Circadian clocks coordinate physiology and behaviour with the 24h solar day to provide temporal homeostasis with the external environment. The circadian clock is the master regulator of mammalian physiology, including daily oscillations of crucial biological processes and behaviours. In recent years, epidemiological studies have provided evidence suggesting a connection between the circadian clock and human health. Some studies revealed that a third of the mammalian genome is under clock control. As such, understanding the way in which biological rhythms are generated throughout the body may provide avenues for temporally-directed therapeutics to improve health and prevent disease. Notably, circadian disruption has recently been identified as an independent risk factor for cancer, and the World Health Organization designated circadian disruption as a likely carcinogen. As reported, circadian regulatory functions play critical roles in several hallmarks of cancer, including cancers of the prostate, breast, colon, liver, pancreas, ovary, lung, and blood. Furthermore, the circadian clock has been described to play important roles in regulating cell proliferation, cell death, DNA repair, and metabolic alteration. While disrupting the circadian rhythm facilitates the establishment of cancer hallmarks which is important for tumour genesis, on the other hand, oncogenic processes directly weaken circadian rhythms. The pharmacological modulation of core clock genes is even a new approach in cancer therapy. Therefore, the integration of circadian biology into cancer research offers new options for making cancer treatment more effective, encompassing the prevention, diagnosis, and treatment of this devastating disease.

Host immunity is an energetically intensive process that requires the coordination of multiple immune cell types to sense, communicate, and respond to a variety of microorganisms. Interestingly, the circadian clock entrains immune cell development, function, and trafficking to environmental light cycles. As is well known, proper coordination of immune cell development, trafficking, and function is essential for the maintenance of host immunity. A growing number of immune responses are now known to be under circadian control or are influenced by circadian clock-driven processes. The malfunction of any aspect of immunity can result in autoimmunity, deleterious immune responses to exogenous antigens, or cancer. With the progress of science, it is becoming clear that circadian clocks have a profound impact on the immune system and the host response to pathogenic microorganisms. An improved understanding of interactions among microorganisms, the immune system, and the circadian clock will undoubtedly uncover new opportunities for improving treatments for human disease including cancer. Recent findings have begun to uncover the molecular impact of circadian disruption on cancer phenotypes. A question that remains is how disrupted diurnal patterns interact with immunity to promote tumour development. Developing a deeper understanding of circadian and immunity-cancer regulation crosstalk holds promise for developing new strategies for cancer interception, prevention, and management.

The purpose of this Special Issue is to collect recent studies to help understand the relevance of circadian clock-immune system crosstalk in cancer, so as to develop new unconventional approaches in systems biology and immunology. Original research and review articles are welcome.

Potential topics include but are not limited to the following:

• The predisposing factors of cancer that are linked to disruption of the circadian clock and immune system
• Original studies focusing on the primary pathogenesis of cancer based on the interaction between the circadian clock and immune system
• Epigenetics and other factors affecting the circadian clock mediating immune dysfunction and the occurrence and development of tumours
• The studies of circadian dysfunction leading to immune disorders, which mediates tumour recurrence and metastasis
• Studies on new diagnosis and treatment methods for cancer based on the immune microenvironment infiltration and the circadian clock
• Single-cell profiling of immune cells on circadian clock involved in cancer
• Identification of new molecular circadian clock-immune-targets of cancer for personalized interventions derived from omics studies
• Investigating the changes of circadian clocks and host immunity after cancer therapy, which includes chemoradiotherapy, surgical treatment, and targeted therapy