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Breaking Frog Defenses
The first line of immune defense against most fungal infections consists of innate immune effector cells, including macrophages and neutrophils. However, Fites et al. (p. 366) have found that the fungus currently decimating the world's amphibia, Batrachochytrium dendrobatidis, is readily engulfed by these cells, but that this does not effectively control the infection. The fungus releases cell-wall components that induce lymphocyte apoptosis and inhibit the proliferation of other nonlymphoid cell types, disarming lymphocyte-mediated responses to infection.
Abstract
The chytrid fungus, Batrachochytrium dendrobatidis, causes chytridiomycosis and is a major contributor to global amphibian declines. Although amphibians have robust immune defenses, clearance of this pathogen is impaired. Because inhibition of host immunity is a common survival strategy of pathogenic fungi, we hypothesized that B. dendrobatidis evades clearance by inhibiting immune functions. We found that B. dendrobatidis cells and supernatants impaired lymphocyte proliferation and induced apoptosis; however, fungal recognition and phagocytosis by macrophages and neutrophils was not impaired. Fungal inhibitory factors were resistant to heat, acid, and protease. Their production was absent in zoospores and reduced by nikkomycin Z, suggesting that they may be components of the cell wall. Evasion of host immunity may explain why this pathogen has devastated amphibian populations worldwide.
