Disease context dictates the cellular targets of IL-17 in inflammatory skin disease

This study reveals that the cellular targets of IL-17 signaling in inflammatory skin diseases are context-dependent, with dermal fibroblasts acting as the primary mediators in hidradenitis suppurativa-like inflammation while both fibroblasts and keratinocytes contribute to psoriasis-like inflammation.

The Gist

The Big Picture: Two Different Fires, Two Different Firefighters

Imagine your skin is a massive city. Sometimes, this city gets attacked by an inflammatory "fire" caused by a specific chemical signal called IL-17. This signal is the alarm bell that tells the body to send in the "firefighters" (white blood cells, specifically neutrophils) to put out the fire.

For a long time, scientists thought there was only one type of fire and one type of firefighter needed to stop it. They believed that the Keratinocytes (the skin cells on the very surface, like the roof of a house) were the main targets of the alarm. If you blocked the alarm from reaching the roof, the fire would stop. This was the standard theory for diseases like Psoriasis.

However, this paper argues that the story is more complicated. There are actually two different types of fires, and they require different strategies to put out.

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The Two Types of Fires

1. The "Roof Fire" (Psoriasis)

• What it looks like: This fire is on the surface. It causes the roof (epidermis) to get thick and scaly.
• The Paper's Finding: To stop this fire, you need to cut the alarm line to both the roof (Keratinocytes) and the foundation (Fibroblasts).
• The Analogy: Imagine a house where the roof is on fire, but the fire is also spreading down to the basement. If you only tell the roof to stop burning, the basement keeps feeding the fire. If you only tell the basement to stop, the roof keeps burning. You have to tell both to stop to put out the fire.
• Conclusion: In Psoriasis, the skin cells on the surface and the structural cells deep down work together as a team. You can't ignore either one.

2. The "Basement Fire" (Hidradenitis Suppurativa / HS)

• What it looks like: This fire is deep underground. It happens in the deep layers of the skin (dermis) and fat, causing painful lumps, tunnels, and abscesses. This is characteristic of Hidradenitis Suppurativa (HS).
• The Paper's Finding: To stop this fire, you only need to cut the alarm line to the foundation (Fibroblasts). The roof (Keratinocytes) doesn't even need to hear the alarm!
• The Analogy: Imagine a fire starting deep in the basement of a skyscraper. The roof is perfectly fine. If you shout at the roof to "stop burning," it won't do anything because the roof isn't involved. The fire is being fueled entirely by the people living in the basement (Fibroblasts). If you tell the basement residents to stop, the fire goes out, even if the roof is still screaming.
• Conclusion: In HS, the deep structural cells are the main drivers. The surface skin cells are just bystanders.

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How They Proved It (The Experiment)

The scientists built two "simulated cities" (mouse models) to test this:

1. The "Roof" Model: They used a cream (Imiquimod) that mimics Psoriasis. When they removed the "alarm receiver" from the roof cells, the fire got smaller. But when they removed it from the basement cells, the fire also got smaller. Result: Both are needed.
2. The "Basement" Model: They injected the alarm chemicals (IL-17 and TNF) directly into the deep skin layers to mimic HS. When they removed the "alarm receiver" from the roof cells, nothing happened. The fire raged on. But when they removed the receiver from the basement cells, the fire was completely extinguished. Result: Only the basement matters here.

Why This Matters

For years, doctors have treated both Psoriasis and HS with the same drugs that block IL-17. While these drugs work, they are like using a sledgehammer to crack a nut—they block the signal everywhere, which can cause side effects.

This paper suggests a new way of thinking:

• Psoriasis is a "Roof and Basement" problem.
• HS is a "Basement-only" problem.

The Takeaway:
The paper proposes that we should stop treating all skin inflammation as if it happens on the surface. For deep skin diseases like HS, we might need to develop new, more precise medicines that specifically target the deep structural cells (Fibroblasts) rather than the surface cells. This could lead to treatments that are more effective and have fewer side effects, because they would be "sniping" the exact cell type causing the problem rather than "carpet bombing" the whole skin.

