Peroxisome Proliferator-Activated Receptor γ (PPARγ) and Immunoregulation: Enhancement of Regulatory T Cells through PPARγ-Dependent and -Independent Mechanisms

Dendritic cell tolerogenicity: a key mechanism in immunomodulation by vitamin D receptor agonists

Dendritic cells (DC) induce or tolerize T cells, and tolerogenic DCs can promote the development of regulatory T cells (Treg) with suppressive activity. Thus, the possibility of manipulating DCs and enhancing their tolerogenic properties using different pharmacologic or biologic agents could be exploited to control a variety of chronic immuno-mediated inflammatory conditions. Among agents able to promote induction of tolerogenic DCs, vitamin D receptor (VDR) agonists have attracted considerable attention, also because of their potential in clinical translation. DCs are key targets for the immunomodulatory effects of VDR agonists, which shape DC phenotype and function, enhancing their tolerogenicity in adaptive immune responses. Tolerogenic DCs induced by a short treatment with VDR agonists promote CD4+CD25+Foxp3+ Treg cells that are able to mediate transplantation tolerance and to arrest the development of autoimmune diseases. VDR agonists not only favor induction of CD4+CD25+ Treg cells, but can also enhance their recruitment at inflammatory sites. The tolerogenic properties induced by VDR agonists in DCs, leading to enhanced Treg cell development, likely contribute to the beneficial activity of these hormone-like molecules in autoimmune disease and graft rejection models, highlighting their applicability to the treatment of chronic inflammatory conditions sustained by autoreactive or alloreactive immune responses.

Purified eicosapentaenoic acid induces prolonged survival of cardiac allografts and generates regulatory T cells

Fish oil, which is rich in eicosapentaenoic acid (EPA), has been found to have immunomodulatory effects. We examined whether administration of purified EPA affected survival of fully mismatched murine cardiac allografts. Hearts from C57BL/10 (H-2(b)) mice were transplanted into CBA (H-2(k)) recipients treated with one intraperitoneal dose of purified EPA the day of transplantation. Untreated CBA recipients and recipients given 0.1 g/kg of EPA rejected C57BL/10 hearts (median survival time [MST], 8 and 13 days, respectively). With a 1.0 g/kg dose of EPA, graft survival was markedly prolonged (MST >100 days). To determine whether regulatory cells were generated, naïve mice (secondary recipients) underwent adoptive transfer of splenocytes from EPA-treated primary recipients and cardiac allograft transplantation. Adoptive transfer of whole, CD4(+) and CD4(+)CD25(+) splenocytes from EPA-treated recipients induced indefinite survival in secondary recipients. Flow cytometry showed that the CD4(+)CD25(+) cells were Foxp3(+). In reverse transcriptase-polymerase chain reaction (RT-PCR) studies, the expression of peroxisome proliferator-activated receptor gamma (PPARgamma) mRNA was upregulated by EPA treatment. A PPARgamma antagonist abrogated the prolongation of graft survival induced by EPA treatment (MST, 13 days). Thus, in our model, purified EPA induced prolonged survival of fully mismatched cardiac allografts and generated regulatory T cells dependent on PPARgamma activation.

PPARgamma regulates retinoic acid-mediated DC induction of Tregs

CD4+ CD25+ Foxp3+ Tregs are critical regulators of immune responses and autoimmune diseases. nTregs are thymically derived; iTregs are converted in the periphery from CD4+ CD25- Foxp3- Teffs. Recent studies reported that GALT CD103+ DCs mediated enhanced iTreg conversion via the secretion of RA. However, the factors regulating RA secretion and hence, the induction of iTregs by DCs are not yet clear. Activation of the nuclear hormone receptor PPARgamma has been shown to induce RA expression in human DCs, and thus, we postulated that PPARgamma activation in DCs may be an important regulator of RA secretion and iTreg generation. Using in vitro and in vivo approaches, we now demonstrate that PPARgamma activation enhances iTreg generation through increased RA synthesis from murine splenic DCs. In addition, we demonstrate that inhibition of DC PPARgamma decreases iTreg generation, suggesting a role for endogenous PPARgamma ligands in this process. Overall, our findings suggest that PPARgamma may be important as a factor that stimulates DCs to produce RA and as a potential mechanism by which PPARgamma ligands ameliorate autoimmunity.

