Circadian rhythms are a feature common to most physiological processes. These rhythms in the body are synchronized to the environment by external cues. Studies have shown that the initial time of day when an organism encounters an inflammatory stimulus determines the strength of the subsequent immune response. This phenomenon is true for both the innate and the adaptive immune system. Despite what is known about rhythmic immunity at steady state and during an acute inflammation, it is still unknown how circadian rhythms affect the influence of the immune system to cancer. The process of cancer immunosurveillance is a mechanism of tumor suppression that can protect the host from cancer development throughout its lifetime. Yet, it is unknown whether the effectiveness of cancer immunosurveillance fluctuates over the course of a single day.

Here, I demonstrate that the initial time-of-day of tumor engraftment dictates the ensuing tumor growth across murine cancer models. Using immunocompromised NSG as well as Rag2 knock-out mice, I could trace this difference to the adaptive immune system. By using a series of antibodies to deplete subpopulations of immune cells, I found dendritic cells (DCs) and CD8 T cells to be the major effector cells in the control of the diurnal tumor growth. More specifically, rhythmicity in the anti-tumor response was abrogated in mice exhibiting lineage-specific loss of circadian rhythmicity in DCs or T cells by deletion of the circadian gene Bmal1. I found that rhythmic trafficking of DCs to the tumor draining lymph node (dLN) governs a circadian response of tumor antigen-specific CD8⁺ T cells. This is dependent on an oscillation in the number and function of DCs. Mechanistically, DCs exhibit a cell-intrinsic circadian expression of the co-stimulatory molecule CD80. Chromatin immunoprecipitation revealed rhythmic binding of BMAL1 to the promoter region of Cd80, indicating a direct control of Cd80 expression by BMAL1 and the circadian clock.

Adjusting the administration time of anti-tumor vaccination enhanced the efficacy in controlling tumor growth, a phenomenon dependent on the DC-intrinsic clock. Administering the vaccine during the day promoted higher levels of antigen-bearing DCs in the draining lymph node, as well as more antigen-specific T cells, compared to night-time vaccination. Additionally, human monocyte derived DCs also presented time-of-day oscillations in CD80, resulting in oscillatory CD8 T cell proliferation.

Consequently, cancer immunotherapy was demonstrated to be more effective when synchronized with rhythmic trafficking of DCs from the tumor to the draining lymph node, as well as peak CD80 expression on DCs, resulting in better CD8 proliferation in mice. In humans, retrospective data also showed a time-of-day dependency for tumor vaccination with respect to the generation of melanoma antigen-specific T cells. These data demonstrate that circadian rhythms of anti-tumor immune components are not only critical for the control of tumor growth, but might also be used to enhance existing therapeutic cancer treatments.