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

## TL;DR

This study shows that immune cell populations in mice vary throughout the day, with some cells more active during the day and others at night.

## Contribution

The study reveals diurnal variations in innate and adaptive immune cell subsets in murine lymph nodes.

## Key findings

- NKT cells and neutrophils are more abundant at the start of the day (ZT0) compared to night (ZT12).
- PD1+ CD4+ and CD8+ T cells are more prevalent during the light phase.
- Dendritic cells, macrophages, and NK cells show no significant temporal variation.

## Abstract

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.

The circadian system is involved in the temporal regulation of the immune system. Our study reveals that two innate immune populations, NKT cells and neutrophils, predominate at the beginning of the day in healthy mice, highlighting how the time of day influences immune responses. These results could have implications for optimizing immunotherapeutic strategies in cancer treatment. Image adapted from Servier Medical Art (https://smart.servier.com/), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)