Mac1-Dependent Copper Sensing Promotes Adaptation to the Phagosome during Adaptive Immunity.

Intracellular pathogens residing within macrophage phagosomes are challenged with recognizing the phagosomal environment and appropriately responding to changing host defense strategies imposed in this organelle. One such phagocyte defense is the restriction of available copper as a form of nutritional immunity during the adaptive immune response to fungal pathogens. The intracellular fungal pathogen Histoplasma capsulatum adapts to this decreased copper through upregulation of the Ctr3 copper transporter. In this study, we show that recognizes the characteristic low-copper phagosomal environment of activated macrophages through the copper-dependent transcriptional regulator Mac1. Multiple -acting regulatory sequences in the promoter control upregulation of transcription under low-copper conditions, and the loss of Mac1 function prevents induction of Ctr3 under low-copper conditions. During adaptive immunity, this loss of copper sensing by Mac1 attenuates virulence more severely than loss of Ctr3 alone, indicating that Mac1 controls the expression of additional mechanisms important for pathogenesis. Transcriptional profiling of yeasts identified a small regulon of Mac1-dependent genes, with the most strongly regulated genes encoding proteins linked to copper, iron, and zinc homeostasis and defenses against reactive oxygen (iron-requiring catalase [CatB] and superoxide dismutase [Sod4]). Accordingly, the loss of Mac1 function increased sensitivity to copper and iron restriction and blocked low-copper-induced expression of extracellular catalase activity. Thus, Mac1-mediated sensing of low-copper signals to yeasts their residence within the activated macrophage phagosome, and in response, yeasts produce factors, including non-copper-dependent factors, to combat the enhanced defenses of activated macrophages. Histoplasma capsulatum is a fungal pathogen that survives and grows within host macrophages. For successful infection, must sense and adapt to a dynamic intracellular environment over the course of an infection. We demonstrate that the copper-dependent transcription factor, Mac1, enables sensing of low copper that characterizes the phagosome environment of activated macrophages. Histoplasma recognition of this state leads not only to upregulation of copper acquisition mechanisms but also to other non-copper-related pathogenesis strategies, including scavenging of other metals and detoxification of reactive oxygen produced by host cells. The limited set of genes regulated by Mac1 compared to those of other fungal pathogens suggests a response that has been tailored specifically for 's life inside the phagosome. Thus, low levels of phagosomal copper serve as a signal to , enabling responses to the enhanced antimicrobial defenses resulting from immune activation of macrophages.