• Fibroblast Activation Protein (FAP) is a type II transmembrane glycoprotein belonging to the serine protease family. It consists of a short N-terminal cytoplasmic domain, a transmembrane domain, and a large extracellular C-terminal region containing a catalytic domain with dipeptidyl peptidase (DPP) and endopeptidase activities. FAP forms homodimers or heterodimers with related proteases like dipeptidyl peptidase IV (DPP4). Its catalytic triad (Ser624, Asp702, His734) enables enzymatic cleavage of substrates such as collagen, fibronectin, and α2-antiplasmin.
• FAP is predominantly expressed by cancer-associated fibroblasts (CAFs) in solid tumors (e.g., pancreatic, breast, colorectal cancers), activated fibroblasts/myofibroblasts in wound healing and fibrotic tissues (e.g., liver cirrhosis, pulmonary fibrosis). FAP is transiently expressed during embryonic development. But it is rarely detected in normal adult tissues, except in regenerative contexts (e.g., placental trophoblasts, bone marrow stroma).
• FAP plays roles in: 1) ECM Remodeling: Degrades extracellular matrix (ECM) components, facilitating tumor invasion and metastasis. 2) Immune Modulation: Suppresses antitumor immunity by cleaving chemokines (e.g., CXCL12) and promoting immunosuppressive microenvironments. 3) Cell Signaling: Processes bioactive peptides (e.g., FGF21) and interacts with integrins to regulate cell adhesion/migration. 4) Fibrosis Promotion: Drives tissue stiffening and fibroblast activation in chronic fibrotic diseases.
• Indications for FAP-targeted therapies including solid tumors, fibrotic diseases.
• In B-NDG hFAP mice, the mouse Fap gene was replaced by human FAP gene. Human FAP was detectable on CD45- cells of tumor tissue.
• Application: This mouse model is suitable for the pharmacodynamics and safety evaluations of drugs targeting human FAP (e.g., anti-hFAP antibodies, FAP-CAR-T cells, radiolabeled FAP inhibitors), tumor microenvironment and fibrosis pathogenesis studies, imaging probe development.

Gene targeting strategy for B-NDG hFAP mice. A chimeric CDS that encodes human FAP extracellular domain, mouse FAP transmembrane and cytoplasmic domain, followed by mouse 3’UTR-STOP is inserted in exon 3 of mouse FAP to replace most of the exon 3 of mouse FAP gene. The chimeric FAP protein expression will be driven by endogenous mouse FAP promoter, while mouse FAP gene transcription and translation will be disrupted.

Strain specific FAP expression analysis in wild-type B-NDG mice (+/+), homozygous humanized B-NDG hFAP mice (H/H) and homozygous B-Fap KO mice (-/-) by flow cytometry. Murine pancreatic ductal adenocarcinoma Pan02 cells (2×10⁷) and melanoma B16-F10 (2×10⁵) were respectively inoculated into the subcutaneous area of B-NDG mice, B-NDG hFAP mice and B-Fap KO mice (female, 6-week-old, n=1). Tumor tissue was collected and analyzed by flow cytometry with species-specific anti-hFAP antibody (R&D, FAB3715A). Human FAP was only detectable in mCD45- cells of tumor tissue from B-NDG hFAP mice, but not detectable in tumor tissues from homozygous B-NDG mice and B-Fap KO mice.