Targeted shedding of extracellular membrane proteins by induced protease recruitment

This study introduces a novel therapeutic strategy using bispecific antibody "Shedders" to recruit the protease ADAM10 to cell-surface proteins, inducing selective extracellular domain shedding that bypasses lysosomal trafficking and effectively reinvigorates immune responses, as demonstrated by the depletion of LAG-3 and other immune receptors.

The Gist

Imagine your body is a bustling city, and the cells are the buildings. On the surface of these buildings, there are "doors" and "signs" (proteins) that control how the buildings talk to each other. Sometimes, a building puts up a "Stop" sign (an inhibitory protein) that tells the immune system's security guards (T-cells) to stand down and stop fighting. In diseases like cancer, these "Stop" signs are often stuck, preventing the security guards from doing their job.

For years, scientists have tried to fix this by either:

1. Blocking the sign: Putting a sticker over the "Stop" sign so the guards can't see it (traditional antibody therapy).
2. Demolishing the building: Dragging the whole door inside the building to a trash compactor (lysosome) to destroy it completely (targeted protein degradation).

This paper introduces a brand new, faster, and smarter way to handle these "Stop" signs.

The New Strategy: The "Scissor-Recruiting" Agent

Instead of blocking the sign or dragging the whole door inside, the scientists created a special tool called a "Shedder."

Think of a "Shedder" as a two-sided magnet:

• Side A grabs onto the specific "Stop" sign you want to remove (like the LAG-3 protein).
• Side B grabs onto a pair of molecular scissors (an enzyme called ADAM10) that naturally lives on the cell surface.

When the Shedder connects these two, it brings the scissors right up to the "Stop" sign. Snip! The scissors cut the sign right off the building.

Why is this better than the old ways?

1. It's a "Snip," not a "Demolition"
Traditional methods often require dragging the protein inside the cell to a trash can (the lysosome). This is like calling a demolition crew to take a whole door off a house, carry it inside, and smash it. It takes time and energy.
The "Shedder" method is like a gardener trimming a hedge. The scissors cut the branch right where it grows, and the trimmed part falls away immediately. It happens outside the cell, very quickly, and doesn't require the cell to do any heavy lifting.

2. The "Cut" Can Be Good News
When you cut a protein off, it doesn't just disappear; it floats away as a "soluble fragment."

• The Analogy: Imagine a security guard (T-cell) is being told to "Stop" by a sign on a building.
  • Old Way (Blocking): You put tape over the sign. The guard still sees the building, but can't read the sign.
  • New Way (Shedding): You cut the sign off. The sign falls to the ground. Now, the guard sees the building clearly and realizes, "Oh, there's no 'Stop' sign here! I can go ahead and attack!"
  • The Bonus: In some cases, the piece that falls off (the soluble fragment) might actually help the immune system fight even harder. The paper found that cutting off the LAG-3 "Stop" sign didn't just stop the signal; it actually supercharged the immune cells, making them release more "fight" chemicals (Interferon-gamma) than just blocking the sign ever could.

3. It's a Universal Tool
The scientists didn't just test this on one "Stop" sign. They built a "Universal Shedder" kit.

• The Metaphor: Imagine you have a master key (a special tag called an "ALFA-tag") that fits any door. You can stick this key on any "Stop" sign you want to remove. Then, you use your Shedder tool to grab that key and bring the scissors to it.
• They tested this on many different targets (like IL-6R, CD62L, and MIC-A) and it worked on all of them. This means scientists can now easily study how cutting any specific protein affects the body.

The Big Picture

This research is like discovering a new way to edit the city's signage. Instead of just covering up bad signs or tearing down whole buildings, we can now precisely trim the specific parts of the cell surface that are causing trouble.

• Speed: It's faster because it's a direct cut, not a long delivery process.
• Precision: It only cuts what you tell it to.
• Power: It can wake up sleeping immune cells more effectively than current drugs.

This opens the door for new therapies that could help the body's natural defenses fight cancer and other diseases more effectively, simply by giving them a clean pair of scissors to trim the obstacles.

Technical Summary

1. Problem Statement

Targeted Protein Degradation (TPD) has revolutionized the treatment of "undruggable" intracellular proteins by recruiting E3 ligases to the proteasome. Recently, this strategy has been extended to the extracellular space (eTPD) to degrade cell-surface proteins. However, current eTPD modalities rely on lysosomal trafficking: bifunctional molecules recruit recycling receptors or membrane-tethered E3 ligases to internalize the Protein of Interest (POI) into the lysosome for complete degradation.

Limitations of existing eTPD:

• Kinetic Bottleneck: Internalization and lysosomal trafficking are multi-step processes, potentially slower than direct enzymatic cleavage.
• Loss of Functionality: Complete degradation destroys the protein, preventing the release of potentially bioactive extracellular domains.
• Mechanistic Gap: While extracellular proteases (sheddases) naturally cleave and shed ectodomains to regulate signaling, there is no engineered platform to induce this specific proteolytic event on demand for therapeutic or research purposes.

2. Methodology

The authors engineered a new modality called "Shedders": bispecific antibodies designed to recruit endogenous extracellular proteases to a specific POI, inducing site-specific ectodomain shedding.

