Cancer-associated fibroblasts (CAFs) are indispensable architects of the tumor microenvironment. They perform the essential functions of extracellular matrix deposition, stromal remodeling, tumor vasculature modulation, modification of tumor metabolism, and participation in crosstalk between cancer and immune cells. In this review, we discuss our current understanding of the principal differences between normal fibroblasts and CAFs, the origin of CAFs, their functions, and ultimately, highlight the intimate connection of CAFs to virtually all of the hallmarks of cancer. We address the remarkable degree of functional diversity and phenotypic plasticity displayed by CAFs and strive to stratify CAF biology among different tumor types into practical functional groups. Finally, we summarize the status of recent and ongoing trials of CAF-directed therapies and contend that the paucity of trials resulting in Food and Drug Administration (FDA) approvals thus far is a consequence of the failure to identify targets exclusive of pro-tumorigenic CAF phenotypes that are mechanistically linked to specific CAF functions. We believe that the development of a unified CAF nomenclature, the standardization of functional assays to assess the loss-of-function of CAF properties, and the establishment of rigorous definitions of CAF subpopulations and their mechanistic functions in cancer progression will be crucial to fully realize the promise of CAF-targeted therapies.

A series of 2-amino-2,3-dihydro-1H-indene-5-carboxamides were designed and synthesized as new selective discoidin domain receptor 1 (DDR1) inhibitors. One of the representative compounds, 7f, bound with DDR1 with a K_d value of 5.9 nM and suppressed the kinase activity with an half-maximal (50%) inhibitory concentration value of 14.9 nM. 7f potently inhibited collagen-induced DDR1 signaling and epithelial–mesenchymal transition, dose-dependently suppressed colony formation of pancreatic cancer cells, and exhibited promising in vivo therapeutic efficacy in orthotopic mouse models of pancreatic cancer.

Immune profiling of tissue through multiplex immunohistochemistry is important for the investigation of immune cell dynamics, and it can contribute to disease prognosis and evaluation of treatment response in cancer patients. However, protocols for mouse formalin-fixed, paraffin-embedded tissue have been less successful. Given that formalin fixation and paraffin embedding remains the most common preparation method for processing mouse tissue, this has limited the options to study the immune system and the impact of novel therapeutics in preclinical models. In an attempt to address this, we developed an improved immunohistochemistry protocol with a more effective Ag-retrieval buffer. We also validated 22 Abs specific for mouse immune cell markers to distinguish B cells, T cells, NK cells, macrophages, dendritic cells, and neutrophils. In addition, we designed and tested novel strategies to identify immune cells for which unique Abs are currently not available. Last, in the 4T1 model of breast cancer, we demonstrate the utility of our protocol and Ab panels in the quantitation and spatial distribution of immune cells.

Metastatic cancer cells adapt to thrive in secondary organs. To investigate metastatic adaptation, we performed transcriptomic analysis of metastatic and non-metastatic murine breast cancer cells. We found that pleiotrophin (PTN), a neurotrophic cytokine, is a metastasis-associated factor that is expressed highly by aggressive breast cancers. Moreover, elevated PTN in plasma correlated significantly with metastasis and reduced survival of breast cancer patients. Mechanistically, we find that PTN activates NF-κB in cancer cells leading to altered cytokine production, subsequent neutrophil recruitment, and an immune suppressive microenvironment. Consequently, inhibition of PTN, pharmacologically or genetically, reduces the accumulation of tumor-associated neutrophils and reverts local immune suppression, resulting in increased T cell activation and attenuated metastasis. Furthermore, inhibition of PTN significantly enhanced the efficacy of immune checkpoint blockade and chemotherapy in reducing metastatic burden in mice. These findings establish PTN as a previously unrecognized driver of a prometastatic immune niche and thus represents a promising therapeutic target for the treatment of metastatic breast cancer.

Cirrhosis is a high-risk state for hepatocellular carcinoma (HCC) development and represents an opportunity to prevent cancer. In the precancerous state of cirrhosis, there is an accumulation of neoantigens that may be specifically targetable through immunotherapy. We asked whether immune checkpoint inhibition could prevent tumorigenesis in a mouse model of diethylnitrosamine and carbon tetrachloride–induced HCC. We found that initiation of anti-PD-1 therapy prior to tumorigenesis could prevent up to 46% of liver tumors. This significant reduction in tumor burden was accompanied by infiltration of CD4⁺ Th cells and CD8⁺ cytotoxic T cells into the liver parenchyma. Importantly, anti-PD-1 therapy did not exacerbate liver dysfunction or worsen overall health in this liver disease model. Given the safety and preservation of quality of life observed with long-term immunotherapy use, an immunotherapy chemoprevention strategy is likely associated with a low risk-to-benefit ratio and high value care in select patients. These results encourage a prevention trial in cirrhotic patients with the highest risk of developing HCC. See related Spotlight by Mohammed et al., p. 897

Recent studies have identified a unique cancer-associated fibroblast (CAF) population termed antigen-presenting CAFs (apCAFs), characterized by the expression of major histocompatibility complex class II molecules, suggesting a function in regulating tumor immunity. Here, by integrating multiple single-cell RNA-sequencing studies and performing robust lineage-tracing assays, we find that apCAFs are derived from mesothelial cells. During pancreatic cancer progression, mesothelial cells form apCAFs by downregulating mesothelial features and gaining fibroblastic features, a process induced by interleukin-1 and transforming growth factor β. apCAFs directly ligate and induce naive CD4⁺ T cells into regulatory T cells (Tregs) in an antigen-specific manner. Moreover, treatment with an antibody targeting the mesothelial cell marker mesothelin can effectively inhibit mesothelial cell to apCAF transition and Treg formation induced by apCAFs. Taken together, our study elucidates how mesothelial cells may contribute to immune evasion in pancreatic cancer and provides insight on strategies to enhance cancer immune therapy. Citation Format: Huocong Huang, Zhaoning Wang, Yuqing Zhang, Rachana N. Pradhan, Debolina Ganguly, Raghav Chandra, Gilbert Murimwa, Steven Wright, Xiaowu Gu, Ravikanth Maddipati, Sören Müller, Shannon J. Turley, Rolf A. Brekken. Mesothelial cell-derived antigen-presenting cancer-associated fibroblasts induce expansion of regulatory T cells in pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer; 2022 Sep 13-16; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2022;82(22 Suppl):Abstract nr C078.