Tumor intrinsic regulation of PD-L1 and of interferon Type I via an SLC25A1-driven mitochondrial pathway, influences the anti-tumor immune response

Immune checkpoint inhibitors (ICIs) have transformed cancer therapy, but variable patient responses highlight the need to better regulators of immune sensitivity. Here, we identify the mitochondrial citrate carrier SLC25A1 as a determinant of anti-PD-L1 antibody therapy responsiveness through a dual regulation of type I interferon (IFN-I) signaling and of PD-L1 expression. SLC25A1 promotes a mitochondrial-to-nuclear retrograde signaling via cytosolic accumulation of mitochondrial DNA, activation of the cGAS-STAT1 axis, and establishment of a virus mimicry state that triggers the IFN-I response. This activation is enriched in cancer stem cell populations, consistent with the role for SLC25A1 in tumor stemness and therapy resistance. Moreover, SLC25A1 also regulates PD-L1 protein levels through a newly identified fumarate-Keap1-PD-L1 axis, whereby fumarate inhibits Keap1, leading to PD-L1 up-regulation. In vivo, tumors expressing high levels of SLC25A1 exhibit an inflammatory environment and increased sensitivity to PD-L1 blockade, but accelerated growth in the absence of anti-PD-L1 treatment. These findings position SLC25A1 as a novel regulator of mitochondrial-driven IFN-I signaling and PD-L1 stability, and suggest that SLC25A1 exploits PD-L1 to evade immune surveillance, while at the same time creating an intrinsic tumor vulnerability to checkpoint blockade. Thus, SLC25A1 may serve both as a biomarker of response and as a target to enhance the efficacy of immunotherapy.