Sleep and immunity are bidirectionally linked. Immune system activation alters sleep, and sleep in turn affects the innate and adaptive arm of our body’s defense system. Stimulation of the immune system by microbial challenges triggers an inflammatory response, which, depending on its magnitude and time course, can induce an increase in sleep duration and intensity, but also a disruption of sleep. Enhancement of sleep during an infection is assumed to feedback to the immune system to promote host defense. Indeed, sleep affects various immune parameters, is associated with a reduced infection risk, and can improve infection outcome and vaccination responses. The induction of a hormonal constellation that supports immune functions is one likely mechanism underlying the immune-supporting effects of sleep. In the absence of an infectious challenge, sleep appears to promote inflammatory homeostasis through effects on several inflammatory mediators, such as cytokines. This notion is supported by findings that prolonged sleep deficiency (e.g., short sleep duration, sleep disturbance) can lead to chronic, systemic low-grade inflammation and is associated with various diseases that have an inflammatory component, like diabetes, atherosclerosis, and neurodegeneration. Here, we review available data on this regulatory sleep-immune crosstalk, point out methodological challenges, and suggest questions open for future research.

Sleep and the circadian system exert a strong regulatory influence on immune functions. Investigations of the normal sleep–wake cycle showed that immune parameters like numbers of undifferentiated naïve T cells and the production of pro-inflammatory cytokines exhibit peaks during early nocturnal sleep whereas circulating numbers of immune cells with immediate effector functions, like cytotoxic natural killer cells, as well as anti-inflammatory cytokine activity peak during daytime wakefulness. Although it is difficult to entirely dissect the influence of sleep from that of the circadian rhythm, comparisons of the effects of nocturnal sleep with those of 24-h periods of wakefulness suggest that sleep facilitates the extravasation of T cells and their possible redistribution to lymph nodes. Moreover, such studies revealed a selectively enhancing influence of sleep on cytokines promoting the interaction between antigen presenting cells and T helper cells, like interleukin-12. Sleep on the night after experimental vaccinations against hepatitis A produced a strong and persistent increase in the number of antigen-specific Th cells and antibody titres. Together these findings indicate a specific role of sleep in the formation of immunological memory. This role appears to be associated in particular with the stage of slow wave sleep and the accompanying pro-inflammatory endocrine milieu that is hallmarked by high growth hormone and prolactin levels and low cortisol and catecholamine concentrations.

Sleep is essential for health. Slow wave sleep (SWS), the deepest sleep stage hallmarked by electroencephalographic slow oscillations (SOs), appears of particular relevance here. SWS is associated with a unique endocrine milieu comprising minimum cortisol and high aldosterone, growth hormone (GH), and prolactin levels, thereby presumably fostering efficient adaptive immune responses. Yet, whether SWS causes these changes is unclear. Here we enhance SOs in men by auditory closed-loop stimulation, i.e., by delivering tones in synchrony with endogenous SOs. Stimulation intensifies the hormonal milieu characterizing SWS (mainly by further reducing cortisol and increasing aldosterone levels) and reduces T and B cell counts, likely reflecting a redistribution of these cells to lymphoid tissues. GH remains unchanged. In conclusion, closed-loop stimulation of SOs is an easy-to-use tool for probing SWS functions, and might also bear the potential to ameliorate conditions like depression and aging, where disturbed sleep coalesces with specific hormonal and immunological dysregulations.

In humans, numbers of circulating naive T cells strongly decline in the morning, which was suggested to be mediated by cortisol, inducing a CXCR4 up-regulation with a subsequent extravasation of the cells. As a systematic evaluation of this assumption is lacking, we investigated in two human placebocontrolled studies the effects of the glucocorticoid receptor (GR) antagonist mifepristone (200 mg orally at 23:00) and of suppressing endogenous cortisol with metyrapone (1 g orally at 04:00) on temporal changes in CXCR4 expression and numbers of different T-cell subsets using flow cytometry. Mifepristone attenuated, and metyrapone completely blocked, the morning increase in CXCR4 expression on naive T cells. In parallel, both substances also hindered the decline in naive T-cell numbers with this effect, however, being less apparent after mifepristone. We identified, and confirmed in additional in vitro studies, a partial agonistic GR effect of mifepristone at night (i.e., between 02:00 and 03:30) that could explain the lower antagonistic efficacy of the substance on CXCR4 expression and naive T-cell counts. CXCR4 expression emerged to be a most sensitive marker of GR signaling. Our studies jointly show that endogenous cortisol, specifically via GR activation, causes the morning increase in CXCR4 expression and the subsequent extravasation of naive T cells, thus revealing an important immunological function of the morning cortisol rise. Besedovsky, L., Born, J., Lange, T. Endogenous glucocorticoid receptor signaling drives rhythmic changes in human T-cell subset numbers and the expression of the chemokine receptor CXCR4. FASEB J. 28, 67-75 (2014). www.fasebj.org

