Characterization of antigens from Aspergillus fumigatus. V. Reactivity in immunodiffusion tests with serums from patients with aspergillosis caused by Aspergillus flavus, A. niger, and A fumigatus

Evaluation of LDBio Aspergillus ICT Lateral Flow Assay for IgG and IgM Antibody Detection in Chronic Pulmonary Aspergillosis

Detecting Aspergillus-specific IgG is critical to diagnosing chronic pulmonary aspergillosis (CPA). Existing assays are often cost- and resource-intensive and not compatible with resource-constrained laboratory settings. LDBio Diagnostics has recently commercialized a lateral flow assay based on immunochromatographic technology (ICT) that detects Aspergillus antibodies (IgG and IgM) in less than 30 min, requiring minimal laboratory equipment. A total of 154 CPA patient sera collected at the National Aspergillosis Centre (Manchester, United Kingdom) and control patient sera from the Peninsula Research Bank (Exeter, United Kingdom) were evaluated. Samples were applied to the LDBio Aspergillus ICT lateral flow assay, and results were read both visually and digitally. Results were compared with Aspergillus IgG titers in CPA patients, measured by ImmunoCAP-specific IgG assays. For proven CPA patients versus controls, sensitivity and specificity for the LDBio Aspergillus ICT were 91.6% and 98.0%, respectively. In contrast, the routinely used ImmunoCAP assay exhibited 80.5% sensitivity for the same cohort (cutoff value, 40 mg of antigen-specific antibodies [mgA]/liter). The assay is easy to perform but challenging to read when only a very faint band is present (5/154 samples tested). The ImmunoCAP Aspergillus IgG titer was also compared with the Aspergillus ICT test line intensity or rate of development, with weak to moderate correlations. The Aspergillus ICT lateral flow assay exhibits excellent sensitivity for serological diagnosis of CPA. Quantifying IgG from test line intensity measurements is not reliable. Given the short run time, simplicity, and limited resources needed, the LDBio Aspergillus ICT is a suitable diagnostic tool for CPA in resource-constrained settings.

Role of Serological Tests in the Diagnosis of Mold Infections

Purpose of Review

To understand the role of antibody detection in the diagnosis of infections caused by filamentous fungi (molds). Rapid and accurate profiling of infection-causing fungal pathogens remains a significant challenge in modern health care. Classical fungal culture and serology continue to be relevant even though over the past few decades, antigen (biomarker) assays such as ELISA and lateral flow devices have been developed and validated.

Recent Findings

This article reviews the current antibody detection systems (serological tests) for the diagnosis of mold infections associated with pulmonary disease and introduces new developments. Classic and more recently developed serological techniques and their performance characteristics, including immunodiffusion, complement fixation, and ELISA.

Summary

The diseases covered are allergic bronchopulmonary aspergillosis, chronic pulmonary aspergillosis, invasive aspergillosis, mucormycosis, diseases caused by filamentous basidiomycetes, infection caused by Talaromyces marneffei and pythiosis. Serology remains a cornerstone for fungal diagnostic testing.

Immunodiagnosis of aspergillosis

Aspergillus species are ubiquitous fungi and have been implicated as the causative agents of a variety of lung disorders in humans. These disorders include allergic, saprophytic, and systemic manifestations. The allergic disorders mainly affect atopic persons, and invasive or systemic diseases affect immunosuppressed individuals. Immunodiagnosis can help the practitioner diagnose these diseases. Demonstration of circulating antibodies is a useful criterion, but the lack of dependable and standardized antigens is a limiting factor in the diagnosis of most Aspergillus-induced diseases. Despite this limitation, however, immunodiffusion and enzyme-linked immunosorbent assays have been widely used for the detection of antibodies in the sera of patients with aspergillosis. Similarly, crude and semipurified antigens are being used to demonstrate skin hypersensitivity in patients, and several methods have been useful in the detection of antigenemia in patients with invasive aspergillosis. With a growing number of reports on the incidence of aspergillosis and an increase in the number of immunosuppressed individuals in the population, more rapid methods and more reliable reagents for immunodiagnosis are needed. With recent attempts at obtaining reliable reagents for through hybridoma technology and molecular biological techniques, substantial progress toward efficient immunodiagnosis may be achieved.

Serology of coccidioidomycosis

Serologic tests have assisted in the diagnosis and prognosis of coccidioidomycosis for a half-century. The causative agent, Coccidioides immitis, is a dimorphic fungus existing in a hyphal form with arthroconidia in nature and in the usual culture. The arthroconidia represent the inhaled infective forms which in vivo and under special laboratory conditions form spherules which endosporulate. The culture filtrate/autolysate (coccidioidin) from the hyphal phase has provided antigens of suitable reliability for currently used serologic tests. These tests are primarily to determine the two major antibody responses: the early immunoglobulin M (IgM) response is useful in the diagnosis of acute primary coccidioidomycosis. Later, IgG is produced and usually outlasts the IgM, persisting in chronic coccidioidomycosis. The IgM is detectable by tube precipitin, a corresponding immunodiffusion, or latex particle agglutination tests. The pertinent antigen(s) is heat stable and pronase resistant and appears to be largely carbohydrate, mainly mannose with some 3-O-methyl mannose. The IgG detectable in the serum and other body fluids by complement fixation and a corresponding immuno-diffusion is useful in diagnosis, and its quantitation provides an indicator of progression of disease (increasing titer) or regression (decreasing titer). The pertinent antigen appears to be a heat-labile, pronase-sensitive protein which in an unreduced form has a molecular weight of 110,000. A third very useful serologic procedure is the exoantigen test for identification of putative cultures of C. immitis.

Comparison of immunodiffusion and enzyme linked immunosorbent assay for antibodies to four Aspergillus species

Antigenic extracts were prepared from Aspergillus fumigatus, A niger, A flavus and A terreus for use in enzyme linked immunosorbent assay (ELISA) and immunodiffusion (ID) tests for Aspergillus antibodies to determine whether the use of antigenic extracts from species other than A fumigatus increased the sensitivity of the ELISA. ELISA titres correlated well with positive ID tests. Patient titres by ELISA were significantly higher than control titres for all species. Patient titres to A niger were also significantly higher than titres to the other species. Total number of ID bands to A fumigatus correlated significantly with anti-A fumigatus ELISA titres. It is concluded that the use of antigenic extracts from species other than A fumigatus improves the sensitivity of the ELISA.