Scott B. Lovitch

SIRT4 Coordinates the Balance between Lipid Synthesis and Catabolism by Repressing Malonyl CoA Decarboxylase

Lipid metabolism is tightly controlled by the nutritional state of the organism. Nutrient-rich conditions increase lipogenesis, whereas nutrient deprivation promotes fat oxidation. In this study, we identify the mitochondrial sirtuin, SIRT4, as a regulator of lipid homeostasis. SIRT4 is active in nutrient-replete conditions to repress fatty acid oxidation while promoting lipid anabolism. SIRT4 deacetylates and inhibits malonyl CoA decarboxylase (MCD), an enzyme that produces acetyl CoA from malonyl CoA. Malonyl CoA provides the carbon skeleton for lipogenesis and also inhibits fat oxidation. Mice lacking SIRT4 display elevated MCD activity and decreased malonyl CoA in skeletal muscle and white adipose tissue. Consequently, SIRT4 KO mice display deregulated lipid metabolism, leading to increased exercise tolerance and protection against diet-induced obesity. In sum, this work elucidates SIRT4 as an important regulator of lipid homeostasis, identifies MCD as a SIRT4 target, and deepens our understanding of the malonyl CoA regulatory axis.

The coinhibitory receptor CTLA-4 Controls B cell Responses by Modulating T Follicular Helper, T Follicular Regulatory and T Regulatory Cells

The receptor CTLA-4 has been implicated in controlling B cell responses, but the mechanisms by which CTLA-4 regulates antibody production are not known. Here we showed deletion of CTLA-4 in adult mice increased Tfh and Tfr cell numbers and augmented B cell responses. In the effector phase, loss of CTLA-4 on Tfh cells resulted in heightened B cell responses, whereas loss of CTLA-4 on Tfr cells resulted in defective suppression of antigen-specific antibody responses. We also found that non-Tfr Treg cells could suppress B cell responses through CTLA-4 and that Treg and/or Tfr cells might downregulate B7-2 on B cells outside germinal centers as a means of suppression. Within the germinal center, however, Tfr cells potently suppress B cells through CTLA-4, but with a mechanism independent of altering B7-1 or B7-2. Thus, we identify multifaceted regulatory roles for CTLA-4 in Tfh, Tfr, and Treg cells, which together control humoral immunity.

T Cells Distinguish MHC-Peptide Complexes Formed in Separate Vesicles and Edited by H2-DM

The peptide spanning residues 48-61 of hen egg white lysozyme (HEL) presented by I-A(k) gives rise to two T cell populations, referred to as type A and B, that distinguish the complex generated intracellularly upon processing of HEL from that formed with exogenous peptide. Here, we ascribe this difference to recognition of distinct conformers of the complex and show that formation of the two complexes results from antigen processing in different intracellular compartments and is dependent upon H2-DM. While the type A complex preferentially formed in a lysosome-like late vesicle, the type B complex failed to form in this compartment; this distinction was abolished in antigen-presenting cells lacking DM. Experiments in vitro indicated that H2-DM acts directly on the complex to eliminate the type B conformation. We conclude that different antigen-processing pathways generate distinct MHC-peptide conformers, priming T cells with distinct specificity that may play unique roles in immunity.

Deletion of CTLA-4 on regulatory T cells during adulthood leads to resistance to autoimmunity

Paterson et al. demonstrate that, in contrast to CTLA-4 germline knockout mice, conditional deletion on T reg cells during adulthood confers protection from EAE and does not increase resistance to tumors.

Cutting edge: H-2DM is responsible for the large differences in presentation among peptides selected by I-Ak during antigen processing

We quantitated the amounts of peptides from hen egg-white lysozyme presented by I-Ak molecules in APC lines. The large chemical gradient of presentation of the four hen egg-white lysozyme epitopes observed in cell lines expressing HLA-DM or H-2DM (referred to in this study as DM) was significantly diminished in the T2.Ak line lacking DM. Differences in levels of presentation between wild-type and DM-deficient APC were observed for all four epitopes, but differences were most evident for the highest affinity epitope. As a result of these quantitative differences in display, presentation of all four epitopes to T cells was impaired in the line lacking DM. The binding affinity of the pool of naturally processed peptides from DM-expressing lines was higher than that from the DM-deficient line. Thus, using a direct biochemical approach in APC, we demonstrate that DM influences the selection of peptides bound to MHC class II by favoring high affinity peptides.

