some mycoses, such as cryptococcosis, it is definitive diagnosis.

Evaluation of a Cryptococcus Capsular Polysaccharide Detection FungiXpert LFA (Lateral Flow Assay) for the Rapid Diagnosis of Cryptococcosis

Cryptococcus is an opportunistic pathogenic fungus and is the major cause of fungal meningitis. The cryptococcal antigen (CrAg) lateral flow assay (LFA) is an immunochromatographic test system that has simplified diagnosis as a point-of-care test. In this study, we evaluated the diagnostic performance of Cryptococcal capsular polysaccharide detection FungiXpert (Genobio Pharmaceutical, Tianjin, China) using serum and cerebrospinal fluid (CSF) samples for the diagnosis of cryptococcosis and investigated the cross-reaction of the assays to pathogenic fungi and bacterium by comparing it to the U.S. Food and Drug Administration (US FDA)-approved IMMY CrAg LFA. Eighty CSF and 119 serum/plasma samples from 158 patients were retrospectively collected to test for qualitative or semi-quantitative detection of CrAg. Cross-reaction of the assays was tested using 28 fungi and 1 bacterium. Compared to IMMY CrAg LFA, the FungiXpert LFA demonstrated 99.1% sensitivity and 98.9% specificity in the qualitative test. In the 96 semi-quantitative CrAg assay results, 39 (40.6%) test titers of FungiXpert LFA were 1-2 dilutions higher than those of IMMY CrAg LFA. The Intraclass Correlation Coefficient of the Semi-quantitative results of CrAg titer tests via the two assays was 0.976. Similar to IMMY CrAg LFA, FungiXpert LFA showed cross-reactivity with Trichosporon asahii. Compared with the IMMY CrAg LFA, the FungiXpert LFA showed an equal, yet, excellent performance. However, it is important to note that these two assays have potential cross-reactivity to T. asahii when diagnosing patients. FungiXpert LFA is a rapid screening method for the effective and practical diagnosis and treatment of cryptococcosis.

A Novel, Inexpensive In-House Immunochromatographic Strip Test for Cryptococcosis Based on the Cryptococcal Glucuronoxylomannan Specific Monoclonal Antibody 18B7

The aim of this study was to develop a novel lateral flow immunochromatoghaphic strip test (ICT) for detecting cryptococcal polysaccharide capsular antigens using only a single specific monoclonal antibody, mAb 18B7. The mAb 18B7 is a well characterized antibody that specifically binds repeating epitopes displayed on the cryptococcal polysaccharide glucuronoxylomannan (GXM). We validated the immunoreactivities of mAb 18B7 against capsular antigens of different cryptococcal serotypes. The mAb 18B7 ICT was constructed as a sandwich ICT strip and the antibody serving in the mobile phase (colloidal gold conjugated mAb 18B7) to bind one of the GXM epitopes while the stationary phase antibody (immobilized mAb18B7 on test line) binding to other remaining unoccupied epitopes to generate a positive visual readout. The lower limit of detection of capsular antigens for each of the Cryptococcus serotypes tested was 0.63 ng/mL. No cross-reaction was found against a panel of antigens isolated from cultures of other pathogenic fungal, except the crude antigen of Trichosporon sp. with the lower limit of detection of 500 ng/mL (~800 times higher than that for cryptococcal GXM). The performance of the mAb 18B7 ICT strip was studied using cerebrospinal fluid (CSF) and serum and compared to commercial diagnostic kits (latex agglutination CALAS and CrAg IMMY). The sensitivity, specificity and accuracy of the mAb18B7 ICT with CSF from patients with confirmed cryptococcal meningitis were 92.86%, 100% and 96.23%, respectively. No false positives were observed with samples from non-cryptococcosis patients. With serum samples, the mAb 18B7 ICT gave a sensitivity, specificity and accuracy of 96.15%, 97.78% and 96.91%, respectively. Our results show that the mAb 18B7 based ICT was reliable, reproducible, and cost-effective as a point-of-care immunodiagnostic test for cryptococcosis. The mAb 18B7 ICT may be particularly useful in countries where commercial kits are not available or affordable.

