Characterization of monoclonal antibodies against cell wall epitopes of the insect pathogenic fungus, Nomuraea rileyi: differential binding to fungal surfaces and cross-reactivity with host hemocytes and basement membrane components.

Monoclonal antibodies (MAbs) were generated against epitopes on yeast-like hyphal bodies and hyphae of the entomopathogenic hyphomycete, Nomuraea rileyi. Two MAbs (4C10, 2H4) bind to epitopes common to both hyphal bodies and hyphae, whereas MAb 4E9 binds only to hyphal surfaces. 4C10 and 2H4 appear to be directed towards carbohydrate portions of cell surface mannoproteins, as evidenced by similarities in staining patterns between these MAbs and Concanavalin A on Western blots of N. rileyi cell wall extracts. These MAbs cross-react with antigens on blastospore and hyphal surfaces of two other entomopathogenic fungi, Beauveria bassiana and Paecilomyces farinosus in fluorescence microscopy assays, but do not cross-react with a non-entomopathogenic strain of Candida albicans or with Saccharomyces cerevisiae yeasts. MAb 4C10 also cross-reacts with immunocompetent granular hemocytes from Spodoptera exigua (beet armyworm) and Trichoplusia ni (cabbage looper) larvae and with S. exigua plasmatocytes. Electron microscopy revealed that this MAb binds to a component in cytoplasmic granules in the hemocytes, and that surface labeling may be due to the release of this MAb-positive component upon degranulation. MAb 2H4 does not cross-react with granular hemocytes, but does bind to plasmatocytes and hemocytes that tightly adhere to the substrate in monolayer assays. Additionally, MAb 4C10 specifically labels a basement membrane epitope on S. exigua fat body, suggesting that this antibody binds to mannose residues on extracellular matrix glycoproteins. Cross-reactivity of these N. rileyi MAbs with insect hemocyte and tissue components indicates that fungal surface epitopes can mimic host surface molecules, which could explain why N. rileyi hyphal bodies are not recognized by granulocytes and are able to circulate freely in the hemolymph without binding to basement membranes lining the hemocoel.