Published in Trends in Immunology, researchers discuss how circadian rhythms affect the severity of immune responses, ranging from allergies to heart attacks.
Researchers in Switzerland compiled studies that looked at the connection between circadian rhythms and immune responses. For example, studies showed that adaptive immune responses, in which highly specialised, pathogen-fighting cells develop over weeks, are under circadian control.
Reacting to circadian rhythms
The body reacts to cues such as light and hormones to anticipate recurring rhythms of sleep, metabolism, and other physiological processes. In both humans and mice, the numbers of white blood cells also oscillate in a circadian manner, this essentially raises the question of whether it might be possible one day to optimise immune response through awareness and utilisation of the circadian clock.
Researchers from separate studies, that compared immune cell time-of-day rhythms under normal conditions, inflammation, and disease, found that:
- Heart attacks in humans are known to strike most commonly in the morning, and research suggests that morning heart attacks tend to be more severe than at night. In mice, the numbers of a type of white blood cell that fights off bacteria, viruses, and fungi (otherwise known as monocytes) are elevated in the blood during the day. At night, monocytes are elevated in infarcted heart tissue, resulting in decreased cardiac protection at that time of day relative to morning
- Parasite infections are time-of-day dependent. Mice infected with the gastrointestinal parasite Trichuris muris in the morning have shown to be able to kill worms significantly faster than mice infected in the evening
- In the lungs of mice, a bacterial toxin tied to pneumonia initiates an inflammatory response. Recruitment of immune cells during lung inflammation displays a circadian oscillation pattern. Separately, more monocytes can be recruited into the peritoneal cavity, spleen, and liver in the afternoon, thus resulting in enhanced bacterial clearance at that time.
- Allergic symptoms follow a time-of-day dependent rhythmicity, generally worse between midnight and early morning. Hence, the molecular clock can physiologically drive innate immune cell recruitment and the outcomes of asthma in humans, or airway inflammation in mice.
The importance of investigating circadian rhythms
“Investigating circadian rhythms in innate and adaptive immunity is a great tool to generally understand the physiological interplay and time-dependent succession of events in generating immune responses,” senior author Christoph Scheiermann, immunologist at the University of Geneva says.
“The challenge lies in how to channel our growing mechanistic understanding of circadian immunology into time-tailored therapies for human patients.”