The cytokine transforming growth factor-beta (TGF-beta) converts naïve T cells into regulatory T (Treg) cells that prevent autoimmunity. However, in the presence of interleukin-6 (IL-6), TGF-beta has also been found to promote the differentiation of naïve T lymphocytes into proinflammatory IL-17 cytokine-producing T helper 17 (T(H)17) cells, which promote autoimmunity and inflammation. This raises the question of how TGF-beta can generate such distinct outcomes. We identified the vitamin A metabolite retinoic acid as a key regulator of TGF-beta-dependent immune responses, capable of inhibiting the IL-6-driven induction of proinflammatory T(H)17 cells and promoting anti-inflammatory Treg cell differentiation. These findings indicate that a common metabolite can regulate the balance between pro- and anti-inflammatory immunity.

IntroductionA progressive reduction in CD4 ϩ T-helper lymphocytes is the main feature of HIV infection and leads to a depression in adaptive immunity. 1 Innate immunity is also important in the host response to HIV infection and can be impaired during the course of this infection. Dendritic cells (DCs) can promote HIV transmission, [2][3][4][5] and DC function 6 and number 7 decline with HIV infection. The effector functions of monocytes and macrophages, including phagocytosis and intracellular oxidative responses, can be found decreased in HIV-infected subjects 8,9 and in cultured cells in the presence of HIV. [10][11][12][13] Superoxide production by neutrophils 14 as well as natural killer cell function as measured by the lymphokineactivated killer activity and responsiveness to interferon-␣ (IFN-␣) 15,16 have been shown to be defective in HIV-infected subjects.An important part of the innate defense against virus is the production of the type I IFNs, IFN-␣, and IFN-␤. 17 IFN-␣/␤ not only directly inhibit HIV replication, 18-20 but also have important adjuvant effects on a variety of immune cell types, such as monocytes, natural killer cells, 21 and T cells. [22][23][24][25][26] The in vitro type I IFN production by total peripheral blood mononuclear cells (PBMCs) was shown to be impaired during the course of HIV infection, and this impairment was associated with the occurrence of opportunistic infections. 27,28 CD4 ϩ CD11c Ϫ lineage marker Ϫ type 2 DC precursors (pre-DC2) were recently shown to be the natural IFN-␣/␤-producing cells in human blood. 29,30 IPCs produce up to 1000 times more IFN-␣ than any other blood cell type in response to viral stimulation. 29 Whether this impairment of IFN-␣/␤ production in HIV-infected individuals is due to a functional defect or to a reduction in number of IPCs is not known.In this study, we show that blood IPCs are severely decreased in AIDS patients but increased in asymptomatic long-term survivors (LTSs). The drop in IPC number and a decrease in their induced IFN production are associated with the presence of opportunistic infections and active Kaposi sarcoma. Our findings bring a new insight into the physiopathology of HIV infection and identify the IPC count as a new parameter to monitor the status of the immune system of HIV-infected subjects. Patients, materials, and methods HIV-infected subjectsFifty-four HIV-infected subjects were recruited from 3 centers: the University of California at San Francisco (UCSF), the San Francisco General Hospital, and the Hospices Civils de Lyon, France. This study was approved by the Committee for Human Research, UCSF. Subjects were enrolled consecutively, and the only inclusion criterion was a confirmed HIVpositive serology and a written informed consent. The following conditions, which can nonspecifically affect blood cell counts, were used as exclusion criteria: previous cytotoxic chemotherapy, splenectomy, hypersplenism, and blood transfusion within the past 4 weeks. After inclusion, a full medical history was taken and physic...

The primary CD8+ T cell response of C57BL/6J mice against the 28 known epitopes of lymphocytic choriomeningitis virus (LCMV) is associated with a clear immunodominance hierarchy whose mechanism has yet to be defined. To evaluate the role of epitope competition in immunodominance, we manipulated the number of CD8+ T cell epitopes that could be recognized during LCMV infection. Decreasing epitope numbers, using a viral variant lacking dominant epitopes or C57BL/6J mice lacking H-2Kb, resulted in minor response increases for the remaining epitopes and no new epitopes being recognized. Increasing epitope numbers by using F1 hybrid mice, delivery by recombinant vaccinia virus, or epitope delivery as a pool in IFA maintained the overall response pattern; however, changes in the hierarchy did become apparent. MHC binding affinity of these epitopes was measured and was found to not strictly predict the hierarchy since in several cases similarly high binding affinities were associated with differences in immunodominance. In these instances the naive CD8+ T cell precursor frequency, directly measured by tetramer staining, correlated with the response hierarchy seen after LCMV infection. Finally, we investigated an escape mutant of the dominant GP33-41 epitope that elicited a weak response following LCMV variant virus infection. Strikingly, dominance loss likely reflects a substantial reduction in frequencies of naive precursors specific for this epitope. Thus, our results indicate that an intrinsic property of the epitope (MHC binding affinity) and an intrinsic property of the host (naive precursor frequency) jointly dictate the immunodominance hierarchy of CD8+ T cell responses.

