Circadian immunometabolic states impart a temporal response to SARS-CoV-2 spike proteins in mammalian macrophages

This study demonstrates that circadian rhythms regulate macrophage responses to SARS-CoV-1 and SARS-CoV-2 spike proteins by driving time-dependent immunometabolic states characterized by central metabolic and mitochondrial changes rather than classical immune activation.

The Gist

The Big Idea: Your Body Has a "Daily Schedule" for Fighting Germs

Imagine your body's immune system isn't just a random army that fights whenever it sees a bad guy. Instead, it's like a 24-hour factory with a strict shift schedule.

This paper explores what happens when that factory gets attacked by the "spike protein" from the SARS-CoV-2 virus (the virus that causes COVID-19). The researchers wanted to know: Does it matter what time of day the virus shows up?

The answer is a resounding YES. The time of day changes how the factory reacts, but not in the way you might expect.

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The Characters: Macrophages (The Security Guards)

The main characters in this story are macrophages. Think of them as the security guards of your body. They patrol your tissues, eat up invaders (like viruses), and sound the alarm if something is wrong.

These guards have an internal clock (a circadian rhythm) that tells them when to be sleepy and when to be hyper-alert. This clock controls their energy levels, their tools, and how they react to danger.

The Experiment: Testing the Guards at Different Times

The scientists took these security guards (from both mice and humans) and synchronized their clocks so they all knew exactly what time it was. Then, they exposed the guards to the SARS-CoV-2 spike protein at different times of the day.

They looked at three things:

1. The Blueprint (Proteins): The tools the guards are holding.
2. The Fuel (Metabolites): The energy in their tanks.
3. The Oil (Lipids): The lubricants keeping their machines running.

The Surprise: It's Not About the Siren; It's About the Engine

Usually, when a security guard sees a burglar, they scream (release inflammatory cytokines) and charge. The scientists expected the guards to scream louder or quieter depending on the time of day.

But they didn't.
The "screaming" (the release of inflammatory chemicals like IL-6 or TNF-alpha) was pretty much the same regardless of the time. The alarm didn't change its volume.

Instead, the change happened in the engine room.
The real difference was in the metabolism (the energy production) and the mitochondria (the power plants inside the cells).

Analogy 1: The Two Shifts

The researchers found that the virus triggered two completely different reactions depending on the shift:

• Shift A (The "Low Energy" Shift): If the virus attacked when the guards were naturally winding down (their "anti-inflammatory" phase), the virus acted like a saboteur. It turned off the power plants (mitochondria), broke the fuel lines, and made the guards sluggish. The guards' energy systems got suppressed.
• Shift B (The "High Energy" Shift): If the virus attacked when the guards were naturally waking up and revving their engines (their "pro-inflammatory" phase), the virus actually boosted the energy production. The guards' power plants worked harder, and their fuel lines opened up.

The Takeaway: The virus didn't just "attack"; it hijacked the guards' internal clock. If the clock said "sleep," the virus put the brakes on. If the clock said "go," the virus hit the gas.

Analogy 2: The Shape-Shifting Power Plant

The scientists also looked at the mitochondria (the power plants). These aren't static; they change shape, fusing together or splitting apart like a school of fish.

• When the virus hit during the "Low Energy" shift, the power plants got tangled and branched out (like a messy knot of wires), trying to recover.
• When the virus hit during the "High Energy" shift, the power plants stretched out and got bigger, ready to produce more power.

Why Does This Matter? (The Vaccine Connection)

You might have heard that getting a vaccine in the morning is better than getting it at night. This paper helps explain why.

If your immune system's "power plant" is naturally running low at night, a vaccine (which uses the spike protein to train the immune system) might not train the guards as effectively. But if you get the vaccine in the morning, when the guards' energy systems are naturally revving up, the training sticks better.

