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

## TL;DR

The study shows that CD8+ T cells in tumors change their distribution based on the time of day, influenced by CXCR4 and TGF-β–SMAD signaling, which impacts immunotherapy effectiveness.

## Contribution

The study reveals a novel mechanism linking circadian rhythms to T cell localization in tumors, affecting immune checkpoint inhibitor response.

## Key findings

- CD8+ T cell distribution in tumors varies diurnally with CXCR4 expression.
- TGF-β–SMAD signaling regulates CXCR4 expression in a time-dependent manner.
- CXCR4 inhibition during high expression phases enhances immunotherapy efficacy.

## 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.

What's new?

In addition to governing daily variations in biological activity, the circadian clock regulates immune cell migration into tumors. Whether this influence extends to immune cell localization within the tumor microenvironment (TME) is unknown. Here, using cell and animal models, the authors investigated immune cell distribution within the TME according to time of day. Experiments show that changes in CD8+ T‐cell localization are regulated by diurnal CXCR4 expression and TGF‐β–SMAD signaling, allowing for precise spatiotemporal control of tumor immune evasion. Critically, this mechanism influences immune checkpoint inhibitor responsiveness, underscoring the importance of timing in optimizing immunotherapy outcomes.