A Survey of Lectin Reactivity to Coccidioides in Infected Human Lung Tissue

Background: Coccidioidomycosis (Valley Fever, VF) is a respiratory disease that occurs primarily in the southwestern United States, the central valley in California, and northern Mexico.  It is caused by the soil-dwelling fungi Coccidioides immitis and Coccidioides posadasii.  Symptoms may range from mild pneumonia to disseminated infection in multiple tissues including the central nervous system.  Since fungal glycosylation is different from mammalian glycosylation, we hypothesized that certain lectins (carbohydrate-binding proteins) would bind to coccidioidal glycoproteins and not to human glycoproteins. Methods: Lectin-based immunohistochemistry was performed using a panel of 21 lectins on formalin-fixed, paraffin-embedded samples of human lung tissue from patients infected with Coccidioides. Positive binding lectins were confirmed through inhibition experiments. Competition ELISAs were utilized to determine if these lectins bound to the same glycan structure in a Coccidioides fungal lysate.  Mass spectrometry was performed on eluates from lectin-affinity chromatography columns to identify the coccidioidal glycoproteins.  Results: Two GlcNAc-binding lectins bound specifically to endospores and spherules in infected human lung tissue, and not to adjacent lung tissue. Other lectins bound to lung tissue but not to endospores or spherules, and some lectins appeared to bind non-specifically to everything. SDS-PAGE analysis of eluates from lectin-affinity chromatography demonstrated that these lectins bind to coccidioidal glycoproteins. Deglycosylation of the lysate using PNGase resulted in loss of lectin binding to these glycoproteins. Competition ELISAs and mass spectrometric identification of the top ten lectin-binding glycoproteins collected by lectin-affinity chromatography confirmed that these lectins bind to the same panel of glycoproteins. Conclusion: This is the first report of lectins that bind specifically to Coccidioides endospores and spherules in infected humans.  This study proves that it is possible to use lectins to purify and characterize glycoproteins from the fungus that causes Valley Fever.