IL-6 Mediates a Switch in Free Fatty Acid Metabolism from the Liver to the Bone Marrow to Support HSC Expansion during Infection

The hematopoietic stem cell (HSC) pool is tightly controlled in both normal and stressed conditions. Under normal conditions, HSCs reside quiescently in their hypoxic niche with minimal mitochondrial activity, instead favouring glycolysis to meet their energy needs. During acute infection HSCs switch to oxidative phosphorylation over glycolysis due to increased energy requirements. This metabolic switch enables HSCs to expand and differentiate into downstream progenitors to increase the immune cell pool. We recently showed that fatty acid (FA) uptake and metabolism play a central role in providing HSCs with sufficient energy for their expansion during acute infection. However, how infection drives changes in available FAs for HSC expansion is not known. This study investigated the role of liver in regulating lipid availability for HSC expansion.

We have previously shown that circulating free fatty acids (FFAs) within the serum of mice increased in response to infection using both lipopolysaccharide (LPS) (6 hours) and S. Typhimurium (72 hours) models. This correlated with an expansion of bone marrow HSCs 16 hours after LPS and 72 hours after S. Typhimurium, confirmed using flow cytometry. To investigate real-time fatty-acid uptake by hematopoietic cells in response to infection, an in vivo transplant model was used. CD45.1 lineage negative, CD117-positive cells tagged with firefly luciferase (LK+FF) were transplanted into CD45.2 mice. After transplantation was confirmed, mice were injected with LPS for 16 hours. Mice were then subsequently injected with a luciferin molecule conjugated to a long-chain FFA (FFA-luc), which is visible via bioluminescent imaging if the FFA-luc is taken up into the transplanted LK+FF engrafted cells. Live animal imaging using this method confirmed that long-chain FFA is taken up by hematopoietic cells in response to LPS in vivo.

As the liver is the master regulator of FA in the serum we isolated livers from control and LPS (6 hour) treated mice. RNAseq revealed down-regulation of genes involved in FA uptake and metabolism as well genes involved in ketogenesis and up-regulation in genes involved in glucose uptake. Further kinetic studies showed that FA uptake and metabolism genes were downregulated within 2 hours of LPS treatment. To understand if this was a direct LPS effect on hepatocytes which express the receptor for LPS (TLR4) or mediated through unknown factors, we isolated primary hepatocytes and treated them with LPS or media conditioned with serum from control or LPS treated mice. Only the media conditioned with serum from LPS treated mice downregulated genes associated with FA uptake and metabolism. To identify the factors driving this metabolic switch in hepatocytes, a cytokine array of serum from control or LPS treated mice (90 minutes) was performed. This proteome cytokine array revealed a significant upregulation of several cytokines and chemokines, including IL-6, TNFα and CXCL1 and CXCL2. Primary mouse hepatocytes were then treated with factors identified in the array for 2 and 4 hours. RT-qPCR confirmed that IL-6 downregulated FA uptake and metabolism genes in primary hepatocytes. To understand if this was a LPS specific response or induce by infection mice were treated with S. Typhimurium for various time points and livers extracted. RT-qPCR confirmed that S. Typhimurium treated mice down-regulated genes associated with FA uptake and metabolism.

To study the role of IL-6 in regulating liver FA metabolism in response to LPS, mice were treated with IL-6 neutralising antibody (MP5-20F3). Pre-treatment of mice with MP5-20F3 inhibited LPS induced downregulation of liver FA uptake and metabolic genes, which lead to reversing LPS-induced increase in serum FA.

In conclusion we have shown a role for IL-6 in switching fatty acid metabolism from the liver to the bone marrow to allow HSC expansion during infection.