Pioneer factor IRF1 unlocks latent enhancers to rewire chromatin and immunometabolism in inflammatory macrophages

This study identifies Interferon Regulatory Factor 1 (IRF1) as a pioneer factor that orchestrates IFN{gamma}-driven macrophage activation by unlocking latent enhancers to remodel chromatin architecture, reprogram immunometabolism toward glycolysis, and establish durable epigenetic memory.

The Gist

The Big Picture: The Macrophage as a "Security Guard"

Imagine your body's immune system is a high-tech security team. The macrophages are the security guards patrolling the building. Usually, they are calm and just doing their rounds (resting state). But when an intruder (like a bacteria or virus) is detected, they need to transform instantly into "special forces" to fight back.

This paper discovers the switch that flips the guards from "calm patrol" to "special forces mode." That switch is a protein called IRF1.

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1. The Locked Room and the Master Key (Pioneer Factors)

Inside the security guard's brain (the cell's nucleus), there are thousands of instruction manuals (genes). Some manuals are open and easy to read; others are locked in a heavy, steel safe (closed chromatin).

Usually, the guards need a specific key to open these safes. The paper shows that IRF1 is a "Master Key" (a Pioneer Factor).

• The Problem: When the alarm sounds (IFNγ signal), the guards need to read instructions for new weapons and energy sources that are currently locked in those steel safes.
• The Solution: IRF1 doesn't just wait for the door to be unlocked. It physically pries the steel doors open. It forces its way into the locked, closed areas of the DNA to find the instructions needed for the fight.

2. The Construction Crew (SWI/SNF Complex)

Prying the door open is hard work. IRF1 realizes it can't do it alone. It calls in a specialized construction crew called SWI/SNF (specifically a member named BRG1).

• The Analogy: Think of IRF1 as the site manager who finds the locked room. Once IRF1 is inside, it signals the construction crew to come in and renovate the room. They knock down the walls (remove repressive marks) and install new windows and lights (add activating marks).
• The Result: The room is now open, bright, and ready for the guards to read the blueprints. If you stop the construction crew (using a drug to block them), the room stays locked, and the guards can't get the instructions they need.

3. The "Motif" Density: Why Some Doors are Harder to Open

The researchers found something interesting about how IRF1 works. It's not just about having the key; it's about how many times the keyhole appears on the door.

• The Analogy: Some doors have just one keyhole. Others have a whole wall covered in keyholes (high-density motif arrays).
• The Discovery: IRF1 loves the doors with many keyholes. It grabs onto them tightly, bringing in more construction crew. This allows it to open the most stubborn, heavily locked rooms (latent enhancers) that no other factor could touch.

4. Rewiring the Power Grid (Metabolic Reprogramming)

To fight a war, the security guards need a massive amount of energy. They can't rely on their usual slow-burning fuel (Oxidative Phosphorylation). They need to switch to a high-octane, fast-burning fuel (Glycolysis).

• The Analogy: Imagine the guards usually drive a hybrid car that runs on electricity. IRF1 forces them to switch to a drag racer that runs on nitrous oxide.
• The Discovery: IRF1 doesn't just open the door to the weapons manuals; it also opens the door to the fuel manuals. It rewrites the instructions to make the guards produce energy faster and create specific chemicals (like itaconate) that help kill bacteria and manage the stress of the fight.
• Without IRF1: The guards try to fight but are stuck in "economy mode." They run out of energy, can't produce the right chemicals, and the infection wins.

5. The "Training" Effect (Epigenetic Memory)

One of the coolest findings is what happens after the fight is over.

• The Analogy: Usually, after a security drill, you reset everything to normal. But with IRF1, the guards remember the drill. Even after the alarm stops and the intruder is gone, the "renovated rooms" (the open enhancers) stay open for days.
• The Result: If a second intruder shows up, the guards don't have to pry the doors open again. They can instantly read the blueprints and react much faster. This is called "Trained Immunity."
• The Catch: This memory only happens if IRF1 was there to do the initial renovation. Without IRF1, the guards forget the drill and have to start from scratch every time.

Summary

This paper tells the story of IRF1, a molecular "Master Key" that:

1. Pries open locked instructions in the DNA that other factors can't reach.
2. Hires a construction crew (SWI/SNF) to remodel the DNA so the instructions can be read.
3. Rewires the energy grid to switch the immune cells from slow-burning to high-speed fuel.
4. Leaves a permanent mark (memory) so the immune system remembers how to fight faster next time.

In short: IRF1 is the architect that redesigns the immune system's headquarters to prepare it for war, ensuring it has the weapons, the fuel, and the memory to win.

Technical Summary

1. Problem Statement

Macrophages must rapidly reprogram their chromatin landscape and metabolic state to mount effective inflammatory responses against pathogens. While lineage-determining transcription factors (LDTFs) like PU.1 establish the baseline macrophage identity, the mechanisms by which signal-dependent transcription factors (SDTFs) actively reshape the epigenome during activation—specifically converting "latent" (closed) chromatin into active enhancers—remain poorly defined. Furthermore, the molecular link connecting inflammatory signaling to the necessary metabolic shift from oxidative phosphorylation (OXPHOS) to aerobic glycolysis (immunometabolism) is not fully understood. This study investigates the role of Interferon Regulatory Factor 1 (IRF1) as a potential pioneer factor that orchestrates these epigenetic and metabolic transitions in response to IFN$\gamma$ stimulation.

