Single-Cell Analysis Reveals Fibroblast Clusters Linked to Immunotherapy Resistance in Cancer

Targeting tumor immunosuppressive microenvironment for pancreatic cancer immunotherapy: Current research and future perspective

Pancreatic cancer is one of the most malignant tumors with increased incidence rate. The effect of surgery combined with chemoradiotherapy on survival of patients is unsatisfactory. New treatment strategy such as immunotherapy need to be investigated. The accumulation of desmoplastic stroma, infiltration of immunosuppressive cells including myeloid derived suppressor cells (MDSCs), tumor associated macrophages (TAMs), cancer‐associated fibroblasts (CAFs), and regulatory T cells (Tregs), as well as tumor associated cytokine such as TGF-β, IL-10, IL-35, CCL5 and CXCL12 construct an immunosuppressive microenvironment of pancreatic cancer, which presents challenges for immunotherapy. In this review article, we explore the roles and mechanism of immunosuppressive cells and lymphocytes in establishing an immunosuppressive tumor microenvironment in pancreatic cancer. In addition, immunotherapy strategies for pancreatic cancer based on tumor microenvironment including immune checkpoint inhibitors, targeting extracellular matrix (ECM), interfering with stromal cells or cytokines in TME, cancer vaccines and extracellular vesicles (EVs) are also discussed. It is necessary to identify an approach of immunotherapy in combination with other modalities to produce a synergistic effect with increased response rates in pancreatic cancer therapy.

Heterotypic signaling of cancer-associated fibroblasts in shaping the cancer cell drug resistance

The context-dependent reciprocal interaction between the cancer cells and surrounding fibroblasts is imperative for regulating malignant potential, metabolic reprogramming, immunosuppression, and ECM deposition. However, recent evidence also suggests that cancer-associated fibroblasts induce chemoresistance in cancer cells to various anticancer regimens. Because of the protumorigenic function of cancer-associated fibroblasts, these stromal cell types have emerged as fascinating therapeutic targets for cancer. However, this notion was recently challenged by studies that targeted cancer-associated fibroblasts and highlighted the underlying heterogeneity by identifying a subset of these cells with tumor-restricting functions. Hence, it is imperative to understand the heterogeneity and heterotypic signaling of cancer-associated fibroblasts to target tumor-promoting signaling processes by sparing tumor-restricting ones. In this review, we discuss the heterogeneity and heterotypic signaling of cancer-associated fibroblasts in shaping drug resistance and also list the cancer-associated fibroblast-targeting therapeutics.

Role and mechanism of fibroblast-activated protein-α expression on the surface of fibroblast-like synoviocytes in rheumatoid arthritis

Fibroblast-activated protein-α (FAP) is a type II integrated serine protease expressed by activated fibroblasts during fibrosis or inflammation. Fibroblast-like synoviocytes (FLSs) in rheumatoid arthritis (RA) synovial sites abundantly and stably overexpress FAP and play important roles in regulating the cellular immune, inflammatory, invasion, migration, proliferation, and angiogenesis responses in the synovial region. Overexpression of FAP is regulated by the initial inflammatory microenvironment of the disease and epigenetic signaling, which promotes RA development by regulating FLSs or affecting the signaling cross-linking FLSs with other cells at the local synovium and inflammatory stimulation. At present, several treatment options targeting FAP are in the process of development. This review discusses the basic features of FAP expressed on the surface of FLSs and its role in RA pathophysiology and advances in targeted therapies.

CAFrgDB: a database for cancer-associated fibroblasts related genes and their functions in cancer

As one of the most essential components of the tumor microenvironment (TME), cancer-associated fibroblasts (CAFs) interact extensively with cancer cells and other stromal cells to remodel TME and participate in the pathogenesis of cancer, which earmarked themselves as new promising targets for cancer therapy. Numerous studies have highlighted the heterogeneity and versatility of CAFs in most cancer types. Thus, the identification and appropriate use of CAF-related genes (CAFGenes) in the context of specific cancer types will provide critical insights into disease mechanisms and CAF-related therapeutic targets. In this study, we collected and curated 5421 CAFGenes identified from small- or large-scale experiments, encompassing 4982 responsors that directly or indirectly participate in cancer malignant behaviors managed by CAFs, 1069 secretions that are secreted by CAFs and 281 regulators that contribute in modulating CAFs in human and mouse, which covered 24 cancer types. For these human CAFGenes, we performed gene expression and prognostic marker-based analyses across 24 cancer types using TCGA data. Furthermore, we provided annotations for CAF-associated proteins by integrating the knowledge of protein-protein interaction(s), drug-target relations and basic annotations, from 9 public databases. CAFrgDB (CAF related Gene DataBase) is free for academic research at http://caf.zbiolab.cn and we anticipate CAFrgDB can be a useful resource for further study of CAFs.

SPARC in cancer-associated fibroblasts is an independent poor prognostic factor in non-metastatic triple-negative breast cancer and exhibits pro-tumor activity

Triple-negative breast cancer (TNBC) is the most aggressive breast cancer subtype and lacks specific targeted therapeutic agents. The current mechanistic evidence from cell-based studies suggests that the matricellular protein SPARC has a tumor-promoting role in TNBC; however, data on the clinical relevance of SPARC expression/secretion by tumor and stromal cells in TNBC are limited. Here, we analyzed by immunohistochemistry the prognostic value of tumor and stromal cell SPARC expression in 148 patients with non-metastatic TNBC and long follow-up (median: 5.4 years). We also quantified PD-L1 and PD-1 expression. We detected SPARC expression in tumor cells (42.4%), cancer-associated fibroblasts (CAFs; 88.1%), tumor-associated macrophages (77.1%), endothelial cells (75.2%) and tumor-infiltrating lymphocytes (9.8%). Recurrence-free survival was significantly lower in patients with SPARC-expressing CAFs. Multivariate analysis showed that SPARC expression in CAFs was an independent prognostic factor. We also detected tumor and stromal cell SPARC expression in TNBC cytosols, and in patient-derived xenografts and cell lines. Furthermore, we analyzed publicly available single-cell mRNA sequencing data and found that in TNBC, SPARC is expressed by different CAF subpopulations, including myofibroblasts and inflammatory fibroblasts that are involved in tumor-related processes. We then showed that fibroblast-secreted SPARC had a tumor-promoting role by inhibiting TNBC cell adhesion and stimulating their motility and invasiveness. Overall, our study demonstrates that SPARC expression in CAFs is an independent prognostic marker of poor outcome in TNBC. Patients with SPARC-expressing CAFs could be eligible for anti-SPARC targeted therapy.

Deciphering breast cancer: from biology to the clinic

Breast cancer remains a leading cause of cancer-related mortality in women, reflecting profound disease heterogeneity, metastasis, and therapeutic resistance. Over the last decade, genomic and transcriptomic data have been integrated on an unprecedented scale and revealed distinct cancer subtypes, critical molecular drivers, clonal evolutionary trajectories, and prognostic signatures. Furthermore, multi-dimensional integration of high-resolution single-cell and spatial technologies has highlighted the importance of the entire breast cancer ecosystem and the presence of distinct cellular "neighborhoods." Clinically, a plethora of new targeted therapies has emerged, now being rapidly incorporated into routine care. Resistance to therapy, however, remains a crucial challenge for the field.

Neutrophil extracellular traps in tumor progression and immunotherapy

Tumor immunity is a growing field of research that involves immune cells within the tumor microenvironment. Neutrophil extracellular traps (NETs) are neutrophil-derived extracellular web-like chromatin structures that are composed of histones and granule proteins. Initially discovered as the predominant host defense against pathogens, NETs have attracted increasing attention due to they have also been tightly associated with tumor. Excessive NET formation has been linked to increased tumor growth, metastasis, and drug resistance. Moreover, through direct and/or indirect effects on immune cells, an abnormal increase in NETs benefits immune exclusion and inhibits T-cell mediated antitumor immune responses. In this review, we summarize the recent but rapid progress in understanding the pivotal roles of NETs in tumor and anti-tumor immunity, highlighting the most relevant challenges in the field. We believe that NETs may be a promising therapeutic target for tumor immunotherapy.

