Evaluation of enzyme immunoassay as a rapid screening test for histoplasmosis and blastomycosis

Blastomycosis: new insights into diagnosis, prevention, and treatment

The basic science and clinical understanding of infection with Blastomyces dermatitidis has been a field of constant evolution and continued revision of hypotheses. This article highlights some areas in which recent progress has the potential for significant impact on the clinical care of patients. Specifically, this article examines the application of modern technology to epidemiologic studies, the development of novel vaccine candidates, emerging populations at risk for the disease, rapid diagnostic tools, and the application of novel antifungal agents in the treatment of blastomycosis.

Pulmonary blastomycosis: an appraisal of diagnostic techniques

OBJECTIVES

Pulmonary blastomycosis often mimics bacterial pneumonia or bronchogenic carcinoma, which may result in delayed therapy or the performance of unnecessary diagnostic procedures. We have reviewed the utilization of diagnostic techniques in the workup of patients with pulmonary blastomycosis, defined their diagnostic yields, and proposed an optimal diagnostic approach for the patient in whom pulmonary blastomycosis is considered.

DESIGN

Retrospective chart review of all patients with the diagnosis of blastomycosis at a major academic medical center.

RESULTS

Of the 119 patients with blastomycosis, 56 (47%) had pulmonary involvement. A total of 92 specimens were obtained by noninvasive means (sputa, 72 specimens; tracheal secretions, 5 specimens; and gastric washings, 15 specimens) in 35 patients. KOH smears were prepared from 22 of those specimens (24%). The diagnostic yield from these culture specimens obtained by noninvasive means was 86% per patient, and 75% per single sample. The diagnostic yields from KOH smears were 46% and 36%, respectively. Flexible bronchoscopy was performed in 24 patients and yielded a diagnosis in 22 (92%). Cultures of bronchial secretions (19 patients) and BAL fluid (6 patients) were positive in 100% and 67% of patients, respectively. The corresponding yields of KOH preparations were 17% (1 of 6 preparations) and 50% (3 of 6 preparations), respectively. Pathology specimens including those from bronchoscopic lung biopsies (nine patients), bronchial brushings (two patients), and bronchoscopic needle aspiration (one patient) were positive in 22%, 50%, and 0% of cases, respectively. Cytology was usually performed to exclude malignancy and was positive for Blastomyces dermatitidis in five patients (sputum, three patients; bronchial washings, two patients). Thoracotomy was performed in 11 cases, and in all patients the procedure yielded a diagnosis. Serology results were available in 25 patients. Immunodiffusion was positive in 10 patients (40%), and complement fixation in 4 patients (16%).

CONCLUSIONS

In patients with pulmonary blastomycosis, the positive yield from respiratory specimen cultures is high, but the confirmation of a diagnosis may take up to 5 weeks. Wet smears and cytology examinations of respiratory specimens provide quicker diagnoses but are underutilized. Their routine use is recommended in endemic areas. Commonly used serologic assays are insensitive and are not useful for diagnostic screening.

Immunological and chemical characterization of glycoproteins in IEF fractions of Blastomyces dermatitidis yeast lysate antigen

After isoelectric focusing (IEF), fractions of a Blastomyces dermatitidis yeast lysate antigen were analysed for the presence of glycoproteins that may lead to cross-reactivity in immunoassays for the diagnosis of blastomycosis. Five major glycoproteins were apparent, two of which showed cross-reactivity when used in Western blots with sera obtained from dogs with histoplasmosis and coccidioidomycosis. These five glycoproteins were characterized for linkage to the proteins using N-glycosidase F (NGF) and for their lectin binding properties. The cross-reactive 235- and 160-kDa glycoproteins were found to possess mainly O-linked, high-mannose-type carbohydrates, and periodate-mediated oxidation of these molecules eliminated cross-reactivity observed with heterologous sera. Thus, the periodate-treated IEF antigens described here may be useful in solid-phase enzyme immunoassays for the diagnosis of blastomycosis.

Molecular cloning and characterization of a recombinant Histoplasma capsulatum antigen for antibody-based diagnosis of human histoplasmosis

Immunological cross-reactivity among fungi has hampered the development of specific serodiagnostic assays for histoplasmosis. We report the molecular cloning and characterization of a Histoplasma capsulatum cDNA (GH17) that encodes an antigen with immunodiagnostic potential. GH17 is an 810-bp cDNA which encodes a protein of 211 amino acid residues. The GH17 sequence has almost no significant homology with other sequences in GenBank. Southern blot analysis suggests that GH17 is confined to a single location in the genomic DNA of H. capsulatum. Immunoblots indicated that the protein product of GH17 (expressed as a 140-kDa beta-galactosidase fusion protein) was recognized by antibodies in 18 of 18 sera from histoplasmosis patients, but not by antibodies in sera from patients or animals infected with other fungi. GH17 was expressed in a prokaryotic expression vector, pPROEX-1, and recombinant protein was purified by preparative electrophoresis. Antibodies raised to this protein bound to a 60-kDa native antigen in immunoblots of H. capsulatum yeast antigen extract. These results suggest that GH17 encodes an H. capsulatum antigen that may be useful for the diagnosis of histoplasmosis in humans.

