RAD51D deficiency enhances IFN-JAK-STAT signaling to upregulate PD-L1 and increase sensitivity to PD-L1 blockade in colorectal cancer

Abstract Homologous recombination (HR) repair deficiency is an important source of genomic instability in cancer and has increasingly been linked to tumor-immune interactions. However, the role of HR repair components in regulating immune checkpoint expression remains poorly defined in colorectal cancer (CRC). Here, we report that deficiency of the HR repair factor RAD51 Homolog D (RAD51D) significantly increases programmed death ligand-1 (PD-L1) expression and reshapes tumor immune responses. Transcriptomic analyses revealed that RAD51D loss is associated with DNA damage accumulation and enhanced interferon-related signaling, particularly the JAK-STAT axis. Mechanistically, RAD51D deficiency impaired HR repair and was associated with cGAS-STING-related signaling and enhanced JAK1-STAT1-IRF1 signaling, leading to increased PD-L1 expression. In vivo, RAD51D-deficient tumors exhibited accelerated growth in an immune-dependent manner and was associated with reduced CD8 + T-cell abundance, impaired effector activity and enhanced exhaustion. Pharmacological inhibition and genetic silencing of STAT1 attenuated RAD51D deficiency-associated PD-L1 upregulation, supporting STAT1 as an important mediator of this phenotype. Importantly, the tumor-growth advantage associated with RAD51D deficiency was abolished in Rag1 -/- mice and following CD8 + T-cell depletion, demonstrating a critical role for adaptive immunity and CD8 + T cells. Despite promoting immune suppression under untreated conditions, RAD51D deficiency markedly enhanced tumor sensitivity to PD-L1 blockade, accompanied by increased CD8 + T-cell infiltration, enhanced effector activity and reduced exhaustion. Pharmacological RAD51 inhibition also recapitulated key molecular features of RAD51D deficiency. These findings establish a link between HR repair deficiency and immune checkpoint regulation in CRC. Our study suggests that RAD51D-associated HR repair defects may have implications for the response to immune checkpoint blockade in CRC.