Activated fibroblasts in cancer: Perspectives and challenges

Activated fibroblasts in tumors, or cancer-associated fibroblasts (CAFs), have become a popular research area over the past decade. As important players in many aspects of tumor biology, with functions ranging from collagen deposition to immunosuppression, CAFs have been the target of clinical and pre-clinical studies that have revealed their potential pro- and anti-tumorigenic dichotomy. In this review, we describe the important role of CAFs in the tumor microenvironment and the technological advances that made these discoveries possible, and we detail the models that are currently available for CAF investigation. Additionally, we present evidence to support the value of encompassing CAF investigation as a future therapeutic avenue alongside immune and cancer cells while highlighting the challenges that must be addressed for successful clinical translation of new findings.

Cancer-associated Fibroblasts in Bladder Cancer: Origin, Biology, and Therapeutic Opportunities

CONTEXT

Bladder cancer (BLCA) is a highly prevalent tumour and a health problem worldwide, especially among men. Recent work has highlighted the relevance of the tumour microenvironment (TME) in cancer biology with translational implications. Cancer-associated fibroblasts (CAFs) are a prominent, heterogeneous population of cells in the TME. CAFs have been associated with tumour development, progression, and poor prognosis in several neoplasms. However, their role in BLCA has not yet been exploited deeply.

OBJECTIVE

To review the role of CAFs in BLCA biology and provide an understanding of CAF origin, subtypes, markers, and phenotypic and functional characteristics to improve patient management.

EVIDENCE ACQUISITION

A PubMed search was performed to review manuscripts published using the terms "cancer associated fibroblast" and "bladder cancer" or "urothelial cancer". All abstracts were reviewed, and the full content of all relevant manuscripts was analysed. In addition, selected manuscripts on CAFs in other tumours were considered.

EVIDENCE SYNTHESIS

CAFs have been studied less extensively in BLCA than in other tumours. Thanks to new techniques, such as single-cell RNA-seq and spatial transcriptomics, it is now possible to accurately map and molecularly define the phenotype of fibroblasts in normal bladder and BLCA. Bulk transcriptomic analyses have revealed the existence of subtypes among both non-muscle-invasive and muscle-invasive BLCA; these subtypes display distinct features regarding their CAF content. We provide a higher-resolution map of the phenotypic diversity of CAFs in these tumour subtypes. Preclinical studies and recent promising clinical trials leverage on this knowledge through the combined targeting of CAFs or their effectors and the immune microenvironment.

CONCLUSIONS

Current knowledge of BLCA CAFs and the TME is being increasingly applied to improve BLCA therapy. There is a need to acquire a deeper understanding of CAF biology in BLCA.

PATIENT SUMMARY

Tumour cells are surrounded by nontumoural cells that contribute to the determination of the behaviour of cancers. Among them are cancer-associated fibroblasts. The "neighbourhoods" established through these cellular interactions can now be studied with much greater resolution. Understanding these features of tumours will contribute to the designing of more effective therapies, especially in relationship to bladder cancer immunotherapy.

From gatekeepers to providers: regulation of immune functions by cancer-associated fibroblasts

Cancer-associated fibroblasts (CAFs) are major protumorigenic components of the tumor microenvironment in solid cancers. CAFs are heterogeneous, consisting of multiple subsets that display diverse functions. Recently, CAFs have emerged as major promoters of immune evasion. CAFs favor T cell exclusion and exhaustion, promote recruitment of myeloid-derived suppressor cells, and induce protumoral phenotypic shifts in macrophages and neutrophils. With the growing appreciation of CAF heterogeneity came the understanding that different CAF subpopulations may be driving distinct immune-regulatory effects, interacting with different cell types, and perhaps even driving opposing effects on malignancy. In this review we discuss the current understanding of CAF-immune interactions, their effect on tumor progression and therapeutic response, and the possibility of exploiting CAF-immune interactions as potential targets for cancer therapy.

Pre-existing TGF-β-specific T-cell immunity in patients with pancreatic cancer predicts survival after checkpoint inhibitors combined with radiotherapy

BACKGROUND

Circulating transforming growth factor-β (TGF-β)-specific T cells that recognize TGF-β-expressing immune regulatory cells have been described in patients with cancer. TGF-β-derived peptide vaccination modulates the tumor microenvironment and has shown clinical effects in animal models of pancreatic cancer (PC). TGF-β-expressing regulatory cells are especially elevated in PC and may prevent the clinical response to immune checkpoint inhibitors (ICIs). Thus, in the present study we investigated the significance of TGF-β-specific T-cell immunity in patients with PC treated with ICI combined with radiotherapy in a randomized phase 2 study (CheckPAC).

METHODS

Immune responses to a TGF-β-derived epitope entitled TGF-β-15 as well as epitopes from Clostridium tetani (tetanus) and influenza were measured in peripheral blood mononuclear cells (PBMCs) with interferon-ɣ enzyme-linked immunospot assays. PBMCs were isolated before and after treatment. Correlations between immune response data and clinical data were evaluated with parametric and non-parametric statistical methods. Survival was analyzed with univariate and multivariate Cox-regression. TGF-β-15 specific T cells were isolated and expanded and examined for recognition of autologous regulatory immune cells by flow cytometry.

RESULTS

PBMCs from 32 patients were analyzed for immune responses to the TGF-β-derived epitope entitled TGF-β-15. Patients with a strong TGF-β-specific immune response at treatment initiation had longer progression-free and overall survival, compared with patients with a weak or no TGF-β-specific immune response. This remained significant in multivariate analysis. Patients with weak and strong TGF-β-specific responses displayed similar responses towards viral antigens. Furthermore, we show that TGF-β-specific T cells from a clinical responder specifically reacted to and lysed autologous, regulatory immune cells. Finally, mimicking a TGF-β-15 vaccination, we showed that repeated stimulations with the TGF-β-15 epitope in vitro enhanced the immune response to TGF-β-15.

CONCLUSION

A strong TGF-β-15 specific immune response was associated with clinical benefit and improved survival after ICI/radiotherapy for patients with PC. Importantly, the lack of TGF-β-specific T cells in some patients was not caused by a general immune dysfunction. TGF-β-specific T cells recognized regulatory immune cells and could be introduced in vitro in patients without spontaneous responses. Taken together, our data suggest that combining TGF-β-based vaccination with ICI/radiotherapy will be beneficial for patients with PC.

Tumor Cell-Intrinsic CD96 Mediates Chemoresistance and Cancer Stemness by Regulating Mitochondrial Fatty Acid β-Oxidation

Targeting CD96 that originates in immune cells has shown potential for cancer therapy. However, the role of intrinsic CD96 in solid tumor cells remains unknown. Here, it is found that CD96 is frequently expressed in tumor cells from clinical breast cancer samples and is correlated with poor long-term prognosis in these patients. The CD96⁺ cancer cell subpopulations exhibit features of both breast cancer stem cells and chemoresistance. In vivo inhibition of cancer cell-intrinsic CD96 enhances the chemotherapeutic response in a patient-derived tumor xenograft model. Mechanistically, CD96 enhances mitochondrial fatty acid β-oxidation via the CD155-CD96-Src-Stat3-Opa1 pathway, which subsequently promotes chemoresistance in breast cancer stem cells. A previously unknown role is identified for tumor cell-intrinsic CD96 and an attractive target in improving the chemotherapeutic response.

Fibroblast Activation Protein Inhibitor Theranostics

Fibroblast activation protein (FAP)-radioligand therapy might be effective in some patients without being curative. FAP-radioligands deliver ionizing radiation directly to FAP⁺ cancer-associated fibroblasts and, in some cancers, to FAP⁺ tumor cells; in addition, they indirectly irradiate FAP^- cells in tumor tissue via cross-fire and bystander effects. Here, we discuss the potential to improve FAP-radioligand therapy through interfering with DNA damage repair, immunotherapy, and co-targeting cancer-associated fibroblasts. As the molecular and cellular effects of FAP-radioligands on the tumor and its microenvironment have not been investigated yet, we call for future research to close this gap in knowledge, which prevents the development of more effective FAP-radioligand therapies.

