Cancer-Associated Fibroblasts: Subtypes, FAP, Fibrosis Link

The Intersection of Tumor Stroma and Organ Fibrosis

"Antifibrotic drugs can work against cancer. CAF-directed therapies may treat fibrosis." This is the emerging paradigm of late-2020s drug development.

Cancer-associated fibroblasts (CAFs) are the dominant cell population of the tumor microenvironment (TME). FAP (fibroblast activation protein) — expressed by more than 90% of epithelial tumors' CAFs — has become a pan-tumor target for radioligand therapy and CAR-T approaches. Remarkably, the same FAP-positive fibroblast population drives collagen deposition in idiopathic pulmonary fibrosis (IPF), liver cirrhosis, and kidney fibrosis. The two fields are now converging. This pillar article summarizes CAF subtypes, their molecular overlap with fibrotic myofibroblasts, and two competing therapeutic strategies — eliminate versus reprogram.

1. Quick Reference: CAF Subtypes vs. Fibrosis Cells

CAF Subtype	Key Markers	Primary Function	Counterpart in Organ Fibrosis
myCAF (myofibroblastic)	α-SMA high, FAP, COL1A1, HAS2	ECM production, invasion support	Myofibroblasts in IPF, cirrhosis, kidney fibrosis
iCAF (inflammatory)	IL-6, IL-8, LIF, CXCL12	Immunosuppression, chemoresistance	Pro-fibrotic M2 macrophage niche
apCAF (antigen-presenting)	MHC II, CD74	T-cell modulation (double-edged)	Tertiary lymphoid structures in SSc, RA
vCAF (perivascular)	PDGFR-β, NG2	Angiogenesis, CAF-pool supply	Pericyte-derived myofibroblasts

Key insight: myCAFs and fibrotic myofibroblasts are molecularly near-identical. This provides the biological rationale for repurposing antifibrotics (nintedanib, pirfenidone) in PDAC and glioblastoma trials, and for redirecting CAF therapeutics toward IPF.

2. What Are CAFs? Origin and Definition

CAFs are defined as the population of activated, proliferating fibroblast-lineage cells residing in tumor stroma. They are heterogeneous — arising from multiple origins — as established by single-cell RNA sequencing over the past decade.

Principal Origins

Source	Contribution	Organ Specificity
Resident fibroblasts	Dominant	All tissues
Hepatic stellate cells (HSC)	High	Liver (incl. PDAC liver metastases)
Pancreatic stellate cells (PSC)	High	Pancreas (PDAC)
Mesenchymal stem cells (MSC)	Moderate	Bone marrow-derived recruitment
Epithelial-mesenchymal transition (EMT)	Low–moderate	Epithelial tumors generally
Endothelial-mesenchymal transition (EndMT)	Low	Lung, kidney, heart
Pericytes	Moderate	Kidney, lung

These origins overlap almost entirely with the known sources of myofibroblasts in organ fibrosis. The emerging consensus: CAFs and fibrotic myofibroblasts are the same cellular platform responding to different stimuli (tumor vs. tissue injury).

3. CAF Subtypes: myCAF / iCAF / apCAF {#subtypes}

The foundational 2017 study by Öhlund et al. (Tuveson lab) proposed a myCAF/iCAF dichotomy in PDAC. Elyada et al. (2019) added apCAF as a third major class. Subsequent spatial transcriptomics has extended this taxonomy further.

3.1 myCAF (Myofibroblastic CAF)

• Location: Clustered adjacent to tumor nests
• Markers: α-SMA high, FAP, COL1A1, HAS2, TAGLN
• Drivers: TGF-β, contact-dependent YAP/TAZ mechanosignaling
• Function: ECM deposition, physical barrier, invasion and metastasis support
• Clinical correlation: High myCAF fraction predicts poor prognosis in PDAC

myCAFs are functionally equivalent to IPF and cirrhosis myofibroblasts, sharing α-SMA expression, contractility, and TGF-β/Smad dependence.

3.2 iCAF (Inflammatory CAF)

• Location: Dispersed, perivascular, distant from tumor nests
• Markers: IL-6, IL-8 (CXCL8), LIF, CXCL12 (SDF-1), PDGFRα
• Drivers: IL-1α/β, NF-κB, JAK/STAT3
• Function: Immunosuppressive TME, MDSC recruitment, chemoresistance
• Clinical correlation: IL-6/STAT3 axis drives gemcitabine and PARP-inhibitor resistance in PDAC and ovarian cancer

iCAFs resemble chronic-inflammation-phase activated fibroblasts in fibrosis, sustaining a fibro-inflammatory loop with recruited macrophages.

3.3 apCAF (Antigen-Presenting CAF)

• Markers: MHC class II, CD74, Saa3
• Function: Antigen presentation to T cells — but lacking co-stimulation, biased toward anergy and Treg induction
• Origin: Initially proposed to arise from iCAFs; later work identified mesothelial origin in PDAC
• Clinical relevance: May modulate immune checkpoint inhibitor (ICI) response

apCAFs are double-edged — context-dependent outcomes range from Treg-mediated suppression to CD4+ T activation.

