C‐X‐C chemokine receptor <scp>CXCR4</scp> mediates diurnal changes in the aggregation and dispersion of <scp>CD8</scp> ⁺ T cells within the tumor microenvironment

Abstract Immune checkpoint inhibitors (ICIs) are widely used to treat various types of cancer; however, their effectiveness varies, with some patients exhibiting resistance. Recent studies have shown that the efficacy of ICIs depends on the localization of immune cells, particularly T cells, within the tumor microenvironment (TME). Although the circadian clock is known to regulate immune cell migration into tumors, its role in orchestrating spatially precise intratumoral localization remains unclear. Here, we found that the distribution of CD8 + T cells within the TME varied according to the time of day, accompanied by diurnal expression of C‐X‐C chemokine receptor type 4 (CXCR4). The amplitude of the Cxcr4 expression rhythm was more pronounced in tumor‐infiltrated T cells than in those from the spleen and was associated with time‐dependent changes in their migration toward CXCL12‐expressing cancer‐associated fibroblasts (CAFs). Reanalysis of single‐cell RNA‐seq data from T cells of lung cancer patients also revealed that upregulation of CXCR4 expression in tumor‐infiltrated CD8 + T cells was linked to TGF‐β‐SMAD signaling. The TGF‐β‐SMAD signaling‐mediated transactivation of Cxcr4 was time‐dependently repressed by SMAD7, resulting in diurnal CXCR4 expression. Consequently, administration of a CXCR4 inhibitor during the circadian phase of elevated CXCR4 expression in tumor‐infiltrated CD8 + T cells promotes their dispersion throughout tumor tissues, thereby enhancing the efficacy of ICIs. Our findings highlight an unrecognized mechanism underlying diurnal changes in the aggregation and dispersion of CD8 + T cells within tumors, offering a novel approach to enhance the anti‐tumor immune effects of ICIs.