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Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a curative option in the treatment of aggressive malignant and non-malignant blood disorders. However, the benefits of allo-HSCT can be compromised by graft-versus-host disease (GvHD), a prevalent and morbid complication of allo-HSCT. GvHD occurs when donor immune cells mount an alloreactive response against host antigens due to histocompatibility differences between the donor and host, which may result in extensive tissue injury. The reprogramming of cellular metabolism is a feature of GvHD that is associated with the differentiation of donor CD4+ cells into the pathogenic Th1 and Th17 subsets along with the dysfunction of the immune-suppressive protective T regulatory cells (Tregs). The activation of glycolysis and glutaminolysis with concomitant changes in fatty acid oxidation metabolism fuel the anabolic activities of the proliferative alloreactive microenvironment characteristic of GvHD. Thus, metabolic therapies such as glycolytic enzyme inhibitors and fatty acid metabolism modulators are a promising therapeutic strategy for GvHD.
Allogeneic hematopoietic stem cell transplantation (allo-HSCT) has the potential to cure multiple hematological disorders, including aggressive malignancies [1]. The curative potential of allo-HSCT, the graft-versus-leukemia (GvL) effect, can be mediated by several immune subsets [2][3][4]. However, these cell populations may also trigger graft-versus-host disease (GvHD), a morbid and prevalent barrier to allo-HSCT [5][6]. CD4+ T cells are at the center of GvHD pathogenesis. Activated CD4+ T cells differentiate into a plethora of cytokine-secreting T helper (Th) cells including Th1, Th2, Th9, Th17, and regulatory T cells (Tregs) [7][8][9]. Pro-inflammatory Th1 and Th17 CD4+ cells direct GvHD targeting the skin, lungs, gastrointestinal tract, and other tissues [4][10][11][12]. By contrast, regulatory CD4+ Forkhead box protein P3 (Foxp3) + cells (Tregs) are capable of ameliorating GvHD through immunosuppressive mechanisms [13][14][15][16][17]. Alloreactivity induces a metabolic shift in immune cells from a quiescent oxidative phosphorylation (OXPHOS) dominant state to an anabolic profile featuring aerobic glycolysis as the main energy source. In addition, glutamine and fatty acid metabolism provide intermediates for the tri-carboxylic acid (TCA) cycle which fuels the OXPHOS activity seen in early T cell activation [10][18][19][20]. The increased glycolytic flux supports the proliferation of the pathogenic Th1 and Th17 cells. The resultant high T effector/Treg ratio facilitates GvHD pathogenesis in target tissues [9][10][13][21].
There are several forms of GvHD, including acute, chronic, late acute, and overlap syndrome, and some of the biological and pathophysiological features are shared by these entities [4][5]. Acute GvHD exhibits systemic inflammation and tissue destruction in the skin, liver, and gastrointestinal tract that involves alloreactive donor T-cell-mediated cytotoxicity [4][11]. The pathophysiology of cGvHD is complex and thought to involve the interplay between dysregulated innate and adaptive immune cell populations, as well as tissue fibrosis and organ damage [6][22]. Acute GvHD is the main risk factor of chronic GvHD and together they negatively impact on the success of allo-HSCT [23][24][25][26].
Standard GvHD prophylactic measures involve peri-transplant immunosuppression directed primarily at the T cells compartment, including anti-thymocyte globulin (ATG), alemtuzumab, alpha-beta depletion, post-transplant cyclophosphamide (PTCy), calcineurin inhibitors, and methotrexate [27][28][29][30][31]. Despite prophylaxis, the prevalence of acute and chronic GvHD is high. First-line therapeutic interventions aim to provide global immune suppression via the administration of systemic corticosteroids with or without calcineurin inhibitors [27][32]. Unfortunately, these measures increase the risk of infection, relapse, and secondary malignancies [32][33]. In addition, systemic corticosteroids have many side effects and are ineffective for many patients with acute and chronic GvHD, with clinical responses to second-line agents being also being limited [34][35]. An ideal GvHD therapeutic intervention would suppress pathogenic allogeneic immune reactivity while preserving the graft-versus-leukemia (GvL) effect and maintain functional anti-infection immunity [2][36]. Given that malignant cells share cellular metabolism features with alloreactive T cells, metabolic reprogramming represents a promising strategy for the therapeutic targeting of GvHD [7][8][37].
