A metabolic gene survey pinpoints fucosylation as a key pathway underlying the suppressive function of regulatory T cells in cancer.

Forkhead box P3 (Foxp3)-expressing regulatory T cells (Tregs) are the guardians of controlled immune reactions and prevent the development of autoimmune diseases. However, in the tumor context, their increased number suppresses antitumor immune responses, indicating the importance of understanding the mechanisms behind their function and stability. Metabolic reprogramming can affect Foxp3 regulation and, therefore, Treg suppressive function and fitness. Here, we performed a metabolic CRISPR/Cas9 screen and pinpointed novel candidate positive and negative metabolic regulators of Foxp3. Among the positive regulators, we revealed that targeting the GDP-fucose transporter Slc35c1, and more broadly fucosylation, in Tregs compromises their proliferation and suppressive function both in vitro and in vivo, leading to alteration of the tumor microenvironment (TME) and impaired tumor progression and pro-tumoral immune responses. Pharmacologic inhibition of fucosylation dampened tumor immunosuppression mostly by targeting Tregs, thus, resulting in reduced tumor growth. In order to substantiate these findings in humans, tumoral Tregs from colorectal cancer (CRC) patients were clustered based on the expression of fucosylation (Fuco)-related genes. FucoLOW Tregs were found to exhibit a more immunogenic profile compared to FucoHIGH Tregs. Furthermore, an enrichment of a FucoLOW signature, mainly derived from Tregs, correlated with better prognosis and response to immune checkpoint blockade in melanoma patients. In conclusion, Slc35c1-dependent fucosylation is able to regulate the suppressive function of Tregs, and measuring its expression in Tregs might pave the way towards a useful biomarker model for cancer patients.