The Human vs. Mouse Connection

The researchers did this in both mice and humans. Even though mice and humans are different, their immune "clocks" and how they react to the virus were surprisingly similar. This suggests that this "time-of-day" rule is a fundamental part of how mammals fight viruses.

Summary in One Sentence

The SARS-CoV-2 virus doesn't just fight your immune system; it plays a game of "follow the leader" with your body's internal clock, either suppressing your immune cells' energy when they are tired or boosting it when they are already awake, rather than just making them scream louder.

The Bottom Line: Timing is everything. When your body meets a virus (or a vaccine), the time of day determines whether your immune cells are running on empty or full throttle.

Technical Summary

1. Problem Statement

Macrophages are critical innate immune cells that orchestrate inflammatory responses and adapt their metabolism to fight pathogens. It is well-established that circadian rhythms regulate immune function and that the timing of vaccination (e.g., morning vs. evening) affects efficacy against SARS-CoV-2. However, the molecular mechanisms underlying how the time of day influences the macrophage response to viral stimuli—specifically the SARS-CoV-2 spike protein—remain poorly understood. Previous studies suggest the spike protein disrupts mitochondrial function and glycolysis, but it is unknown if these disruptions are temporally regulated by the circadian clock or if they follow classical inflammatory polarization pathways.

2. Methodology

The study employed a multi-omics approach using both murine and human primary macrophages to investigate temporal responses to SARS spike proteins.

• Cell Models:
  • Murine: Bone marrow-derived macrophages (BMDMs) from male PER2::LUC C57BL/6J mice, allowing for real-time monitoring of circadian rhythms via luciferase.
  • Human: Primary macrophages derived from peripheral blood mononuclear cells (PBMCs) of a male donor.
• Experimental Design:
  • Synchronization: Cells were synchronized using a serum shock protocol (50% FBS for 2 hours) followed by starvation.
  • Stimulation: Cells were exposed to the S1 subunit of SARS-CoV-1, SARS-CoV-2 (WT), SARS-CoV-2 (D614G), and SARS-CoV-2 (Alpha) at seven distinct time points across the circadian day (every 4 hours).
  • Duration: Exposure lasted 6 hours to capture acute molecular changes.
• Multi-Omics Analysis (MPLEx):
  • A single cell pellet was processed to extract proteins, metabolites, and lipids simultaneously using the MPLEx method.
  • Proteomics: TMT-labeled LC-MS/MS (16-plex).
  • Metabolomics: GC-MS/MS.
  • Lipidomics: LC-ESI-MS/MS (positive and negative modes).
  • Data Processing: Data were imputed and batch-corrected using LIMBR. Circadian oscillations were modeled using ECHO (period 20–28 hours, BH-adjusted p < 0.05). Differential expression was analyzed using edgeR.
• Functional Assays:
  • Mitochondrial Membrane Potential (MMP): Measured via JC-10 dye ratio (590/525 nm).
  • Mitochondrial Morphology: Confocal microscopy with MitoTracker Red, analyzed via the MitochondriaAnalyzer plugin in FIJI/ImageJ.
  • Cytokine Profiling: ELISA for IL-6, TNF-α, IL-10, and TGF-β1.
  • Clock Tracking: Long-term bioluminescence recording of PER2::LUC expression.

3. Key Contributions

• First Multi-Omics Circadian Map: The study provides the first comprehensive, synchronized multi-omics (proteome, metabolome, lipidome) map of circadian regulation in both mouse and human macrophages, revealing conserved temporal patterns in central metabolism.
• Decoupling of Immune and Metabolic Responses: It demonstrates that the temporal response to SARS spike proteins is driven primarily by immunometabolic shifts (central metabolism and mitochondria) rather than classical inflammatory cytokine release or core clock disruption.
• Cross-Species Conservation: It establishes that the circadian timing of immunometabolic responses to SARS spike proteins is conserved between mice and humans, despite species-specific differences in ACE2 binding affinity.
• Mechanistic Insight into Vaccine Timing: The findings offer a molecular explanation for why the time of day affects vaccine efficacy, linking it to the circadian state of macrophage metabolism.

