Pyruvate dehydrogenase complex integrates the metabolome and epigenome in memory T cell differentiation in vitro

Background

Modulation of metabolic flux through pyruvate dehydrogenase complex (PDC) plays an important role in T cell activation and differentiation. PDC sits at the transition between glycolysis and the tricarboxylic acid cycle and is a major producer of acetyl-CoA, marking it as a potential metabolic and epigenetic node.

Methods

To understand the role of pyruvate dehydrogenase complex in T cell differentiation, we generated mice deficient in T cell pyruvate dehydrogenase E1A (Pdha) subunit using a CD4-cre recombinase-based strategy. To control for the contribution of exogenous metabolites in vivo, we conducted our T cell functional studies in vitro. T cells were differentiated into memory and effector T cells using standardized protocols. Cells were analyzed using stable isotopic tracing studies, metabolomics, RNAseq, ATACseq, ChIPseq and histone proteomics.

Results

Herein, we show that genetic ablation of PDC activity in T cells (TPdh-/-) leads to marked perturbations in glycolysis, the tricarboxylic acid cycle, and OXPHOS. Due to depressed OXPHOS, TPdh-/-T cells became dependent upon substrate level phosphorylation via glycolysis. Due to the block of PDC activity, histone acetylation was reduced, as were most other types of post translational modifications. Transcriptional and functional profiling revealed abnormal CD8+ memory T cell differentiation in vitro.

Conclusions

Collectively, our data indicate that PDC integrates the metabolome and epigenome in memory T cell differentiation. Targeting this metabolic and epigenetic node can have widespread ramifications on cellular function.

Tissue acylcarnitine status in a mouse model of mitochondrial β-oxidation deficiency during metabolic decompensation due to influenza virus infection

Despite judicious monitoring and care, patients with fatty acid oxidation disorders may experience metabolic decompensation due to infection which may result in rhabdomyolysis, cardiomyopathy, hypoglycemia and liver dysfunction and failure. Since clinical studies on metabolic decompensation are dangerous, we employed a preclinical model of metabolic decompensation due to infection. By infecting mice with mouse adapted influenza and using a pair-feeding strategy in a mouse model of long-chain fatty acid oxidation (Acadvl-/-), our goals were to isolate the effects of infection on tissue acylcarnitines and determine how they relate to their plasma counterparts. Applying statistical data reduction techniques (Partial Least Squares-Discriminant Analysis), we were able to identify critical acylcarnitines that were driving differentiation of our experimental groups for all the tissues studied. While plasma displayed increases in metabolites directly related to mouse VLCAD deficiency (e.g. C16 and C18), organs like the heart, muscle and liver also showed involvement of alternative pathways (e.g. medium-chain FAO and ketogenesis), suggesting adaptive measures. Matched correlation analyses showed strong correlations (r > 0.7) between plasma and tissue levels for a small number of metabolites. Overall, our results demonstrate that infection as a stress produces perturbations in metabolism in Acadvl-/- that differ greatly from WT infected and Acadvl-/- pair-fed controls. This model system will be useful for studying the effects of infection on tissue metabolism as well as evaluating interventions aimed at modulating the effects of metabolic decompensation.

Sensitivity to Restimulation-Induced Cell Death Is Linked to Glycolytic Metabolism in Human T Cells

Restimulation-induced cell death (RICD) regulates immune responses by restraining effector T cell expansion and limiting nonspecific damage to the host. RICD is triggered by re-engagement of the TCR on a cycling effector T cell, resulting in apoptosis. It remains unclear how RICD sensitivity is calibrated in T cells derived from different individuals or subsets. In this study we show that aerobic glycolysis strongly correlates with RICD sensitivity in human CD8⁺ effector T cells. Reducing glycolytic activity or glucose availability rendered effector T cells significantly less sensitive to RICD. We found that active glycolysis specifically facilitates the induction of proapoptotic Fas ligand upon TCR restimulation, accounting for enhanced RICD sensitivity in highly glycolytic T cells. Collectively, these data indicate that RICD susceptibility is linked to metabolic reprogramming, and that switching back to metabolic quiescence may help shield T cells from RICD as they transition into the memory pool.

Acute metabolic decompensation due to influenza in a mouse model of ornithine transcarbamylase deficiency

The urea cycle functions to incorporate ammonia, generated by normal metabolism, into urea. Urea cycle disorders (UCDs) are caused by loss of function in any of the enzymes responsible for ureagenesis, and are characterized by life-threatening episodes of acute metabolic decompensation with hyperammonemia (HA). A prospective analysis of interim HA events in a cohort of individuals with ornithine transcarbamylase (OTC) deficiency, the most common UCD, revealed that intercurrent infection was the most common precipitant of acute HA and was associated with markers of increased morbidity when compared with other precipitants. To further understand these clinical observations, we developed a model system of metabolic decompensation with HA triggered by viral infection (PR8 influenza) using spf-ash mice, a model of OTC deficiency. Both wild-type (WT) and spf-ash mice displayed similar cytokine profiles and lung viral titers in response to PR8 influenza infection. During infection, spf-ash mice displayed an increase in liver transaminases, suggesting a hepatic sensitivity to the inflammatory response and an altered hepatic immune response. Despite having no visible pathological changes by histology, WT and spf-ash mice had reduced CPS1 and OTC enzyme activities, and, unlike WT, spf-ash mice failed to increase ureagenesis. Depression of urea cycle function was seen in liver amino acid analysis, with reductions seen in aspartate, ornithine and arginine during infection. In conclusion, we developed a model system of acute metabolic decompensation due to infection in a mouse model of a UCD. In addition, we have identified metabolic perturbations during infection in the spf-ash mice, including a reduction of urea cycle intermediates. This model of acute metabolic decompensation with HA due to infection in UCD serves as a platform for exploring biochemical perturbations and the efficacy of treatments, and could be adapted to explore acute decompensation in other types of inborn errors of metabolism.

