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Fibroblast activation protein-α (FAP) is a type-II transmembrane serine protease expressed almost exclusively to pathological conditions including fibrosis, arthritis, and cancer. Across most cancer types, elevated FAP is associated with worse clinical outcomes. Despite the clear association between FAP and disease severity, the biological reasons underlying these clinical observations remain unclear. Here we review basic FAP biology and FAP’s role in non-oncologic and oncologic disease. We further explore how FAP may worsen clinical outcomes via its effects on extracellular matrix remodeling, intracellular signaling regulation, angiogenesis, epithelial-to-mesenchymal transition, and immunosuppression. Lastly, we discuss the potential to exploit FAP biology to improve clinical outcomes.

Cancer-associated fibroblasts (CAFs), the principle component of the tumor-associated stroma, form a highly protumorigenic and immunosuppressive microenvironment that mediates therapeutic resistance. Co-targeting CAFs in addition to cancer cells may therefore augment the antitumor response. Fibroblast activation protein-α (FAP), a type 2 dipeptidyl peptidase, is expressed on CAFs in a majority of solid tumors making it an attractive immunotherapeutic target. To target FAP-positive CAFs in the tumor-associated stroma, we genetically modified T cells to express a FAP-specific chimeric antigen receptor (CAR). The resulting FAP-specific T cells recognized and killed FAP-positive target cells as determined by proinflammatory cytokine release and target cell lysis. In an established A549 lung cancer model, adoptive transfer of FAP-specific T cells significantly reduced FAP-positive stromal cells, with a concomitant decrease in tumor growth. Combining these FAP-specific T cells with T cells that targeted the EphA2 antigen on the A549 cancer cells themselves significantly enhanced overall antitumor activity and conferred a survival advantage compared to either alone. Our study underscores the value of co-targeting both CAFs and cancer cells to increase the benefits of T-cell immunotherapy for solid tumors.

Background Cancer-associated fibroblasts (CAFs) are a dominant cell type in the stroma of non-small cell lung cancer (NSCLC). Fibroblast heterogeneity reflects subpopulations of CAFs, which can influence prognosis and treatment efficacy. We describe the subtypes of CAFs in NSCLC. Methods Primary human NSCLC resections were assessed by flow cytometry and multiplex immunofluorescence for markers of fibroblast activation which allowed identification of CAF subsets. Survival data were analysed for our NSCLC cohort consisting of 163 patients to understand prognostic significance of CAF subsets. Results We identified five CAF populations, termed CAF S1-S5. CAF-S5 represents a previously undescribed population, and express FAP and PDPN but lack the myofibroblast marker αSMA, whereas CAF-S1 populations express all three. CAF-S5 are spatially further from tumour regions then CAF-S1 and scRNA data demonstrate an inflammatory phenotype. The presence of CAF-S1 or CAF-S5 is correlated to worse survival outcome in NSCLC, despite curative resection, highlighting the prognostic importance of CAF subtypes in NSCLC. TCGA data suggest the predominance of CAF-S5 has a poor prognosis across several cancer types. Conclusion This study describes the fibroblast heterogeneity in NSCLC and the prognostic importance of the novel CAF-S5 subset where its presence correlates to worse survival outcome.

Membrane-bound proteases have recently emerged as critical mediators of tumorigenesis, angiogenesis, and metastasis. However, the mechanisms by which they regulate these processes remain unknown. As the cell surface serine protease fibroblast activation protein (FAP) is selectively expressed on tumor-associated fibroblasts and pericytes in epithelial tumors, we set out to investigate the role of FAP in mouse models of epithelial-derived solid tumors. In this study, we demonstrate that genetic deletion and pharmacologic inhibition of FAP inhibited tumor growth in both an endogenous mouse model of lung cancer driven by the K-rasG12D mutant and a mouse model of colon cancer, in which CT26 mouse colon cancer cells were transplanted into immune competent syngeneic mice. Interestingly, growth of only the K-rasG12D-driven lung tumors was also attenuated by inhibition of the closely related protease dipeptidyl peptidase IV (DPPIV). Our results indicate that FAP depletion inhibits tumor cell proliferation indirectly, increases accumulation of collagen, decreases myofibroblast content, and decreases blood vessel density in tumors. These data provide proof of principle that targeting stromal cell-mediated modifications of the tumor microenvironment may be an effective approach to treating epithelial-derived solid tumors.

