Diurnal control of adaptive immune responses by adrenergic nerves
Keywords: Diurnal rhythm, Adrenergic nerves, Lymphocyte trafficking, Adaptive immunity

A group of researchers led by Kazuhiro Suzuki (Associate Professor, WPI Immunology Frontier
Research Center [IFReC], Osaka University) has revealed that neural inputs to β2-adrenergic
receptors (β2ARs) expressed on lymphocytes generate the diurnal variation in the frequency oflymphocyte egress from lymph nodes, which is reflected in the magnitude of the adaptive immuneresponse.

It has long been proposed that various aspects of immune responses are influenced by nervous
system activity. However, the cellular and molecular basis for neural regulation of immunity arelargely unclear. Adrenergic nerves constitute the efferent arc of the sympathetic nervous systemand play important roles in coordinating organ functions through the release of a neurotransmitter,
noradrenaline. Like other vital organs, lymphoid organs, including the bone marrow, thymus,
spleen and lymph nodes (LNs), receive a rich supply of adrenergic nerves. We recently reported
that inputs from adrenergic nerves control lymphocyte egress from LNs through β2ARs.
Activation of lymphocyte β2ARs enhances the responsiveness of chemokine receptors that
promote LN retention of lymphocytes, and consequently inhibits their LN egress (Nakai et al., J.
Exp. Med. 2014). However, the physiological role of this mechanism in adaptive immune
responses has been unclear.

The activity of adrenergic nerves displays a circadian rhythm that is synchronized with the restactivity cycle of the species. The noradrenaline release from adrenergic nerves increases during
the daytime in humans, whereas it reaches a peak at night in rodents. Indeed, we found that thenoradrenaline content in LNs was elevated toward the night time in mice. Given this observation,
we interrogated the physiological relevance of the β2AR-mediated control of lymphocyte
trafficking in relation to the circadian oscillation of adrenergic nerve activity. The night time surgeof adrenergic nerve activity in LNs was accompanied by an increase of lymphocyte numbers inLNs and their reciprocal decrease in lymph and blood. Consistent with these observations, wefound that lymphocyte egress from LNs was restricted during the night time in mice. The diurnalvariation of LN egress was dependent on neural inputs to lymphocyte β2ARs.

Lymphocytes specific for a given antigen are very rare (estimated to be less than 10 cells per LN)
and the population size dictates the magnitude of responses. Therefore, we hypothesized that theaccumulation of lymphocytes in LNs during the period of high adrenergic nerve activity may
increase the chance of antigen encounter and potentiate adaptive immune responses. We found inmice that immunization in the night time, when lymphocyte numbers in LNs were high, induced
more robust humoral immune responses than immunization in the daytime. The diurnal variationof humoral immune responses was dependent on β2AR-mediated neural signals and diminishedby stopping lymphocyte circulation through LNs. These findings suggest that the β2AR-mediatedcontrol of lymphocyte trafficking contributes to the daily fluctuation of adaptive immune
responses (Figure).

Previous studies from other groups showed that the innate immune system is prepared to sense
pathogens more efficiently during periods of activity. Our present study has demonstrated that theadaptive immune system is also poised to mount higher responses in LNs during the active phase.
The synchronization of both arms of the immune system may have evolved to maximize theefficiency of host defense when encounters with pathogens are more likely to occur. The timedependent differences in immune responses can be exploited in clinical settings, such thatvaccination during the immunologically active phase might ensure potent protection against
infectious diseases.

Abstract
Various aspects of the immune system display circadian rhythms. Although lymphocyte
trafficking has been suggested to show diurnal variations, the mechanisms and influences on
immune responses are unclear. Here, we show in mice that inputs from adrenergic nerves
contribute to the diurnal variation of lymphocyte recirculation through lymph nodes (LNs), whichis reflected in the magnitude of the adaptive immune response. Neural inputs to β 2-adrenergic
receptors (β2ARs) expressed on lymphocytes reduced the frequency of lymphocyte egress fromLNs at night, which was accompanied by an increase of lymphocyte numbers in LNs.
Immunization during the period of lymphocyte accumulation in LNs enhanced antibody

responses. The diurnal variation of the humoral immune response was dependent on β2ARmediated neural signals and diminished by stopping lymphocyte recirculation through LNs. This
study reveals the physiological role of adrenergic control of lymphocyte trafficking in adaptive
immunity and establishes a novel mechanism that generates diurnal rhythmicity in the immunesystem.

Figure. Adrenergic control of adaptive immunity by diurnal lymphocyte
trafficking. During the period of high adrenergic nerve activity, lymphocyte
egress from LNs is restricted, which leads to an increase of lymphocyte
numbers in LNs. Immunization during the period of lymphocyte accumulation
in LNs promote adaptive immune responses. This diurnal variation oflymphocyte trafficking may have evolved to maximize the efficiency of hostdefense against pathogens.