In short: The same alarm bell (IL-17) can start a fire in the attic or the basement. To put it out, you have to know exactly where the fire is and tell the right people to stop.

Technical Summary

1. Problem Statement

Therapeutic blockade of Interleukin-17 (IL-17) and Tumor Necrosis Factor (TNF) effectively treats chronic inflammatory skin diseases like psoriasis and hidradenitis suppurativa (HS). However, a fundamental mechanistic question remains unresolved: How can the same cytokines drive two fundamentally distinct patterns of tissue inflammation?

• Psoriasis: Characterized by epidermal inflammation, hyperplasia, and superficial neutrophil infiltration.
• Hidradenitis Suppurativa (HS): Characterized by deep dermal inflammation, abscess formation, and dense neutrophil infiltration in the deep dermis and subcutaneous tissue.

Current conceptual models, largely derived from the topical imiquimod (IMQ) mouse model of psoriasis, posit keratinocytes (KCs) as the dominant effector cells responding to IL-17. While fibroblasts (FBs) are increasingly recognized as active immune participants, their relative contribution compared to KCs, and whether this contribution varies by disease context (epidermal vs. dermal), is unknown. Furthermore, no experimental system has successfully modeled the robust dermal type 17 inflammation seen in HS.

2. Methodology

The authors integrated human transcriptomics with novel murine experimental systems and genetic perturbations to dissect cell-type-specific signaling.

• Murine Experimental Models:
  • Epidermal Model: Topical application of Imiquimod (IMQ) to mouse dorsal skin (standard psoriasis model).
  • Dermal Model: Intradermal (i.d.) injection of recombinant IL-17A and TNF. The authors hypothesized this would mimic HS-like deep dermal inflammation.
• Genetic Models (Conditional Knockouts):
  • Generated mice with cell-type-specific deletion of the IL-17 Receptor A (IL-17RA) using Cre-Lox technology:
    • $Pdgfra^{\Delta Il17ra}$: IL-17RA deleted specifically in fibroblasts.
    • $Krt14^{\Delta Il17ra}$: IL-17RA deleted specifically in keratinocytes.
    • $Adipoq^{\Delta Il17ra}$: IL-17RA deleted specifically in adipocytes (to rule out adipocyte contribution).
• Omics and Analysis:
  • scRNA-seq: Performed on mouse skin and human HS/psoriasis lesions.
  • Network Analysis: Used CellChat to map cell-cell communication hubs.
  • Bulk RNA-seq & qPCR: Analyzed gene expression profiles in primary cell cultures (FBs vs. KCs) and tissue samples.
  • Flow Cytometry & Histology: Quantified neutrophil infiltration (Ly6G+), epidermal thickness, and spatial distribution of inflammation.
• In Vitro Assays: Primary mouse FBs and KCs were cultured with IL-17A/TNF to identify direct transcriptional responses. Neutrophil migration assays were conducted using conditioned media.

3. Key Contributions

1. Establishment of a Dermal Inflammation Model: The authors validated that intradermal injection of IL-17/TNF in mice recapitulates the deep dermal neutrophilic inflammation and gene expression profile of HS, distinct from the epidermal inflammation of IMQ-induced psoriasis.
2. Redefinition of Cellular Targets: The study challenges the dogma that KCs are the sole or primary target of IL-17 in all skin inflammation. It demonstrates that the cellular target of IL-17 is dictated by the anatomical context (epidermal vs. dermal).
3. Discovery of Fibroblast Dominance in Dermal Disease: The study identifies dermal fibroblasts as the critical, non-redundant drivers of dermal type 17 inflammation (HS-like), whereas both fibroblasts and keratinocytes contribute equally to epidermal inflammation (psoriasis-like).