3.4 Emerging Subtypes

• vCAF (vascular CAF): PDGFR-β high, pericyte-like, supports angiogenesis
• dCAF (developmental CAF): SOX9+, embryonic fibroblast program
• meCAF (metabolic CAF): Glycolysis/lipid metabolism elevated, supplies substrates to tumor

Identified through spatial transcriptomics in 2023–2025, these subsets point toward subtype-selective future therapeutics.

4. CAF Markers: Diagnostic and Therapeutic Handles

Marker	CAF Subset Enriched	Expression in Fibrosis	Drug-Development Utility
FAP	All CAFs (esp. myCAF)	High in IPF, MASH, kidney fibrosis	FAPI PET imaging, 177Lu-FAP RLT, FAP CAR-T
α-SMA	myCAF	Myofibroblasts	Pharmacodynamic marker
PDGFR-β	vCAF, myCAF	Pericyte-derived myofibroblasts	PDGFR-β inhibition
Periostin (POSTN)	myCAF	IPF, cardiac fibrosis	Biomarker
S100A4 (FSP-1)	iCAF-leaning	EMT-derived cells	Lineage tracing
PDPN (podoplanin)	General CAF	Also lymphatic endothelium	CAR-T candidate

The centrality of FAP: Absent in healthy adult tissue (except fetal development and wound healing), FAP is selectively upregulated on CAFs and activated fibroblasts — making it a universal molecular handle for both tumor and fibrosis targeting.

5. Shared Molecular Axes in TME and Organ Fibrosis

5.1 TGF-β/Smad Signaling

TGF-β is the master regulator of both CAF activation and myofibroblast differentiation. Systemic TGF-β blockade has historically failed due to cardiotoxicity (valvulopathy), inflammatory bowel disease, and loss of immune tolerance. Current strategies favor receptor-selective inhibitors (Vactosertib/TEW-7197, Galunisertib, LY3200882) and CAF-selective interventions (αv integrin inhibitors). For a detailed analysis of why so many TGF-β inhibitors have failed clinically, see our deep-dive on TGF-β/Smad.

5.2 YAP/TAZ Mechanotransduction

Tumor stroma is 2–10× stiffer than healthy tissue. This stiffness self-amplifies CAF activation — YAP/TAZ nuclear translocation is essential for the myCAF phenotype and is equally active in fibrotic tissue. YAP/TAZ inhibitors are therefore being developed in parallel for both indications.

5.3 Hedgehog/GLI Pathway

Hedgehog signaling drives PDAC desmoplastic stroma and IPF fibroproliferative activation. The contrasting outcomes — Vismodegib failing in PDAC while ENV-101/Taladegib showed positive Phase 2 in IPF — highlight that the same pathway has disease-specific intervention windows.

5.4 PDGF and FGF Signaling

PDGF/FGF signaling drives pericyte-derived CAF and myofibroblast proliferation. Nintedanib (multi-kinase inhibitor of PDGFR, FGFR, VEGFR) is approved for IPF and SSc-ILD; its stroma-normalizing effects are under evaluation in lung and ovarian cancer models — a prime example of antifibrotic-to-oncology repurposing.

5.5 NF-κB Inflammatory Axis

iCAF programs and chronic inflammation-to-fibrosis transitions share the NF-κB/IL-6/STAT3 axis. Tocilizumab (anti-IL-6R) is approved for SSc-ILD and widely used in oncology for CAR-T CRS management — a bi-directional exemplar of axis reuse.

6. Cancer-Specific CAF Profiles

Cancer	Stromal Fraction	Dominant Subtype	Representative Targets
Pancreatic (PDAC)	60–90%	myCAF + iCAF	FAP, Hedgehog, IL-6
Breast (esp. TNBC)	30–50%	All subtypes	FAP, CXCL12, Periostin
Colorectal (CRC)	20–50%	CAF-S1/S4	TGF-β, IL-11
Lung adenocarcinoma	20–40%	myCAF	FAP, POSTN
Gastric (diffuse type)	40–60%	myCAF + iCAF	RHOA-mutant driven
Ovarian	30–60%	iCAF-dominant	IL-6, FGF

PDAC is the bellwether of CAF drug development. As of 2025, FAP CAR-T, FAPI radioligand therapy, and αv integrin inhibitors are all in simultaneous clinical evaluation.

7. Therapeutic Strategy: Eliminate or Reprogram? {#fap-targeting}

Early-2010s approaches aimed to eliminate FAP+ stroma — but resulted in cachexia, colitis, anemia, and paradoxical loss of anti-tumor immunity (through apCAF depletion). The field has since diversified.

7.1 Selective Elimination

Targeting FAP+ CAFs specifically:

• FAPI radioligand therapy (FAP-RLT): 177Lu-FAPI-46, 177Lu-EB-FAPI, 68Ga-FAPI-46 — in early-phase trials for pancreatic, biliary, and sarcoma cancers. Off-target uptake in normal wound-healing tissue is the key limitation.
• FAP CAR-T: Early FAP CAR-T (2013) caused cachexia and bone-marrow toxicity by targeting FAP+ multipotent stromal cells. The field re-opened with Rurik et al. (Science 2022), showing that transient CD5-LNP-mRNA in vivo CAR-T reversed cardiac fibrosis without cachexia, and Yan et al. (Nat Commun 2025), extending the approach to bleomycin-induced pulmonary fibrosis. Lung and cardiac fibrosis are now the lead clinical-translation targets.
• Bispecific antibodies: anti-FAP × anti-CD3 BsAbs with 4-1BB agonism.