The advent of immune stimulating CAFs in cancer

The theory that cancer-associated fibroblasts (CAFs) are immunosuppressive cells has prevailed throughout the past decade. However, recent high-throughput, high-resolution mesenchyme-directed single-cell studies have harnessed computational advances to functionally characterize cell states, highlighting the existence of immunostimulatory CAFs. Our group and others have uncovered and experimentally substantiated key functions of cancer antigen-presenting CAFs in T cell immunity, both in vitro and in vivo, refuting the conventional assumption that CAFs impede adaptive immune rejection of tumours. In this Perspective, I unify the follicular and non-follicular, non-endothelial stroma of tumours under the 'peripheral adaptive immune mesenchyme' framework and position subsets of CAFs as direct positive regulators of the adaptive immune system. Building on the understanding of cancer antigen presentation by CAFs and the second touch hypothesis, which postulates that full T cell polarization requires interaction with antigen-presenting cells in the non-lymphoid tissue where the antigen resides, I re-design the 'cancer-immunity cycle' to incorporate intratumoural activation of cancer-specific CD4⁺ T cells. Lastly, a road map to therapeutic harnessing of immunostimulatory CAF states is proposed.

Review of biomarkers for response to immunotherapy in HNSCC microenvironment

Head and neck squamous cell carcinoma are one of the most common types of cancer worldwide. Although a variety of treatment methods such as surgery, radiotherapy, chemotherapy, and targeted therapy are widely used in diagnosing and treating HNSCC, the survival prognosis of patients has not been significantly improved in the past decades. As an emerging treatment approach, immunotherapy has shown exciting therapeutic effects in R/M HNSCC. However, the current screening methods are still insufficient, and there is a significant need for reliable predictive biomarkers for personalized clinical management and new therapeutic strategies. This review summarized the application of immunotherapy in HNSCC, comprehensively analyzed the existing bioinformatic studies on immunotherapy in HNSCC, evaluated the current methods of tumor immune heterogeneity and immunotherapy, and aimed to screen molecular markers with potential predictive significance. Among them, PD-1 has obvious predictive relevance as the target of existing immune drugs. Clonal TMB is a potential biomarker for HNSCC immunotherapy. The other molecules, including IFN-γ, CXCL, CTLA-4, MTAP, SFR4/CPXM1/COL5A1, TILs, CAFs, exosomes, and peripheral blood indicators, may have suggestive significance for tumor immune microenvironment and prognosis of immunotherapy.

Periostin secreted from podoplanin-positive cancer-associated fibroblasts promotes metastasis of gastric cancer by regulating cancer stem cells via AKT and YAP signaling pathway

Cancer-associated fibroblasts (CAFs) are heterogeneous stromal cells present in the tumor microenvironment (TME), which play a critical role in gastric cancer (GC) progression. Here, we examined a subset of CAFs with high podoplanin (PDPN) expression, which is correlated with tumor metastasis and poor survival in GC patients. Animal models of gastric cancer liver metastasis monitored by micro-PET/CT confirmed that periostin (POSTN) derived from PDPN(+) CAFs regulated CAFs' pro-migratory ability. Mechanistically, PDPN(+) CAFs secreted POSTN to modulate cancer stem cells (CSCs) through FAK/AKT phosphorylation. Furthermore, POSTN could also activate FAK/YAP signaling in GC cells to produce increased amounts of IL-6, which in turn induced phosphorylation of PI3K/AKT in PDPN(+) CAFs. Prolonged PI3K/AKT pathway activation in PDPN(+) CAFs maintains the production of POSTN and the effect on CSC enrichment and GC cell migration. In conclusion, our study demonstrated a positive feedback loop between PDPN(+) CAFs and CSCs during GC progression and suggested a selective target for GC treatment.

Roles of cancer-associated fibroblasts (CAFs) in anti- PD-1/PD-L1 immunotherapy for solid cancers

In recent years, breakthroughs have been made in tumor immunotherapy. However, tumor immunotherapy, particularly anti-PD-1/PD-L1 immune checkpoint inhibitors, is effective in only a small percentage of patients in solid cancer. How to improve the efficiency of cancer immunotherapy is an urgent problem to be solved. As we all know, the state of the tumor microenvironment (TME) is an essential factor affecting the effectiveness of tumor immunotherapy, and the cancer-associated fibroblasts (CAFs) in TME have attracted much attention in recent years. As one of the main components of TME, CAFs interact with cancer cells and immune cells by secreting cytokines and vesicles, participating in ECM remodeling, and finally affecting the immune response process. With the in-depth study of CAFs heterogeneity, new strategies are provided for finding targets of combination immunotherapy and predicting immune efficacy. In this review, we focus on the role of CAFs in the solid cancer immune microenvironment, and then further elaborate on the potential mechanisms and pathways of CAFs influencing anti-PD-1/PD-L1 immunotherapy. In addition, we summarize the potential clinical application value of CAFs-related targets and markers in solid cancers.

Identification and Biological Validation of a Chemokine/Chemokine Receptor-Based Risk Model for Predicting Immunotherapeutic Response and Prognosis in Head and Neck Squamous Cell Carcinoma

Over 80% of head and neck squamous cell carcinoma (HNSCC) patients failed to respond to immunotherapy, which can likely be attributed to the tumor microenvironment (TME) remolding mediated by chemokines/chemokine receptors (C/CR). This study aimed to establish a C/CR-based risk model for better immunotherapeutic responses and prognosis. After assessing the characteristic patterns of the C/CR cluster from the TCGA-HNSCC cohort, a six-gene C/CR-based risk model was developed to stratify patients by LASSO Cox analysis. The screened genes were multidimensionally validated by RT-qPCR, scRNA-seq, and protein data. A total of 30.4% of patients in the low-risk group had better responses to anti-PD-L1 immunotherapy. A Kaplan-Meier analysis showed that patients in the low-risk group had longer overall survival. A time-dependent receiver operating characteristic curve and Cox analyses indicated that risk score served as an independent predictive indicator. The robustness of the immunotherapy response and prognosis prediction was also validated in independent external datasets. Additionally, the TME landscape revealed that the low-risk group was immune activated. Furthermore, the cell communication analysis on the scRNA-seq dataset revealed that cancer-associated fibroblasts were the main communicators within the C/CR ligand-receptor network of TME. Collectively, The C/CR-based risk model simultaneously predicted immunotherapeutic response and prognosis, potentially optimizing personalized therapeutic strategies of HNSCC.

Cell-specific expression of the FAP gene is regulated by enhancer elements

Fibroblast activation protein (FAP) is an integral membrane serine protease that acts as both dipeptidyl peptidase and collagenase. In recent years, FAP has attracted considerable attention due to its specific upregulation in multiple types of tumor cell populations, including cancer cells in various cancer types, making FAP a potential target for therapy. However, relatively few papers pay attention to the mechanisms driving the cell-specific expression of the FAP gene. We found no correlation between the activities of the two FAP promoter variants (short and long) and the endogenous FAP mRNA expression level in several cell lines with different FAP expression levels. This suggested that other mechanisms may be responsible for specific transcriptional regulation of the FAP gene. We analyzed the distribution of known epigenetic and structural chromatin marks in FAP-positive and FAP-negative cell lines and identified two potential enhancer-like elements (E1 and E2) in the FAP gene locus. We confirmed the specific enrichment of H3K27ac in the putative enhancer regions in FAP-expressing cells. Both the elements exhibited enhancer activity independently of each other in the functional test by increasing the activity of the FAP promoter variants to a greater extent in FAP-expressing cell lines than in FAP-negative cell lines. The transcription factors AP-1, CEBPB, and STAT3 may be involved in FAP activation in the tumors. We hypothesized the existence of a positive feedback loop between FAP and STAT3, which may have implications for developing new approaches in cancer therapy.

The Evolution of Single-Cell RNA Sequencing Technology and Application: Progress and Perspectives

As an emerging sequencing technology, single-cell RNA sequencing (scRNA-Seq) has become a powerful tool for describing cell subpopulation classification and cell heterogeneity by achieving high-throughput and multidimensional analysis of individual cells and circumventing the shortcomings of traditional sequencing for detecting the average transcript level of cell populations. It has been applied to life science and medicine research fields such as tracking dynamic cell differentiation, revealing sensitive effector cells, and key molecular events of diseases. This review focuses on the recent technological innovations in scRNA-Seq, highlighting the latest research results with scRNA-Seq as the core technology in frontier research areas such as embryology, histology, oncology, and immunology. In addition, this review outlines the prospects for its innovative application in traditional Chinese medicine (TCM) research and discusses the key issues currently being addressed by scRNA-Seq and its great potential for exploring disease diagnostic targets and uncovering drug therapeutic targets in combination with multiomics technologies.

