Obesity-related IL-18 Impairs T-Regulatory Cell Function and Promotes Lung Ischemia-Reperfusion Injury.

Rationale: Primary graft dysfunction (PGD) is a severe form of acute lung injury, leading to increased early morbidity and mortality after lung transplant. Obesity is a major health problem, and recipient obesity is one of the most significant risk factors for developing PGD. Objectives: We hypothesized that T-regulatory cells (Tregs) are able to dampen early ischemia-reperfusion events and thereby decrease the risk of PGD, whereas that action is impaired in obese recipients. Methods: We evaluated Tregs, T cells, and inflammatory markers, plus clinical data, in 79 lung transplant recipients and 41 liver or kidney transplant recipients and studied two groups of mice on a high-fat diet (HFD), which did ("inflammatory" HFD) or did not ("healthy" HFD) develop low-grade inflammation with decreased Treg function. Measurements and Main Results: We identified increased levels of IL-18 as a previously unrecognized mechanism that impairs Tregs' suppressive function in obese individuals. IL-18 decreases levels of FOXP3, the key Treg transcription factor, decreases FOXP3 di- and oligomerization, and increases the ubiquitination and proteasomal degradation of FOXP3. IL-18-treated Tregs or Tregs from obese mice fail to control PGD, whereas IL-18 inhibition ameliorates lung inflammation. The IL-18-driven impairment in Tregs' suppressive function before transplant was associated with an increased risk and severity of PGD in clinical lung transplant recipients. Conclusions: Obesity-related IL-18 induces Treg dysfunction that may contribute to the pathogenesis of PGD. Evaluation of Tregs' suppressive function together with evaluation of IL-18 levels may serve as a screening tool to identify obese individuals with an increased risk of PGD before transplant.

Legionnaires' Disease Mortality in Guinea Pigs Involves the p45 Mobile Genomic Element.

BACKGROUND: Legionella can cause Legionnaires' disease, a potentially fatal form of pneumonia that occurs as sporadic epidemics. Not all strains display the same propensity to cause disease in humans. Because Legionella pneumophila serogroup 1 is responsible for >85% of infections, the majority of studies have examined this serogroup, but there are 3 commonly used laboratory strains: L pneumophila serogroup 1 Philadelphia (Phil-1)-derived strains JR32 and Lp01 and 130b-derived strain AA100. METHODS: We evaluated the ability of Phil-1, JR32, Lp01, and AA100 to cause disease in guinea pigs. RESULTS: We found that, although Phil-1, JR32, and AA100 cause an acute pneumonia and death by 4 days postinfection (100%), strain Lp01 does not cause mortality (0%). We also noted that Lp01 lacks a mobile element, designated p45, whose presence correlates with virulence. Transfer of p45 into Lp01 results in recovery of the ability of this strain to cause mortality, leads to more pronounced disease, and correlates with increased interferon-γ levels in the lungs and spleens before death. CONCLUSIONS: These observations suggest a mechanism of Legionnaires' disease pathogenesis due to the presence of type IVA secretion systems that cause higher mortality due to overinduction of a proinflammatory response in the host.

Nuclear Coregulatory Complexes in Tregs as Targets to Promote Anticancer Immune Responses.

T-regulatory (Treg) cells display considerable heterogeneity in their responses to various cancers. The functional differences among this cell type are heavily influenced by multiprotein nuclear complexes that control their gene expression. Many such complexes act mechanistically by altering epigenetic profiles of genes important to Treg function, including the forkhead P3 (Foxp3) transcription factor. Complexes that form with certain members of the histone/protein deacetylase (HDAC) class of enzymes, like HDACs 1, 2, and 3, along with histone methyltransferase complexes, are important in the induction and stabilization of Foxp3 and Treg identity. The functional behavior of both circulating and intratumoral Tregs greatly impacts the antitumor immune response and can be predictive of patient outcome. Thus, targeting these regulatory complexes within Tregs may have therapeutic potential, especially in personalized immunotherapies.

A Biological Circuit Involving Mef2c, Mef2d, and Hdac9 Controls the Immunosuppressive Functions of CD4+Foxp3+ T-Regulatory Cells.