Naïve T cells primarily depend on oxidative phosphorylation. Following allo-HSCT, donor T cells are stimulated by mismatched recipient cell antigens to undergo pro-glycolytic metabolic reprogramming and form allogeneic T effector cells (Teffs) [38][39][40]. During this phase, pyruvate is directed towards lactate synthesis under the influence of pyruvate dehydrogenase kinase 1 (PDHK1) [18][41]. The upregulation of glycolysis supports cell proliferation as glycolytic intermediates feed anabolic pathways, including the pentose phosphate pathway (PPP) [9][42][43]. T cell receptor (TCR) antigen-presenting cell (APC) interaction stimulates the phosphoinositide 3-kinases (PI3K)-protein kinase B-mechanistic target of rapamycin (mTOR) pathway to stabilize the glycolytic microenvironment. Furthermore, protein kinase B (a.k.a. Akt) activates glycolytic enzymes such as hexokinase (HK) and phosphofructokinase (PFK) through phosphorylation [44][45]. Akt also upregulates the expression of the transmembrane glucose transporters (Glut)1 and Glut3 in alloreactive T cells [46][47].
Concomitantly, pyruvate is decarboxylated into acetyl-CoA which is relayed into the TCA cycle to facilitate NADH and flavin adenine dinucleotide (FADH2) synthesis. In the presence of oxygen, NADH and FADH2 fuel ATP generation by shuttling electrons along the electron transport chain in OXPHOS [43][48]. Studies show that both glycolysis and OXPHOS are upregulated in alloreactive cells [20][49]. However, the increase in OXPHOS activity is transient and may characterize early events in GvHD pathogenesis and T cell activation [49][50], perhaps preceding the development of tissue hypoxia. The alloreactive metabolism and cell proliferation form a hypoxic environment that is characterized by the elevated expression of hypoxia-induced factor 1 alpha (HIF-1α) [51][52]. HIF-1α is a transcription factor, which modulates the expression of glycolytic enzymes, such as HK1, and glucose transporters, namely Glut1, and Glut3 [19][51]. The messenger ribonucleic acid (mRNA) expression of other glycolytic enzymes and transporters, such as glyceraldehyde 3-phosphate dehydrogenase (Gapdh), lactate dehydrogenase (Ldh)-a, monocarboxylate transporter (Mtc4) and phosphoglycerate kinase (Pgk1) have also been found to be elevated in alloreactive T cells in GvHD [19][53].
The metabolic reprogramming from OXPHOS towards glycolysis sustains the proliferation and secretion of pro-inflammatory cytokines in alloreactive T cells [6][8][43]. Our group confirmed through transcriptomic, protein, and metabolic analyses that the CD4 T effector memory (Tem) cells, a pathogenic subset that mediates GvHD in target tissues, is highly glycolytic [53], providing a further rationale for testing glycolytic inhibitors in GvHD. Pharmacological inhibitors of glycolytic enzymes have been previously tested in an aGvHD pre-clinical model [19] and identified glycolysis as an important metabolic target for re-programming alloreactive T cells in aGvHD (Figure 1).
Macrophages (MCs) are phagocytic antigen-presenting cells with dynamic profiles that reciprocally differentiate into pro-inflammatory M1 and anti-inflammatory M2 subsets [98]. The M1 macrophages are induced by the detection of foreign antigens and pro-inflammatory cytokines (TNF-α, INF-γ), leading to cytokine secretion and the activation of Th1 cells, and the release of reactive nitrogen and oxygen species [99]. By contrast, M2 cells are induced by inflammatory cytokines (IL-4 and IL-13) and Th2 cells to mostly induce immune regulation and the resolution of inflammation [100]. M1 and M2 MCs feature distinct metabolic profiles [98]. The M1 profile is dependent on glycolysis for energy generation, and enforces two breaks in the TCA cycle to produce citrate and succinate [101]. The accumulated citrate generates itaconate, an important antimicrobial agent [102]. Succinate stabilizes HIF-1α, which sustains the glycolytic metabolism of M1 cells [68][103]. Glycolysis feeds the PPP to generate NADPH, which acts as a co-factor of iNOS in NO production [104]. Acetyl-CoA generated by glycolysis is used in FAS. Enhanced FAS is an important feature of M1 metabolism. HIF-1α also suppresses OXPHOS in M1 macrophages, while M2 macrophages rely on OXPHOS for ATP generation. Additionally, in contrast to M1, M2 MCs rely on the TCA cycle [105]. In M2 MCs, FAO and glutamine metabolism are upregulated to provide intermediates for the TCA cycle. Glycolysis and, subsequently, the PPP, are downregulated in M2 macrophages [99].