A metabolic peace process

In humans, loss-of-function mutations in the key regulatory T (T_reg) cell transcription factor FOXP3 lead to the severe autoimmune disease IPEX (imunodysregulation, polyendocrinopathy, enteropathy, X-linked). A recent study suggests that T_reg cell function could be restored in such patients by targeting the mTOR pathway.

Although T_reg cells still develop in the thymus of FOXP3-deficient patients and mice, they show defective suppressive activity and acquire effector-like functions. Charbonnier et al. reasoned this could be linked to altered cell metabolism, as effector T cells show several metabolic changes, including an increase in aerobic glycolysis and oxidative phosphorylation (OXPHOS). The authors generated a Foxp3^ΔEGFPiCre knock-in mouse system that allowed for the co-deletion of other molecules in FOXP3-deficient T_reg cells. Similarly to other FOXP3-deficient strains, male mice hemizygous for the Foxp3^ΔEGFPiCre allele were runted and died early from autoimmune lymphoproliferative disease. Compared with control T_reg cells, the FOXP3-deficient T_reg cells from Foxp3^ΔEGFPiCre mice showed increased activation of the mTOR pathway, which is crucial for supporting effector T cell metabolism and function.

The mTOR kinase is the catalytic subunit of two distinct complexes; mTORC1 and mTORC2. The authors therefore generated mice in which FOXP3-deficient T_reg cells also lacked RAPTOR and/ or RICTOR, which are key components of mTORC1 and mTORC2, respectively. Mice with T_reg cells deficient in both FOXP3 and RICTOR still showed lymphoproliferation but had less tissue inflammation, increased body weight and better survival compared with mice with FOXP3-deficient T_reg cells. By contrast, deletion of RAPTOR or both RAPTOR and RICTOR in FOXP3-deficient T_reg cells did not ameliorate disease. However, mice with RAPTOR-deficient FOXP3-deficient T_reg cells showed a reversal of the T_reg cell population expansion seen in Foxp3^ΔEGFPiCre mice. Therefore, mTORC1 seems to support the population expansion of FOXP3-deficient T_reg cells in Foxp3^ΔEGFPiCre mice, whereas mTORC2 drives the dysregulation of FOXP3-deficient T_reg cells that causes inflammatory disease.

Indeed, RICTOR deficiency in FOXP3-deficient T_reg cells restored their suppressor functions and reversed the upregulation in T helper 1 (T_H1) cell-type effector activity. RICTOR deficiency also enhanced regulatory functions in FOXP3-sufficient T_reg cells but did not cause effector T cells to acquire suppressive functions.

Transcriptomics indicated that RICTOR deficiency in FOXP3-deficient T_reg cells is associated with the upregulation of a core set of T_reg cell-associated genes, including Il10, and with the suppression of genes linked to effector T cell function. Notably, neutralization of IL-10 abrogated the improved suppressor activity of RICTOR-deficient FOXP3-deficient T_reg cells. Further experiments indicated that the improved suppressor functions seen in RICTOR-deficient FOXP3-deficient T_reg cells were largely dependent on the activity of FOXO1, which is an important negative regulator of the T_H1 cell programme.

Metabolic analyses indicated that FOXP3-deficient T_reg cells had increased expression of enzymes linked with the glycolytic and pentose phosphate pathways and showed an increase in glycolysis and OXPHOS; concurrent RICTOR deficiency reversed most of these changes. Moreover, deletion of the bifunctional enzyme PFKFB3 — a potent stimulator of glycolysis — in FOXP3-deficient T_reg cells had a similar effect to RICTOR deletion in restoring their capacity to suppress T_H1 cell-type immune responses, although unlike RICTOR deficiency it did not restore the capacity of FOXP3-deficient T_reg cells to suppress effector T cell proliferation. Loss of both PFKFB3 and RICTOR from FOXP3-deficient T_reg cells did not have an additive effect, confirming aerobic glycolysis as a common target of both interventions; therefore, mTORC2-dependent metabolic dysregulation, including increases in glycolysis and OXPHOS, may drive distinct facets of the regulatory dysfunction in FOXP3-deficient T_reg cells.

targeting mTOR could improve T_reg cell functions not only in patients with IPEX but in patients with other autoimmune or inflammatory diseases

Importantly, the authors found that T_reg cells from patients with IPEX also showed increased glycolytic activity. Although T_reg cells from these patients do not normally suppress effector T cell proliferation in vitro, pretreatment with an mTOR inhibitor enabled this suppressive capacity. Furthermore, mTOR inhibitors were found to heighten the suppressive functions of FOXP3-sufficient T_reg cells from control individuals. Therefore, targeting mTOR could improve T_reg cell functions not only in patients with IPEX but in patients with other autoimmune or inflammatory diseases not associated with FOXP3 deficiency.