The role of microRNAs in the modulation of cancer-associated fibroblasts activity during pancreatic cancer pathogenesis

Pancreatic ductal adenocarcinoma (PDAC) is the deadliest of the common cancers. A major hallmark of PDAC is an abundant and dense fibrotic stroma, the result of a disproportionate deposition of extracellular matrix (ECM) proteins. Cancer-associated fibroblasts (CAFs) are the main mediators of PDAC desmoplasia. CAFs represent a heterogenous group of activated fibroblasts with different origins and activation mechanisms. microRNAs (miRNAs) are small non-coding RNAs with critical activity during tumour development and resistance to chemotherapy. Increasing evidence has revealed that miRNAs play a relevant role in the differentiation of normal fibroblasts into CAFs in PDAC. In this review, we discuss recent findings on the role of miRNAs in the activation of CAFs during the progression of PDAC and its response to therapy, as well as the potential role that PDAC-derived exosomal miRNAs may play in the activation of hepatic stellate cells (HSCs) and formation of liver metastasis. Since targeting of CAF activation may be a viable strategy for PDAC therapy, and miRNAs have emerged as potential therapeutic targets, understanding the biology underpinning miRNA-mediated tumour cell-CAF interactions is an important component in guiding rational approaches to treating this deadly disease.

Combined Single-Cell and Spatial Transcriptomics Reveal the Metabolic Evolvement of Breast Cancer during Early Dissemination

Breast cancer is now the most frequently diagnosed malignancy, and metastasis remains the leading cause of death in breast cancer. However, little is known about the dynamic changes during the evolvement of dissemination. In this study, 65 968 cells from four patients with breast cancer and paired metastatic axillary lymph nodes are profiled using single-cell RNA sequencing (scRNA-seq) and spatial transcriptomics. A disseminated cancer cell cluster with high levels of oxidative phosphorylation (OXPHOS), including the upregulation of cytochrome C oxidase subunit 6C and dehydrogenase/reductase 2, is identified. The transition between glycolysis and OXPHOS when dissemination initiates is noticed. Furthermore, this distinct cell cluster is distributed along the tumor's leading edge. The findings here are verified in three different cohorts of breast cancer patients and an external scRNA-seq dataset, which includes eight patients with breast cancer and paired metastatic axillary lymph nodes. This work describes the dynamic metabolic evolvement of early disseminated breast cancer and reveals a switch between glycolysis and OXPHOS in breast cancer cells as the early event during lymph node metastasis.

Single-cell analysis reveals prognostic fibroblast subpopulations linked to molecular and immunological subtypes of lung cancer

Fibroblasts are poorly characterised cells that variably impact tumour progression. Here, we use single cell RNA-sequencing, multiplexed immunohistochemistry and digital cytometry (CIBERSORTx) to identify and characterise three major fibroblast subpopulations in human non-small cell lung cancer: adventitial, alveolar and myofibroblasts. Alveolar and adventitial fibroblasts (enriched in control tissue samples) localise to discrete spatial niches in histologically normal lung tissue and indicate improved overall survival rates when present in lung adenocarcinomas (LUAD). Trajectory inference identifies three phases of control tissue fibroblast activation, leading to myofibroblast enrichment in tumour samples: initial upregulation of inflammatory cytokines, followed by stress-response signalling and ultimately increased expression of fibrillar collagens. Myofibroblasts correlate with poor overall survival rates in LUAD, associated with loss of epithelial differentiation, TP53 mutations, proximal molecular subtypes and myeloid cell recruitment. In squamous carcinomas myofibroblasts were not prognostic despite being transcriptomically equivalent. These findings have important implications for developing fibroblast-targeting strategies for cancer therapy.

Cancer-educated mammary adipose tissue-derived stromal/stem cells in obesity and breast cancer: spatial regulation and function

BACKGROUND

Breast cancer is the most frequently diagnosed cancer and a common cause of cancer-related death in women. It is well recognized that obesity is associated with an enhanced risk of more aggressive breast cancer as well as reduced patient survival. Breast adipose tissue-derived mesenchymal stromal/stem cells (bASCs) are crucial components of the tumor microenvironment. A key step initially involved in this process might be the de-differentiation of bASCs into tumor supporting phenotypes.

METHODS

In the present work, we isolated bASCs from adipose tissues adjacent to the tumor (aT bASCs) from lean- (ln-aT bASCs, BMI ≤ 25) and breast cancer patients with obesity (ob-aT bASCs, BMI ≥ 35), and analyzed their phenotypes with functional assays and RNA sequencing, compared to their counterparts isolated from adipose tissues distant from the tumor (dT bASCs).

RESULTS

We show that ln-aT bASCs are susceptible to be transformed into an inflammatory cancer-associated phenotype, whereas ob-aT bASCs are prone to be cancer-educated into a myofibroblastic phenotype. Both ln-aT- and ob-aT bASCs compromise their physiological differentiation capacity, and upregulate metastasis-promoting factors. While ln-aT bASCs stimulate proliferation, motility and chemoresistance by inducing epithelial-mesenchymal transition of low malignant breast cancer cells, ob-aT bASCs trigger more efficiently a cancer stem cell phenotype in highly malignant breast cancer cells.

CONCLUSION

Breast cancer-associated bASCs are able to foster malignancy of breast cancer cells by multiple mechanisms, especially, induction of epithelial-mesenchymal transition and activation of stemness-associated genes in breast cancer cells. Blocking the de-differentiation of bASCs in the tumor microenvironment could be a novel strategy to develop an effective intervention for breast cancer patients.

SIGNIFICANCE

This study provides mechanistic insights into how obesity affects the phenotype of bASCs in the TME. Moreover, it highlights the molecular changes inside breast cancer cells upon cell-cell interaction with cancer-educated bASCs.

Cancer-associated fibroblasts: The chief architect in the tumor microenvironment

Stromal heterogeneity of tumor microenvironment (TME) plays a crucial role in malignancy and therapeutic resistance. Cancer-associated fibroblasts (CAFs) are one of the major players in tumor stroma. The heterogeneous sources of origin and subsequent impacts of crosstalk with breast cancer cells flaunt serious challenges before current therapies to cure triple-negative breast cancer (TNBC) and other cancers. The positive and reciprocal feedback of CAFs to induce cancer cells dictates their mutual synergy in establishing malignancy. Their substantial role in creating a tumor-promoting niche has reduced the efficacy of several anti-cancer treatments, including radiation, chemotherapy, immunotherapy, and endocrine therapy. Over the years, there has been an emphasis on understanding CAF-induced therapeutic resistance in order to enhance cancer therapy results. CAFs, in the majority of cases, employ crosstalk, stromal management, and other strategies to generate resilience in surrounding tumor cells. This emphasizes the significance of developing novel strategies that target particular tumor-promoting CAF subpopulations, which will improve treatment sensitivity and impede tumor growth. In this review, we discuss the current understanding of the origin and heterogeneity of CAFs, their role in tumor progression, and altering the tumor response to therapeutic agents in breast cancer. In addition, we also discuss the potential and possible approaches for CAF-mediated therapies.

Mechanisms driving the immunoregulatory function of cancer cells

Tumours display an astonishing variation in the spatial distribution, composition and activation state of immune cells, which impacts their progression and response to immunotherapy. Shedding light on the mechanisms that govern the diversity and function of immune cells in the tumour microenvironment will pave the way for the development of more tailored immunomodulatory strategies for the benefit of patients with cancer. Cancer cells, by virtue of their paracrine and juxtacrine communication mechanisms, are key contributors to intertumour heterogeneity in immune contextures. In this Review, we discuss how cancer cell-intrinsic features, including (epi)genetic aberrations, signalling pathway deregulation and altered metabolism, play a key role in orchestrating the composition and functional state of the immune landscape, and influence the therapeutic benefit of immunomodulatory strategies. Moreover, we highlight how targeting cancer cell-intrinsic parameters or their downstream immunoregulatory pathways is a viable strategy to manipulate the tumour immune milieu in favour of antitumour immunity.

