Pyruvate kinase M2-deficiency in T cells leads to exacerbation of ConA hepatitis and alterations of T cell polarization

Introduction:

In autoimmune liver disease, impaired homeostasis results from a dysbalance between regulatory T cells (Tregs) and T effector cells (Teff). Activated Teff cells undergo metabolic re-programming to preferential aerobic glycolysis known as Warburg Effect. Aerobic glycolysis is required for T cell growth and effector function. Contrary, Tregs rely on fatty acid oxidation. The Warburg effect and glycolytic switch are promoted by dimerization of pyruvate kinase M2 that acts as a cofactor for HIF-1a to induce transcription of inflammatory cytokines and glycolytic enzymes. These effects can be counterbalanced by allosteric tetramerization the PKM2. The cytosolic PKM2 tetramer has high enzymatic activity and ensures an intact glycolytic flux.

Objectives:

Deletion or inactivation of Pkm2 in T cells should alter T cell polarization and T cell-mediated inflammation. Moreover, small molecule-induced tetramerization of PKM2 could restore immune homeostasis.

Materials & methods:

CD4crexPkm2fl/fl mice (PKM2^ΔCD4 mice) and controls were subjected to Concanavalin A (ConA)-induced hepatitis. ALT elevation and histology/immune histochemistry were employed to gauge disease outcome. Metabolism was analyzed with a Seahorse analyzer. In cocultures (hepatocytes (HC) + CD4⁺ T cells), Treg induction was analyzed by FACS. Th1 and Th17 polarization of T cells was analyzed using FACS with and without the addition of PKM2 activators (TEPP46, stabilizes the PKM2 tetramer) and inhibitors (shikonin).

Results:

PKM2^ΔCD4 mice exhibit exacerbation of ConA hepatitis. PKM2^ΔCD4 Th1 and Th17 cells showed alteration in glycolytic activity. In HC-T cell cocultures and livers, enhanced Treg formation was observed in T cells/livers from PKM2^ΔCD4 mice, whereas TEPP46 enhanced formation of CD73⁺ Tregs in vitro. PKM2^ΔCD4 Teff exhibited reduced CD73 expression. In Th17 cells, RORγt expression was reduced, whereas TEPP treatment restored RORγt expression in vitro.

Conclusion:

PKM2 is required for immune homeostasis by controlling the glycolytic switch in immune cell activation. PKM2 expression and activity affects Treg induction and Th17 cell polarization. The role of PKM2 activity and the different isomers in Treg induction and their suppressive properties and polarization of inflammatory Th17 cells is currently further investigated.