Free Fatty Acid Uptake By Hematopoietic Stem and Progenitor Cells Drives Immune Cell Expansion in Response to Salmonella Typhimurium infection

Haematopoietic stem and progenitor cells (HSPC) are tightly regulated in a balance between self-renewal and differentiation and undergo rapid expansion in response to infection. In the context of acute bacterial infection, the hematopoietic system needs energy to drive the immune response necessary for host survival. In an adult human, bone marrow adipose tissue accounts for up to 70% of bone marrow and we have previously shown that free fatty acids are transferred from adipocytes to AML blasts providing the AML blasts more energy for proliferation and survival (Shafat, Blood 2017). Therefore, we hypothesis that during an acute bacterial infection, the HSPC population uptakes free fatty acids (FFA) to support and sustain the rapid response to bacterial infection. To determine if HSPC acquired FFA we infected C57BL/6 with Salmonella Typhimurium (S.typhimurium) or treated C57BL/6 with lipopolysaccharide (LPS) for 2 and 16 hours. Using both models we show that the HSPC populations (HSC (haematopoietic stem cell), MPP (multipotent progenitor) and LSK (LN-, CD117+, Sca1+)) all accumulate FFA as measured by staining with neutral lipid-specific BODIPY 493/503 dye. To determine if this was uptake or accumulation of fatty acid within the HSPC of animals infected with S.typhimurium we used BODIPY®-dodecanoic acid fluorescent fatty acid to trace FFA uptake. HSPC populations from animals treated with LPS had a higher uptake of BODIPY®-dodecanoic acid when compared to the control treated mice. To assess metabolic changes within HSC population in response to infection, cells were sorted (LN-, CD117+, Sca1+, CD48-, CD150+ and CD34+) and analysed for gene expression changes. In HSC, increases were observed in several metabolic genes including FABP4, CD36 and CPT1 in response to S.typhimurium as well as LPS at 16 hours post stimulation. Moreover, following cell sorting, the LSK (LN-, CD117+, Sca1+) population were assessed for metabolic activity for different substrates. The LSK population from LPS treated animals had an increased dependency on beta oxidation when compared to the control. Moreover, etoximir (a beta-oxidation inhibitor) inhibited LSK basal metabolism and reduced LSK expansion (as measured by colony forming cell assay and Ki-67 staining) in animals treated with LPS. Our data indicates that infection drives uptake of FFA in different HSPC populations. The FFA are metabolised by beta oxidation which provides the energy for the expansion of leukocytes.