Single-cell dissection of cellular and molecular features underlying human cervical squamous cell carcinoma initiation and progression

Cervical squamous cell carcinoma (CESC) is a prototypical human cancer with well-characterized pathological stages of initiation and progression. However, high-resolution knowledge of the transcriptional programs underlying each stage of CESC is lacking, and important questions remain. We performed single-cell RNA sequencing of 76,911 individual cells from 13 samples of human cervical tissues at various stages of malignancy, illuminating the transcriptional tumorigenic trajectory of cervical epithelial cells and revealing key factors involved in CESC initiation and progression. In addition, we found significant correlations between the abundance of specific myeloid, lymphoid, and endothelial cell populations and the progression of CESC, which were also associated with patients' prognosis. Last, we demonstrated the tumor-promoting function of matrix cancer-associated fibroblasts via the NRG1-ERBB3 pathway in CESC. This study provides a valuable resource and deeper insights into CESC initiation and progression, which is helpful in refining CESC diagnosis and for the design of optimal treatment strategies.

Single-Cell Profiling Comparisons of Tumor Microenvironment between Primary Advanced Lung Adenocarcinomas and Brain Metastases and Machine Learning Algorithms in Predicting Immunotherapeutic Responses

Brain metastasis (BM) occurs commonly in patients with lung adenocarcinomas. Limited evidence indicates safety and efficacy of immunotherapy for this metastatic tumor, though immune checkpoint blockade has become the front-line treatment for primary advanced non-small cell lung cancer. We aim to comprehensively compare tumor microenvironments (TME) between primary tumors (PT) and BM at single-cell resolution. Single-cell RNA transcriptomics from tumor samples of PT (N = 23) and BM (N = 16) and bulk sequencing data were analyzed to explore potential differences in immunotherapeutic efficacy between PT and BM of lung adenocarcinomas. Multiple machine learning algorithms were used to develop and validate models that predict responses to immunotherapy using the external cohorts. We found obviously less infiltration of immune cells in BM than PT, characterized specifically by deletion of anti-cancer CD8+ Trm cells and more dysfunctional CD8+ Tem cells in BM tumors. Meanwhile, macrophages and dendritic cells within BM demonstrated more pro-tumoral and anti-inflammatory effects, represented by distinct distribution and function of SPP1+ and C1Qs+ tumor-associated microphages, and inhibited antigen presentation capacity and HLA-I gene expression, respectively. Besides, we also found the lack of inflammatory-like CAFs and enrichment of pericytes within BM tumors, which may be critical factors in shaping inhibitory TME. Cell communication analysis further revealed mechanisms of the immunosuppressive effects associated with the activation of some unfavorable pathways, such as TGFβ signaling, highlighting the important roles of stromal cells in the anti-inflammatory microenvironment, especially specific pericytes. Furthermore, pericyte-related genes were identified to optimally predict immunotherapeutic responses by machine learning models with great predictive performance. Overall, various factors contribute to the immunosuppressive TME within BM tumors, represented by the lack of critical anti-cancer immune cells. Meanwhile, pericytes may help shape the TME and targeting the associated mechanisms may enhance immunotherapy efficacy for BM tumors in patients with lung adenocarcinomas.

PVRIG Expression Is an Independent Prognostic Factor and a New Potential Target for Immunotherapy in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is a frequent and deadly cancer in need of new treatments. Immunotherapy has shown promising results in several solid tumors. The TIGIT/DNAM-1 axis gathers targets for new immune checkpoint inhibitors (ICIs). Here, we aimed at highlighting the potential of this axis as a new therapeutic option for HCC. For this, we built a large transcriptomic database of 683 HCC samples, clinically annotated, and 319 normal liver tissues. We interrogated this database for the transcriptomic expression of each member of the TIGIT/DNAM-1 axis and tested their prognostic value for survival. We then focused on the most discriminant one for these criteria, i.e., PVRIG, and analyzed the clinical characteristics, the disease-free and overall survivals, and biological pathways associated with PVRIG High tumors. Among all members of the TIGIT/DNAM-1 axis, PVRIG expression was higher in tumors than in normal liver, was heterogeneous across tumors, and was the only member with independent prognostic value for better survival. PVRIG High tumors were characterized by a higher lymphocytic infiltrate and enriched for signatures associated with tertiary lymphoid structures and better anti-tumor immune response. These results suggest that patients with PVRIG High tumors might be good candidates for immune therapy involving ICIs, notably ICIs targeting the TIGIT/DNAM-1 axis. Further functional and clinical validation is urgently required.

The promise of targeting heme and mitochondrial respiration in normalizing tumor microenvironment and potentiating immunotherapy

Cancer immunotherapy shows durable treatment responses and therapeutic benefits compared to other cancer treatment modalities, but many cancer patients display primary and acquired resistance to immunotherapeutics. Immunosuppressive tumor microenvironment (TME) is a major barrier to cancer immunotherapy. Notably, cancer cells depend on high mitochondrial bioenergetics accompanied with the supply of heme for their growth, proliferation, progression, and metastasis. This excessive mitochondrial respiration increases tumor cells oxygen consumption, which triggers hypoxia and irregular blood vessels formation in various regions of TME, resulting in an immunosuppressive TME, evasion of anti-tumor immunity, and resistance to immunotherapeutic agents. In this review, we discuss the role of heme, heme catabolism, and mitochondrial respiration on mediating immunosuppressive TME by promoting hypoxia, angiogenesis, and leaky tumor vasculature. Moreover, we discuss the therapeutic prospects of targeting heme and mitochondrial respiration in alleviating tumor hypoxia, normalizing tumor vasculature, and TME to restore anti-tumor immunity and resensitize cancer cells to immunotherapy.

Therapeutic Targeting of Cancer-Associated Fibroblasts in the Non-Small Cell Lung Cancer Tumor Microenvironment

Lung cancer is the most frequently diagnosed cancer and the leading cause of cancer death worldwide. The most common lung cancer is non-small cell lung cancer (NSCLC), with an overall 5-year survival rate of around 20% because NSCLC is a metastatic disease. A better understanding of the mechanism underlying lung cancer metastasis is therefore urgently needed. The tumor microenvironment involves different types of stromal cells and functions as key components in the progression of NSCLC. Through epithelial-mesenchymal transition (EMT), in which epithelial cells lose their polarity and acquire mesenchymal potential, cancer cells acquire metastatic abilities, as well as cancer stem-cell-like potential. We previously reported that cancer-associated fibroblasts (CAFs) interact with lung cancer cells to allow for the acquisition of malignancy and treatment resistance by paracrine loops via EMT signals in the tumor microenvironment. Furthermore, CAFs regulate the cytotoxic activity of immune cells via various cytokines and chemokines, creating a microenvironment of immune tolerance. Regulation of CAFs can therefore affect immune responses. Recent research has shown several roles of CAFs in NSCLC tumorigenesis, owing to their heterogeneity, so molecular markers of CAFs should be elucidated to better classify tumor-promoting subtypes and facilitate the establishment of CAF-specific targeted therapies. CAF-targeted cancer treatments may suppress EMT and regulate the niche of cancer stem cells and the immunosuppressive network and thus may prove useful for NSCLC treatment through multiple mechanisms.

Resistance mechanisms of immune checkpoint inhibition in lymphoma: Focusing on the tumor microenvironment

Immune checkpoint inhibitors (ICIs) have revolutionized the therapeutic strategies of multiple types of malignancies including lymphoma. However, efficiency of ICIs varies dramatically among different lymphoma subtypes, and durable response can only be achieved in a minority of patients, thus requiring unveiling the underlying mechanisms of ICI resistance to optimize the individualized regimens and improve the treatment outcomes. Recently, accumulating evidence has identified potential prognostic factors for ICI therapy, including tumor mutation burden and tumor microenvironment (TME). Given the distinction between solid tumors and hematological malignancies in terms of TME, we here review the clinical updates of ICIs for lymphoma, and focus on the underlying mechanisms for resistance induced by TME, which play important roles in lymphoma and remarkably influence its sensitivity to ICIs. Particularly, we highlight the value of multiple cell populations (e.g., tumor infiltrating lymphocytes, M2 tumor-associated macrophages, and myeloid-derived suppressor cells) and metabolites (e.g., indoleamine 2, 3-dioxygenase and adenosine) in the TME as prognostic biomarkers for ICI response, and also underline additional potential targets in immunotherapy, such as EZH2, LAG-3, TIM-3, adenosine, and PI3Kδ/γ.

