Sleep may strengthen our immune system

For more than a century, science has known sleep supports retention of memories, facts and events. Later it was shown deep sleep is important for transforming fragile, recent memories into stable, long-term ones. Now, research proposes deep sleep may strengthen our immune system by retaining our memory of pathogens.

The following article is edited from an Opinion article published September 29 in Cell: Trends in Neurosciences, Neuroimmunology.

Our immune system "remembers" an encounter with a bacteria or virus by collecting fragments from the bug to create memory T cells. These T cells last for months or years and help the body recognize a previous infection so as to quickly respond. They appear to abstract "gist information" about the pathogens, as only T cells that store gist information can ever elicit a response to such tiny fragments.

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. Taken together, the findings support the view that slow-wave sleep contributes to formation of long-term memory of abstract or generalized information. Slow-wave sleep appears to lead to behavioral and immunological responses. The next obvious implication is that sleep deprivation could put your body at risk.

Born goes on to clarify: "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 effective sleep in all organismic systems is - in our view - entirely new.

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

Born says that future research should examine what information is selected for storage in long-term memory during sleep, and how this selection is achieved. In the end, this research could have important clinical implications.

"In order to design effective vaccines against HIV, malaria, and tuberculosis, which are based on immunological memory, the correct memory model must be available," adds Born. "It is our hope that by comparing the concepts of neuronal and immunological memory, a model of immunological memory can be developed...and serves as a helpful basis for vaccine development."

Abstract
Sleep benefits the consolidation of psychological memory, and there are hints that sleep likewise supports immunological memory formation. Comparing psychological and immunological domains, we make the case for active system consolidation that is similarly established in both domains and partly conveyed by the same sleep-associated processes. In the psychological domain, neuronal reactivation of declarative memory during slow-wave sleep (SWS) promotes the redistribution of representations initially stored in hippocampal circuitry to extra-hippocampal circuitry for long-term storage. In the immunological domain, SWS seems to favor the redistribution of antigenic memories initially held by antigen-presenting cells, to persisting T cells serving as a long-term store. Because storage capacities are limited in both systems, system consolidation presumably reduces information by abstracting ‘gist’ for long-term storage.

Trends
Although responding to different environmental events, the central nervous system and the immune system share basic functions of memory.

Sleep benefits the consolidation of psychological and immunological memory.

In the psychological domain, neuronal reactivation of declarative memory during sleep promotes the redistribution of representations initially stored in hippocampal circuitry towards the neocortex and striatum for long-term storage.

In the immunological domain, sleep promotes the redistribution of antigenic memories initially held by antigen-presenting cells, to persisting T cells serving as a long-term store.

In both systems, the consolidation of memory is mediated by slow-wave sleep that suppresses cholinergic and cortisol activity, and enhances proinflammatory signals.

Long-term memory formation in both systems is associated with information reduction by abstracting gist memory.

This work was primarily supported by the Deutsche Forschungsgemeinschaft.

Trends in Neurosciences, Westermann et al.: "System Consolidation during Sleep--A Common Principle Underlying Psychological and Immunological Memory Formation" http://dx.doi.org/10.1016/j.tins.2015.07.007

Trends in Neurosciences, published by Cell Press, is a monthly review journal that brings together research covering all disciplines of the neurosciences, allowing researchers, students, and teachers to keep up with the latest developments, insights, and future directions in the field. For more information, please visit http://www.cell.com/trends/neurosciences. To receive media alerts for this or other Cell Press journals, please contact press@cell.com.

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A model of memory formation in the central nervous system (upper section)
and the immune system (lower section) (credit: )

Image Credit: Westermann et al./Trends in Neurosciences 2015