SIRT3 (sirtuin 3) modulates respiration via the deacetylation of lysine residues in electron transport chain proteins. Whether mitochondrial protein acetylation is controlled by a counter-regulatory program has remained elusive. In the present study we identify an essential component of this previously undefined mitochondrial acetyltransferase system. We show that GCN5L1 [GCN5 (general control of amino acid synthesis 5)-like 1; also known as Bloc1s1] counters the acetylation and respiratory effects of SIRT3. GCN5L1 is mitochondrial-enriched and displays significant homology with a prokaryotic acetyltransferase. Genetic knockdown of GCN5L1 blunts mitochondrial protein acetylation, and its reconstitution in intact mitochondria restores protein acetylation. GCN5L1 interacts with and promotes acetylation of SIRT3 respiratory chain targets and reverses global SIRT3 effects on mitochondrial protein acetylation, respiration and bioenergetics. The results of the present study identify GCN5L1 as a critical prokaryote-derived component of the mitochondrial acetyltransferase programme.

Certain glycolipid Ags for Vα14i NKT cells can direct the overall cytokine balance of the immune response. Th2-biasing OCH has a lower TCR avidity than the most potent agonist known, α-galactosylceramide. Although the CD1d-exposed portions of OCH and α-galactosylceramide are identical, structural analysis indicates that there are subtle CD1d conformational differences due to differences in the buried lipid portion of these two Ags, likely accounting for the difference in antigenic potency. Th1-biasing C-glycoside/CD1d has even weaker TCR interactions than OCH/CD1d. Despite this, C-glycoside caused a greater downstream activation of NK cells to produce IFN-γ, accounting for its promotion of Th1 responses. We found that this difference correlated with the finding that C-glycoside/CD1d complexes survive much longer in vivo. Therefore, we suggest that the pharmacokinetic properties of glycolipids are a major determinant of cytokine skewing, suggesting a pathway for designing therapeutic glycolipids for modulating invariant NKT cell responses.

Mitochondria are highly dynamic cellular organelles, with the ability to change size, shape and position over the course of a few seconds. Many of these changes are related to the ability of mitochondria to undergo the highly co-ordinated processes of fission (division of a single organelle into two or more independent structures) or fusion (the opposing reaction). These actions occur simultaneously and continuously in many cell types, and the balance between them regulates the overall morphology of mitochondria within any given cell. Fission and fusion are active processes which require many specialized proteins, including mechanical enzymes that physically alter mitochondrial membranes, and adaptor proteins that regulate the interaction of these mechanical proteins with organelles. Although not fully understood, alterations in mitochondrial morphology appear to be involved in several activities that are crucial to the health of cells. In the present chapter we discuss the mechanisms behind mitochondrial fission and fusion, and discuss the implications of changes in organelle morphology during the life of a cell.

SIRT3 is the primary mitochondrial deacetylase that modulates mitochondrial metabolic and oxidative stress regulatory pathways. However, its role in response to nutrient excess remains unknown. Thus, we investigated SIRT3 regulation of the electron transfer chain and evaluated the role of SIRT3 in hepatic lipotoxic stress. SIRT3 depleted HepG2 cells shows diffuse disruption in mitochondrial electron transfer chain functioning, a concurrent reduction in the mitochondrial membrane potential, and excess basal reactive oxygen species levels. As this phenotype may predispose to increased lipotoxic hepatic susceptibility we evaluated the expression of SIRT3 in murine liver following chronic high-fat feeding. In this nutrient-excess model SIRT3 transcript and protein levels are downregulated in parallel with increased hepatic fat storage and oxidative stress. Palmitate was used to investigate lipotoxic susceptibility in SIRT3 knockout mouse primary hepatocytes and SIRT3 siRNA depleted HepG2 cells. Under SIRT3 deficient conditions palmitate enhances reactive oxygen species and increases hepatocyte cell death. Reconstitution of SIRT3 levels and/or treatment with N-acetylcysteine ameliorates these adverse effects. In conclusion SIRT3 functions to ameliorate hepatic lipotoxicity, although paradoxically, exposure to high-fat downregulates this adaptive program in the liver. This SIRT3-dependent lipotoxic susceptibility is possibly modulated, in part, by SIRT3 mediated control of electron transfer chain flux.

Background:The balance between mitochondrial biogenesis and autophagy controls cellular mitochondrial content. Results: Mitochondrial deacetylation-induced mitophagy evokes concurrent and interdependent up-regulation of TFEB and PGC-1␣ to sustain cellular mitochondrial content. Conclusion: Mitochondrial content is coordinately regulated by the mitochondrial and lysosome biogenesis programs under GCN5L1 control. Significance: Counter-regulatory interdependent programs function to sustain mitochondrial content/homeostasis in a nutrient-sensing, acetylation-dependent manner.