Regulatory T (Treg) cells, possess a strategic role in the maintenance of immune homeostasis, and their function has been closely linked to development of diverse pathologies including autoimmunity and cancer. Comprehensive studies in various disease contexts revealed an increased plasticity as a characteristic of Treg cells. Although Treg cell plasticity comes in various flavors, the major categories enclose the loss of Foxp3 expression, which is the master regulator of Treg cell lineage, giving rise to “ex-Treg” cells and the “fragile” Treg cells in whichFOXP3expression is retained but accompanied by the engagement of an inflammatory program and attenuation of the suppressive activity. Treg cell plasticity possess a tremendous therapeutic potential either by inducing Treg cell de-stabilization to promote anti-tumor immunity, or re-enforcing Treg cell stability to attenuate chronic inflammation. Herein, we review the literature on the Treg cell plasticity with lessons learned in autoimmunity and cancer and discuss challenges and open questions with potential therapeutic implications.

Cancer-associated fibroblasts (CAFs) have emerged as a dominant non-hematopoietic cell population in the tumour microenvironment, serving diverse functions in tumour progression. However, the mechanisms via which CAFs influence the anti-tumour immunity remain poorly understood. Here, using multiple tumour models and biopsies from cancer patients, we report that α-SMA⁺ CAFs can form immunological synapses with Foxp3⁺ regulatory T cells (Tregs) in tumours. Notably, α-SMA⁺ CAFs can phagocytose and process tumour antigens and exhibit a tolerogenic phenotype which instructs movement arrest, activation and proliferation in Tregs in an antigen-specific manner. Moreover, α-SMA⁺ CAFs display double-membrane structures resembling autophagosomes in their cytoplasm. Single-cell transcriptomic data showed an enrichment in autophagy and antigen processing/presentation pathways in α-SMA-expressing CAF clusters. Conditional knockout of Atg5 in α-SMA⁺ CAFs promoted inflammatory re-programming in CAFs, reduced Treg cell infiltration and attenuated tumour development. Overall, our findings reveal an immunosuppressive mechanism entailing the formation of synapses between α-SMA⁺ CAFs and Tregs in an autophagy-dependent manner.

Cancer-associated fibroblasts (CAFs) have emerged as a dominant non-hematopoietic cell population in the tumor microenvironment, serving diverse functions in tumor progression, invasion, matrix remodeling and resistance to therapy; yet, the precise mechanisms via which CAFs imprint on the anti-tumor immunity remain poorly understood. Extensive molecular characterization revealed an increased heterogeneity in the CAF compartment and proposed an interaction between CAFs and tumor-infiltrating immune cells, which may shape tumor immune evasion. Herein, we describe a synapse formation between α-SMA ⁺ CAFs and regulatory T cells (Tregs) in the TME. Foxp3 ⁺ Tregs were localized close to α-SMA ⁺ CAFs in diverse types of tumor models as well as biopsies from melanoma and colorectal cancer patients. Notably, phenotypically tolerogenic α-SMA ⁺ CAFs demonstrated the ability to phagocytose and process tumor antigens, instructing Treg movement arrest, activation and proliferation, in an antigen-specific manner. Of interest, α-SMA ⁺ CAFs were characterized by the presence of double-membrane structures, resembling autophagosomes, in their cytoplasm, while analysis of single-cell transcriptomic data pointed autophagy and antigen processing/presentation pathways to be enriched in α-SMA-expressing CAF clusters. In a mechanistic view, conditional knockout of the autophagy pathway in α-SMA ⁺ CAFs promoted an inflammatory re-programming of CAFs, reduced Treg infiltration, attenuated tumor development, and potentiated the efficacy of immune checkpoint inhibitor immunotherapy. Overall, our findings reveal an immunosuppressive mechanism operating in the TME, which entails the formation of synapses between α-SMA ⁺ CAFs and Tregs in an autophagy-dependent fashion and raises the potential for the development of CAF-targeted therapies in cancer. One Sentence Summary: α-SMA ⁺ Cancer Associated Fibroblasts process and present tumor antigens via autophagy to form immunological synapses with Foxp3 ⁺ T regulatory cells in the tumor microenvironment, promoting tumor development.