A genome-wide CRISPR Screening identifies targets that drive Tolerogenic Dendritic Cells

This study utilizes a genome-wide CRISPR screen with CD86 as a functional readout to identify UBE2L6 as a novel regulator of tolerogenic dendritic cells that promotes immune tolerance by suppressing T cell activation through an ISGylation-dependent pathway.

The Gist

The Big Picture: Fixing a Broken Security System

Imagine your body's immune system is a highly trained security force (the T-cells) designed to attack invaders like viruses and bacteria. Usually, this force is smart; it knows not to attack your own body (like your skin or joints).

However, in autoimmune diseases (like Type 1 Diabetes or Rheumatoid Arthritis), the security system gets confused. It starts thinking your own body is the enemy and launches an attack, causing damage.

The goal of this study was to find a way to "retrain" the immune system to stop attacking itself. The researchers focused on a specific type of cell called a Dendritic Cell (DC). Think of DCs as the security guards or scouts at the front gate. Their job is to show a picture of the "enemy" to the security force.

• Normal DCs: Show a picture of a bad guy and shout, "ATTACK!" (This causes inflammation).
• Tolerogenic DCs (TolDCs): Show a picture of a bad guy but whisper, "Stand down, this is a friend," or "Don't attack." This calms the immune system.

The researchers wanted to figure out exactly how to turn a normal, aggressive security guard into a calm, peace-loving one, and find the "switches" inside the cell that control this behavior.

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Step 1: Finding the "Stop Sign" (The CD86 Marker)

The first challenge was: How do we know if a security guard is actually calm?

Previously, scientists had to run long, complicated tests to see if a cell was "tolerogenic." It was like trying to guess if a person is angry by watching them for hours.

The researchers discovered a simple, reliable Stop Sign on the surface of these cells called CD86.

• Aggressive Guard: Has a big, bright CD86 sign. It screams "ATTACK!"
• Calm Guard (TolDC): The CD86 sign is dim or missing.

They tested three different "calming agents" (Vitamin D3, Dexamethasone, and Rapamycin) and found that all of them successfully dimmed the CD86 sign.

• The Analogy: They realized that if you want to know if a security guard is on "peace mode," you just need to check if their "Attack" sign (CD86) is turned off. This became their "readout" or measurement tool for the rest of the study.

Crucial Discovery: They didn't just use CD86 as a sign; they proved it was the cause of the behavior. When they used a tool to physically block or delete the CD86 sign, the cells automatically became calm and stopped the security force from attacking. So, CD86 isn't just a badge; it's the remote control for the attack.

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Step 2: The Genome-Wide "Search Party" (CRISPR Screen)

Now that they had a reliable way to measure "calmness" (checking the CD86 sign), they wanted to find the internal switches inside the cell that control it.

They used a technology called CRISPR-Cas9, which acts like a molecular pair of scissors. They took a library of scissors that could cut out every single gene in the human/mouse genome (about 20,000 genes).

The Experiment:

1. They took millions of security guards (Dendritic Cells).
2. They gave each guard a random pair of scissors to cut out one random gene.
3. They then tried to turn these guards into "calm" guards using the calming agents.
4. They looked for the guards that failed to become calm (their CD86 sign stayed bright).

The Result:
If a guard kept their "Attack" sign (CD86) on even after trying to be calm, it meant the gene they lost was essential for making them calm. By finding which guards failed, they identified the genes responsible for peacekeeping.

This massive search uncovered hundreds of genes, but one stood out as a major new discovery: UBE2L6.

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Step 3: The New Star Player (UBE2L6)

The researchers found that UBE2L6 is a critical manager inside the cell.

The Analogy of the "Tagging System":
Imagine the cell has a tagging system.

• UBE2L6 is a worker who puts a specific "tag" (called ISG15) on other proteins.
• USP18 is a worker who removes that tag.

