Comparative multi-omics of the macrophage response to infection with Mycobacterium tuberculosis complex bacteria reveals pathogen-driven epigenomic reprogramming

This study employs a comparative multi-omics approach to demonstrate that *Mycobacterium bovis* drives distinct epigenomic reprogramming in bovine alveolar macrophages compared to other *M. tuberculosis* complex bacteria, revealing specific host-pathogen interaction mechanisms and identifying candidate genes that could inform breeding strategies for enhanced tuberculosis resilience in cattle.

The Gist

Imagine your body's immune system as a highly trained security force. The alveolar macrophages are the "front-line guards" stationed in your lungs, whose job is to spot invaders, swallow them, and destroy them.

This paper is like a detective story where scientists investigated what happens when these guards face four different types of "intruders" from the same criminal family (the Mycobacterium tuberculosis complex). The goal was to see how the guards' "brain" (their genetic instructions) and their "security protocols" (their epigenetic settings) change when they fight these different enemies.

Here is the breakdown of the investigation using simple analogies:

1. The Four Intruders

The scientists set up a training simulation with four different versions of the bacteria to see how the guards reacted:

• The Master Criminal (M. bovis): This is the main villain that causes tuberculosis in cows. It's fully alive, very strong, and adapted specifically to cows.
• The Foreign Spy (M. tuberculosis): This is the cousin of the master criminal. It causes TB in humans. It's also alive and dangerous, but it's not perfectly adapted to cows.
• The Attenuated Spy (M. bovis BCG): This is the "vaccine" version. It's alive but has been weakened (like a criminal who had their weapons taken away). It can't cause serious disease but still looks like the enemy.
• The Dead Dummy (Gamma-irradiated M. bovis): This is the master criminal, but it's been killed by radiation. It still looks like the enemy and has the same "uniform," but it can't move or fight back.

2. The Investigation: Reading the Guards' "Diaries"

The scientists didn't just watch the guards fight; they looked inside the guards' control rooms using three high-tech tools:

• RNA-seq (The Transcriptome): This reads the active to-do list. It shows which genes the guards are currently shouting about (e.g., "Attack!" or "Build a wall!").
• ATAC-seq (The Open Doors): This checks which doors in the control room are unlocked. If a door is open, the guards can quickly access the instructions inside.
• ChIP-seq (The Sticky Notes): This looks at the sticky notes placed on the instruction manuals. Some notes say "Read this loudly" (activating a gene), while others say "Ignore this" (silencing a gene).

3. The Big Discovery: The "Master Criminal" Rewrites the Rules

The most surprising finding was that the Master Criminal (M. bovis) did something the others didn't.

• The Others (Human TB, Vaccine, Dead Dummy): When the guards fought these, they sounded the alarm and changed their to-do lists a bit. It was like a standard security drill. They opened a few doors and put a few sticky notes on the manuals.
• The Master Criminal (M. bovis): When this one attacked, it didn't just trigger the alarm; it completely rewired the security system.
  • It forced the guards to unlock hundreds of new doors (opening up massive amounts of DNA).
  • It slapped thousands of new sticky notes on the manuals, changing how the guards read their own instructions.
  • It essentially hacked the guards' operating system.

The Analogy: Imagine the other three intruders are like burglars who break a window and steal a TV. The house alarm goes off, and the police come. But the M. bovis is like a hacker who breaks in, takes over the house's smart-home system, changes the locks, and reprograms the security cameras to look the other way. It's a much deeper, more chaotic takeover.

4. Why Does This Matter?

The scientists found that M. bovis is so good at this "hacking" because it has evolved specifically to trick cow immune cells. It knows exactly which "sticky notes" to move to make the guards less effective at killing it.

• The "Dead Dummy" Surprise: Interestingly, the dead bacteria (the dummy) caused a stronger reaction than the human TB or the vaccine. Why? Because the live bacteria are smart—they have secret weapons (virulence factors) that actively suppress the guards' panic. The dead dummy has no weapons, so the guards scream and panic at full volume because nothing is stopping them. The live M. bovis, however, manages to calm the guards down while secretly taking over their control room.