Evaluation of a Cryptococcal Antigen Lateral Flow Assay and Cryptococcal Antigen Positivity at a Large Public Hospital in Atlanta, Georgia

BACKGROUND

Cryptococcus neoformans is a major cause of morbidity and mortality among human immunodeficiency virus (HIV)-infected persons worldwide, and there are scarce recent data on cryptococcal antigen (CrAg) positivity in the United States We sought to determine the frequency of cryptococcal disease and compare the performance of a CrAg lateral flow assay (LFA) versus latex agglutination (LA) test.

METHODS

All patients from Grady Health System in Atlanta who had a serum or cerebrospinal fluid (CSF) sample sent for CrAg testing as part of clinical care from November 2017 to July 2018 were included. Percentage positivity and test agreement were calculated.

RESULTS

Among 467 patients, 557 diagnostic tests were performed; 413 on serum and 144 on CSF. The mean age was 44 years, and most were male (69%) and had HIV (79%). Twenty-four (6.4%, 95% confidence interval [CI] = 4.1-9.4) patients were serum CrAg positive, and 8 (5.8%, 95% CI = 2.6-11.2) individuals tested positive for CSF CrAg. Although overall agreement between the LA and LFA was substantial to high for CSF (κ = 0.71, 95% CI = 0.51-0.91) and serum (κ = 0.93, 95% CI = 0.86-1.00), respectively, there were important discrepancies. Five patients had false-positive CSF LA tests that affected clinical care, and 4 patients had discordant serum tests.

CONCLUSIONS

We found a moderately high proportion of cryptococcal disease and important discrepancies between the LA test and LFA. Clinical implications of these findings include accurate detection of serum CrAg and averting unnecessary treatment of meningitis with costly medications associated with high rates of adverse events.

Exopolysaccharide from Cryptococcus heimaeyensis S20 induces autophagic cell death in non-small cell lung cancer cells via ROS/p38 and ROS/ERK signalling

OBJECTIVES

Cryptococcus heimaeyensis S20 is found in Antarctica and can produce exopolysaccharides (CHEPS). Here, we explore the anti-tumour effects of CHEPS on non-small cell lung cancer (NSCLC).

MATERIALS AND METHODS

Cell viability was assessed by CCK8 and colony formation assays. Flow cytometry was used to analyse the cell cycle, cell apoptosis and reactive oxygen species (ROS). Cell autophagy was detected by EGFP-LC3 puncta assay, Lyso-Tracker Red staining and transmission electron microscopy. mRNA and protein levels were analysed by qRT-PCR and Western blot. Related mechanisms were confirmed using appropriate inhibitors or shRNA. In vitro results were further confirmed by a tumour xenograft study.

RESULTS

CHEPS inhibited the proliferation of NSCLC cells by inducing S- and G2/M-phase arrest and autophagic cell death, but not apoptosis. CHEPS was less toxic to normal human embryonic lung fibroblasts. CHEPS activated the MAPK pathway in NSCLC cells, and p38 and ERK promoted CHEPS-induced cell death. Further studies showed that p38 and ERK promoted CHEPS-induced NSCLC cell autophagy and ERK promoted CHEPS-induced S- and G2/M-phase arrest. ROS were induced by CHEPS. A ROS scavenger attenuated CHEPS-induced p38 and ERK activation, autophagy and cell death. Finally, CHEPS reduced orthotopic lung tumour growth without organ-related toxicity. CHEPS also induced ROS, activated p38 and ERK, and triggered autophagy in vivo.

CONCLUSIONS

CHEPS induces autophagic cell death and S- and G2/M-phase arrest in NSCLC cells via ROS/p38 and ROS/ERK signalling.

