Daylight Enhances Immune Response by Regulating Neutrophil Activity

A study led by researchers at the University of Auckland has provided new insights into how daylight influences the immune system’s ability to combat infections. The focus of the research was on neutrophils, the most common type of white blood cell, which act as the body’s first responders to infection by targeting and killing bacteria.

To investigate this, the team used zebrafish as a model organism. Previous studies had shown that immune responses in zebrafish were strongest in the morning, coinciding with the fish’s most active period. According to senior author Christopher Hall, “We think this represents an evolutionary response such that during daylight hours the host is more active so more likely to encounter bacterial infections.”

The new research, published in Science Immunology, revealed that neutrophils contain a circadian clock that responds to daylight, enhancing their antibacterial activity during the day. Most cells possess circadian clocks, which are primarily regulated by light, to coordinate bodily functions with the external environment.

The study found that under constant light, the core clock component Per2 increased in neutrophils and was necessary for their bactericidal function and for host survival. During daylight, Per2 regulated the infection-responsive gene hmgb1a, boosting the cells’ ability to kill bacteria. Hall noted, “Given that neutrophils are the first immune cells to be recruited to sites of inflammation, our discovery has very broad implications for therapeutic benefit in many inflammatory diseases.”

The team is now focusing on the mechanisms by which light influences the neutrophil circadian clock.