Memory retrieval is impaired at very low as well as very high cortisol levels, but not at intermediate levels. This inverted-U-shaped relationship between cortisol levels and memory retrieval may originate from different roles of the mineralocorticoid (MR) and glucocorticoid receptor (GR) that bind cortisol with distinctly different affinity. Here, we examined the role of MRs and GRs in human memory retrieval using specific receptor antagonists. In two double-blind within-subject, cross-over designed studies, young healthy men were asked to retrieve emotional and neutral texts and pictures (learnt 3 days earlier) between 0745 and 0915 hours in the morning, either after administration of 400 mg of the MR blocker spironolactone vs placebo (200 mg at 2300 hours and 200 mg at 0400 hours, Study I) or after administration of the GR blocker mifepristone vs placebo (200 mg at 2300 hours, Study II). Blockade of MRs impaired free recall of both texts and pictures particularly for emotional material. In contrast, blockade of GRs resulted in better memory retrieval for pictures, with the effect being more pronounced for neutral than emotional materials. These findings indicate indeed opposing roles of MRs and GRs in memory retrieval, with optimal retrieval at intermediate cortisol levels likely mediated by high MR but concurrently low GR activation.

SignificanceAssessing antigen-specific T cells is crucial for our understanding of immune reactions against pathogens and tumors, and for evaluating immunotherapies in patients. Existing techniques to evaluate the functionality of T lymphocytes all rely on de novo expression of proteins, typically intracellular cytokines, and therefore require elaborated protocols and reagents. We have established a simple flow cytometry-based method to assess the functionality of CD8+ T cells by identifying immediate changes in the conformation and valency of cell surface integrins that occur within minutes following antigenic stimulation. Because of its robustness, sensitivity, and broad applicability, the assay can be rapidly implemented for the measurement and isolation of functional T cells for basic research and in the clinical setting.

Memory formation is a selective process in which reward contingencies determine which memory is maintained and which is forgotten. Sleep plays a pivotal role in maintaining information for the long term and has been shown to specifically benefit memories that are associated with reward. Key to memory consolidation during sleep is a neuronal reactivation of newly encoded representations. However, it is unclear whether preferential consolidation of memories associated with reward requires the reactivation of dopaminergic circuitry known to mediate reward effects at encoding. In a placebo-controlled, double-blind, balanced crossover experiment, we show that the dopamine D2-like receptor agonist pramipexole given during sleep wipes out reward contingencies. Before sleep, 16 men learned 160 pictures of landscapes and interiors that were associated with high or low rewards, if they were identified between new stimuli at retrieval 24 hr later. In the placebo condition, the participants retained significantly more pictures that promised a high reward. In the pramipexole condition, this difference was wiped out, and performance for the low reward pictures was as high as that for high reward pictures. Pramipexole did not generally enhance memory consolidation probably because of the fact that the dopaminergic agonist concurrently suppressed both SWS and REM sleep. These results are consistent with the concept that preferential consolidation of reward-associated memories relies on hippocampus-driven reactivation within the dopaminergic reward system during sleep, whereby during sleep reward contingencies are fed back to the hippocampus to strengthen specific memories, possibly, through dopaminergic facilitation of long-term potentiation.

Efficient T cell responses require the firm adhesion of T cells to their targets, e.g., virus-infected cells, which depends on T cell receptor (TCR)–mediated activation of β2-integrins. Gαs-coupled receptor agonists are known to have immunosuppressive effects, but their impact on TCR-mediated integrin activation is unknown. Using multimers of peptide major histocompatibility complex molecules (pMHC) and of ICAM-1—the ligand of β2-integrins—we show that the Gαs-coupled receptor agonists isoproterenol, epinephrine, norepinephrine, prostaglandin (PG) E2, PGD2, and adenosine strongly inhibit integrin activation on human CMV- and EBV-specific CD8+ T cells in a dose-dependent manner. In contrast, sleep, a natural condition of low levels of Gαs-coupled receptor agonists, up-regulates integrin activation compared with nocturnal wakefulness, a mechanism possibly underlying some of the immune-supportive effects of sleep. The findings are also relevant for several pathologies associated with increased levels of Gαs-coupled receptor agonists (e.g., tumor growth, malaria, hypoxia, stress, and sleep disturbances).