Sleep may strengthen long-term memories of immune system

Deep sleep may strengthen the immune system's memory of an encounter with a bacteria or virus, helping the body recognise a previous infection and respond quickly, researchers suggest.
"While it has been known for a long time that sleep supports long-term memory formation in the psychological domain, the idea that long-term memory formation is a function of sleep effective in all organismic systems is in our view entirely new," said Jan Born of the University of Tuebingen in Germany.
The immune system "remembers" an encounter with a bacteria or virus by collecting fragments from the bug to create memory T cells, which last for months or years and help the body recognise a previous infection and quickly respond.

These memory T cells appear to abstract "gist information" about the pathogens, as only T cells that store information about the tiniest fragments ever elicit a response.
The selection of gist information allows memory T cells to detect new pathogens that are similar, but not identical, to previously encountered bacteria or viruses.
Studies in humans have shown that long-term increases in memory T cells are associated with deep slow-wave sleep on the nights after vaccination.

The findings support the view that slow-wave sleep contributes to the formation of long-term memories of abstract, generalised information, which leads to adaptive behavioural and immunological responses.

"If we didn't sleep, then the immune system might focus on the wrong parts of the pathogen," Born said in an article published in the journal Trends in Neurosciences.
"For example, many viruses can easily mutate some parts of their proteins to escape from immune responses. If too few antigen-recognising cells (the cells that present the fragments to T cells) are available, then they might all be needed to fight off the pathogen," Born said.
"In addition to this, there is evidence that the hormones released during sleep benefit the crosstalk between antigen-presenting and antigen-recognising cells, and some of these important hormones could be lacking without sleep," Born said.

Born said that future research should examine what information is selected during sleep for storage in long-term memory, and how this selection is achieved.
"In order to design effective vaccines against HIV, malaria, and tuberculosis, which are based on immunological memory, the correct memory model must be available," he said.
"It is our hope that by comparing the concepts of neuronal and immunological memory, a model of immunological memory can be developed which integrates the available experimental data and serves as a helpful basis for vaccine development," he said.