Mechanisms Underlying the Exquisite Sensitivity of Candida albicans to Combinatorial Cationic and Oxidative Stress That Enhances the Potent Fungicidal Activity of Phagocytes
Authors: Kaloriti Despoina , Jacobsen Mette , Yin Zhikang , Patterson Miranda , Tillmann Anna , Smith Deborah A. , Cook Emily , You Tao , Grimm Melissa J. , Bohovych Iryna , Grebogi Celso , Segal Brahm H. , Gow Neil A. R. , Haynes Ken , Quinn Janet , Brown Alistair J. P. 
Published: July 2014

ABSTRACT

Immune cells exploit reactive oxygen species (ROS) and cationic fluxes to kill microbial pathogens, such as the fungus
Candida albicans
. Yet,
C. albicans
is resistant to these stresses
in vitro.
Therefore, what accounts for the potent antifungal activity of neutrophils? We show that simultaneous exposure to oxidative and cationic stresses is much more potent than the individual stresses themselves and that this combinatorial stress kills
C. albicans
synergistically
in vitro
. We also show that the high fungicidal activity of human neutrophils is dependent on the combinatorial effects of the oxidative burst and cationic fluxes, as their pharmacological attenuation with apocynin or glibenclamide reduced phagocytic potency to a similar extent. The mechanistic basis for the extreme potency of combinatorial cationic plus oxidative stress—a phenomenon we term stress pathway interference—lies with the inhibition of hydrogen peroxide detoxification by the cations. In
C. albicans
this causes the intracellular accumulation of ROS, the inhibition of Cap1 (a transcriptional activator that normally drives the transcriptional response to oxidative stress), and altered readouts of the stress-activated protein kinase Hog1. This leads to a loss of oxidative and cationic stress transcriptional outputs, a precipitous collapse in stress adaptation, and cell death. This stress pathway interference can be suppressed by ectopic catalase (Cat1) expression, which inhibits the intracellular accumulation of ROS and the synergistic killing of
C. albicans
cells by combinatorial cationic plus oxidative stress. Stress pathway interference represents a powerful fungicidal mechanism employed by the host that suggests novel approaches to potentiate antifungal therapy.

IMPORTANCE
The immune system combats infection via phagocytic cells that recognize and kill pathogenic microbes. Human neutrophils combat
Candida
infections by killing this fungus with a potent mix of chemicals that includes reactive oxygen species (ROS) and cations. Yet,
Candida albicans
is relatively resistant to these stresses
in vitro
. We show that it is the combination of oxidative plus cationic stresses that kills yeasts so effectively, and we define the molecular mechanisms that underlie this potency. Cations inhibit catalase. This leads to the accumulation of intracellular ROS and inhibits the transcription factor Cap1, which is critical for the oxidative stress response in
C. albicans
. This triggers a dramatic collapse in fungal stress adaptation and cell death. Blocking either the oxidative burst or cationic fluxes in human neutrophils significantly reduces their ability to kill this fungal pathogen, indicating that combinatorial stress is pivotal to immune surveillance.