Evaluating near-infrared photoimmunotherapy for targeting fibroblast activation protein-α expressing cells in vitro and in vivo

Photoimmunotherapy (PIT), carried out using an Ab conjugated to the near infrared dye IRDye700DX, is achieving significant success in target-specific elimination of cells. Fibroblast activation protein alpha (FAP-α) is an important target in cancer because of its expression by cancer-associated fibroblasts (CAFs) as well as by some cancer cells. Cancer-associated fibroblasts that express FAP-α have protumorigenic and immune suppressive functions. Using immunohistochemistry of human breast cancer tissue microarrays, we identified an increase of FAP-α⁺  CAFs in invasive breast cancer tissue compared to adjacent normal tissue. We found FAP-α expression increased in fibroblasts cocultured with cancer cells. In proof-of-principle studies, we engineered human FAP-α overexpressing MDA-MB-231 and HT-1080 cancer cells and murine FAP-α overexpressing NIH-3T3 fibroblasts to evaluate several anti-FAP-α Abs and selected AF3715 based on its high binding affinity with both human and mouse FAP-α. After conjugation of AF3715 with the phthalocyanine dye IR700, the resultant Ab conjugate, FAP-α-IR700, was evaluated in cells and tumors for its specificity and effectiveness in eliminating FAP-α expressing cell populations with PIT. Fibroblast activation protein-α-IR700-PIT resulted in effective FAP-α-specific cell killing in the engineered cancer cells and in two patient-derived CAFs in a dose-dependent manner. Following an intravenous injection, FAP-α-IR700 retention was three-fold higher than IgG-IR700 in FAP-α overexpressing tumors, and two-fold higher compared to WT tumors. Fibroblast activation protein-α-IR700-PIT resulted in significant growth inhibition of tumors derived from FAP-α overexpressing human cancer cells. A reduction of endogenous FAP-α⁺  murine CAFs was identified at 7 days after FAP-α-IR700-PIT. Fibroblast activation protein-α-targeted near infrared PIT presents a promising strategy to eliminate FAP-α⁺  CAFs.

Signaling pathways in cancer-associated fibroblasts: recent advances and future perspectives

As a critical component of the tumor microenvironment (TME), cancer-associated fibroblasts (CAFs) play important roles in cancer initiation and progression. Well-known signaling pathways, including the transforming growth factor-β (TGF-β), Hedgehog (Hh), Notch, Wnt, Hippo, nuclear factor kappa-B (NF-κB), Janus kinase (JAK)/signal transducer and activator of transcription (STAT), mitogen-activated protein kinase (MAPK), and phosphoinositide 3-kinase (PI3K)/AKT pathways, as well as transcription factors, including hypoxia-inducible factor (HIF), heat shock transcription factor 1 (HSF1), P53, Snail, and Twist, constitute complex regulatory networks in the TME to modulate the formation, activation, heterogeneity, metabolic characteristics and malignant phenotype of CAFs. Activated CAFs remodel the TME and influence the malignant biological processes of cancer cells by altering the transcriptional and secretory characteristics, and this modulation partially depends on the regulation of signaling cascades. The results of preclinical and clinical trials indicated that therapies targeting signaling pathways in CAFs demonstrated promising efficacy but were also accompanied by some failures (e.g., NCT01130142 and NCT01064622). Hence, a comprehensive understanding of the signaling cascades in CAFs might help us better understand the roles of CAFs and the TME in cancer progression and may facilitate the development of more efficient and safer stroma-targeted cancer therapies. Here, we review recent advances in studies of signaling pathways in CAFs and briefly discuss some future perspectives on CAF research.

Ecological niches for colorectal cancer stem cell survival and thrival

To date, colorectal cancer is still ranking top three cancer types severely threatening lives. According to cancer stem cell hypothesis, malignant colorectal lumps are cultivated by a set of abnormal epithelial cells with stem cell-like characteristics. These vicious stem cells are derived from intestinal epithelial stem cells or transformed by terminally differentiated epithelial cells when they accumulate an array of transforming genomic alterations. Colorectal cancer stem cells, whatever cell-of-origin, give rise to all morphologically and functionally heterogenous tumor daughter cells, conferring them with overwhelming resilience to intrinsic and extrinsic stresses. On the other hand, colorectal cancer stem cells and their daughter cells continuously participate in constructing ecological niches for their survival and thrival by communicating with adjacent stromal cells and circulating immune guardians. In this review, we first provide an overview of the normal cell-of-origin populations contributing to colorectal cancer stem cell reservoirs and the niche architecture which cancer stem cells depend on at early stage. Then we survey recent advances on how these aberrant niches are fostered by cancer stem cells and their neighbors. We also discuss recent research on how niche microenvironment affects colorectal cancer stem cell behaviors such as plasticity, metabolism, escape of immune surveillance as well as resistance to clinical therapies, therefore endowing them with competitive advantages compared to their normal partners. In the end, we explore therapeutic strategies available to target malignant stem cells.

CCL18 signaling from tumor-associated macrophages activates fibroblasts to adopt a chemoresistance-inducing phenotype

The heterogeneity of cancer-associated fibroblasts (CAFs) might be ascribed to differences in origin. CD10 and GPR77 have been reported to identify a chemoresistance-inducing CAF subset in breast cancer. However, the precise mechanism for the formation of the CD10⁺GPR77⁺ CAFs remains unknown. In this study, we found that CCL18 expression was positively correlated with the density of CD10⁺GPR77⁺ CAFs in breast cancer and associated with a poor response to chemotherapy. Moreover, CCL18 secreted by tumor-associated macrophages (TAMs) activated a CD10⁺GPR77⁺ CAF phenotype in normal breast-resident fibroblasts (NBFs), which could then enrich cancer stem cells (CSCs) and induce chemoresistance in breast cancer cells. Mechanistically, CCL18 activated NF-κB signaling via PITPNM3 and thus enhanced the production of IL-6 and IL-8. Furthermore, intratumoral CCL18 injection significantly induced the activation of NBFs and the chemoresistance of xenografts in vivo. In addition, targeting CCL18 by anti-CCL18 antibody could inhibit the formation of CD10⁺GPR77⁺ CAFs and recover the chemosensitivity in vivo, leading to effective tumor control. Collectively, these findings reveal that inflammatory signaling crosstalk between TAMs and fibroblasts is responsible for the formation of the CD10⁺GPR77⁺ CAFs, suggesting CCL18-PITPNM3 signaling is a potential therapeutic target to block the activation of this specific CAF subtype and tumor chemoresistance.

myCAFs are better than yours: targeting myofibroblasts potentiates immunotherapy

New findings (Krishnamurty et al.) implicate a subset of cancer-associated fibroblasts (CAFs) that express leucine-rich repeat containing 15 (LRRC15) in promoting tumor growth in pancreatic adenocarcinoma (PDAC), by suppressing the antitumor immunity of cytotoxic T cells. Genetic ablation of LRRC15+ CAFs resulted in better response to immune checkpoint blockade, suggesting they may be a novel target for therapy.

Promising immunotherapeutic targets in lung cancer based on single-cell RNA sequencing

Immunotherapy has made great strides in the treatment of lung cancer, but a significant proportion of patients still do not respond to treatment. Therefore, the identification of novel targets is crucial to improving the response to immunotherapy. The tumor microenvironment (TME) is a complex niche composed of diverse pro-tumor molecules and cell populations, making the function and mechanism of a unique cell subset difficult to understand. However, the advent of single-cell RNA sequencing (scRNA-seq) technology has made it possible to identify cellular markers and understand their potential functions and mechanisms in the TME. In this review, we highlight recent advances emerging from scRNA-seq studies in lung cancer, with a particular focus on stromal cells. We elucidate the cellular developmental trajectory, phenotypic remodeling, and cell interactions during tumor progression. Our review proposes predictive biomarkers and novel targets for lung cancer immunotherapy based on cellular markers identified through scRNA-seq. The identification of novel targets could help improve the response to immunotherapy. The use of scRNA-seq technology could provide new strategies to understand the TME and develop personalized immunotherapy for lung cancer patients.