Normally, these two balance each other out. But the researchers found that when they removed UBE2L6 (cut the gene):

1. The "tag remover" (USP18) went into overdrive and piled up.
2. This pile of USP18 acted like a brake on the cell's internal alarm system (the NF-κB pathway).
3. Because the alarm was silenced, the cell couldn't turn on the "Attack" sign (CD86).
4. The cell became a Tolerogenic DC (a peacekeeper).

In simple terms: Removing UBE2L6 causes a chain reaction that slams the brakes on the immune system's aggression, forcing the cell to become a pacifist.

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Why This Matters

1. A New Tool: They created a fast, reliable way to find drugs that can turn immune cells into peacekeepers. Instead of guessing, scientists can now look for drugs that dim the CD86 sign.
2. New Targets: They found UBE2L6 as a new target. If we can develop a drug that temporarily blocks UBE2L6, we might be able to "hack" a patient's immune cells to stop attacking their own body, potentially curing or managing autoimmune diseases without needing lifelong, broad-spectrum immunosuppressants (which weaken the whole immune system).
3. Connecting the Dots: They linked these cellular switches to real-world diseases. Many of the genes they found are associated with genetic risks for diseases like Type 1 Diabetes, suggesting that fixing these switches could address the root cause of these conditions.

Summary

The researchers found the "Off Switch" (CD86) for immune aggression. They then used a massive genetic search to find the internal managers that control that switch. They discovered that UBE2L6 is a key manager; when you take it away, the cell automatically becomes a peacekeeper. This opens the door to new, smarter therapies for autoimmune diseases that teach the body's immune system to stop fighting itself.

Technical Summary

1. Problem Statement

• Clinical Need: Autoimmune diseases arise from a breakdown in immune tolerance. Current treatments rely on broad immunosuppression, which often causes long-term side effects and fails to restore true immune tolerance. 30–50% of patients do not respond to existing therapies.
• Therapeutic Gap: Tolerogenic Dendritic Cells (TolDCs) offer a promising cell-based therapy to restore immune tolerance by suppressing effector T cells and promoting regulatory T cells (Tregs). However, their clinical application is hindered by:
  • Lack of Consensus Markers: Different agents (e.g., Vitamin D3, Dexamethasone, Rapamycin) induce TolDCs via distinct pathways, making it difficult to define a universal phenotypic profile.
  • Unreliable Assays: Current evaluation methods (phenotyping and allogeneic T cell proliferation assays) are time-consuming and often unreliable.
  • Unknown Mechanisms: The molecular pathways governing the differentiation and maintenance of the tolerogenic phenotype remain elusive.
  • Technical Barriers: Applying large-scale functional genomics (like CRISPR screens) to primary dendritic cells is difficult due to their limited proliferative capacity and resistance to genetic manipulation.

2. Methodology

The authors developed a comprehensive workflow to identify novel regulators of TolDC function:

• Model Systems: Used both human monocyte-derived dendritic cells (moDCs) and murine bone marrow-derived dendritic cells (BMDCs).
• TolDC Induction: Generated TolDCs using three benchmark agents: Vitamin D3 (VitD3), Dexamethasone (Dex), and Rapamycin (Rapa).
• Biomarker Identification:
  • Screened a panel of co-stimulatory and tolerogenic markers (CD40, CD80, CD86, IDO-1, MHCII/HLA-DR).
  • Selection: Identified CD86 as the most consistent and robust marker downregulated across all TolDC conditions.
  • Validation: Confirmed CD86's functional role using CRISPR-mediated knockout (KO) and blocking antibodies, demonstrating that CD86 loss is required for T cell suppression.
• Genome-Wide CRISPR-Cas9 Screen:
  • Library: Pooled lentiviral library containing 80,600 sgRNAs targeting 21,300 protein-coding genes.
  • Strategy: Transduced primary BMDCs (MOI = 0.1) to maintain library representation.
  • Readout: Cells were stimulated with LPS, then sorted based on CD86 surface expression (Top 20% vs. Bottom 20%).
  • Analysis: Deep sequencing of sgRNAs in sorted populations to identify genes whose knockout enriched for low CD86 (positive regulators of tolerance) or high CD86 (negative regulators).
• Validation & Mechanistic Studies:
  • Arrayed Screening: Validated hits in human moDCs to ensure translational relevance.
  • Proteomics: Performed quantitative mass spectrometry (LC-MS/MS) on UBE2L6 KO vs. Control cells under basal and LPS-stimulated conditions.
  • GWAS Integration: Mapped screen hits to human autoimmune disease risk loci using the Open Targets Locus-to-Gene (L2G) model and data from FinnGen/UK Biobank.