The Mads/Mef2 (Mef2a/b/c/d) family of transcription factors (TFs) regulates differentiation of muscle cells, neurons and hematopoietic cells. By functioning in physiological feedback loops, Mef2 TFs promote the transcription of their repressor, Hdac9, thereby providing temporal control of Mef2-driven differentiation. Disruption of this feedback is associated with the development of various pathologic states, including cancer. Beside their direct involvement in oncogenesis, Mef2 TFs indirectly control tumor progression by regulating antitumor immunity. We recently reported that in CD4+CD25+Foxp3+ T-regulatory (Treg) cells, Mef2d is required for the acquisition of an effector Treg (eTreg) phenotype and for the activation of an epigenetic program that suppresses the anti-tumor immune responses of conventional T and B cells. We now report that as with Mef2d, the deletion of Mef2c in Tregs switches off the expression of Il10 and Icos and leads to enhanced antitumor immunity in syngeneic models of lung cancer. Mechanistically, Mef2c does not directly bind the regulatory elements of Icos and Il10, but its loss-of-function in Tregs induces the expression of the transcriptional repressor, Hdac9. As a consequence, Mef2d, the more abundant member of the Mef2 family, is converted by Hdac9 into a transcriptional repressor on these loci. This leads to the impairment of Treg suppressive properties in vivo and to enhanced anti-cancer immunity. These data further highlight the central role played by the Mef2/Hdac9 axis in the regulation of CD4+Foxp3+ Treg function and adds a new level of complexity to the analysis and study of Treg biology.

Kynurenine induces T cell fat catabolism and has limited suppressive effects in vivo.

BACKGROUND: L-kynurenine is a tryptophan-derived immunosuppressive metabolite and precursor to neurotoxic anthranilate and quinolinate. We evaluated the stereoisomer D-kynurenine as an immunosuppressive therapeutic which is hypothesized to produce less neurotoxic metabolites than L-kynurenine. METHODS: L-/D-kynurenine effects on human and murine T cell function were examined in vitro and in vivo (homeostatic proliferation, colitis, cardiac transplant). Kynurenine effects on T cell metabolism were interrogated using [13C] glucose, glutamine and palmitate tracing. Kynurenine was measured in tissues from human and murine tumours and kynurenine-fed mice. FINDINGS: We observed that 1 mM D-kynurenine inhibits T cell proliferation through apoptosis similar to L-kynurenine. Mechanistically, [13C]-tracing revealed that co-stimulated CD4+ T cells exposed to L-/D-kynurenine undergo increased ß-oxidation depleting fatty acids. Replenishing oleate/palmitate restored effector T cell viability. We administered dietary D-kynurenine reaching tissue kynurenine concentrations of 19 µM, which is close to human kidney (6 µM) and head and neck cancer (14 µM) but well below the 1 mM required for apoptosis. D-kynurenine protected Rag1-/- mice from autoimmune colitis in an aryl-hydrocarbon receptor dependent manner but did not attenuate more stringent immunological challenges such as antigen mismatched cardiac allograft rejection. INTERPRETATION: Our dietary kynurenine model achieved tissue concentrations at or above human cancer kynurenine and exhibited only limited immunosuppression. Sub-suppressive kynurenine concentrations in human cancers may limit the responsiveness to indoleamine 2,3-dioxygenase inhibition evaluated in clinical trials. FUNDING: The study was supported by the NIH, the Else Kröner-Fresenius-Foundation, Laffey McHugh foundation, and American Society of Nephrology.

MEF2D sustains activation of effector Foxp3+ Tregs during transplant survival and anticancer immunity.

The transcription factor MEF2D is important in the regulation of differentiation and adaptive responses in many cell types. We found that among T cells, MEF2D gained new functions in Foxp3+ T regulatory (Treg) cells due to its interactions with the transcription factor Foxp3 and its release from canonical partners, like histone/protein deacetylases. Though not necessary for the generation and maintenance of Tregs, MEF2D was required for the expression of IL-10, CTLA4, and Icos, and for the acquisition of an effector Treg phenotype. At these loci, MEF2D acted both synergistically and additively to Foxp3, and downstream of Blimp1. Mice with the conditional deletion in Tregs of the gene encoding MEF2D were unable to maintain long-term allograft survival despite costimulation blockade, had enhanced antitumor immunity in syngeneic models, but displayed only minor evidence of autoimmunity when maintained under normal conditions. The role played by MEF2D in sustaining effector Foxp3+ Treg functions without abrogating their basal actions suggests its suitability for drug discovery efforts in cancer therapy.

Legionella pneumophila p45 element influences host cell entry and sensitivity to sodium.

Legionella pneumophila are environmental bacteria found ubiquitously in both natural and man-made water reservoirs, sometimes as constituents of biofilm communities, but mostly intracellularly within protozoal hosts. In the event that Legionella become aerosolized in water droplets and inhaled by humans, they can cause a potentially fatal form of pneumonia called Legionnaires' disease. Strains of L. pneumophila have highly plastic genomes that harbor numerous inter- and intra-genomic elements, enhancing their ability to live under diverse environmental conditions. One such mobile genomic element, p45 carries ~45 kbp of genes, including the Lvh (Legionella Vir homolog) type IVa secretion system. This element was evaluated for its contribution to L. pneumophila environmental resilience and virulence-related characteristics by comparing clinically isolated strain Philadelphia-1 that carries p45, Lp01 that lacks p45, and Lp01 with p45 reintroduced, Lp01+p45. We found that the p45 element impacts host cell entry and resistance to sodium, both virulence-related characteristics in Legionella species.