Development of circadian immune regulation in early life

Circadian rhythms temporally regulate immune cell trafficking, abundance, and immune responsiveness in adults, yet how this rhythmic organization emerges during postnatal development remains unknown. Here, we systematically investigated the establishment and development of circadian immune regulation across multiple biological levels. Leukocyte rhythmicity was not present at birth but was gradually established and strengthened during postnatal development in a leukocyte subset-dependent manner. In parallel, promigratory markers displayed dynamic and heterogeneous developmental trajectories, indicating progressive coordination of rhythmic promigratory programs. At the systemic level, peripheral clocks matured and synchronized during development, with tissue- and clock-gene-dependent differences in the acquisition timing, peak phase, and amplitude of circadian oscillations. This multi-level analysis identifies postnatal development as a dynamic period during which circadian immunity is progressively consolidated through leukocyte rhythmicity, regulation of promigratory factors, and peripheral clocks.