Day/night variations of myeloid and lymphoid cell subsets in the murine inguinal lymph node.

Circadian rhythms orchestrate physiological processes, including immune function, across a 24-h cycle. This study investigates the temporal distribution of immune cell populations in healthy mice entrained to a 12:12-h light-dark cycle. Inguinal lymph node (iLN) samples were collected at zeitgeber times (ZT) 0 (lights on) and ZT12 (lights off) to assess immune cell composition. A significantly higher proportion of natural killer T (NKT) cells and neutrophils was observed at ZT0 compared to ZT12, while dendritic cells, macrophages, and natural killer (NK) cells showed no significant temporal variation. Additionally, adaptive immune cells, particularly Programmed cell Death protein 1+ (PD1+) Cluster of differentiation 4+ (CD4+) and PD1+ Cluster of differentiation 8+ (CD8+) T cells, were more abundant during the light phase. These findings suggest a diurnal pattern in immune readiness, with implications for optimizing immunotherapeutic interventions based on circadian timing.