β-1,6-linked Galactofuranose- rich peptidogalactomannan of Fusarium oxysporum is important in the activation of macrophage mechanisms and as a potential diagnostic antigen

A peptidogalactomannan (PGM) from Fusarium oxysporum was structurally characterized by a combination of chemical and spectroscopic methods, including one and two-dimensional nuclear magnetic resonance (1D and 2D NMR). The galactomannan component consists of a main chain containing (1→6)-linked β-D-galactofuranose residues with side chains containing (1→2)-linked α-D-Glcp, (1→2)-linked -β-D-Manp (1→2) and β-D-Manp terminal nonreducing end units and differs from that of Aspergillus fumigatus and Cladosporium resinae that present a main chain containing (1→6)-linked α-D-Manp residues presenting β-D-Galf as side chains of 3-4 units that are (1→5)-interlinked. The importance of the carbohydrate moiety of the F. oxysporum PGM was demonstrated. Periodate oxidation abolished much of the PGM antigenic activity. A strong decrease in reactivity was also observed with de-O-glycosylated PGM. In addition, de-O-glycosylated PGM was not able to inhibit F. oxysporum phagocytosis, suggesting that macrophages recognize and internalize F. oxysporum via PGM. F. oxysporum PGM triggered TNF-α release by macrophages. Chemical removal of O-linked oligosaccharides from PGM led to a significant increase of TNF-α cytokine levels, suggesting that their removal could exposure another PGM motifs able to induce a higher secretion of TNF-α levels. Interestingly, F. oxysporum conidia, intact and de-O-linked PGM were not able to induce IL-10 cytokine release. The difference in patient serum reativity using a PGM from F. oxysporum characterized in the present study as compared with a PGM from C. resinae, that presents the same epitopes recognized by serum from patients with aspergillosis, could be considered a potential diagnostic antigen and should be tested with more sera.

A Glucuronoxylomannan Epitope Exhibits Serotype-Specific Accessibility and Redistributes towards the Capsule Surface during Titanization of the Fungal Pathogen Cryptococcus neoformans

Disseminated infections with the fungal species Cryptococcus neoformans or, less frequently, Cryptococcus gattii are an important cause of mortality in immunocompromised individuals. Central to the virulence of both species is an elaborate polysaccharide capsule that consists predominantly of glucuronoxylomannan (GXM). Due to its abundance, GXM is an ideal target for host antibodies, and several monoclonal antibodies (mAbs) have previously been derived using purified GXM or whole capsular preparations as antigens. In addition to their application in the diagnosis of cryptococcosis, anti-GXM mAbs are invaluable tools for studying capsule structure. In this study, we report the production and characterization of a novel anti-GXM mAb, Crp127, that unexpectedly reveals a role for GXM remodeling during the process of fungal titanization. We show that Crp127 recognizes a GXM epitope in an O-acetylation-dependent, but xylosylation-independent, manner. The epitope is differentially expressed by the four main serotypes of Cryptococcus neoformans and C. gattii, is heterogeneously expressed within clonal populations of C. gattii serotype B strains, and is typically confined to the central region of the enlarged capsule. Uniquely, however, this epitope redistributes to the capsular surface in titan cells, a recently characterized morphotype where haploid 5-μm cells convert to highly polyploid cells of >10 μm with distinct but poorly understood capsular characteristics. Titan cells are produced in the host lung and critical for successful infection. Crp127 therefore advances our understanding of cryptococcal morphological change and may hold significant potential as a tool to differentially identify cryptococcal strains and subtypes.

Cryptococcus neoformans: Diagnostic Dilemmas, Electron Microscopy and Capsular Variants

Two cases of cryptococcal meningitis went undetected by a cryptococcal antigen (CrAg) lateral flow assay on blood in a reflex CrAg screen-and-treat programme in South Africa, although Cryptococcus neoformans was identified by culturing the cerebrospinal fluid specimens. Further investigations into these discordant diagnostic results included multilocus sequence typing (which showed no mutations in the CAP59 gene) and transmission electron microscopy using a capsule-staining protocol (which revealed a >50% reduction in capsular material in both cases, relative to a control culture). A multi-disciplinary approach for resolving discordant diagnostic test results is recommended.

