Daily variation in macrophage phagocytosis is clock-independent and dispensable for cytokine production.

Innate immune responses vary in a circadian manner, and more recent investigations aim to understand the underlying molecular mechanisms. Cytokine production varies significantly over the course of a day depending on the time of stimulation by pathogens or Toll-like receptor ligands, and multiple signaling pathways linked to the cell-autonomous circadian clock modulate innate immunity. Recognition of foreign material, especially by innate immune cells, engages a myriad of receptors, which trigger inflammatory responses, as well as endocytosis and degradation and/or processing for antigen presentation. Because of the close connection between particle engulfment and inflammation, it has been proposed that phagocytic uptake may drive cytokine production in phagocytes. Here we show that bacterial particle ingestion by mouse peritoneal macrophages displays temporal variation, but is independent of the cell-intrinsic circadian clock in an ex vivo setting. Although cytokine production is dependent on phagocytosis, uptake capacity across 12 hr does not translate into 24-hr rhythms in cytokine production. In vivo, time-of-day variations in phagocytic capacity are not found, whereas a time of day and clock-dependent cytokine response is maintained. These data show that efficiency of bacterial phagocytosis and the 24-hr rhythmicity of cytokine production by macrophages are independent of one another and should be studied separately.