IRE1α promotes phagosomal calcium flux to enhance macrophage fungicidal activity.

The mammalian endoplasmic reticulum (ER) stress sensor inositol-requiring enzyme 1α (IRE1α) is essential for cellular homeostasis and plays key roles in infection responses, including innate immunity and microbicidal activity. While IRE1α functions through the IRE1α-XBP1S axis are known, its XBP1S-independent roles are less well understood, and its functions during fungal infection are still emerging. We demonstrate that Candida albicans activates macrophage IRE1α via C-type lectin receptor signaling independent of protein misfolding, suggesting non-canonical activation. IRE1α enhances macrophage fungicidal activity by promoting phagosome maturation, which is crucial for containing C. albicans hyphae. IRE1α facilitates early phagosomal calcium flux post-phagocytosis, which is required for phagolysosomal fusion. In macrophages lacking the IRE1α endoribonuclease domain, defective calcium flux correlates with fewer ER-early endosome contact sites, suggesting a homeostatic role for IRE1α-promoting membrane contact sites. Overall, our findings illustrate non-canonical IRE1α activation during infection and a function for IRE1α in supporting organelle contact sites to safeguard against rapidly growing microbes.