Histoplasma antigens as novel players for the development of new enzyme immunoassays for the serodiagnosis of histoplasmosis: A comparative study of their analytical performance

Definitive diagnosis of histoplasmosis relies on culture and/or cytology/histopathology; however, these procedures have limited sensitivity and cultures are time-consuming. Antibodies detection by immunodiffusion has low sensitivity in immunocompromised individuals and uses histoplasmin (HMN), a crude antigenic extract, as reagent. Novel protein antigen candidates have been recently identified and produced by DNA-recombinant techniques to obtain standardized and specific reagents for diagnosing histoplasmosis. To compare the analytical performance of novel enzyme-linked immunosorbent assays (ELISAs) for antibodies testing for diagnosing histoplasmosis using different Histoplasma capsulatum antigens as reagents. The H. capsulatum 100 kDa protein (Hcp100), the M antigen and its immunoreactive fragment F1 were produced by DNA-recombinant techniques. Galactomannan was purified from both the yeast and mycelial cell walls (yGM and mGM, respectively). The analytical performance of the ELISA tests for the serological detection of antibodies against these antigens was evaluated and compared with those obtained using HMN as reagent. Antibodies detection by the Hcp100 ELISA demonstrated 90.0% sensitivity and 92.0% specificity, versus 43.3% sensitivity and 95.0% specificity of the M ELISA, 33.3% sensitivity and 84.0% specificity of the F1 ELISA, 96.7% sensitivity and 94.0% specificity of the yGM ELISA, 83.3% sensitivity and 88.0% specificity of the mGM ELISA, and 70.0% sensitivity and 86.0% specificity for the HMN ELISA. In summary, Hcp100 is proposed as the most promising candidate for the serodiagnosis of histoplasmosis. The primary immunoreactive element in HMN proved to be GM rather than the M antigen. Nevertheless, a higher incidence of cross-reactions was noted with GM compared to M.

Clinical Significance and Histologic Characterization of Histoplasma Granulomas

OBJECTIVES

To clarify the clinical significance and degree of resolution (ie, grade) of Histoplasma granulomas in routinely reviewed surgical pathology specimens and the clinical outcomes of patients with this diagnosis, with an emphasis on those not receiving antifungal therapy.

METHODS

We performed a retrospective medical record, laboratory data, and surgical pathology slide review of patients with Histoplasma granulomas following institutional review board approval.

RESULTS

Clinical, pathologic, and laboratory data from 62 patients with Histoplasma granulomas were available for review. Of these, 1 of 19 (5%) fungal cultures, 4 of 12 (33%) fungal serologic studies, 0 of 9 Histoplasma urinary antigen tests, and 0 of 2 Histoplasma serum antigen tests were positive. All but 3 of the Histoplasma granulomas were either in the resolving (grade 2) or resolved (grade 3) stage of resolution. None of the patients, including those who did not receive antifungal therapy after the histologic diagnosis, developed progressive or disseminated histoplasmosis.

CONCLUSIONS

These findings, which are supportive of clinical guidelines, suggest that patients with old, hyalinized Histoplasma granulomas do not benefit from further laboratory studies or antifungal therapy. The proposed grading of Histoplasma granulomas informs clinicians of the stage of resolution of an excised lesion, which informs therapeutic decisions and thus is recommended.