Circadian disruption in lung fibroblasts enhances NF-κB activity to exacerbate neutrophil recruitment.

Fibroblasts are stromal cells abundant throughout tissues, including the lungs. Fibroblasts are integral coordinators of immune cell recruitment through chemokine secretion. Circadian rhythms direct the recruitment of immune cells to the lung, which in turn impacts response to infection and survival. Although fibroblasts display robust circadian rhythms, the contribution of the fibroblast molecular clock to lung-specific migration of immune cells and recruitment remains to be established. Mice challenged intranasally with lipopolysaccharide (LPS) at dusk showed increased expression of the pro-inflammatory cytokine IL-1ß and chemokine CXCL5 in the lung, which was accompanied by increased neutrophil recruitment. Primary lung fibroblasts with knockdown of the core clock gene Bmal1 and immortalized Bmal1-/- lung fibroblasts also displayed increased Cxcl5 expression under IL-1ß stimulation. Conditioned media obtained from IL-1ß-stimulated Bmal1-/- immortalized fibroblasts-induced greater neutrophil migration compared with Bmal1+/+ lung fibroblast controls. Phosphorylation of the NF-κB subunit, p65, was enhanced in IL-1ß-stimulated Bmal1-/- lung fibroblasts, and pharmacological inhibition of NF-κB attenuated the enhanced CXCL5 production and neutrophil recruitment observed in these cells. Collectively, these results demonstrate that Bmal1 represses NF-κB activity in lung fibroblasts to control chemokine expression and immune cell recruitment during an inflammatory response.

The circadian clock influences T cell responses to vaccination by regulating dendritic cell antigen processing.

Dendritic cells play a key role in processing and presenting antigens to naïve T cells to prime adaptive immunity. Circadian rhythms are known to regulate many aspects of immunity; however, the role of circadian rhythms in dendritic cell function is still unclear. Here, we show greater T cell responses when mice are immunised in the middle of their rest versus their active phase. We find a circadian rhythm in antigen processing that correlates with rhythms in both mitochondrial morphology and metabolism, dependent on the molecular clock gene, Bmal1. Using Mdivi-1, a compound that promotes mitochondrial fusion, we are able to rescue the circadian deficit in antigen processing and mechanistically link mitochondrial morphology and antigen processing. Furthermore, we find that circadian changes in mitochondrial Ca2+ are central to the circadian regulation of antigen processing. Our results indicate that rhythmic changes in mitochondrial calcium, which are associated with changes in mitochondrial morphology, regulate antigen processing.

Intertwining roles of circadian and metabolic regulation of the innate immune response.

It has emerged that an interconnected relationship exists between metabolism, circadian rhythms, and the immune system. The relationship between metabolism and circadian rhythms is not that surprising given the necessity to align rhythms of feeding/fasting with activity/rest. Recently, our understanding of the importance of metabolic pathways in terms of immune function, termed immunometabolism, has grown exponentially. It is now appreciated that the time of day during which the innate immune system is challenged strongly conditions the subsequent response. Recent observations have found that many individual components that make up the circadian clock also control aspects of metabolism in innate immune cells to modulate inflammation. This circadian/metabolic axis may be a key factor driving rhythmicity of immune function and circadian disruption is associated with a range of chronic inflammatory diseases such as atherosclerosis, obesity, and diabetes. The field of "circadian immunometabolism" seeks to reveal undiscovered circadian controlled metabolic pathways that in turn regulate immune responses. The innate immune system has been intricately linked to chronic inflammatory diseases, and within the immune system, individual cell types carry out unique roles in inflammation. Therefore, circadian immunometabolism effects are unique to each innate immune cell.

The Circadian Clock Protein BMAL1 Acts as a Metabolic Sensor In Macrophages to Control the Production of Pro IL-1ß.

The transcription factor BMAL1 is a clock protein that generates daily or circadian rhythms in physiological functions including the inflammatory response of macrophages. Intracellular metabolic pathways direct the macrophage inflammatory response, however whether the clock is impacting intracellular metabolism to direct this response is unclear. Specific metabolic reprogramming of macrophages controls the production of the potent pro-inflammatory cytokine IL-1ß. We now describe that the macrophage molecular clock, through Bmal1, regulates the uptake of glucose, its flux through glycolysis and the Krebs cycle, including the production of the metabolite succinate to drive Il-1ß production. We further demonstrate that BMAL1 modulates the level and localisation of the glycolytic enzyme PKM2, which in turn activates STAT3 to further drive Il-1ß mRNA expression. Overall, this work demonstrates that BMAL1 is a key metabolic sensor in macrophages, and its deficiency leads to a metabolic shift of enhanced glycolysis and mitochondrial respiration, leading to a heightened pro-inflammatory state. These data provide insight into the control of macrophage driven inflammation by the molecular clock, and the potential for time-based therapeutics against a range of chronic inflammatory diseases.

Alterations in mitochondrial morphology as a key driver of immunity and host defence.

Mitochondria are dynamic organelles whose architecture changes depending on the cell's energy requirements and other signalling events. These structural changes are collectively known as mitochondrial dynamics. Mitochondrial dynamics are crucial for cellular functions such as differentiation, energy production and cell death. Importantly, it has become clear in recent years that mitochondrial dynamics are a critical control point for immune cell function. Mitochondrial remodelling allows quiescent immune cells to rapidly change their metabolism and become activated, producing mediators, such as cytokines, chemokines and even metabolites to execute an effective immune response. The importance of mitochondrial dynamics in immunity is evident, as numerous pathogens have evolved mechanisms to manipulate host cell mitochondrial remodelling in order to promote their own survival. In this review, we comprehensively address the roles of mitochondrial dynamics in immune cell function, along with modulation of host cell mitochondrial morphology during viral and bacterial infections to facilitate either pathogen survival or host immunity. We also speculate on what the future may hold in terms of therapies targeting mitochondrial morphology for bacterial and viral control.