Brief Report: Point of Care Cryptococcal Antigen Screening: Pipetting Finger-Prick Blood Improves Performance of Immunomycologics Lateral Flow Assay

BACKGROUND

Cryptococcal antigen (CrAg) screening at the point of care could improve cryptococcal meningitis prevention where laboratory resources are limited. We evaluated the accuracy of Immunomycologics (IMMY, Norman, OK) CrAg lateral flow assay (LFA) using different techniques at point of care.

SETTING

Two tertiary-level hospitals in Johannesburg and a community health clinic in Soweto, South Africa.

METHODS

A case-control diagnostic validation study and a prospective clinic-based implementation study using the IMMY CrAg LFA on finger-prick blood. Accuracy, using direct application of LFA to sample, or pipette to transfer sample to diluent, and reading after 10 and 20 minutes, was compared with laboratory-based plasma testing.

RESULTS

The validation study tested 64 CrAg-positive and 152 CrAg-negative patients with no symptoms or signs of meningitis, identified by routine laboratory screening, recruited by convenience sampling. Consecutively diagnosed HIV-infected adults (n = 654) were included in the implementation study. Sensitivity was 82% and 20% when the LFA was read 10 minutes after direct application to finger-prick blood in the validation and implementation studies, respectively. Using a pipette to transfer blood and reading after 20 minutes improved sensitivity to 100%, while retaining 100% specificity, in both studies.

CONCLUSIONS

Although the IMMY CrAg LFA performs well when applied directly to finger-prick blood for diagnosing cryptococcal meningitis, this technique may not provide adequate volume to detect low concentrations of CrAg when screening asymptomatic patients. Using a pipette to transfer larger volumes of blood to diluent before CrAg LFA testing and reading results after 20 minutes is a more reliable point-of-care method.

UDP-Glucuronic Acid Transport Is Required for Virulence of Cryptococcus neoformans

Glycans play diverse biological roles, ranging from structural and regulatory functions to mediating cellular interactions. For pathogens, they are also often required for virulence and survival in the host. In Cryptococcus neoformans, an opportunistic pathogen of humans, the acidic monosaccharide glucuronic acid (GlcA) is a critical component of multiple essential glycoconjugates. One of these glycoconjugates is the polysaccharide capsule, a major virulence factor that enables this yeast to modulate the host immune response and resist antimicrobial defenses. This allows cryptococci to colonize the lung and brain, leading to hundreds of thousands of deaths each year worldwide. Synthesis of most glycans, including capsule polysaccharides, occurs in the secretory pathway. However, the activated precursors for this process, nucleotide sugars, are made primarily in the cytosol. This topological problem is resolved by the action of nucleotide sugar transporters (NSTs). We discovered that Uut1 is the sole UDP-GlcA transporter in C. neoformans and is unique among NSTs for its narrow substrate range and high affinity for UDP-GlcA. Mutant cells with UUT1 deleted lack capsule polysaccharides and are highly sensitive to environmental stress. As a result, the deletion mutant is internalized and cleared by phagocytes more readily than wild-type cells are and is completely avirulent in mice. These findings expand our understanding of the requirements for capsule synthesis and cryptococcal virulence and elucidate a critical protein family.IMPORTANCECryptococcus neoformans causes lethal meningitis in almost two hundred thousand immunocompromised patients each year. Much of this fungal pathogen's ability to resist host defenses and cause disease is mediated by carbohydrate structures, including a complex polysaccharide capsule around the cell. Like most eukaryotic glycoconjugates, capsule polysaccharides are made within the secretory pathway, although their precursors are generated in the cytosol. Specific transporters are therefore required to convey these raw materials to the site of synthesis. One precursor of particular interest is UDP-glucuronic acid, which donates glucuronic acid to growing capsule polysaccharides. We discovered a highly specific, high-affinity transporter for this molecule. Deletion of the gene encoding this unusual protein abolishes capsule synthesis, alters stress resistance, and eliminates fungal virulence. In this work, we have identified a novel transporter, elucidated capsule synthesis and thereby aspects of fungal pathogenesis, and opened directions for potential antifungal therapy.