Chemokine switch regulated by TGF-β1 in cancer-associated fibroblast subsets determines the efficacy of chemo-immunotherapy

Combining immunogenic cell death-inducing chemotherapies and PD-1 blockade can generate remarkable tumor responses. It is now well established that TGF-β1 signaling is a major component of treatment resistance and contributes to the cancer-related immunosuppressive microenvironment. However, whether TGF-β1 remains an obstacle to immune checkpoint inhibitor efficacy when immunotherapy is combined with chemotherapy is still to be determined. Several syngeneic murine models were used to investigate the role of TGF-β1 neutralization on the combinations of immunogenic chemotherapy (FOLFOX: 5-fluorouracil and oxaliplatin) and anti-PD-1. Cancer-associated fibroblasts (CAF) and immune cells were isolated from CT26 and PancOH7 tumor-bearing mice treated with FOLFOX, anti-PD-1 ± anti-TGF-β1 for bulk and single cell RNA sequencing and characterization. We showed that TGF-β1 neutralization promotes the therapeutic efficacy of FOLFOX and anti-PD-1 combination and induces the recruitment of antigen-specific CD8⁺ T cells into the tumor. TGF-β1 neutralization is required in addition to chemo-immunotherapy to promote inflammatory CAF infiltration, a chemokine production switch in CAF leading to decreased CXCL14 and increased CXCL9/10 production and subsequent antigen-specific T cell recruitment. The immune-suppressive effect of TGF-β1 involves an epigenetic mechanism with chromatin remodeling of CXCL9 and CXCL10 promoters within CAF DNA in a G9a and EZH2-dependent fashion. Our results strengthen the role of TGF-β1 in the organization of a tumor microenvironment enriched in myofibroblasts where chromatin remodeling prevents CXCL9/10 production and limits the efficacy of chemo-immunotherapy.

Parallel single-cell and bulk transcriptome analyses reveal key features of the gastric tumor microenvironment

BACKGROUND

The tumor microenvironment (TME) has been shown to strongly influence treatment outcome for cancer patients in various indications and to influence the overall survival. However, the cells forming the TME in gastric cancer have not been extensively characterized.

RESULTS

We combine bulk and single-cell RNA sequencing from tumors and matched normal tissue of 24 treatment-naïve GC patients to better understand which cell types and transcriptional programs are associated with malignant transformation of the stomach. Clustering 96,623 cells of non-epithelial origin reveals 81 well-defined TME cell types. We find that activated fibroblasts and endothelial cells are most prominently overrepresented in tumors. Intercellular network reconstruction and survival analysis of an independent cohort imply the importance of these cell types together with immunosuppressive myeloid cell subsets and regulatory T cells in establishing an immunosuppressive microenvironment that correlates with worsened prognosis and lack of response in anti-PD1-treated patients. In contrast, we find a subset of IFNγ activated T cells and HLA-II expressing macrophages that are linked to treatment response and increased overall survival.

CONCLUSIONS

Our gastric cancer single-cell TME compendium together with the matched bulk transcriptome data provides a unique resource for the identification of new potential biomarkers for patient stratification. This study helps further to elucidate the mechanism of gastric cancer and provides insights for therapy.

A Novel Molecular Signature of Cancer-Associated Fibroblasts Predicts Prognosis and Immunotherapy Response in Pancreatic Cancer

Cancer-associated fibroblasts (CAFs), a prominent population of stromal cells, play a crucial role in tumor progression, prognosis, and treatment response. However, the relationship among CAF-based molecular signatures, clinical outcomes, and tumor microenvironment infiltration remains largely elusive in pancreatic cancer (PC). Here, we collected multicenter PC data and performed integrated analysis to investigate the role of CAF-related genes (CRGs) in PC. Firstly, we demonstrated that α-SMA⁺ CAFs were the most prominent stromal components and correlated with the poor survival rates of PC patients in our tissue microarrays. Then, we discriminated two diverse molecular subtypes (CAF clusters A and B) and revealed the significant differences in the tumor immune microenvironment (TME), four reported CAF subpopulations, clinical characteristics, and prognosis in PC samples. Furthermore, we analyzed their association with the immunotherapy response of PC patients. Lastly, a CRG score was constructed to predict prognosis, immunotherapy responses, and chemosensitivity in pancreatic cancer patients. In summary, these findings provide insights into further research targeting CAFs and their TME, and they pave a new road for the prognosis evaluation and individualized treatment of PC patients.

ATM Regulates Differentiation of Myofibroblastic Cancer-Associated Fibroblasts and Can Be Targeted to Overcome Immunotherapy Resistance

Myofibroblastic cancer-associated fibroblast (myoCAF)-rich tumors generally contain few T cells and respond poorly to immune-checkpoint blockade. Although myoCAFs are associated with poor outcome in most solid tumors, the molecular mechanisms regulating myoCAF accumulation remain unclear, limiting the potential for therapeutic intervention. Here, we identify ataxia-telangiectasia mutated (ATM) as a central regulator of the myoCAF phenotype. Differentiating myofibroblasts in vitro and myoCAFs cultured ex vivo display activated ATM signaling, and targeting ATM genetically or pharmacologically could suppress and reverse differentiation. ATM activation was regulated by the reactive oxygen species-producing enzyme NOX4, both through DNA damage and increased oxidative stress. Targeting fibroblast ATM in vivo suppressed myoCAF-rich tumor growth, promoted intratumoral CD8 T-cell infiltration, and potentiated the response to anti-PD-1 blockade and antitumor vaccination. This work identifies a novel pathway regulating myoCAF differentiation and provides a rationale for using ATM inhibitors to overcome CAF-mediated immunotherapy resistance.

SIGNIFICANCE

ATM signaling supports the differentiation of myoCAFs to suppress T-cell infiltration and antitumor immunity, supporting the potential clinical use of ATM inhibitors in combination with checkpoint inhibition in myoCAF-rich, immune-cold tumors.

Interferon-dependent SLC14A1+ cancer-associated fibroblasts promote cancer stemness via WNT5A in bladder cancer

Cancer-associated fibroblasts (CAFs) play a role in response to cancer treatment and patient prognosis. CAFs show phenotypic and functional heterogeneity and differ widely in tumors of different tissue origin. Here, we use single-cell RNA sequencing of bladder cancer (BC) patient samples and report a CAF subpopulation characterized by overexpression of the urea transporter SLC14A1. This population is induced by interferon signaling and confers stemness to BC cells via the WNT5A paracrine pathway. Activation of cGAS-STING signaling in tumor cells drives interferon production, thereby revealing a link between cGAS-STING signaling and SLC14A1+ CAF differentiation. Furthermore, the inhibition of SLC14A1+ CAF formation via targeting of STAT1 or STING sensitizes tumor cells to chemotherapy. More important, BC patients with high proportions of intratumoral SLC14A1+ CAFs show cancer stage-independent poor outcome and a worse response rate to neoadjuvant chemotherapy or immunotherapy.

CAF-immune cell crosstalk and its impact in immunotherapy

Tumour cells do not exist as isolated entities. Instead, they are surrounded by a variety of cells and extracellular matrix, which form the tumour microenvironment (TME). The interaction between cancer cells and their microenvironment is increasingly acknowledged as essential in dictating the outcome of the patients. The TME includes everything that surrounds tumour cells and is often highjacked by the latter to promote their growth, invasion, and immune escape. Immune cells and cancer-associated fibroblasts (CAFs) are essential components of the TME, and there is increasing evidence that their interaction constitutes a major player not only for tumour progression but also for therapy response.

Recent work in the field of immuno-oncology resulted in the development of novel therapies that aim at activating immune cells against cancer cells to eliminate them. Despite their unprecedented success, the lack of response from a large portion of patients highlights the need for further progress and improvement. To achieve its ultimate goal, the interaction between cancer cells and the TME needs to be studied in-depth to allow the targeting of mechanisms that are involved in resistance or refractoriness to therapy. Moreover, predictive and prognostic biomarkers for patient stratification are still missing. In this review, we focus on and highlight the complexity of CAFs within the TME and how their interaction, particularly with immune cells, can contribute to treatment failure. We further discuss how this crosstalk can be further dissected and which strategies are currently used to target them.

