Researchers at Stanford University have found that the immune system's fight against invading bacteria reaches its peak activity at night and is lowest during the day.
While conducting a study with the laboratory model organism, Drosophila melanogaster, researchers found that the specific immune response known as phagocytosis oscillates with the body's circadian rhythm.
"These results suggest that immunity is stronger at night, consistent with the hypothesis that circadian proteins upregulate restorative functions such as specific immune responses during sleep, when animals are not engaged in metabolically costly activities," said Mimi Shirasu-Hiza of Stanford University.
For the study, Shirasu-Hiza and her colleague David Schneider turned to the fruit fly, Drosophila melanogaster, as the model system to help them define the relationship between innate immunity and circadian rhythm, which is the oscillating protein clock or timing mechanism in cells.
Circadian rhythm paces the human body as well as the fruit fly through its days and nights, setting the rest/activity cycle that cues when to eat, sleep and mate over a 24-hour cycle.
In phagocytosis, the innate immune response targeted by the researchers, specific immune cells engulf and destroy the bacteria invading the body.
In humans, immune responses such as phagocytosis not only are involved in clearing bacterial infection but also are implicated in a growing number of human diseases, including cancer and neurodegenerative disorders.
In previous experiments, the researchers noted that flies sick with bacterial infection lost their circadian rhythm and that flies lacking circadian rhythm were highly susceptible to infection.
The flies were infected with two different bacterial pathogens, Listeria monocytogenes and Streptococcus pneumoniae.
To find out whether circadian proteins regulate immunity, the researchers infected flies with these pathogens at different times of day or night.
The researchers found that the flies infected at night had a better chance of surviving than did the flies infected during the day.
The study has been presented at the American Society for Cell Biology (ASCB) 48th Annual Meeting, in San Francisco.