5. The "Genetic Treasure Map"

Finally, the scientists combined their findings with a massive database of cow genetics (like a family tree for millions of cows). They looked for specific genetic "glitches" in cows that made them naturally better or worse at fighting this infection.

They found four specific genes (ERBB4, LRCH1, MRTFA, and RNPC3) that act like the "weak points" in the security system. If a cow has certain versions of these genes, it might be better at resisting the "hacker" (M. bovis).

The Bottom Line

This study tells us that the battle between cows and tuberculosis isn't just a simple fight; it's a complex game of genetic chess. The cow's immune system tries to adapt, but the M. bovis bacteria is a master strategist that rewrites the rules of the game to survive.

Why is this good news?
By understanding exactly how the bacteria hacks the cow's system, scientists can now:

1. Breed better cows: We can select cows with the "stronger locks" (the good versions of those four genes) to naturally resist the disease.
2. Develop better treatments: We can design drugs that stop the bacteria from moving those "sticky notes" and hacking the system.

In short, the scientists found the "cheat codes" the bacteria uses, and now we know how to patch the security system.

Technical Summary

1. Problem Statement

Bovine tuberculosis (bTB), caused primarily by Mycobacterium bovis (M. bovis), is a chronic infectious disease causing significant economic losses in the global livestock industry and posing zoonotic risks. While alveolar macrophages (bAM) are the primary host cells targeted during early infection, the precise mechanisms by which M. bovis manipulates the host epigenome to ensure survival and transmission remain poorly understood. Furthermore, it is unclear how the host response differs between the bovine-adapted M. bovis, the human-adapted M. tuberculosis (M. tb), the attenuated vaccine strain M. bovis BCG, and non-viable bacteria. Understanding these differences is critical for developing genome-enabled breeding strategies to enhance disease resilience in cattle.

2. Methodology

The study employed a comprehensive comparative multi-omics approach using bovine alveolar macrophages (bAM) derived from six Holstein-Friesian calves.

• Experimental Design:
  • Infection Models: bAMs were challenged in vitro for 24 hours with four distinct conditions:
    1. Virulent M. bovis (MBO).
    2. Virulent M. tuberculosis (MTU).
    3. Attenuated M. bovis BCG (BCG).
    4. Gamma-irradiated (killed) M. bovis (IRR).
    5. Non-challenged control (CON).
  • Assays: Three functional genomics assays were performed in parallel:
    • RNA-seq: To profile transcriptomic changes (Differentially Expressed Genes - DEGs).
    • ChIP-seq: To map histone modifications (H3K4me1, H3K4me3, H3K27ac, H3K27me3) and CTCF binding.
    • ATAC-seq: To assess chromatin accessibility (Differential Open Chromatin Regions - DOCRs).
• Bioinformatics & Integration:
  • Data was aligned to the Bos taurus ARS-UCD1.2 reference genome.
  • Differential analysis was performed using DESeq2, DiffBind, and MACS2.
  • Multi-omics Integration: DEGs, Differential Affinity Binding Sites (DABS), and DOCRs were integrated to construct regulatory networks.
  • GWAS Integration: The resulting multi-omics gene set was integrated with a published Genome-Wide Association Study (GWAS) dataset for M. bovis infection susceptibility in Holstein-Friesian cattle using the gwinteR tool.
  • Pathway Analysis: Ingenuity Pathway Analysis (IPA) and g:Profiler were used for functional enrichment.

3. Key Contributions

• First Comparative Multi-omics Study: This is the first study to simultaneously compare the transcriptomic and epigenomic landscapes of bAMs in response to live virulent, live attenuated, human-adapted, and killed mycobacteria.
• Discovery of Pathogen-Specific Reprogramming: The study demonstrates that M. bovis drives a unique and extensive epigenomic reprogramming distinct from other MTBC members, suggesting a specific host-adaptation mechanism.
• Identification of Novel Susceptibility Genes: By integrating multi-omics data with GWAS, the study identified four candidate genes (ERBB4, LRCH1, MRTFA, RNPC3) associated with bTB resilience that were previously undetected by GWAS alone.
• Mechanistic Insight into Immune Evasion: The research elucidates how M. bovis manipulates histone marks (specifically H3K4me1 and H3K27ac) to modulate innate immune pathways like TLR-MyD88-NF-κB and RIG-I-like receptor signaling.