4. Key Results

A. Distinct Inflammatory Patterns in Mouse Models

• IMQ (Epidermal): Induced epidermal hyperplasia and neutrophil accumulation primarily in the papillary dermis and epidermis.
• i.d. IL-17/TNF (Dermal): Induced robust inflammation restricted to the reticular (deep) dermis and subcutaneous adipose tissue, with minimal epidermal hyperplasia initially. Neutrophil chemokines (e.g., CXCL1) were restricted to the deep dermis, mirroring HS pathology.
• Transcriptomics: The i.d. IL-17/TNF model showed upregulation of FB-biased genes (Lcn2, Cxcl12, Cxcl5, Timp1), while IMQ upregulated KC-biased genes (Il36a/b/g, S100a8/9).

B. Human Disease Correlation

• Cell-Cell Communication: scRNA-seq analysis of human HS skin revealed fibroblasts as the dominant signaling hub. In contrast, psoriasis skin showed strong, balanced communication between fibroblasts and keratinocytes.
• Gene Expression: Human HS lesions were enriched for the FB-biased gene signature (e.g., LCN2, CXCL12), while psoriasis lesions were enriched for the KC-biased signature (S100A8/9, IL36G).

C. Genetic Perturbation Studies (The Core Finding)

• Epidermal Context (IMQ Challenge):
  • Deletion of IL-17RA in either fibroblasts ($Pdgfra^{\Delta Il17ra}$) or keratinocytes ($Krt14^{\Delta Il17ra}$) resulted in a similar, significant reduction in epidermal thickness and neutrophil infiltration.
  • Conclusion: IL-17 signaling in both cell types is equally required for epidermal inflammation. There is significant bidirectional crosstalk; loss of signaling in one cell type dampens the response in the other.
• Dermal Context (i.d. IL-17/TNF Challenge):
  • Deletion of IL-17RA in fibroblasts ($Pdgfra^{\Delta Il17ra}$) completely abolished neutrophil recruitment and reduced epidermal thickening.
  • Deletion of IL-17RA in keratinocytes ($Krt14^{\Delta Il17ra}$) had no detectable effect on neutrophil recruitment or inflammation.
  • Conclusion: Dermal type 17 inflammation is strictly dependent on fibroblast IL-17 sensing and is independent of keratinocyte IL-17 sensing.
• Adipocyte Role: Deletion of IL-17RA in adipocytes ($Adipoq^{\Delta Il17ra}$) did not significantly reduce inflammation, confirming that the effect in $Pdgfra^{\Delta Il17ra}$ mice is driven by fibroblasts (which are progenitors of adipocytes) rather than mature adipocytes.

D. Molecular Mechanisms

• Fibroblast-Specific Response: Upon IL-17 stimulation, fibroblasts uniquely upregulate Lcn2 (Lipocalin-2), Cxcl12, and Cxcl5. Lcn2 was identified as a critical mediator of neutrophil recruitment in the dermal context.
• Keratinocyte-Specific Response: Keratinocytes uniquely upregulate Il36 family members and S100 proteins, which are more relevant to epidermal barrier disruption and psoriasis.

5. Significance

• Paradigm Shift: This work overturns the "Keratinocyte-Centric" model of type 17 skin inflammation. It establishes that disease context determines the effector cell:
  • Psoriasis (Epidermal): Requires coordinated signaling in both KCs and FBs.
  • HS (Dermal): Driven primarily by FBs; KCs are dispensable for the initial inflammatory cascade.
• Therapeutic Implications: The findings suggest that broad cytokine blockade (e.g., anti-IL-17 antibodies) works by hitting both targets, but future precision biologics could potentially target fibroblast-specific pathways (e.g., FB-specific IL-17 signaling or downstream chemokines like LCN2) to treat HS with fewer side effects or greater efficacy.
• Experimental Utility: The authors provide a validated intradermal IL-17/TNF mouse model that accurately mimics the dermal inflammation of HS, filling a critical gap in preclinical research tools.
• General Principle: The study suggests that deep-tissue inflammation in other dermatoses may also follow a fibroblast-centered paradigm, shifting the focus of therapeutic development toward the stromal compartment.