Diurnal variations in inflammatory monocyte recruitment help control infection and metabolic disease, reports a recent study in Science (doi:10.1126/science.1240636).

Ajay Chawla and his colleagues found that in mice kept on a 12-h light-dark cycle, the number of inflammatory monocytes recruited to the inflamed peritoneum showed oscillatory changes, peaking 8 h after lights are turned on then decreasing thereafter. The authors found that when they infected mice with Listeria monocytogenes at this time point of 8 h, when monocyte numbers are at a peak, there was a reduced bacterial burden and a more robust adaptive immune response than in mice infected at 0 h of the cycle.

Credit: David McCarthy / Science Source

The researchers then generated mice with myeloid-specific deletion of the circadian gene Bmal1. This disrupted the oscillatory variation in monocyte recruitment, resulting in increased recruitment of monocytes to inflammatory sites and a higher mortality rate in response to L. monocytogenes infection compared to that observed in wild-type mice. When these myeloid-specific Bmal1-knockout mice were fed a high-fat diet, they gained more weight and had a greater accumulation of macrophages and adaptive immune cells in adipose tissue than wild-type mice. Together, these findings suggest that diurnal variations in inflammatory monocyte recruitment are important for both acute and chronic inflammatory conditions.