The metabolite BH4 controls T cell proliferation in autoimmunity and cancer

Enhancing BH4 production, either genetically or pharmacologically, results in hyper-activated T cells displaying stronger effector functions.

“Furthermore, by enhancing BH4 production, either genetically or pharmacologically, we observed an opposing phenotype, where the T cells are hyper-activated, displaying stronger effector functions.”

T cell development or homeostasis as well as B cell and regulatory T cell functions are unaffected by inhibition of the BH4 pathway.

“However, interestingly, T cell development or homeostasis as well as B cell and regulatory T cell (Treg) functions are unaffected by inhibition of this pathway.”

When T cells become stimulated through TCR engagement, the increased metabolic needs of the dividing, cytokine-producing effector cell require enhanced Fe2+-dependent mechanisms for efficient ETC during mitochondrial respiration and increased ATP synthesis.

“These data suggested that when T cells become stimulated through TCR engagement, the increased metabolic needs of the dividing, cytokine-producing effector cell requires enhanced Fe2+-dependent mechanisms for, among many others, efficient electron transfer chain (ETC) during mitochondrial respiration and increased ATP synthesis.”

BH4-deficient T cells show increases at the protein level of Mitoferrin, HO-1, ferritin, and frataxin.

“We indeed observed increases at the protein level of Mitoferrin, the mitochondrial Fe2+ importer; HO-1, which breaks heme into Fe2+; ferritin, the storage regulator of Fe3+, as well as frataxin, responsible for Fe-S biogenesis.”

The mitochondrial respiration defect in Gch1-ablated T cells after stimulation can be rescued by supplying reduced cytochrome-C-Fe2+ directly to the mitochondria.

“We could rescue the mitochondrial respiration defect by supplying reduced cytochrome-C-Fe2+ directly to the mitochondria of Gch1-ablated T cells after stimulation.”

BH4-deficient T cells after activation show overall depletion of total iron.

“We observed an overall depletion of total iron in T cells after activation under BH4-deficient conditions.”

Genetic inactivation of GCH1 or pharmacological inhibition of SPR results in severely impaired proliferation of T cells.

“Using multiple T cell specific ablation strategies, we demonstrated that genetic inactivation of GCH1 or pharmacological inhibition of the terminal enzyme in the biosynthetic pathway of BH4, sepiapterin reductase (SPR), results in severely impaired proliferation of T cells (Cronin et al., 2018).”

In orthotopic cancer transfer models, BH4-overproducing T cells have increased anti-cancer immunity, resulting in greatly reduced cancer burden.

“In orthotopic cancer transfer models, BH4-overproducing T cells have increased anti-cancer immunity, resulting in greatly reduced cancer burden (Cronin et al., 2018).”

Our laboratory has recently uncovered a novel role for a metabolite, linking iron regulation to T cell effector function.

“Our laboratory has recently uncovered a novel role for a metabolite, linking iron regulation to T cell effector function (Cronin et al., 2018).”

GCH1 is expressed in T cells but only after TCR stimulation.

“We and others have shown that GCH1 is expressed in T cells but only after TCR stimulation (Chen et al., 2011; Cronin et al., 2018).”

In genetic inactivation models, there were no differences in iNOS expression or NO production in T cells, nor detectable signs of activated T cells producing biogenic amine neurotransmitters that require co-factor BH4.

“In genetic inactivation models, we did not detect any differences in iNOS expression or NO production in T cells, nor any detectable signs of activated T cells producing biogenic amine neurotransmitters that require co-factor BH4.”

In various models of T cell development autoimmunity or asthma, BH4 blockage significantly reduces T cell effector function and infiltration into affected tissues.

“In various models of T cell development autoimmunity or asthma, BH4 blockage significantly reduces T cell effector function and infiltration into affected tissues.”

Under BH4 deficiency, cytochrome-C reduction is compromised, resulting in dysfunctional mitochondrial respiration, increased superoxide formation, and diminished ATP production.

“Under conditions of BH4 deficiency, as in our Gch1-ablated T cells, cytochrome-C reduction is compromised, resulting in dysfunctional mitochondrial respiration, increased superoxide formation and, ultimately diminished ATP production, all of which we observed (Cronin et al., 2018).”

Gene expression profiling comparing activated wild-type T cells to those lacking GCH1 pointed to iron regulation.

“Gene expression profiling comparing activated wild-type T cells to those lacking GCH1 (and hence displaying BH4 deficiency) pointed instead to iron regulation.”

Targeting the BH4 pathway offers a novel therapeutic avenue for the treatment of autoimmunity.