4. Key Results

A. Basal Circadian Regulation

• Oscillating Entities: In mouse macrophages, 36% of proteins, 13% of metabolites, and 36% of lipids exhibited circadian oscillations. Human macrophages showed similar proportions (29% proteins, 21% metabolites, 28% lipids).
• Metabolic Phasing: Central metabolic pathways (Glycolysis, TCA cycle, OXPHOS) showed distinct temporal peaks.
  • Glycolysis/TCA: Peaked during the "anti-inflammatory" phase (approx. CT6–CT10).
  • OXPHOS: Peaked during the "pro-inflammatory" phase (approx. CT18).
• Lipidomics: Lipids associated with mitochondrial stability (e.g., cardiolipins, phosphatidylcholines) oscillated in phase with OXPHOS proteins.

B. Response to Spike Proteins

• No Core Clock Disruption: Unlike classical stimuli (LPS/IL-4), SARS spike proteins did not alter the period, amplitude, or phase of core clock proteins (BMAL1, PER1, CRY1) in either species.
• Limited Cytokine Response: Spike protein exposure induced only modest increases in pro-inflammatory cytokines (IL-6, TNF-α) with no significant temporal trend. This contrasts sharply with the strong rhythmic cytokine responses seen with LPS.
• Temporal Immunometabolic Shifts: The response to spike proteins was highly dependent on the time of day:
  • Anti-inflammatory Phase (e.g., CT8/HPS20): Exposure led to suppression of central metabolism. Glycolysis and TCA cycle proteins/metabolites were downregulated, and mitochondrial membrane potential (MMP) decreased.
  • Pro-inflammatory Phase (e.g., CT16/HPS28): Exposure led to modest activation or maintenance of metabolic pathways. Glycolysis and TCA cycle proteins were upregulated or maintained.
• Mitochondrial Morphology:
  • Spike protein exposure induced time-dependent changes in mitochondrial morphology that differed from LPS/IL-4.
  • At CT8 (suppression phase), mitochondria showed increased branching (fission).
  • At CT20, mitochondria showed elongation (fusion) and increased surface area.

C. Human vs. Mouse Comparison

• Conservation: Both species exhibited rhythmic suppression of metabolism at specific times and activation at others.
• Nuance: In human macrophages, the metabolic repression occurred slightly later (HPS20–24) and was accompanied by the downregulation of specific immune processing proteins (e.g., macropinocytosis activator PKCδ and ROS scavenger SOD2), suggesting a reduced capacity to internalize/process the virus at that specific time.

5. Significance

• Mechanism for Vaccine Efficacy: The study provides a biological basis for the observation that morning vaccination yields better immune responses. It suggests that macrophages are in a "metabolically primed" state (high glycolysis/TCA) during specific circadian windows, allowing for a more robust initial response to the spike antigen/adjuvant.
• Redefining Viral Pathogenesis: The findings challenge the view that SARS-CoV-2 spike protein primarily acts through classical inflammatory cascades. Instead, it acts as a metabolic disruptor, hijacking the circadian regulation of energy production to potentially facilitate viral replication or dysregulate immune clearance.
• Therapeutic Implications: Understanding that the spike protein's impact is temporally gated suggests that therapeutic interventions (e.g., metabolic modulators or timing of antiviral administration) could be optimized based on the host's circadian state to mitigate hyperinflammation or enhance viral clearance.
• Model for Chrono-Immunology: The work establishes a robust framework for studying how circadian rhythms dictate immune-metabolic crosstalk, applicable to other pathogens and immune cell types.

In conclusion, the paper demonstrates that the circadian clock does not merely modulate the intensity of the immune response to SARS-CoV-2 spike proteins but fundamentally alters the nature of the response, shifting it between states of immunometabolic suppression and activation, largely independent of classical inflammatory signaling.