Arid3b suppresses CD8+ T cell infiltration and function in microsatellite stable colorectal cancer via Runx3

Microsatellite stable/proficient mismatch repair (MSS/pMMR) colorectal cancer (CRC) is characterized by a cold tumor microenvironment, marked by sparse CD8⁺ T cell infiltration and resistance to immune checkpoint inhibitors (ICIs). To uncover regulators that limit CD8⁺ T cell infiltration in MSS CRC, we conducted an in vivo CRISPR/Cas9 screen targeting genes in CD8⁺ T cells using a CMT93 cell-derived tumor model. This screen identified Arid3b as a key negative regulator of CD8⁺ T cell tumor infiltration and antitumor activity. Genetic ablation of Arid3b in CD8⁺ T cells significantly enhanced their intratumoral accumulation and mediated robust tumor control. Mechanistically, Arid3b deficiency upregulated Runx3, promoting a tissue-resident memory phenotype and effector function. Notably, the benefits conferred by Arid3b knockout were abrogated by Runx3 deletion, indicating a Runx3-dependent mechanism. In summary, targeting ARID3B could offer a promising strategy to reshape the tumor microenvironment and sensitize MSS CRC to immunotherapy.