“Our work therefore provides strong evidence that targeting this pathway offers a novel therapeutic avenue for the treatment of autoimmunity.”

Gch1-deficient T cells show enhanced secretion of IFNγ following antigen receptor-mediated activation.

“Interestingly, we also observed enhanced secretion of IFNγ in Gch1-deficient T cells following antigen receptor-mediated activation.”

BH4-deficient T cells attempt to increase import of Fe2+ into mitochondria for Fe-S biogenesis and other iron-dependent processes by increasing levels of mitoferrin, HO-1, and frataxin.

“By increasing the levels of mitoferrin, HO-1, and frataxin, the BH4-deficent T cells attempt to increase the import of Fe2+ into the mitochondria for Fe-S biogenesis and other iron-dependent processes.”

BH4 is a metabolite induced in antigen receptor-stimulated T cells.

“We identified BH4 as a metabolite induced in antigen receptor-stimulated T cells.”

QM385 blocks both mouse and human T cell proliferation at nanomolar potency.

“To this end, we developed a novel BH4 inhibitor, QM385, which targets SPR, and blocks both mouse and human T cell proliferation at nanomolar potency with good oral bioavailability and no observable side-effects at immune suppressant exposures (Cronin et al., 2018).”

Enhancing BH4 genetically or pharmacologically with BH4 itself causes T cells to display heightened proliferation and increased anti-tumor activity.

“We also showed that by enhancing BH4 genetically, or pharmacologically with BH4 itself, T cells displayed heightened proliferation and, importantly, increased anti-tumor activity (Cronin et al., 2018).”

QM385-mediated inhibition of BH4 diminishes the proliferative capacity of healthy human donor T cells.

“We also demonstrated that human peripheral blood mononuclear cells (PBMCs) when stimulated through the T cell receptor (TCR) produce BH4 and that QM385-mediated inhibition of BH4 diminished the proliferative capacity of healthy human donor T cells (Cronin et al., 2018).”

QM385 is a novel BH4 inhibitor that targets SPR.

“To this end, we developed a novel BH4 inhibitor, QM385, which targets SPR, and blocks both mouse and human T cell proliferation at nanomolar potency with good oral bioavailability and no observable side-effects at immune suppressant exposures (Cronin et al., 2018).”

Human peripheral blood mononuclear cells when stimulated through the T cell receptor produce BH4.

“We also demonstrated that human peripheral blood mononuclear cells (PBMCs) when stimulated through the T cell receptor (TCR) produce BH4 and that QM385-mediated inhibition of BH4 diminished the proliferative capacity of healthy human donor T cells (Cronin et al., 2018).”

In several different models of T cell-mediated autoimmunity, such as intestinal inflammation, EAE, psoriasis, and T cell-driven asthma, genetic or pharmacologic inhibition of the BH4 pathway markedly abrogated the severity of autoimmunity.

“Importantly, in several different models of T cell-mediated autoimmunity, such as intestinal inflammation, experimental allergic encephalitis (EAE), psoriasis, and T cell-driven asthma, the genetic or pharmacologic inhibition of the BH4 pathway markedly abrogated the severity of autoimmunity.”

QM385 has good oral bioavailability and no observable side-effects at immune suppressant exposures.

“To this end, we developed a novel BH4 inhibitor, QM385, which targets SPR, and blocks both mouse and human T cell proliferation at nanomolar potency with good oral bioavailability and no observable side-effects at immune suppressant exposures (Cronin et al., 2018).”

In activated T cells, BH4 regulates both the reduction of Fe3+ and increases the availability of Fe2+ in the cell, which aid activated T cells in coordinating the increased metabolic needs for effector functions.

“In such activated T cells, BH4 regulates both the reduction of Fe3+ and increases the availability of Fe2+ in the cell, which aid activated T cells in coordinating the increased metabolic needs for effector functions.”

With increased BH4, mitochondrial respiration and ATP production are amplified.

“We observed that with increased BH4, mitochondrial respiration and ATP production were also amplified, however, we did not see a surge in total intracellular iron contents.”

Targeting the BH4 pathway offers a novel therapeutic and new mode-of-action opportunity to control T cell proliferation, and in this way help to control autoimmunity or enhance anti-cancer immunity.

“Targeting this pathway offers a novel therapeutic and new mode-of-action opportunity to control T cell proliferation, and in this way help to control autoimmunity or enhance anti-cancer immunity.”

Increased BH4 did not increase total intracellular iron contents.

“We observed that with increased BH4, mitochondrial respiration and ATP production were also amplified, however, we did not see a surge in total intracellular iron contents.”