Xylose donor transport is critical for fungal virulence

Cryptococcus neoformans, an AIDS-defining opportunistic pathogen, is the leading cause of fungal meningitis worldwide and is responsible for hundreds of thousands of deaths annually. Cryptococcal glycans are required for fungal survival in the host and for pathogenesis. Most glycans are made in the secretory pathway, although the activated precursors for their synthesis, nucleotide sugars, are made primarily in the cytosol. Nucleotide sugar transporters are membrane proteins that solve this topological problem, by exchanging nucleotide sugars for the corresponding nucleoside phosphates. The major virulence factor of C. neoformans is an anti-phagocytic polysaccharide capsule that is displayed on the cell surface; capsule polysaccharides are also shed from the cell and impede the host immune response. Xylose, a neutral monosaccharide that is absent from model yeast, is a significant capsule component. Here we show that Uxt1 and Uxt2 are both transporters specific for the xylose donor, UDP-xylose, although they exhibit distinct subcellular localization, expression patterns, and kinetic parameters. Both proteins also transport the galactofuranose donor, UDP-galactofuranose. We further show that Uxt1 and Uxt2 are required for xylose incorporation into capsule and protein; they are also necessary for C. neoformans to cause disease in mice, although surprisingly not for fungal viability in the context of infection. These findings provide a starting point for deciphering the substrate specificity of an important class of transporters, elucidate a synthetic pathway that may be productively targeted for therapy, and contribute to our understanding of fundamental glycobiology.

Laboratory Diagnostics for Fungal Infections: A Review of Current and Future Diagnostic Assays

This article reviews the current diagnostic approaches, both serologic and molecular, for the detection of fungi associated with pulmonary disease. Classic serologic techniques, including immunodiffusion and complement fixation, both of which remain a cornerstone for fungal diagnostic testing, are reviewed and their performance characteristics presented. More recent advances in this field, including novel lateral-flow assays for fungal antigen detection, are also described. Molecular techniques for fungal identification both from culture and directly from patient specimens, including nucleic acid probes, mass spectrometry-based methods, nucleic acid amplification testing, and traditional and broad-range sequencing, are discussed and their performance evaluated.

Nonculture Diagnostics in Fungal Disease

Fungal diagnostics that utilize antibody, antigen or nucleic acid detection offer several advantages that supplement traditional culture-based methods. As a group, nonculture assays can help identify patients with invasive fungal infection (IFI) sooner than is possible with culture, are often more sensitive, and can be used to guide early interventions. Challenges associated with these techniques include the possibility for contamination or cross-reactivity as well as the potential for false negative tests. This review summarized the test characteristics and clinical utility of nonculture-based laboratory methods.

Equine Pulmonary Cryptococcosis: A Comparative Literature Review and Evaluation of Fluconazole Monotherapy

Cryptococcus is the most common fungal respiratory pathogen in Australian horses, manifesting primarily as pulmonary granulomas. Disease severity at presentation is dependent on the athletic use of the horse. The diagnosis and estimation of disease severity are centred around clinical findings, cytological evaluation of respiratory tract secretions, diagnostic imaging, and antigen titre testing. Both the lateral flow assay and the latex cryptococcal antigen titre are used, and important similarities and differences between species are discussed. Cryptococcus gattii occurs with greater frequency than Cryptococcus neoformans in equine pulmonic cryptococcosis and can be successfully treated with enteral fluconazole monotherapy, with disease severity determining treatment length.

