Use of CD42b immunohistochemical stain for the detection of Histoplasma.

In pathologic specimen, Histoplasma capsulatum can frequently be identified by morphology and special stains such as GMS and PAS. Incidentally, we noted unusual staining of the platelet associated marker CD42b/GP1b expressed on the surface of fungal organisms. Evaluation of additional cases demonstrated that a majority of histoplasmosis cases (15/18 cases; 83%) showed positive staining with CD42b/GP1b, comparable to GMS stain results. Other platelet associated markers such as Factor VIII and CD61 showed no or rare expression (1/18 cases with Factor VIII). Studies have shown that 14-3-3 proteins bind directly to cytoplasmic domain of CD42b/GP1b. Significant homology is seen between fungal and human 14-3-3 proteins which may represent a molecular basis for our observation. Our study demonstrated that CD42b/GP1b staining by immunohistochemistry can aid in detection of Histoplasma organisms. Further studies with organisms with similar morphologic features such as Blastomyces and Leishmania may demonstrate a diagnostic utility in speciating organisms.