Leukocyte numbers in the blood and release of hematopoietic progenitor cells from the bone marrow are known to undergo circadian oscillations. In Immunity, Frenette and colleagues show that the recruitment of leukocytes to tissues in the steady state also has circadian rhythms orchestrated by sympathetic nerves. The rhythmic recruitment of leukocytes to the bone marrow and skeletal muscles peaks at night and correlates with fluctuation in the expression of adhesion mediators (ICAM-1 and CCL2) on epithelial cells in muscle or of rolling and adhesion mediators (selectins and VCAM-1) in the bone marrow. Oscillations in the expression of selectins and integrin receptors are induced through local adrenaline signaling via β-adrenoreceptors and are ablated by changes in the light cycle. Because humoral factors are also known to regulate circadian rhythms, neural and hormonal pathways probably act together to fine tune these responses.

Immunity 37, 290–301 (2012)