Conformational isomers of a peptide–class II major histocompatibility complex

Summary:  The relative plasticity of peptide binding to class II major histocompatibility complex (MHC) molecules permits formation of multiple conformational isomers by the same peptide and MHC molecule; such conformers are specifically recognized by distinct subsets of T cells. Here, we review current knowledge and recent advances in our understanding of peptide–class II MHC conformational isomerism and the mechanisms that generate distinct MHC–peptide conformers. We focus on our studies of two T‐cell subsets, type A and B, which recognize distinct conformers of the dominant epitope of hen egg white lysozyme presented by I‐Ak. These conformers form via different pathways and in distinct intracellular vesicles: the type A conformer forms in late endosomes upon processing of native protein, while the more flexible type B conformer forms in early endosomes and at the cell surface. In this process, H2‐DM acts as a conformational editor, eliminating the type B conformer in late endosomes. Type B T cells constitute a significant component of the naïve T‐cell repertoire; furthermore, self‐reactive type B T cells escape negative selection and are present in abundance in the periphery. Ongoing studies should elucidate the role of type B T cells in immunity to pathogens and in autoimmune pathology.

Amino-Terminal Flanking Residues Determine the Conformation of a Peptide–Class II MHC Complex

The peptide spanning residues 48–62 of hen egg white lysozyme presented by I-Ak molecules gives rise to two T cell populations, types A and B, that recognize distinct conformers of the complex generated in late and recycling endosomes. The class II–like accessory molecule H2-DM functions as a conformational editor, eliminating the type B conformer in late endosomes. Here, we show that the conformation of the complex, and its susceptibility to editing by H2-DM, are determined by peptide amino-terminal flanking residues. Elimination of these residues abolished editing, permitting formation of the type B conformer in late endosomes. Substitutions at P(−2) affected the stability of the type B conformer, preventing its formation and/or editing, without hindering peptide binding or formation of the type A conformer of the complex. We conclude that interactions involving amino-terminal flanking residues stabilize peptide-MHC conformers and confer resistance to editing by H2-DM, influencing the nature of the T cell repertoire.

Programmed death ligand-1 expression on donor T cells drives graft-versus-host disease lethality

Programmed death ligand-1 (PD-L1) interaction with PD-1 induces T cell exhaustion and is a therapeutic target to enhance immune responses against cancer and chronic infections. In murine bone marrow transplant models, PD-L1 expression on host target tissues reduces the incidence of graft-versus-host disease (GVHD). PD-L1 is also expressed on T cells; however, it is unclear whether PD-L1 on this population influences immune function. Here, we examined the effects of PD-L1 modulation of T cell function in GVHD. In patients with severe GVHD, PD-L1 expression was increased on donor T cells. Compared with mice that received WT T cells, GVHD was reduced in animals that received T cells from Pdl1-/- donors. PD-L1-deficient T cells had reduced expression of gut homing receptors, diminished production of inflammatory cytokines, and enhanced rates of apoptosis. Moreover, multiple bioenergetic pathways, including aerobic glycolysis, oxidative phosphorylation, and fatty acid metabolism, were also reduced in T cells lacking PD-L1. Finally, the reduction of acute GVHD lethality in mice that received Pdl1-/- donor cells did not affect graft-versus-leukemia responses. These data demonstrate that PD-L1 selectively enhances T cell-mediated immune responses, suggesting a context-dependent function of the PD-1/PD-L1 axis, and suggest selective inhibition of PD-L1 on donor T cells as a potential strategy to prevent or ameliorate GVHD.

APCs Present Aβk-Derived Peptides That Are Autoantigenic to Type B T Cells

Type B T cells recognize peptide provided exogenously but are ignorant of the same epitope derived from intracellular processing. In this study, we demonstrate the existence of type B T cells to an abundant autologous peptide derived from processing of the I-Ak β-chain. T cell hybridomas raised against this peptide fail to recognize syngeneic APC despite abundant presentation of the naturally processed epitope but react in a dose-dependent manner to exogenous peptide. Moreover, these hybridomas respond to Aβk peptide extracted from the surface of I-Ak-expressing APC. This peptide was isolated from B cell lines where it was found in high abundance; it was also present in lines lacking HLA-DM, but in considerably lower amounts. Therefore, type B T cells exist in the naive repertoire to abundant autologous peptides. We discuss the implications of these findings to the potential biological role of type B T cells in immune responses and autoimmune pathology.

Activation of Type B T Cells after Protein Immunization Reveals Novel Pathways of In Vivo Presentation of Peptides

Type B T cells recognize a peptide-MHC conformer generated in recycling endosomes and eliminated by H2-DM in late endosomes; as a result, they recognize exogenous peptide, but fail to respond to the identical epitope generated from the native protein. To investigate the behavior of these cells in vivo, we generated mice transgenic for a type B TCR recognizing the 48-62 epitope of hen egg white lysozyme (HEL) presented by I-Ak. Type B T cells responded only to peptide ex vivo, but responded in vivo to immunization with either protein or peptide in the presence of Freund’s adjuvant or LPS. Presentation of the type B conformer was MyD88-independent, evident within 24 h after HEL immunization, and restricted to the CD11b/c+ APC subset. Immunization with listeriolysin O, a potent inducer of cell death, also primed type B T cells in vivo, and transfer of HEL-bearing allogeneic dendritic cells activated type B T cells. We conclude that a number of conditions in vivo, some of which induce inflammation and cell death, lead to peptide presentation through mechanisms distinct from the classical pathways involving H-2DM molecules.