SLC19A1 transports immunoreactive cyclic dinucleotides

The accumulation of DNA in the cytosol serves as a key immunostimulatory signal associated with infections, cancer and genomic damage(1,2). Cytosolic DNA triggers immune responses by activating the cGAS/STING pathway(3). The binding of DNA to the cytosolic enzyme cGAMP synthase (cGAS), activates its enzymatic activity, leading to the synthesis of a second messenger, cyclic[G(2’,5’)pA(3’,5’)] (2’3’-cGAMP)(4-7). 2’3’-cGAMP, a cyclic dinucleotide (CDN), activates the protein ‘stimulator of interferon genes’ (STING)(8), which in turn activates the transcription factors IRF3 and NF-κB promoting the transcription of genes encoding type I interferons and other cytokines and mediators that stimulate a broader immune response. Exogenous 2’3’-cGAMP produced by malignant cells(9) and other CDNs, including CDNs produced by bacteria(10-12) and synthetic CDNs used in cancer immunotherapy(13,14), must traverse the cell membrane to activate STING in target cells. How these charged CDNs pass through the lipid bilayer is unknown. Here we used a genome-wide CRISPR interference screen to identify the reduced folate carrier SLC19A1, a folate-organic phosphate antiporter, as the major transporter for CDNs. CDN uptake and functional responses are inhibited by depleting SLC19A1 from human cells and enhanced by overexpressing SLC19A1. In both human cell lines and primary cells ex vivo, CDN uptake is inhibited by folates, as well as two medications approved for treatment of inflammatory diseases, sulfasalazine and the antifolate methotrexate. The identification of SLC19A1 as the major transporter of CDNs into cells has implications for the immunotherapeutic treatment of cancer(13), host responsiveness to CDN-producing pathogenic microorganisms(11), and potentially in certain inflammatory diseases.