Accumulated evidence has demonstrated that the microenvironment of a given tumor is important in determining its drug resistance, tumorigenesis, progression and metastasis. These microenvironments, like tumor cells, are vital targets for cancer therapy. The cross-talk between tumor cells and cancer-associated fibroblasts (CAFs, alternatively termed activated fibroblasts) is crucial in regulating the drug resistance, tumorigenesis, neoplastic progression, angiogenesis, invasion and metastasis of a tumor. Fibroblast activation protein α (FAPα) is a transmembrane serine protease and is highly expressed on CAFs present in >90% of human epithelial neoplasms. FAPα activity, alongside that of gelatinase and type I collagenase, has become increasingly important in cancer therapy due to its effectiveness in modulating tumor behavior. In this review, recent progression in the knowledge of the role of FAPα in tumor microenvironments is discussed.

Cancer-associated fibroblasts (CAF) are activated fibroblasts in the cancer stroma and play an important role in cancer progression. Some reports have indicated the correlation between the expression of CAF markers and adverse prognosis in several cancers. However, no reports have studied CAF phenotype and its clinical relevance in esophageal squamous cell carcinoma (ESCC). We investigated CAF phenotype of ESCC based on histology and immunohistochemical expressions of five CAF markers such as fibroblast activation protein (FAP), smooth muscle actin (SMA), fibroblast-specific protein-1 (FSP1), platelet-derived growth factor receptor (PDGFRα), and PDGFRβ in 116 ESCC tissue samples. Besides, we also examined the correlation of the CAF phenotype with clinical relevance as well as other cancer-microenvironment related factors. Histologically immature CAF phenotype was correlated with poor prognosis (p<0.001) and associated with increased microvessel density, increased tumor associated macrophages, and epithelial to mesenchymal transition. CAF markers were characteristically expressed in stromal fibroblast close to tumor cells and the expression pattern of 5 CAF markers was highly heterogeneous in every individual cases. Of five CAF markers, SMA, FSP1, and PDGFRα were unfavorable prognostic indicators of ESCC. The number of positive CAF markers was greater in ESCC with immature CAFs than in those with mature ones. Our results demonstrate that histologic classification of CAF phenotype is a reliable and significant prognostic predictor in ESCC. CAF markers have the potential to be diagnostic and therapeutic targets in ESCC.

Tumor thrombus (TT) worsens prognosis and complicates surgery in renal cell carcinoma (RCC), yet its formation mechanisms remain unclear. Here, we perform integrative single-cell and spatial transcriptomic analyses on 71 tissues and 48 sections from RCC patients with or without TT. The cellular and spatial atlas reveals distinct TT-associated tumor microenvironment remodeling characterized by the enrichment of FAP + fibroblasts. These FAP + fibroblasts are spatially contiguous to aggressive cancer cells and promote their malignant phenotypes in vitro. Their abundance inversely correlates with functional NK cells, suggesting roles in tumor invasion and immune evasion. Furthermore, single-cell multiomics analysis identifies tumor pericytes as a source of FAP + fibroblasts and delineates transcription factor dynamics underlying pericyte-fibroblast transition. Finally, high levels of FAP + fibroblasts are associated with poor prognosis and predict a weaker response to anti-VEGF-based therapy. In conclusion, our study highlights FAP + fibroblasts as drivers of aggressive RCC with TT, suggesting potential therapeutic targets. Tumour thrombus (TT) complicates renal cell carcinoma (RCC) treatment. Here, authors conduct an integrative single-cell RNA sequencing and spatial transcriptomic analyses on RCC patients with and without TT and identify FAP + fibroblasts as drivers for TT formation.