Pitfalls in Serological Diagnosis of Cryptococcus gattii Infections

The detection of cryptococcal antigen by latex agglutination tests (LATs), enzyme-linked immunoassays (ELISA), or lateral flow assay (LFA) is an important tool for diagnosis of a Cryptococcus infection. Cerebrospinal fluid and/or serum samples of 10 patients with cryptococcosis due to Cryptococcus gattii or a hybrid of Cryptococcus neoformans and C. gattii were examined by three LATs (the IMMY Latex-Crypto(®) test, the Pastorex(TM) Crypto Plus, and the Remel Cryptococcus Antigen Test Kit) and the LFA made by Immuno-Mycologics. LATs based on monoclonal antibodies (mAbs) like the Pastorex(TM) Crypto Plus or the Remel Cryptococcus Antigen Test Kit turned out to have an insufficient sensitivity to detect four out of 10 C. gattii infections, including one infection by a hybrid between C. gattii and C. neoformans. Reflecting the ongoing expansion of C. gattii in geographical zones outside of tropical and subtropical areas like Mediterranean countries, Vancouver Island (British Columbia, Canada) and the Pacific Northwest region (USA), these findings are alarming because of the risk of delayed diagnosis of infections caused by C. gattii. Therefore, the preliminary serological screening for cryptococcal antigen in the case of a suspected Cryptococcus infection should be performed by using an assay with a broad range specificity and sensitivity for C. neoformans and C. gattii, including their hybrids.

The cryptococcal antigen lateral flow assay: A point-of-care diagnostic at an opportune time

Cryptococcal meningitis is a devastating HIV-related opportunistic infection, affecting nearly 1 million individuals and causing over 500 000 deaths each year. The burden of disease is greatest in sub-Saharan Africa and Southeast Asia, where cryptococcal disease is the most common cause of meningitis. Rapid, accurate and affordable diagnosis of cryptococcal disease has been lacking in many of the most heavily affected areas. Here, we review a point-of-care assay for cryptococcal disease, the dipstick-formatted cryptococcal antigen lateral flow assay (LFA) (IMMY, Norman, OK). In comparison to culture, the assay is 99.5% sensitive and 98% specific. In comparison to other commercially available tests for cryptococcal antigen, the LFA has equal or superior sensitivity and specificity in CSF, plasma and serum samples. We discuss potential applications for the use of the assay in resource-limited settings, including what is likely to be an important role of the LFA in screening for early cryptococcal infection before clinical disease and in evaluating pre-emptive treatment.

Case report: false negative serum cryptococcal latex agglutination test in a patient with disseminated cryptococcal disease

A case of false-negative serum latex agglutination cryptococcal antigen (CRAG) test in a 45-year-old HIV-positive male with Cryptococcus-positive culture is described. The patient was presented to a hospital in Botswana, with breathlessness and a diffuse papular rash. His CD4 count was 25 cells/μL. Despite the suspicion for disseminated cryptococcal disease, an initial serum CRAG latex test was negative. Results of subsequent Indian ink staining, culture of cerebrospinal fluid and skin scrapings, and serum lateral flow immunoassay (LFA) were all positive for Cryptococcus neoformans. There are several possible explanations for the false-negative CRAG latex test. Given the positive LFA result, we speculate that disease may have been caused by Cryptococcus gattii, which is estimated to be responsible for between 15% and 30% of all cryptococcal diseases in Botswana. Reduced sensitivity of CRAG latex assays for detecting C gattii may lead to underdiagnosis of cryptococcal infection.

