The Assessment of Circadian Rhythms Within the Immune System.

In recent years, circadian rhythms have been observed in many aspects of the immune system, both for the innate immunity (the first line of defense against pathogens) and the adaptive immunity (a more specific set of responses, which lead to immune memory). Here, to illustrate principles to be taken into account when working on circadian rhythms in immunology experiments, two protocols will be presented. The first one aims to analyze immune parameters in blood sampled from human subjects at different times over the day: counts of different cell types among the peripheral blood mononuclear cells and cytokine secretion by monocytes and T cells after ex vivo stimulation. The second protocol describes how to follow the response of CD8+ T cells after immunization of mice with antigen presenting cells loaded with a peptide antigen. These two protocols are optimized for circadian experiments, and outcome measures are mainly based on flow cytometry, which allows analysis of different parameters in the same cells.

The circadian clock of CD8 T cells modulates their early response to vaccination and the rhythmicity of related signaling pathways.

Circadian variations of various aspects of the immune system have been described. However, the circadian control of T cells has been relatively unexplored. Here, we investigated the role of circadian clocks in regulating CD8 T cell response to antigen presentation by dendritic cells (DCs). The in vivo CD8 T cell response following vaccination with DCs loaded with the OVA257-264 peptide antigen (DC-OVA) leads to a higher expansion of OVA-specific T cells in response to vaccination done in the middle of the day, compared to other time points. This rhythm was dampened when DCs deficient for the essential clock gene Bmal1 were used and abolished in mice with a CD8 T cell-specific Bmal1 deletion. Thus, we assessed the circadian transcriptome of CD8 T cells and found an enrichment in the daytime of genes and pathways involved in T cell activation. Based on this, we investigated early T cell activation events. Three days postvaccination, we found higher T cell activation markers and related signaling pathways (including IRF4, mTOR, and AKT) after a vaccination done during the middle of the day compared to the middle of the night. Finally, the functional impact of the stronger daytime response was shown by a more efficient response to a bacterial challenge at this time of day. Altogether, these results suggest that the clock of CD8 T cells modulates the response to vaccination by shaping the transcriptional program of these cells and making them more prone to strong and efficient activation and proliferation according to the time of day.