UC San Diego

Circadian rhythms are physiological processes that occur with a repeated period (in humans around 24 hours) that coordinate internal physiology with external stimuli. These changes are coordinated by a molecular transcriptional-translational feedback loop that includes high amplitude fluctuations in activity and expression of key molecular components. Several lines of evidence have indicated that the circadian clock may play a role in inflammation and immunity. A diurnal difference in the adaptive immune response has been observed suggesting it is influenced by the circadian clock. However, studies have not yet been done about the circadian regulation of regulatory T cells (Tregs), which are an essential immune cell required for maintaining immune homeostasis. Tregs are a suppressive subset of effector CD4+ T cells expressing the transcription factor Foxp3, which are essential for preventing the development of autoimmune responses. Here we identified Cryptochrome (Cry), the main circadian transcriptional repressor, has important for Treg development and function. Cry deficiency in Tregs resulted in a marked competitive disadvantage in homeostatic proliferation compared to their wildtype Treg counterparts. This demonstrated that cryptochromes are required for optimal peripheral homeostasis of Tregs. Furthermore we identified Cry as important for Treg function; Cry deficient Tregs have compromised suppressive capacity both in vitro and in vivo. Mice with Cry deficient Tregs developed symptoms of immunopathology due to increased activation of CD4+ and CD8+ T cells as well as increased Interferon gamma (IFN-g) production by CD4+ T cells. RNA sequencing revealed the biological processes that were the most significantly altered in CryDKO Tregs; processes involved in rhythmic processes and mitochondrial respiration. Further analysis revealed that CryDKO Tregs had increased mitochondrial respiratory capacity resulting from increased mitochondrial membrane potential. These results have identified a novel component necessary for Treg cell homeostasis and reveals that there are many more complexities waiting to be discovered in Treg biology in regards to the role of the circadian clock and cellular metabolism.