Weakly migratory metastatic breast cancer cells activate fibroblasts via microvesicle-Tg2 to facilitate dissemination and metastasis

Cancer cell migration is highly heterogeneous, and the migratory capability of cancer cells is thought to be an indicator of metastatic potential. It is becoming clear that a cancer cell does not have to be inherently migratory to metastasize, with weakly migratory cancer cells often found to be highly metastatic. However, the mechanism through which weakly migratory cells escape from the primary tumor remains unclear. Here, utilizing phenotypically sorted highly and weakly migratory human breast cancer cells, we demonstrate that weakly migratory metastatic cells disseminate from the primary tumor via communication with stromal cells. While highly migratory cells are capable of single cell migration, weakly migratory cells rely on cell-cell signaling with fibroblasts to escape the primary tumor. Weakly migratory cells release microvesicles rich in tissue transglutaminase 2 (Tg2) which activate murine fibroblasts and lead weakly migratory cancer cell migration in vitro. These microvesicles also induce tumor stiffening and fibroblast activation in vivo and enhance the metastasis of weakly migratory cells. Our results identify microvesicles and Tg2 as potential therapeutic targets for metastasis and reveal a novel aspect of the metastatic cascade in which weakly migratory cells release microvesicles which activate fibroblasts to enhance cancer cell dissemination.

An integrated map of fibroblastic populations in human colon mucosa and cancer tissues

Fibroblasts and myofibroblasts are major mesenchymal cells in the lamina propria of colon mucosa and in colon cancer tissues. Detailed insight into the highly specific populations of fibroblasts and myofibroblasts is required to understand the integrity and homeostasis of human colon mucosa and colon cancer. Based on gene expression profiles of single cells, we identified fibroblast populations that produce extracellular matrix components, Wnt ligand- and BMP-secreting fibroblasts, chemokine- and chemokine ligand-generating fibroblasts, highly activated fibroblasts, immune-modulating fibroblasts, epithelial cell-modulating myofibroblasts, stimuli-responsive myofibroblasts, proliferating myofibroblasts, fibroblast-like myofibroblasts, matrix producing myofibroblasts, and contractile myofibroblasts in human colon mucosa. In colon cancer tissue, the compositions of fibroblasts and myofibroblasts were highly altered, as were the expressing patterns of genes including BMPs, Wnt ligands, chemokines, chemokine ligands, growth factors and extracellular matrix components in fibroblasts and myofibroblasts. Our work expands the working atlas of fibroblasts and myofibroblasts and provides a framework for interrogating the complexity of stromal cells in human healthy colon mucosa and colon cancer tissues.

Combined MEK and STAT3 Inhibition Uncovers Stromal Plasticity by Enriching for Cancer-Associated Fibroblasts With Mesenchymal Stem Cell-Like Features to Overcome Immunotherapy Resistance in Pancreatic Cancer

BACKGROUND & AIMS

We have shown that reciprocally activated rat sarcoma (RAS)/mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK) and Janus kinase/signal transducer and activator of transcription 3 (STAT3) pathways mediate therapeutic resistance in pancreatic ductal adenocarcinoma (PDAC), while combined MEK and STAT3 inhibition (MEKi+STAT3i) overcomes such resistance and alters stromal architecture. We now determine whether MEKi+STAT3i reprograms the cancer-associated fibroblast (CAF) and immune microenvironment to overcome resistance to immune checkpoint inhibition in PDAC.

METHODS

CAF and immune cell transcriptomes in MEKi (trametinib)+STAT3i (ruxolitinib)-treated vs vehicle-treated Ptf1aCre/+;LSL-KrasG12D/+;Tgfbr2flox/flox (PKT) tumors were examined via single-cell RNA sequencing (scRNAseq). Clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeats associated protein 9 silencing of CAF-restricted Map2k1/Mek1 or Stat3, or both, enabled interrogation of CAF-dependent effects on immunologic remodeling in orthotopic models. Tumor growth, survival, and immune profiling via mass cytometry by time-of-flight were examined in PKT mice treated with vehicle, anti-programmed cell death protein 1 (PD-1) monotherapy, and MEKi+STAT3i combined with anti-PD1.

RESULTS

MEKi+STAT3i attenuates Il6/Cxcl1-expressing proinflammatory and Lrrc15-expressing myofibroblastic CAF phenotypes while enriching for Ly6a/Cd34-expressing CAFs exhibiting mesenchymal stem cell-like features via scRNAseq in PKT mice. This CAF plasticity is associated with M2-to-M1 reprogramming of tumor-associated macrophages, and enhanced trafficking of cluster of differentiation 8+ T cells, which exhibit distinct effector transcriptional programs. These MEKi+STAT3i-induced effects appear CAF-dependent, because CAF-restricted Mek1/Stat3 silencing mitigates inflammatory-CAF polarization and myeloid infiltration in vivo. Addition of MEKi+STAT3i to PD-1 blockade not only dramatically improves antitumor responses and survival in PKT mice but also augments recruitment of activated/memory T cells while improving their degranulating and cytotoxic capacity compared with anti-PD-1 monotherapy. Importantly, treatment of a patient who has chemotherapy-refractory metastatic PDAC with MEKi (trametinib), STAT3i (ruxolitinib), and PD-1 inhibitor (nivolumab) yielded clinical benefit.

CONCLUSIONS

Combined MEKi+STAT3i mitigates stromal inflammation and enriches for CAF phenotypes with mesenchymal stem cell-like properties to overcome immunotherapy resistance in PDAC.

Targeting cancer-associated fibroblasts: Challenges, opportunities and future directions

Cancer-associated fibroblasts (CAFs) are a common cell in the tumour microenvironment with diverse tumour-promoting functions. Their presence in tumours is commonly associated with poor prognosis making them attractive therapeutic targets, particularly in the context of immunotherapy where CAFs have been shown to promote resistance to checkpoint blockade. Previous attempts to inhibit CAFs clinically have not been successful, however, in part due to a lack of understanding of CAF heterogeneity and function, with some fibroblast populations potentially being tumour suppressive. Recent single-cell transcriptomic studies have advanced our understanding of fibroblast phenotypes in normal tissues and cancers, allowing for a more precise characterisation of CAF subsets and providing opportunities to develop new therapies. Here we review recent advances in the field, focusing on the evolving area of therapeutic CAF targeting.

Integrin α11β1 in tumor fibrosis: more than just another cancer-associated fibroblast biomarker?

There is currently an increased interest in understanding the role of the tumor microenvironment (TME) in tumor growth and progression. In this context the role of integrins in cancer-associated fibroblasts (CAFs) will need to be carefully re-evaluated. Fibroblast-derived cells are not only in the focus in tumors, but also in tissue fibrosis as well as in inflammatory conditions. The recent transcriptional profiling of what has been called "the pan-fibroblast cell lineage" in mouse and human tissues has identified novel transcriptional biomarker mRNAs encoding the secreted ECM proteins dermatopontin and collagen XV as well as the phosphatidylinositol-anchored membrane protein Pi16. Some of the genes identified in these fibroblasts scRNA-seq datasets will be useful for rigorous comparative characterizations of fibroblast-derived cell subpopulations. At the same time, it will be a challenge in the coming years to validate these transcriptional mRNA datasets at the protein-(expression) and at tissue-(distribution) levels and to find useful protein biomarker reagents that will facilitate fibroblast profiling at the cell level. In the current review we will focus on the role of the collagen-binding integrin α11β1 in CAFs, summarizing our own work as well as published datasets with information on α11 mRNA expression in selected tumors. Our experimental data suggest that α11β1 is more than just another biomarker and that it as a functional collagen receptor in the TME is playing a central role in regulating collagen assembly and matrix remodeling, which in turn impact tumor growth and metastasis.

The tumor microenvironment and triple-negative breast cancer aggressiveness: shedding light on mechanisms and targeting

INTRODUCTION

In contrast to other breast cancer subtypes, there are currently limited options of targeted therapies for triple-negative breast cancer (TNBC). Immense research has demonstrated that not only cancer cells but also stromal cells and immune cells in the tumor microenvironment (TME) play significant roles in the progression of TNBC. It is thus critical to understand the components of the TME of TNBC and the interactions between the various cell populations.

AREAS COVERED

The components of the TME of TNBC identified by single-cell technologies are reviewed. Furthermore, the molecular interactions between the cells and the potential therapeutic targets contributing to the progression of TNBC are discussed.

EXPERT OPINION

Single-cell omics studies have contributed to the classification of cells in the TME and the identification of important cell types involved in the progression and the treatment of the tumor. The interactions between cancer cells and stromal cells/immune cells in the TME have led to the discovery of potential therapeutic targets. Experimental data with spatial and temporal resolution will further boost the understanding of the TME of TNBC.