CIRCADIAN COORDINATION OF IMMUNE PROCESSES SHAPES OLFACTORY BULB HOMEOSTASIS

Multiple systems underlie brain tissue homeostasis, but the immune system’s role in these processes is poorly understood. Circadian rhythms are critical for brain health, and understanding the role of biological timing in the regulation of brain immune function is important. The olfactory bulb, which experiences high levels of neuronal turnover, is regularly exposed to inhaled stimuli, and contains a robust circadian clock, offers a powerful model for examining how daily timing coordinates cellular maintenance, immune processes, and tissue repair. Using Nanostring transcriptomics, we identified a temporally organized sequence of biological processes across the circadian cycle. Pathways associated with apoptosis peaked around the middle of the resting phase. As animals transitioned into the active phase, immune transcripts, including microglial sensome components, reached maximal expression. By the start of the resting phase, signatures related to growth, development, and neuronal signaling became dominant. Structural and functional features of microglia also followed this pattern. Immunohistochemistry with Sholl analysis demonstrated greater microglial morphological complexity at active phase onset. Using spectral flow cytometry, we identified three microglial subpopulations distinguished by intrinsic fluorescence signatures and differential expression of CD11b, CD45, P2RY12, and CD68. Intriguingly, CD11b and CD45 were highest at resting phase onset. Additionally, time of day altered the response of olfactory bulb microglia subpopulations to intranasal poly(I:C) (a viral mimic). Together, this suggests olfactory bulb microglia and associated cellular pathways follow a circadian sequence that orchestrates apoptosis, immune processes, and growth, revealing a fundamental temporal framework for maintaining brain tissue homeostasis.