Tumor-associated fibroblasts are key regulators of tumorigenesis. In contrast to tumor cells, which are genetically unstable and mutate frequently, the presence of genetically more stable fibroblasts in the tumor-stromal compartment makes them an optimal target for cancer immunotherapy. These cells are also the primary source of collagen type I, which contributes to decreased chemotherapeutic drug uptake in tumors and plays a significant role in regulating tumor sensitivity to a variety of chemotherapies. To specifically kill tumor-associated fibroblasts, we constructed an oral DNA vaccine targeting fibroblast activation protein (FAP), which is specifically overexpressed by fibroblasts in the tumor stroma. Through CD8+ T cell-mediated killing of tumor-associated fibroblasts, our vaccine successfully suppressed primary tumor cell growth and metastasis of multidrug-resistant murine colon and breast carcinoma. Furthermore, tumor tissue of FAP-vaccinated mice revealed markedly decreased collagen type I expression and up to 70% greater uptake of chemotherapeutic drugs. Most importantly, pFap-vaccinated mice treated with chemotherapy showed a 3-fold prolongation in lifespan and marked suppression of tumor growth, with 50% of the animals completely rejecting a tumor cell challenge. This strategy opens a new venue for the combination of immuno- and chemotherapies.

Pancreatic ductal adenocarcinoma (PDAC) is characterized by a fibrotic stroma with a poor lymphocyte infiltrate, in part driven by cancer-associated fibroblasts (CAFs). CAFs, which express fibroblast activation protein (FAP), contribute to immune escape via exclusion of anti-tumor CD8+ T cells from cancer cells, upregulation of immune checkpoint ligand expression, immunosuppressive cytokine production, and polarization of tumor infiltrating inflammatory cells. FAP is a post-proline peptidase selectively expressed during tissue remodeling and repair, such as with wound healing, and in the tumor microenvironment by cancer-associated fibroblasts. We targeted FAP function using a novel small molecule inhibitor, UAMC-1110, and mice with germline knockout of FAP and concomitant knock-in of E. coli beta-galactosidase. We depleted CAFs by adoptive transfer of anti-βgal T cells into the FAP knockout animals. Established syngeneic pancreatic tumors in immune competent mice were targeted with these 3 strategies, followed by focal radiotherapy to the tumor. FAP loss was associated with improved antigen-specific tumor T cell infiltrate and enhanced collagen deposition. However, FAP targeting alone or with tumor-directed radiation did not improve survival even when combined with anti-PD1 therapy. Targeting of CAFs alone or in combination with radiation did not improve survival. We conclude that targeting FAP and CAFs in combination with radiation is capable of enhancing anti-tumor T cell infiltrate and function, but does not result in sufficient tumor clearance to extend survival.

Immunotherapy has become a primary and secondary treatment for gastric cancer (GC) patients with mismatch repair deficiency (dMMR), and is used in both perioperative and advanced stages. The tumor immune microenvironment (TiME) is crucial for immunotherapy efficacy, yet the impact of MMR status on TiME remains understudied. We employed single-cell RNA sequencing (scRNA-seq) to analyze 33 fresh tissue samples from 25 patients, which included 10 normal tissues, 6 dMMR tumor tissues, and 17 pMMR tumor tissues, aiming to characterize the cellular and molecular components of the TiME. The proficient mismatch repair (pMMR) group displayed a significantly higher prevalence of a specific GC cell type, termed GC2, characterized by increased hypoxia, epithelial-mesenchymal transition (EMT), and angiogenic activities compared to the dMMR group. GC2 cells overexpressed BEX3 and GPC3, and they significantly correlated with poorer survival. The pMMR group also showed increased infiltration of SPP1 + macrophages and FAP + fibroblasts, exhibiting strong hypoxic and pro-angiogenic features. Furthermore, a higher proportion of E2 endothelial cells, involved in extracellular matrix (ECM) remodeling and showing heightened VEGF pathway, HIF pathway, and angiogenesis activity, were identified in pMMR patients. Intercellular communication analyses revealed that GC2 cells, SPP1 + macrophages, FAP + fibroblasts, and E2 endothelial cells interact through VEGF, SPP1, and MIF signals, forming a TiME characterized by hypoxia, pro-angiogenesis, and ECM remodeling. This study uncovered TiME heterogeneity among GC patients with different MMR states, highlighting that the pMMR TiME is distinguished by hypoxia, pro-angiogenesis, and ECM remodeling, driven by the presence of GC2 cells, SPP1 + macrophages, FAP + fibroblasts, and E2 endothelial cells. These findings are pivotal for developing targeted immunotherapies for GC patients with pMMR.