7.2 Reprogramming

Returning activated CAFs to quiescent, non-fibrotic states:

• Vitamin D receptor agonists (Paricalcitol): Induces pancreatic stellate-cell quiescence, enhancing gemcitabine efficacy (Sherman, Cell 2014). Ongoing Phase 2 combinations.
• ATRA (all-trans retinoic acid): Quiescence in PSCs; tested in the STARPAC trial on top of gemcitabine/nab-paclitaxel.
• Minnelide: Triptolide prodrug — stroma depletion plus direct tumor cell kill.

7.3 ECM Normalization

Correcting ECM-related CAF dysfunction rather than eliminating cells:

• LOXL2 inhibition (Simtuzumab): Negative in IPF and PDAC trials — under redesign.
• Hyaluronidase (PEGPH20): Negative HALO trial in PDAC, discontinued.
• Antifibrotic repurposing: Pirfenidone and nintedanib combination trials in multiple solid tumors (Phase 1/2).

7.4 iCAF/Inflammatory Axis Intervention

• IL-6/IL-6R inhibition (Tocilizumab, Siltuximab): Under evaluation with chemotherapy.
• JAK/STAT inhibition (Ruxolitinib): MPN-approved drug being repurposed.
• CXCR4 inhibition (Plerixafor/AMD3100): Blocks iCAF-derived CXCL12.

8. Bridging Fibrosis Research to CAF Biology — Practical Implications

8.1 Preclinical Model Transferability

Models developed for fibrosis research map directly onto CAF biology:

• 3D co-culture (stromal spheres/organoids + CAFs): Lessons from PCLS (precision-cut lung slices) apply.
• Mechanosensing platforms: AFM, micropatterned substrates for CAF activation studies.
• FAP knock-in reporter mice: Lineage tracing shared between CAFs and activated fibroblasts.

8.2 Shared Biomarkers

Biomarker	Cancer Indication	Fibrosis Indication
Periostin (POSTN)	Prognostic (breast, lung, cholangio)	IPF disease activity
PRO-C3	HCC fibrotic niche	MASH screening
Serum FAP (sFAP)	PDAC prognosis	IPF progression prediction

8.3 Repurposing Track Record

• Success: Hedgehog inhibition (Vismodegib → ENV-101, IPF); IL-4/13 blockade (dupilumab → SSc-ILD exploration)
• Failure: LOXL2 inhibition (Simtuzumab) in both IPF and PDAC
• Under evaluation: Nintedanib stroma-normalization trials in PDAC/CRC; pirfenidone for radiation pneumonitis and CAR-T hospital inflammation prophylaxis

Conclusion: The Antifibrotic-Oncology Convergence

CAFs are not passive stromal scaffolding — they are active drivers of cancer progression, sharing their activation biology with organ fibrosis. This recognition is reshaping late-2020s drug development through (1) antifibrotic repurposing in oncology, (2) FAP as a pan-stromal target, and (3) the shift from "eliminate" to "reprogram."

Molecular pharmacology validated in fibrosis (selective TGF-β inhibition, mechanosensor control, ECM normalization) is being transplanted into tumor stroma treatment. Conversely, cancer immunotherapy insights (CAR-T, ICI response modulation) are returning to fibrosis medicine. This bidirectional knowledge transfer will accelerate next-generation antifibrotic and anti-tumor drug development.

This cluster now includes three sibling deep-dives:

• Keloid fibroblasts as the skin's benign CAF analogue
• Cardiac fibroblasts as CAF analogues: FAP-CAR-T & FAPI-PET
• Vascular CAF analogues: vCAF and adventitial fibroblasts

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Related Articles

• Keloid fibroblasts as the skin's CAF analogue — sibling (skin)
• Cardiac fibroblasts as CAF analogues — sibling (heart)
• Vascular CAF analogues — sibling (vessel wall)
• Myofibroblast origins and activation — cell biology shared with CAFs
• TGF-β/Smad pathway — dominant CAF-activation and antifibrotic driver
• YAP/TAZ mechanotransduction — stiffness-driven CAF self-amplification
• Hedgehog/GLI pathway — divergent outcomes in PDAC and IPF
• Macrophage polarization — iCAF-interacting immune compartment

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References

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2. Elyada E, et al. Cross-species single-cell analysis of pancreatic ductal adenocarcinoma reveals antigen-presenting cancer-associated fibroblasts. Cancer Discov. 2019;9(8):1102-1123. PMID: 31197017
3. Sherman MH, et al. Vitamin D receptor-mediated stromal reprogramming suppresses pancreatitis and enhances pancreatic cancer therapy. Cell. 2014;159(1):80-93. PMID: 25259922
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