4. Key Results

A. Transcriptional Responses (RNA-seq)

• Magnitude of Response: M. bovis infection induced the most profound transcriptional changes, with 8,852 DEGs (4,513 up, 4,339 down). This was significantly higher than M. tuberculosis (2,312 DEGs), gamma-irradiated M. bovis (3,320 DEGs), and BCG (586 DEGs).
• Specificity: Only 419 DEGs were common across all four challenge groups. The majority of changes were specific to M. bovis infection.
• Pathway Activation: M. bovis uniquely activated cytosolic sensing pathways (RIG-I-like receptors) and Type I interferon responses more robustly than other strains. Notably, TLR2 and TLR4 were upregulated in M. bovis and killed M. bovis but not in M. tuberculosis or BCG challenges.

B. Epigenomic Remodeling (ChIP-seq & ATAC-seq)

• Chromatin Accessibility: Significant Differential Open Chromatin Regions (DOCRs) were almost exclusively observed in the M. bovis vs. Control contrast (226 DOCRs). In contrast, M. tuberculosis, BCG, and killed M. bovis showed negligible DOCRs (0–1).
• Histone Modifications:
  • M. bovis infection resulted in 878 Differential Affinity Binding Sites (DABS), a ten-fold increase compared to other live strains.
  • H3K4me3 (Active): Increased at promoters of immune genes.
  • H3K27ac (Active Enhancers): Significantly increased in M. bovis infections.
  • H3K4me1 (Repressive in Macrophages): Significantly decreased in M. bovis and killed M. bovis infections. The study confirms the hypothesis that H3K4me1 acts as a repressive mark in macrophages; its removal correlates with the upregulation of innate immune genes (e.g., CD274, CXCL3, CD82).
• Network Complexity: Interaction networks revealed a highly complex regulatory web for M. bovis (919 nodes, 1,104 interactions) compared to M. tuberculosis (57 nodes) and BCG (16 nodes).

B. Gene-Specific Findings

• CD274 (PDL1): Showed increased expression driven by reduced H3K4me1 and increased H3K27ac/ATAC signal specifically in M. bovis infection, suggesting a mechanism for T-cell evasion.
• IRF1 & LDLR: These genes exhibited epigenomic reconfiguration (increased H3K27ac/ATAC) and upregulation exclusively in M. bovis infected cells.
• TMPRSS2: Upregulated in both M. bovis and M. tuberculosis infections, with a corresponding DOCR.

C. GWAS Integration

• Integration of the 928 M. bovis-specific DABS/DOCR genes with cattle GWAS data identified 64 SNPs within four genes significantly associated with M. bovis infection susceptibility:
  1. ERBB4: Epidermal growth factor receptor (innate immunity).
  2. LRCH1: Negative regulator of T-cell signaling.
  3. MRTFA: Coactivator of serum response factor (immunodeficiency link in humans).
  4. RNPC3: Spliceosome component.

5. Significance

• Host Adaptation Mechanism: The study provides strong evidence that M. bovis has evolved specific mechanisms to drive extensive epigenetic reprogramming in bovine macrophages, distinct from the human-adapted M. tuberculosis. This reprogramming facilitates pathogen survival by modulating key immune checkpoints (e.g., PD1/PDL1 pathway) and inflammatory responses.
• Biomarker Discovery: The identification of specific histone mark patterns (e.g., loss of H3K4me1) serves as a potential biomarker for active M. bovis infection and virulence.
• Breeding Applications: The integration of functional genomics with GWAS offers a novel strategy for identifying causal variants for disease resilience. The four candidate genes (ERBB4, LRCH1, MRTFA, RNPC3) provide new targets for genome-enabled breeding programs to reduce bTB prevalence in cattle.
• Therapeutic Implications: Understanding the specific epigenetic drivers of M. bovis survival could inform the development of host-directed therapies that reverse these reprogramming events to restore macrophage antimicrobial function.