3. Key Contributions

• Establishment of CD86 as a Screening Readout: The study validates CD86 not just as a phenotypic marker, but as a functional, mechanistic regulator of tolerance, providing a scalable readout for high-throughput screening in primary DCs.
• First Genome-Wide Screen in TolDCs: Overcame technical barriers to perform the first pooled CRISPR screen specifically targeting the tolerogenic state in primary dendritic cells (previous screens focused on immunogenic activation).
• Discovery of the UBE2L6–ISG15–USP18 Axis: Identified UBE2L6 (UbcH8) as a novel, critical regulator. Its loss drives a tolerogenic phenotype.
• Mechanistic Insight: Elucidated that UBE2L6 deficiency leads to the upregulation of ISG15 and USP18, which subsequently suppresses NF-κB signaling and CD86 expression.

4. Key Results

• Phenotypic Confirmation: TolDCs induced by VitD3, Dex, or Rapa consistently showed reduced T cell proliferation and inflammatory cytokine production (IL-2, IFN-γ, TNF-α).
• CD86 Dependency:
  • CD86 was the only marker consistently downregulated across all TolDC conditions.
  • CRISPR KO or antibody blocking of CD86 in DCs recapitulated the tolerogenic phenotype (reduced T cell proliferation, increased IL-10), confirming its functional necessity.
• Screening Outcomes:
  • Identified 450 positive regulators (knockout reduces CD86) and 345 negative regulators (knockout increases CD86).
  • Recovered known immune pathways (NF-κB, TLR, MAPK, JAK-STAT) and novel pathways (Ubiquitin binding, Ubiquitin-like ligase activity).
  • GWAS Correlation: A significant subset of screen hits showed strong genetic associations with autoimmune diseases (e.g., Type 1 Diabetes, Rheumatoid Arthritis), validating the clinical relevance of the identified targets.
• UBE2L6 Mechanism:
  • Proteomics: UBE2L6 KO cells showed specific upregulation of ISG15 and USP18 (a deubiquitinase and negative regulator of Type I IFN signaling).
  • Signaling: Loss of UBE2L6 led to reduced NF-κB activity (modest IκB changes) and decreased levels of key transcriptional regulators (KLF4, CREBBP) and cytokines (IL12B).
  • Function: UBE2L6-deficient DCs failed to activate T cells, resulting in reduced proliferation and cytokine secretion.
  • Genetic Link: While UBE2L6 itself wasn't a GWAS hit, it is regulated by a Type 1 Diabetes-associated SNP (rs3184504) via a trans-eQTL effect.

5. Significance

• Therapeutic Target Identification: The study provides a new class of therapeutic targets (e.g., UBE2L6 inhibitors) that could be used to pharmacologically induce immune tolerance without the need for complex ex vivo cell manufacturing.
• Methodological Advancement: Demonstrates the feasibility of performing pooled genome-wide CRISPR screens in primary, non-immortalized immune cells, setting a precedent for future functional genomics in primary cell types.
• Mechanistic Clarity: Defines a specific post-translational modification pathway (ISGylation) involving UBE2L6, ISG15, and USP18 as a critical checkpoint for controlling DC immunostimulatory capacity.
• Clinical Translation: By linking screen hits directly to human autoimmune disease genetics, the study bridges the gap between basic mechanistic discovery and clinical application, offering a roadmap for developing next-generation immunotherapies for autoimmune diseases.