Cloning and characterization of bys1, a temperature-dependent cDNA specific to the yeast phase of the pathogenic dimorphic fungus Blastomyces dermatitidis

The pathogenic dimorphic fungal organism Blastomyces dermatitidis exists as a budding yeast at 37 degrees C and as a mycelium at 25 degrees C. While the conversion of one morphological phase of B. dermatitidis to another has long been known to be a thermally dependent process, little of the accompanying biochemical or genetic events controlling the phase transition has been elucidated. Using differential cDNA library screening, we have identified one transcript, bys1, in B. dermatitidis that is expressed at very high levels in the yeast phase but whose levels diminish rapidly when yeast cells are transferred to 25 degrees C to promote conversion to the mycelial phase. Although the 0.95-kb bys1 transcript is absent in B. dermatitidis mycelia maintained at 25 degrees C, transfer of mycelial cultures to 37 degrees C results in the reappearance of bys1 within 12 h. bys1 codes for a protein of 18.6 kDa that contains multiple putative phosphorylation sites, a hydrophobic N terminus, and two 34-amino-acid domains with similarly spaced nine-amino-acid degenerative repeating motifs. Although the nature of the thermal dependency of bys1 expression and the function of the bys1 protein are unknown, the strong expression of this transcript specifically in the yeast phase of B. dermatitidis may prove to be very useful in the development of more specific and sensitive diagnostic methods for blastomycosis.

Relación entre la prueba intradermica de histoplasmina y los niveles de anticuerpos detectables por ELISA e immunodifusion

Se realizó un estudio prospectivo en 40 trabajadores de una granja avícola (grupo 1) considerados con riesgo de exposición a Histoplasma capsulatum, agente etiológico de la histoplasmosis, y en 16 individuos sin riesgo profesional de exposición a dicho agente (grupo 2). En ambos grupos se aplicó la prueba intradérmica de histoplasmina y se obtuvo el suero antes de su aplicación y a los 30 y 180 días después de realizada dicha prueba. Se determinó el nivel de anticuerpos anti-H. capsulatum mediante las técnicas de ELISA e inmunodifusión doble. En los dos grupos de población estudiados la aplicación intradérmica de histoplasmina, aún en los casos en que la respuesta fue positiva, no constituyó un estímulo antigénico suficiente para provocar un aumento en los niveles de anticuerpos anti-H. capsulation detectables por las técnicas serológicas empleadas. Los resultados obtenidos contribuyen a la mejor interpretación de la prueba de ELISA en el diagnóstico de la histoplasmosis.

Experimental histoplasmosis in the mouse. Immunoglobulin class response, total immunoglobulin A levels, immune complex formation, and occurrence of antigen in serum and urine

Biotin-avidin enzyme-linked immunosorbent assays (ELISA) were developed to study the appearance of free antibody of the IgM, G, and A classes of immune complexes (IC) containing those classes and of antigen in the serum and urine of mice with experimentally induced, disseminated histoplasmosis (histo) over a period ranging from 4 to 64 days after infection. Free IgM was detected 4 days after infection, remained at low levels, and disappeared on Day 64, whereas free IgG was first detected on Day 7 and rose to very high levels before declining on Day 64. Free IgA was detected only once, on Day 21. IC containing IgM were seen first on Day 4, remained at low levels, and became undetectable on Day 64. IC containing IgM were detected on Day 7, peaked on Day 14, and declined through Day 64. IC containing IgA wee seen at low levels on Days 7, 14, and 21. Estimation of total IgA levels, done by single radial immunodiffusion, showed a statistically significant decrease on Day 14, followed by a slightly significant increase on Days 21 and 30. Antigen was detected in as much as 80% of serum specimens and 100% of urine samples during the first 2 wk postinfection but rarely afterwards. We conclude that ELISA provides a highly sensitive way to study antibody, IC, and antigen in host body fluids during histo infection and that IgM and antigen detection can be very early indices of infection. Measurements of IC and total IgA seem to be of relatively less importance in detection of infection.

Serological techniques for diagnosis of fungal infection

This review summarizes recent developments in the serodiagnosis of candidiasis, aspergillosis, cryptococcosis, histoplasmosis, blastomycosis, coccidioidomycosis, mucormycosis and sporotrichosis. A number of studies have substantiated the presence of circulating antigens in invasive candidiasis, invasive aspergillosis, disseminated histoplasmosis and coccidioidomycosis, and immunoassays for antigen detection provide moderate sensitivity but high specificity for disease. Improved detection may result mainly from repeated serum or concentrated urine samplings rather than from the development of more sensitive immunoassays. Immunoblot analysis of the serological response is a useful tool for the identification of immunogenic fungal components that elicit a specific antibody response in invasive disease. This method, and others, have been successfully applied to the study of the immune response to several fungi, including Candida, Aspergillus and Rhizopus.

Pulmonary mycoses. New concepts and new therapy

The pathogenic and opportunistic fungi that commonly infect the human respiratory system cause diseases such as histoplasmosis, blastomycosis, aspergillosis, and invasive candidiasis. The increase in the incidence and severity of mycotic infections due to more aggressive immunosuppressive therapy and the spread of acquired immunodeficiency syndrome (AIDS) has had an impact on diagnosis and treatment of these conditions. Several therapeutic agents are currently available or undergoing clinical trials in the treatment of the pulmonary mycoses; among the agents with the greatest potential usefulness are the oral imidazoles, ketoconazole, itraconazole, and fluconazole.