Immune system genes may change with the season

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Genes suppressing inflammation are more active in the Australian summer, while those promoting inflammation are more active in winter.

    

Our mood, metabolism and sex lives are dependent on the seasons, and now it seems, so is our immune system.

In a study published today, British and German researchers find almost a quarter of human genes are more or less active depending on the season.

The researchers say the discovery could help explain why people tend to be healthier in summer and diseases known to be seasonal such as cardiovascular disease and rheumatoid arthritis are more evident in winter.

"It helps explain why so many diseases, from heart disease to mental illness, are much worse in the winter months, but no one had appreciated the extent to which this actually occurred," says senior author Professor John Todd of the University of Cambridge.

The international team of researchers analysed blood and fat tissue samples from more than 16,000 people living in the northern and southern hemispheres including in the UK, the US, Iceland, Australia and the Republic of the Gambia.

The study, published in Nature Communications , found 23 per cent of human genes -- 5136 out of 22,822 genes tested -- change their expression levels depending on the season.

Genes that promoted inflammation were more active in the European winter months of December, January and February were less active in the same months in Australia, but more highly expressed in the southern hemisphere winter months of June, July and August. Meanwhile, genes that suppressed inflammation were more active in the summer months of each hemisphere.

Seasonality also affects the make-up of blood and fat tissue.

One particular gene known as ARNTL, which in mice suppresses inflammation, peaked in the summer months, as did the vitamin D receptor, VDR.

If the ARNTL gene has the same function in humans as in mice then levels of inflammation would be higher during winter, say the researchers.

They suggest their findings could inform treatment such as using drugs in winter months that target inflammation.

Some vaccination programs might be more effective if carried out during winter months when the immune system is already 'primed' to respond, they suggest.

Seasonality also was seen in the immune cells of people from The Gambian, but in this case immunity appeared to peak during the rainy season from June through October.

This could be because the immune system faces an increased infectious disease threat in the wet season, such as a higher risk of malaria infection, the researchers say.

Raises questions

Professor Stephen Nutt, at the Walter and Eliza Hall Institute in Melbourne, says it is interesting that Australia and the northern hemisphere are inverse in the data.

However, he says the unknown question is why this seasonal phenomenon exists.

Nutt says the authors propose it might have evolved to ensure the immune system was heightened in winter when more diseases were about.

"But it is also feasible that the reverse is true and more bugs in the environment and greater time indoors may lead to more exposure to microbes and thus prime the immune system.

"It is a classic chicken-and-egg scenario when one thinks about causality for the phenomenon."

Professor Merlin Thomas, Diabetic Complications head at Baker IDI Heart and Diabetes Institute, says seasonal variations in immunity are very common in nature.

"We might have air-conditioning in summer and central heating in winter, but it appears we still change with the seasons like most other animals on the planet," he says.

However, he points out seasonality is not unique to immune response.

"It is also clear that our mood, our physical activity, our metabolism, our appetite and even our sex lives are seasonal," he says.

"Whether these variations can explain why some illnesses are more common at some times of the year than others is unclear. All illnesses are multifactorial. The time of the year may be one small component in the puzzle."

Professor Stuart Tangye, Immunology Division head at the Garvan Institute of Medical Research, agrees and says there are many factors that can impact on the immune system such as latitude.

He points out that in Australia the risk factor for developing multiple sclerosis is 10 times greater in Tasmania than in Far North Queensland and this is related to sun exposure.