Characterization of the antigenicity of Cpl1, a surface protein of Cryptococcus neoformans var. neoformans

Cryptococcus neoformans var. neoformans is an important fungal pathogen. The capsule is a well established virulence factor and a target site for diagnostic tests. The CPL1 gene is required for capsular formation and virulence. The protein product Cpl1 has been proposed to be a secreted protein, but the characteristics of this protein have not been reported. Here we sought to characterize Cpl1. Phylogenetic analysis showed that the Cpl1 of C. neoformans var. neoformans and the Cpl1 orthologs identified in C. neoformans var. grubii and C. gattii formed a distinct cluster among related fungi; while the putative ortholog found in Trichosporon asahii was distantly related to the Cryptococcus cluster. We expressed Cpl1 abundantly as a secreted His-tagged protein in Pichia pastoris. The protein was used to immunize guinea pigs and rabbits for high titer mono-specific polyclonal antibody that was shown to be highly specific against the cell wall of C. neoformans var. neoformans and did not cross react with C. gattii, T. asahii, Aspergillus spp., Candida spp. and Penicillium spp. Using the anti-Cpl1 antibody, we detected Cpl1 protein in the fresh culture supernatant of C. neoformans var. neoformans and we showed by immunostaining that the Cpl1 protein was located on the surface. The Cpl1 protein is a specific surface protein of C. neoformans var. neoformans.

Immunodetection of fungal and oomycete pathogens: established and emerging threats to human health, animal welfare and global food security

Filamentous fungi (moulds), yeast-like fungi, and oomycetes cause life-threatening infections of humans and animals and are a major constraint to global food security, constituting a significant economic burden to both agriculture and medicine. As well as causing localized or systemic infections, certain species are potent producers of allergens and toxins that exacerbate respiratory diseases or cause cancer and organ damage. We review the pathogenic and toxigenic organisms that are etiologic agents of both animal and plant diseases or that have recently emerged as serious pathogens of immunocompromised individuals. The use of hybridoma and phage display technologies and their success in generating monoclonal antibodies for the detection and control of fungal and oomycete pathogens are explored. Monoclonal antibodies hold enormous potential for the development of rapid and specific tests for the diagnosis of human mycoses, however, unlike plant pathology, their use in medical mycology remains to be fully exploited.

Serotype sensitivity of a lateral flow immunoassay for cryptococcal antigen

To meet the needs of a global community, an immunoassay for cryptococcal antigen (CrAg) must have high sensitivity for CrAg of all major serotypes. A new immunoassay for CrAg in lateral flow format was evaluated and found to have a high sensitivity for detection of serotypes A, B, C, and D.

Large-scale evaluation of the immuno-mycologics lateral flow and enzyme-linked immunoassays for detection of cryptococcal antigen in serum and cerebrospinal fluid

Cryptococcosis is a systemic infection caused by the pathogenic yeasts Cryptococcus neoformans and C. gattii. Detection of cryptococcal capsular antigen (CrAg) in serum and cerebrospinal fluid (CSF) plays an important diagnostic role. We prospectively compared the new Immuno-Mycologics Inc. (IMMY) lateral flow assay (LFA) and enzyme immunoassay (EIA) to our current CrAg test (Premier EIA; Meridian Bioscience Inc.). Discordant samples were retested with the latex-Cryptococcus antigen test (IMMY) and using serotype-specific monoclonal antibodies (MAbs). A total of 589 serum and 411 CSF specimens were tested in parallel. Qualitative agreement across assays was 97.7%. In all, 56 (41 serum and 15 CSF) samples were positive and 921 (527 serum and 394 CSF) samples were negative by all three assays. The 23 discrepant specimens were all Meridian EIA negative. Of 23 discordant specimens, 20 (87.0%) were positive by both the IMMY LFA and EIA, 2 were LFA positive only, and 1 was EIA positive only. Eleven discrepant specimens had adequate volume for latex agglutination (LA) testing; 8 were LA positive, and 3 were LA negative. LA-negative samples (2 CSF samples and 1 serum) had low IMMY LFA/EIA titers (≤1:10). Serotype-specific MAb analysis of the LA-positive samples suggested that these specimens contained CrAg epitopes similar to those of serotype C strains. In conclusion, the IMMY assays showed excellent overall concordance with the Meridian EIA. Assay performance differences were related to issues of analytic sensitivity and possible serotype bias. Incomplete access to patient-level data combined with low specimen volumes limited our ability to fully resolve discrepant results.

