Chytridiomycosis — an infectious disease caused by the fungal pathogen Batrachochytrium dendrobatidis — is a key factor in the ongoing global decline in amphibian populations; however, the molecular mechanisms that are involved in the pathogenesis of B. dendrobatidis are unclear. In a recent study published in Science, Fites et al. now show that B. dendrobatidis releases soluble mycotoxins that inhibit the proliferation of lymphocytes from the African clawed frog Xenopus laevis by inducing caspase-mediated apoptosis.

Credit: NPG

The authors found that co-culturing with live B. dendrobatidis cells inhibited the proliferative capacity of X. laevis splenocytes. This inhibitory effect was also observed when the splenocytes were separated from B. dendrobatidis by a cell-impermeable membrane or when splenocytes were incubated with B. dendrobatidis culture supernatants. Further dissecting the mechanism involved, the authors showed that B. dendrobatidis supernatants activated apoptosis signalling in splenocytes, and this was decreased by a pan-caspase inhibitor. A caspase activity assay showed that B. dendrobatidis supernatants induce both intrinsic and extrinsic caspase signalling pathways in splenocytes.

Importantly, B. dendrobatidis supernatants retained their capacity to inhibit lymphocyte proliferation after heat, acid or proteinase K treatment, which suggests that the stimulatory factors involved are not proteins. This, together with the finding that B. dendrobatidis zoospores (which lack cell walls) did not inhibit lymphocyte proliferation, led the authors to hypothesize that the released inhibitory factors may be cell wall components. In agreement with this, inhibiting B. dendrobatidis cell wall synthesis significantly decreased the negative effect of the fungus on lymphocyte proliferation. However, treatment of B. dendrobatidis supernatants with chitinases or β-glucanases had no effect on proliferating lymphocytes.

the released factors are broad-spectrum cytotoxins

As proliferation of mammalian epithelial cells is also inhibited by B. dendrobatidis supernatants, the authors conclude that the released factors are broad-spectrum cytotoxins; however, the identity of these inhibitory factors remains to be determined.