• Design Strategy:

  • Protease Recruitment Arm: Targets ADAM10, a ubiquitous membrane-bound sheddase. The authors used clone 11G2 (binding the disintegrin domain of ADAM10), which does not interfere with catalytic activity. A second binder (8C7) was also tested.
  • POI Targeting Arm: Targets the specific protein of interest.
    • Primary Target: LAG-3 (an immune checkpoint receptor), using the binding arm from the clinically approved antibody Relatlimab (targeting Domain 1).
    • Universal Platform: An anti-ALFA-tag nanobody to recruit the shedder to any tagged substrate.
  • Formats: Bispecific constructs included scFv-scFv (Shedder 1) and scFv-Fab (Shedder 2) formats, with silenced Fc regions to prevent Fc-mediated effector functions.

• Experimental Models:

  • Cell Lines: HEK293T, HeLa, and Jurkat cells engineered to express FLAG-tagged LAG-3 or ALFA-tagged substrates (IL-6Rα, MIC-A, CD62L, LDLR, LRP8).
  • Primary Cells: Human T cells isolated from PBMCs, activated via anti-CD3/CD28 or Staphylococcal enterotoxin B (SEB) to induce endogenous LAG-3 expression.
  • Assays: Flow cytometry, Western blot, Immunoprecipitation (IP) of supernatants, confocal microscopy (using LysoLight to track lysosomal entry), siRNA knockdown, pharmacological inhibition (GI254023X), and mass spectrometry-based proteomics.

3. Key Contributions

1. Novel Mechanism: Established "Induced Shedding" as a distinct paradigm from lysosomal degradation. It utilizes direct enzymatic cleavage at the cell surface rather than endocytic trafficking.
2. Universal Assay Platform: Developed an ALFA-tag-based "universal shedder" system, allowing rapid screening of diverse cell-surface proteins for ADAM10-mediated shedding compatibility.
3. Functional Insight: Demonstrated that induced shedding can yield superior functional outcomes compared to simple receptor blockade (e.g., Relatlimab) by altering downstream signaling networks and releasing bioactive fragments.
4. Broad Substrate Scope: Validated the platform across immune receptors (LAG-3, IL-6Rα, CD62L, MIC-A) and metabolic receptors (LDLR, LRP8).

4. Key Results

A. Mechanism of Action

• Efficient Depletion: Treatment with LAG-3 Shedders (1 and 2) caused rapid, dose-dependent depletion of surface LAG-3 (observed within 1 hour, plateauing at 6 hours) in both engineered and primary cells.
• Extracellular Specificity:
  • Confocal microscopy and LysoLight assays showed minimal lysosomal internalization for Shedder 2, confirming the mechanism is extracellular.
  • Shedder 1 (scFv-scFv) showed some internalization at high doses, likely due to diabody-like clustering, but Shedder 2 (scFv-Fab) remained strictly extracellular.
• Protease Dependence:
  • ADAM10 Knockdown: siRNA-mediated depletion of ADAM10 abolished LAG-3 shedding.
  • Inhibition: Pharmacological inhibition of ADAM10 (GI254023X) blocked shedding.
  • Site Specificity: A LAG-3 mutant with a deleted cleavage site (LAG-3ESCP) was resistant to shedding, proving the requirement for specific proteolytic motifs.

B. Biological Impact on T Cells

• Proteomic Signature: Mass spectrometry revealed that induced LAG-3 shedding produced a distinct proteomic profile compared to Relatlimab (blocking antibody). Shedding uniquely downregulated immunosuppressive proteins (CEACAM1, KLF12, PTGER4, RBP4) and upregulated migration regulators (CTTN).
• Enhanced Immune Activation: In primary T cells stimulated with SEB, Shedder-treated cells secreted significantly higher levels of IFN-γ compared to cells treated with Relatlimab or isotype controls. This suggests that removing the receptor via cleavage (rather than just blocking it) more effectively reinvigorates T cell function.

C. Universal Platform Validation

• ALFA-Tag System: The anti-ALFA shedder successfully induced shedding of IL-6Rα, MIC-A, CD62L, LDLR, and LRP8.
• Bioactivity of Fragments: Soluble IL-6Rα generated by induced shedding remained biologically active, forming "trans-signaling" complexes with IL-6 that successfully activated STAT3 in reporter cells.
• Epitope Flexibility: Shedding efficacy was maintained regardless of whether the POI arm bound the N-terminus (ALFA-tag) or a distal epitope (Tocilizumab binding site on IL-6Rα), and regardless of which ADAM10 epitope was targeted (11G2 vs. 8C7).

5. Significance

• Therapeutic Potential: Induced shedding offers a faster, catalytic alternative to lysosomal degradation. For immune checkpoints like LAG-3, it provides a mechanism to not only remove the inhibitory receptor but also potentially modulate the immune microenvironment via released soluble fragments or altered signaling stubs.
• Research Tool: The platform serves as a powerful tool to study the functional consequences of specific extracellular proteolytic events, which are often difficult to isolate using genetic knockouts or broad pharmacological inhibitors.
• Drug Discovery: The "universal shedder" approach allows for the rapid identification of new substrates for ADAM-family proteases, expanding the toolkit for targeting "undruggable" surface proteins that are amenable to cleavage but not necessarily internalization.

In summary, this work introduces Shedders as a transformative modality that hijacks endogenous ADAM10 to achieve rapid, specific, and catalytic removal of cell-surface proteins, offering distinct functional advantages over traditional antibody blockade or lysosomal degradation.