How does sleep stimulate the immune system?

It is already well known that sleep is good for the immune system, with scientists having previously shown that in people who slept after a vaccination, the immune response was twice as strong on average as in people who did not sleep during the night after the vaccination. But what are the biological reasons for this? According to a German research team, sleep promotes the potential of immune cells — so-called T cells — to migrate towards lymph nodes.

The research team, led by Professor Luciana Besedovsky from LMU Munich, repeatedly examined the concentration of various subgroups of T cells in the blood of a cohort of healthy men and women over the course of two 24-hour sessions. In one of the two test conditions, the participants were allowed to sleep at night for eight hours, while in the other they relaxed in bed at night but stayed awake. A forearm catheter connected to an adjacent room by means of a tube enabled blood collection without disturbing the participants’ sleep.

Analysis of the blood samples revealed significant differences between the test conditions. According to Besedovsky, “Our results show that sleep promotes the migratory potential of various T-cell subpopulations.”

As the researchers demonstrated, sleep increases the directed migration of T cells towards a signalling protein, the so-called ‘homing’ chemokine CCL19. This molecule mediates the migration of T cells, which possess the corresponding receptor for CCL19, to the lymph nodes, where the T cell immune defences are ‘trained’ by being presented with antigens — for example, after a vaccination.

In further experiments, the researchers showed that incubating T cells with blood plasma taken from sleeping participants likewise increased the migratory potential. As stated by Besedovsky, “This demonstrates that soluble factors that are elevated in blood plasma during sleep mediate the effect of sleep on T-cell migration. So we can in a way recreate the effect of sleep in the lab using the blood plasma of sleeping persons.”

The scientists identified growth hormone and prolactin as the decisive factors for this migration behaviour. Both hormones showed sleep-dependent changes in concentration in the plasma, with higher values among the participants who slept during the night.

“Our results also have potential clinical implications,” Besedovsky said. “Thus, growth hormone and prolactin could be considered as new adjuvants to promote immune responses following vaccination, especially in aged people, who typically display reduced levels of these hormones during sleep.”

Overall, the authors see the study as an important step for better understanding why sleep supports immune responses — for example, after vaccination — and why vaccines are often less effective in older people. Their findings have been published in the journal Brain, Behavior, and Immunity.

Image credit: iStock.com/fizkes