Liver-directed adeno-associated virus serotype 8 gene transfer rescues a lethal murine model of citrullinemia type 1

Citrullinemia type 1 (CTLN1) is an autosomal recessive disorder of metabolism caused by a deficiency of argininosuccinate synthetase. Despite optimal management, CTLN1 patients still suffer from lethal metabolic instability and experience life threatening episodes of acute hyperammonemia. A murine model of CTLN1 (fold/fold) that displays lethality within the first 21 days of life was used to determine the efficacy of adeno-associated viral (AAV) gene transfer as a potential therapy. An AAV serotype 8 (AAV8) vector was engineered to express the human ASS1 cDNA under the control of a liver-specific promoter (thyroxine binding globulin, TBG), AAV8-TBG-hASS1, and delivered to 7–10 day old mice via intraperitoneal injection. Greater than 95% of the mice were rescued from lethality and survival was extended beyond 100 days after receiving a single dose of vector. AAV8-TBG-hASS1 treatment resulted in liver specific expression of hASS1, increased ASS1 enzyme activity, reduction in plasma ammonia and citrulline concentrations, and significant phenotypic improvement of the fold/fold growth and skin phenotypes. These experiments highlight a gene transfer approach using AAV8 vector for liver targeted gene therapy that could serve as a treatment for CTLN1.

SHIP deficient macrophages promote inflammation by supporting the generation of Th17 cells (91.5)

SHIP is an SH2-containing inositol phosphatase expressed in all hematopoietic cells and proposed as negative regulator of the immune response. SHIP-null mice show increased mortality due to a myeloproliferative-like syndrome. To better understand the function of SHIP we used a conditional disruption of SHIP in the macrophage-monocyte lineage (LysMcreSHIPfl/fl), which partially recapitulates the myeloproliferative phenotype of the null deletion. Macrophages from LysMcre SHIPfl/fl mice display an M2 phenotype likely due to enhanced signaling through the GM-CSF receptor. Deletion of SHIP in macrophages not only leads to myeloid hyperproliferation, but also to lymphocyte activation as a secondary effect. LysMcreSHIPfl/fl conditional mice develop splenomegaly with a large number of activated lymphocytes. We determined that T cells in these mice are biased towards the Th17 phenotype and expand the regulatory population. LysMcreSHIPfl/fl mice show higher mortality and greater induction of Th17 cells in the autoimmune model of EAE. Taken together, we have uncovered a specific regulatory role of SHIP signaling in macrophages that has important consequences in negatively regulating macrophage function directly and T cell development indirectly.

Spontaneous apoptosis and expression of cell surface heat-shock proteins in cultured EL-4 lymphoma cells

The expression of heat-shock proteins (HSPs) is enhanced in stressed cells and can protect cells from stress-induced injury. However, existing data about the relationship between apoptosis and HSP expression is contradictory. In this paper, a mouse lymphoma cell death model system is used to detect simultaneously both the process of apoptosis and the level of HSP expression. The model was established after discovering that spontaneous apoptosis and spontaneous cell surface HSP expression occurs in EL-4 mouse lymphoma cells during normal optimal culture conditions. The data show that apoptotic EL-4 cells had higher levels of hsp25, hsp60, hsp70 and hsp90 exposed on the plasma membrane surface than viable cells. The level of surface HSPs was found to increase through several stages of early and late apoptotic death as measured by flow cytometry, with the highest levels observed during the loss of cell membrane phospholipid asymmetry. Heat shock and actinomycin D significantly increased the proportion of apoptotic cells in culture. However, hyperthermia only stimulated a weak and temporary increase in surface HSP expression, whereas actinomycin D strongly elevated the level of surface and intracellular HSPs, particularly in live cells. These results show an associative relationship between apoptosis and HSP expression. The relationship between the progression of cell death and HSP expression suggests a role for membrane HSP expression in programmed cell death.

Splenic cytotoxic cells recognize surface HSP70 on culture-adapted EL-4 mouse lymphoma cells

Heat shock proteins (HSPs) are intracellular proteins which function as molecular chaperones. At the same time, translocation of HSPs to the cell surface has been observed in stressed, infected and transformed cells. It seems plausible that surface HSPs may represent molecular targets for recognition and elimination of 'altered' cells by cytotoxic lymphocytes. Previously we demonstrated that EL-4 mouse lymphoma cells growing in vitro express HSPs on their plasma membrane. In this study, we tested the hypothesis that surface HSPs present on EL-4 cells may mediate their recognition and killing by cytotoxic lymphocytes. We have found that susceptibility of culture-adapted EL-4 cells to in vitro lysis by syngeneic and allogeneic splenocytes correlated with the expression of HSP70 on EL-4 cells. Moreover, cytotoxicity was blocked by pretreatment of EL-4 target cells with anti-HSP70 antibody, whereas antibodies to MHC class I molecules and Thy1 did not have such effect. Cytotoxicity against EL-4 lymphoma was not MHC class I-restricted, and was not decreased after depletion of CD8(+) cells from the effector cell population. We conclude that in vitro killing of EL-4 cells is mediated, at least in part, by NK cells via recognition of HSPs present on the surface of tumor cells. Thus, cytotoxic response against EL-4 lymphoma should serve as a good model to study the role of HSPs in anti-tumor immunity.