Divalent metal cations potentiate the predatory capacity of amoeba for Cryptococcus neoformans

Abstract

Among the best studied interaction between soil phagocytic predators and a human pathogenic fungus is that of Acanthamoeba castellanii and Cryptococcus neoformans. The experimental conditions used in amoeba-fungal confrontation assays can have major effects on whether the fungus or the protozoan is ascendant in the interaction. In the presence of Mg²⁺ and Ca²⁺ in PBS, C. neoformans was consistently killed when incubated with A. castellanii. A. castellanii survived better in the presence of Mg²⁺ and Ca²⁺, even when incubated with C. neoformans. In the absence of Mg²⁺ and Ca²⁺, C. neoformans survived when incubated with A. castellanii, and the percentage of dead amoeba was higher than when incubated without yeast cells. These results show that the presence of Mg²⁺ and Ca²⁺ can make a decisive contribution toward tilting the outcome of the interaction in favor of amoeba. Of the two metals Mg²⁺ had a stronger effect than Ca²⁺. Cations enhanced A. castellanii activity against C. neoformans through enhanced phagocytosis, which is the major mechanism for amoeba to kill fungal cells. We found no evidence that amoeba uses extracellular killing mechanisms in their interactions with C. neoformans. In summary, the presence of Mg²⁺ and Ca²⁺ enhanced cell adhesion on surface and motility of amoeba, thus increasing the chance for contact of C. neoformans and the frequency of phagocytosis. Our findings imply that divalent cation concentration in soils could be an important variable for whether amoeba can control C. neoformans in the environment.