Comparison of four assays for the detection of cryptococcal antigen

We compared the performance of four assays for the detection of cryptococcal antigen in serum samples (n = 634) and cerebrospinal fluid (CSF) samples (n = 51). Compared to latex agglutination, the sensitivity and specificity of the Premier enzyme immunoassay (EIA), Alpha CrAg EIA, and CrAg lateral flow assay (LFA) were 55.6 and 100%, 100 and 99.7%, and 100 and 99.8%, respectively, from serum samples. There was 100% agreement among the four tests for CSF samples, with 18 samples testing positive by each of the assays.

CrAg lateral flow assay for cryptococcosis

Importance of field: Cryptococcal meningitis is a leading cause of death globally among people with AIDS. In sub-Saharan Africa, cryptococcosis is estimated to kill more people than tuberculosis. Cryptococcosis is also an important infectious disease among immunosuppressed patients in countries with advanced medical care. Early diagnosis is the key to effective treatment, particularly in patients in resource-limited settings. A new lateral flow immunoassay (LFA) for cryptococcal antigen (CrAg) allows for rapid and inexpensive diagnosis of cryptococcosis at or near the point of patient contact.

AREAS COVERED

This article reviews the need for improved diagnostics for cryptococcal meningitis and describes the features of an ideal diagnostic. The design of a new LFA for CrAg is described as well as the results of initial clinical evaluation of the CrAg LFA.

EXPERT OPINION

The CrAg LFA is recommended for use with serum, plasma or CSF for diagnosis of cryptococcal meningitis or non-meningeal cryptococcal disease in symptomatic patients. There is a need for further evaluation of LFA for screening of asymptomatic patients. However, the LFA is emerging as a valuable tool for screening of serum or plasma in ART-naive adults with CD4 counts less than 100 cells/mm(3) in geographic regions with a high prevalence of cryptococcal antigenemia. CrAg screening has the potential to identify patients with asymptomatic cryptococcal infection who should receive preemptive antifungal therapy.

Evaluation of a novel point-of-care cryptococcal antigen test on serum, plasma, and urine from patients with HIV-associated cryptococcal meningitis

BACKGROUND

Many deaths from cryptococcal meningitis (CM) may be preventable through early diagnosis and treatment. An inexpensive point-of-care (POC) assay for use with urine or a drop of blood would facilitate early diagnosis of cryptococcal infection in resource-limited settings. We compared cryptococcal antigen (CRAG) concentrations in plasma, serum, and urine from patients with CM, using an antigen-capture assay for glucuronoxylomannan (GXM) and a novel POC dipstick test.

METHODS

GXM concentrations were determined in paired serum, plasma, and urine from 62 patients with active or recent CM, using a quantitative sandwich enzyme-linked immunosorbent assay (ELISA). A dipstick lateral-flow assay developed using the same monoclonal antibodies for the sandwich ELISA was tested in parallel. Correlation coefficients were calculated using Spearman rank test.

RESULTS

All patients had detectable GXM in serum, plasma, and urine using the quantitative ELISA. Comparison of paired serum and plasma showed identical results. There were strong correlations between GXM levels in serum/urine (r(s) = 0.86; P < .001) and plasma/urine (r(s) = 0.85; P < .001). Levels of GXM were 22-fold lower in urine than in serum/plasma. The dipstick test was positive in serum, plasma, and urine in 61 of 62 patients. Dipstick titers correlated strongly with ELISA. Correlations between the methods were 0.93 (P < .001) for serum, 0.94 (P < .001) for plasma, and 0.94 (P < .001) for urine.

CONCLUSIONS

This novel dipstick test has the potential to markedly improve early diagnosis of CM in many settings, enabling testing of urine in patients presenting to health care facilities in which lumbar puncture, or even blood sampling, is not feasible.