2. Methodology

The authors employed an integrated multi-omics approach using primary murine bone marrow-derived macrophages (BMDMs) from Wild-Type (WT) and Irf1-knockout (KO) mice. The experimental design included:

• Time-Course Stimulation: Cells were stimulated with IFN$\gamma$ (400 U/mL) over a 0–48 hour time course.
• Chromatin Accessibility: ATAC-seq was performed to map dynamic changes in chromatin accessibility.
• Transcription Factor Binding: ChIP-seq was conducted for IRF1, histone modifications (H3K4me1, H3K27ac, H3K4me3, H3K27me3, H3K9me2), and the chromatin remodeler BRG1.
• Chromatin Architecture: Hi-ChIP (using IRF1 antibody) was used to map long-range enhancer-promoter interactions.
• Transcriptomics: SLAM-seq (metabolic labeling with 4-thiouridine) was used to distinguish nascent RNA from total RNA, allowing for precise measurement of transcriptional kinetics.
• Metabolomics: Targeted GC-MS and LC-MS were used to quantify metabolites in glycolysis, the pentose phosphate pathway (PPP), and the TCA cycle.
• Functional Assays:
  • OCR Measurement: Real-time oxygen consumption rates were measured using the Resipher platform to assess metabolic switching.
  • Pharmacological Inhibition: The SWI/SNF ATPase inhibitor BRM014 was used to test the dependency of IRF1 function on chromatin remodeling.
  • Epigenetic Memory: A "wash-and-rest" experiment (24h IFN$\gamma$ followed by 6 days of rest) was performed to assess the durability of chromatin changes.

3. Key Contributions & Results

A. IRF1 Acts as a Pioneer Factor for Latent Enhancers

• Discovery of Latent Enhancers: ATAC-seq clustering revealed that ~35% of IFN$\gamma$-responsive regions are dependent on IRF1. Specifically, Cluster 1 (C1) sites lacked baseline accessibility and PU.1 occupancy (the canonical macrophage pioneer) but gained accessibility only upon IFN$\gamma$ stimulation in an IRF1-dependent manner.
• Pioneering Mechanism: IRF1 ChIP-seq showed that IRF1 binds to these closed C1 regions as early as 0.5 hours, preceding the gain in chromatin accessibility (ATAC-seq signal) and histone acetylation (H3K27ac) which occur around 3 hours.
• Motif Density: High-density arrays of IRF1 motifs were found at C1 sites. The study demonstrated that motif density correlates with IRF1 occupancy levels, chromatin opening efficiency, and the frequency of enhancer-promoter looping.

B. Recruitment of SWI/SNF Chromatin Remodelers

• Mechanistic Link: IRF1 binding recruits the BRG1-containing SWI/SNF complex to latent enhancers.
• Dependency: In Irf1-KO cells, BRG1 recruitment to these sites is abolished. Conversely, pharmacological inhibition of BRG1 (using BRM014) prevents IRF1-dependent chromatin opening and gene induction.
• Feed-Forward Loop: IRF1 also transcriptionally upregulates genes encoding chromatin remodelers (e.g., Smarca4/BRG1, Kat2b, Kmt2c), creating a feed-forward mechanism that sustains the open chromatin state.

C. Transcriptional Rewiring and Epigenetic Memory

• Target Genes: IRF1-driven latent enhancers control genes involved in host defense (e.g., Gbp family), oxidative stress response, and cell cycle regulation.
• Epigenetic Memory: Following a 24-hour IFN$\gamma$ pulse and a 6-day washout, IRF1-dependent enhancers retained H3K4me1 (a priming mark) and lost repressive H3K9me2. Upon re-stimulation, these "trained" enhancers rapidly reacquired H3K27ac, demonstrating a durable epigenetic memory reminiscent of trained immunity.

D. Orchestration of Immunometabolism

• Metabolic Switch: IRF1 is essential for the IFN$\gamma$-induced shift from OXPHOS to aerobic glycolysis. Irf1-KO macrophages fail to downregulate OXPHOS and show impaired glycolytic flux.
• Direct Regulation: IRF1 binds directly to enhancers and promoters of key metabolic genes (e.g., Hk1, Acod1, Sdhd, G6pdx).
• Metabolite Profiling:
  • Glycolysis: WT macrophages accumulate late glycolytic intermediates (e.g., phosphoenolpyruvate), which is absent in KO cells.
  • PPP & Redox: IRF1 drives the Pentose Phosphate Pathway (PPP), leading to increased NADPH production and a higher GSH:GSSG ratio (redox balance) in WT cells.
  • TCA Cycle: IRF1 drives the accumulation of immunometabolites like itaconate (via Acod1 induction) and succinate, linking metabolic flux to antimicrobial defense and inflammation control.

4. Significance

This study fundamentally advances the understanding of macrophage biology in three key areas:

1. Redefining Pioneer Activity: It establishes IRF1 not just as a signal-dependent factor, but as a bona fide pioneer transcription factor capable of binding nucleosomal DNA in the absence of PU.1 to unlock latent enhancers. This expands the known repertoire of factors that can initiate de novo enhancer formation.
2. Mechanistic Link to Metabolism: It provides a direct molecular mechanism connecting inflammatory signaling (IFN$\gamma$) to metabolic reprogramming. IRF1 acts as the central node that simultaneously opens chromatin at metabolic genes and recruits the machinery (SWI/SNF) required to execute the metabolic switch necessary for effective immunity.
3. Epigenetic Basis of Trained Immunity: The findings offer a mechanistic explanation for "trained immunity" (innate immune memory). By establishing durable H3K4me1 marks at latent enhancers, IRF1 primes macrophages for a faster and stronger response to secondary challenges, linking transient cytokine exposure to long-term epigenetic and metabolic adaptation.

In summary, the paper demonstrates that IRF1 is a central integrator that couples chromatin remodeling, transcriptional control, and metabolic adaptation, enabling macrophages to transition from a resting state to a highly active, metabolically reprogrammed, and epigenetically "trained" state.