Nutrient Sensing in Immune Function: A Systems Biology Approach

Nutrient deprivation through caloric restriction or time-controlled fasting has a profound impact on reducing risk factors and symptoms of inflammatory disease. Studies have highlighted the immune-modulatory and anti-inflammatory effects of nutrient deprivation, suggesting a critical role for metabolic and mitochondrial function on immune activity. To explore the mechanisms underlying these effects, we have carried out a clinical study to assess the immunologic effects of 24 hours of fasting followed by a period of re-feeding on peripheral blood mononuclear cells (PBMCs), monocytes, and CD4+ T cells in normal volunteers. Preliminary results from these studies show (i) a fasting-state decrease in activation of the NLRP3 inflammasome, a component of the innate immune system and a mediator of sterile inflammation, as well as (ii) a decrease in activation of CD4+ T cells. Furthermore, serum collected in the fasting and re-feeding states recapitulates these immune-modulatory effects in human macrophages, monocytes, and CD4+ T cells. Together, these findings suggest that nutrient level-dependent circulating proteins or metabolites, through putative paracrine signaling, confer these effects on innate and adaptive immune cell functioning. We hypothesize that circulating metabolites play a pivotal role in regulating nutrient-dependent immune function. To identify these metabolites and elucidate the underlying immune-modulatory mechanisms, we have pursued a systems biological approach that uses multivariate modeling of metabolomics and lipidomics data to describe changes in the circulating metabolome of fasting. Further, integration with PBMC RNAseq data has revealed key metabolite-sensing G protein-coupled receptors involved in fasting-mediated immune modulation. Through functional characterization of identified metabolites and metabolite-sensing pathways, we have identified a novel signaling role for the fatty amide N-arachidonylglycine in decreasing the activation of pro-inflammatory Th1 and Th17 CD4+ T cells during fasting. Additionally, we have characterized the counter-regulatory role of the short-chain fatty acid propionate in blunting the CD4+ T cell inflammation associated with re-feeding. Finally, we have found that N-arachidonylglycine and propionate reduce the elevated inflammatory cytokines from CD4+ T cells collected from volunteers with obesity to the level of those from lean volunteers. Further exploration of these anti-inflammatory signaling effects will contribute to our understanding of the complex mechanisms of fasting-mediated immune modulation and shed new light on the role of nutrient intake on inflammatory pathophysiology and treatment of metabolic disease