Seasonal rhythms within the immune system may explain the infection rate – study | Immunology

Clocks and calendars within our immune systems can make us more susceptible to infection and injury at certain times of the day or months of the year, a new study suggests. A better understanding of these rhythms can have implications for the prevention and treatment of diseases such as Covid-19. It may also help explain why certain illnesses, such as the flu, tend to strike in the winter, while the symptoms of other illnesses, such as multiple sclerosis, often worsen in the summer.

While recent studies have hinted that there may be seasonal or daily ‘circadian’ rhythms in our immune function, this has not been confirmed in large numbers of people so far.

“It has been clear for centuries that people are more susceptible to certain diseases in winter, but we still don’t really understand the role our bodies play in that,” said Dr. Cathy Wyse, a postdoctoral fellow at the Royal College of Surgeons in Ireland. , who led the new investigation.

To investigate this, Wyse and colleagues turned to blood collected from about 329,261 participants in the UK Biobank study – which has tracked the health of about half a million Britons for more than a decade – to see if the time of the day or season in which these samples were collected had some effect on the immune cells containing them.

They found marked fluctuations in the number of white blood cells and markers of inflammation in the blood, suggesting that our immune function may be stronger or weaker depending on the time of day or season. “It supports the idea that there may be endogenous clocks and calendars in the immune system,” said Wyse, whose research has been published as prepress and has not yet been peer-reviewed.

Importantly, these variations were not related to environmental or lifestyle factors, or vitamin D levels: “This suggests that such changes in our immune system are due to our body clocks – the innate mechanisms that allow us to keep track of time” said Dr. Rachel Edgar of Imperial College London, who is researching how viruses can use circadian rhythms to promote their spread.

Indeed, the daily fluctuations observed in this study reflect patterns that Edgar had previously documented in mice, with white blood cells congregating in the lymph nodes at the beginning of the mice’s active period and entering the blood as the mice progressed to their resting phase. What happens in those lymph nodes is the key to our immune response to vaccines and viruses like Sars CoV-2: “In mice, the time at which they are challenged [with a virus] has profound implications for the immune response days later, ”said Edgar. “This, coupled with evidence that the seasonal influenza vaccine appears to be more effective when administered in the morning or afternoon, strongly suggests that certain Sars-CoV-2 vaccines work better when administered early in the day.”

This theory has not been tested, and Wyse cautions not to overinterpret her results. However, from an evolutionary perspective, it makes sense to align the activities of your immune cells according to when you are most likely to be active. The simplest interpretation is that the immune system has evolved to better recognize and fight potential pathogens during the day. because this is when humans are most likely to come into contact with pathogens, ”said Dr. John O’Neill of the Molecular Biology Laboratory of the Medical Research Council in Cambridge, who previously showed that cells involved in regeneration of tissue damage, migrate to wounds. faster during the day.

“These findings are exciting confirmation of how biological timing is fundamental to human health,” said Prof. Tami Martino, senior chair in molecular cardiovascular research at the University of Guelph in Ontario, Canada. “From a lifestyle perspective, we can use this new understanding of biological timing to guide our behavior, perhaps by limiting our activity at night or in winter, to minimize our exposure to pathogens at times when we are least resilient. “

In a separate study published last week, Martino found that the composition of the gut microbiome in mice also follows a day-night rhythm, and that feeding the mice during their rest period – which disrupted these microbial rhythms – the mice’s ability to of the heart. damage.

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