Immunometabolic responses of adult and umbilical cord macrophages to stimulation with Mycobacterium tuberculosis or Lipopolysaccharide

Abstract

Tuberculosis (TB) has been the biggest infectious killer in the world in the last decade and young children are among the most vulnerable groups. Mycobacterium tuberculosis (Mtb), the bacteria that causes TB, is phagocytosed by macrophages. Macrophages can mount an effective immune response and kill the intracellular bacteria, or, in certain circumstances, the macrophage response is suboptimal and allows intracellular bacterial growth, causing active TB disease. The Warburg effect, defined as increased glycolysis and decreased oxidative phosphorylation (OXPHOS), occurs in murine macrophages following lipopolysaccharide (LPS) stimulation and is required for activation. It was hypothesised that immunometabolic responses in human macrophages are different to murine responses and that umbilical cord derived macrophages have an altered immunometabolic response compared with adult macrophages, which may make infants and children particularly vulnerable to TB. The Warburg effect was demonstrated in adult monocyte derived macrophages (MDM) immediately in response to stimulation with LPS or Mtb. Cord blood MDM however did not decrease OXPHOS. At 24 hours post stimulation, glycolysis remains elevated in both adult and cord blood MDM, however LPS stimulated adult MDM have increased OXPHOS. Cord blood MDM secreted less TNF following Mtb stimulation and more IL-6 following LPS stimulation compared with adult MDM. The effects of IFN-γ or IL-4 on human macrophage immunometabolic phenotype and function were investigated. IFN-γ increased glycolysis and OXPHOS and IL-4 resulted in a marked decline in glycolysis. IFN-γ equalised cord and adult TNF production in response to Mtb. IL-4 caused a decrease in IL-1β production in both adult and cord MDM stimulated with Mtb. A consequence of increased glycolysis is an increase in extracellular lactate. The addition of exogenous lactate was found to have an immediate effect on metabolism, causing a decrease in glycolysis and an increase in OXPHOS. Lactate significantly reduced the concentrations of TNF and IL-1β produced by human macrophages in response to Mtb. In addition, lactate significantly improved bacillary clearance in human macrophages infected with Mtb. These data indicate that key differences exist in the kinetics of the immunometabolic response to stimulation in human macrophages compared with that which is established in the literature in mice. Furthermore, adult and cord blood macrophages exhibit distinct immunometabolic function upon stimulation which may underlie their differential ability to respond to infection. These data may help to inform therapeutic strategies for host-directed therapies for TB.