In our recent study, published in Nature Communications, we dissected the CAF heterogeneity in melanoma and non-melanoma  skin cancer (basal cell carcinoma, BCC, and squamous cell carcinoma, SCC), and explored their abundance, distribution, and potential role in skin cancer progression.

Setting the Stage: Understanding CAF Subtypes in Skin Cancer
CAFs are a diverse group of cells within the tumor microenvironment that influence cancer growth, immune evasion, and metastasis. Our study uncovered three main CAF populations in human skin cancers, including their interaction with other cells of the TME such as epithelial, mesenchymal and immune cells:

1. Myofibroblast-like RGS5+ CAFs, that show a close transcriptomic similarity to both myofibroblasts and pericytes, and likely correspond to previously described ACTA2+ myoCAFs,
2. Matrix CAFs (mCAFs), which are characterized by elevated expression of extracellular matrix (ECM)-associated genes, and
3. Immunomodulatory CAFs (iCAFs) expressing elevated levels of cytokines and chemokines, matrix metalloproteinases (MMPS) as well as immunomodulatory factors.

Key Findings from Our Study

1. Distinct localization of CAF subsets: Intriguingly, large-field spatial visualization of the CAF subpopulations in tumor tissue of a large cohort of patients with different skin cancer subtypes revealed that the localization of the three CAF subtypes was different. While RGS5+ CAFs were detected throughout the tumors, mCAFs were predominately located at the tumor-stroma border, ensheathing tumor nests, and iCAFs were pervading the tumors in patches. The strong tumor-border association of mCAFs suggested a barrier function for this subtype. Indeed, image analysis showed that a dense mCAF barrier surrounding tumor nests prevented T cell infiltration, despite the presence of large numbers of T cells in the surrounding stroma, suggesting that mCAFs play a key role in immune cell marginalization within tumors.
2. Increased iCAF Abundance in Malignant Tumors: Most strikingly, we discovered a change in the CAF patterns from lower to higher malignancy, along with a higher overall CAF density in the aggressive variants of the respective skin cancer subtypes. Notably, the presence of iCAFs was particularly pronounced in infiltrative basal cell carcinoma (BCC) and aggressive melanomas. These cancers, known for their invasive and destructive behavior, appear to leverage the inflammatory milieu created by iCAFs to support their growth and spread.
3. iCAFs modulate immune cell infiltration and surveillance: Our study also revealed that iCAFs synthesize the majority of cytokines and chemokines, and express many immunomodulatory factors that bind to receptors expressed primarily on neutrophils, T cells and NK cells, highlighting that iCAFs are key players in immune cell recruitment and activation.
   Notably, exposure of normal healthy dermal fibroblasts to the secretome of skin cancer cells in vitro induced an iCAF-like, but not mCAF-like, phenotype, characterized by the upregulation of chemokines and cytokines that were also characteristic for the iCAF population in our single cell RNA sequencing screen. These induced iCAFs were capable of activating naïve T cells in an in vitro co-culture assay.

Implications for Cancer Research and Therapy
Our findings highlight the pivotal role of CAFs, particularly iCAFs, in shaping the tumor microenvironment. Their immunomodulatory nature and association with more aggressive skin cancer types suggest that they could serve as both biomarkers for prognosis and targets for therapeutic intervention. For instance, strategies aimed at modulating the inflammatory signals produced by iCAFs or their immunomodulatory functions might help disrupt the supportive niche they provide for tumor cells.

Moreover, understanding the distinct roles of all CAF subtypes underscores the importance of personalized approaches to therapy. While iCAFs may drive inflammation or induce T cell exhaustion, mCAFs could contribute to mechanical resistance. A deeper knowledge of these subtypes could inform the design of treatments that not only target cancer cells but also their supportive stromal partners.

Our findings suggest that specifically targeting one or the other CAF population might affect tumor progression and improve therapy outcome. Especially patients resistant to immunotherapies could benefit from targeting the immunomodulatory CAF subsets.

Future Directions          
While our study provides valuable insights, it also raises intriguing questions. What drives the variability in iCAF abundance among different cancer types? How do the three CAF subtypes interact within the tumor microenvironment? Are there specific signaling pathways or genetic alterations that preferentially recruit or activate certain CAF subtypes? And importantly, how can we effectively target these cells without disrupting their normal physiological functions?

To address these questions, future studies must incorporate larger datasets, advanced stratification methods, and integrative approaches that combine single-cell analysis, spatial transcriptomics, and functional assays. Such efforts will be instrumental in unraveling the complex interplay between CAFs and cancer cells.

In advanced cancer, the tumor microenvironment often shows immune evasion and dysfunction, including T cell exhaustion, where T cells lose their tumor-fighting ability. While we demonstrated that iCAFs can stimulate initial T cell activation, they might also induce T cell exhaustion or affect other immune cells like macrophages and regulatory T cells, potentially aiding tumor progression. Future research is needed to understand the dual role of iCAFs in balancing immune activation and suppression in advanced skin cancer.

Reflections on the Research Journey

This work was a team effort of our whole lab, and was made possible through a fantastic collaboration with the lab of Maria Kasper at the Karolinska Institute in Stockholm (https://kasperlab.org/), whose invaluable input for the single-cell RNA sequencing (scRNAseq) analysis significantly enhanced our understanding of CAF biology. Their expertise and insights were crucial in unraveling the complexities of the tumor microenvironment and advancing the study.

Conducting this study was both challenging and rewarding. One of the most exciting aspects was uncovering the consistent association between iCAFs and aggressive cancer phenotypes. These findings reinforce the notion that the tumor microenvironment is not merely a passive bystander but an active participant in cancer progression.

At the same time, this work underscored the complexities of cancer research. The subtle differences we observed in SCCs, for example, remind us that biological phenomena rarely conform to neat categories. Instead, they reflect a spectrum of interactions that require careful and nuanced interpretation.

Final Thoughts

As researchers, our ultimate goal is to translate scientific discoveries into concrete benefits for patients. By shedding light on the role of CAF subtypes in skin cancer, we hope to contribute to a growing body of knowledge that will inform more effective and personalized therapeutic strategies.

We invite you to explore our paper, published in Nature Communications, and join us in this endeavor to decode the mysteries of the tumor microenvironment. Together, we can advance our understanding of cancer and pave the way for innovations in treatment.