Targeting PPAR as a therapy to treat multiple sclerosis

BACKGROUND

Multiple sclerosis (MS) is a neurological disorder that causes chronic paralysis and immense socio-economic problem among young adults. The etiology of MS is not known but it is generally viewed as an autoimmune inflammatory disease of the CNS. Over the past decade, several anti-inflammatory drugs have been developed to control MS symptoms but there is no medical cure.

OBJECTIVE

To evaluate the use and mechanism of action of agonists of PPAR, a family of nuclear receptor transcription factors that regulate inflammation, in treatment of MS.

METHODS

There are several reports showing beneficial effects of PPAR agonists in treating MS-like disease in animal models. We review recent advances in this field.

RESULTS/CONCLUSIONS

PPAR agonists regulate MS-like disease in animal models by blocking inflammatory signaling pathways, suggesting their use in treatment of MS. Current human trials are likely to confirm the safety and efficacy of PPAR agonists for MS treatment.

Rosiglitazone preserves islet beta-cell function of adult-onset latent autoimmune diabetes in 3 years follow-up study

The newly developed insulin sensitizer-thiazolidinediones have the potential to downregulate inflammation and autoimmune response. The objective of this study was to observe the beneficial effects on beta-cell function in the LADA patients treated with rosiglitazone. 54 LADA patients were assigned to oral hypoglycemic agents group (GAD-Ab<175 U/mL and FCP>0.3 nmol/L) or early insulin administration group (GAD-Ab>or=175 U/mL or GAD-Ab<175 U/mL and FCP<or=0.3 nmol/L). Then, those patients were randomly assigned to receive sulfonylureas (SUs group) or rosiglitazone (RSG group) therapy, or to receive insulin alone (INS group) or rosiglitazone plus insulin (INS+RSG group). Plasma glucose, HbA1c, fasting C-peptide (FCP) and C-peptide after 2h 75-g glucose load (PCP) were determined every 6 months. The levels of PCP and delta CP were higher in RSG group compared with those in SUs group after the 18th month. The PCP level (after the 12th month) and delta CP level (after the 18th month) in INS+/-RSG group were higher than those in INS group. Rosiglitazone combined with insulin wherever or not preserved beta-cell function in LADA patients after 3 years.

Induction of tolerogenic dendritic cells by vitamin D receptor agonists

Dendritic cells induce and regulate T cell responses, and tolerogenic dendritic cells (DCs) can promote the development of regulatory T cells with suppressive activity. Thus, the possibility to manipulate DCs using different pharmacological or biological agents enables them to exert tolerogenic activities, could be exploited to better control a variety of chronic inflammatory conditions, from autoimmune diseases to allograft rejection. A variety of both biological and pharmacological agents can induce tolerogenic DCs, and several in vitro studies have demonstrated that human regulatory T cells can be induced by DCs manipulated to acquire and/or enhance tolerogenic properties, with in vivo data also accumulating. Within this context, we have explored the immunoregulatory activities of vitamin D receptor (VDR) agonists, secosteroid hormones able to induce tolerogenic DCs and regulatory T cells. Tolerogenic DCs induced by a short treatment with VDR agonists promote CD4(+) CD25(+) Foxp3(+) suppressor T cells that are able to mediate transplantation tolerance and to arrest the development of autoimmune diseases. VDR agonists not only favour the induction of CD4(+) CD25(+) regulatory T cells, but can also enhance their recruitment to inflammatory sites. VDR agonists have been proven effective and safe drugs in a variety of autoimmune disease and graft rejection models, highlighting their potential applicability to chronic inflammatory conditions sustained by autoreactive or alloreactive immune responses. In addition to the topical treatment of psoriasis, a Th1-mediated autoimmune disease of the skin where VDR agonists are the most used topical drugs; these agents might eventually find a broader application in the treatment of inflammatory conditions, where their modulatory effects on DCs enhancing T cells with regulatory functions could turn out to be quite beneficial.

The many faces of PPARgamma: anti-inflammatory by any means?

The peroxisome proliferator-activated receptor gamma (PPARgamma) is a member of the nuclear receptor superfamily, a group of transcription factors that regulate expression of their target genes upon ligand binding. As endogenous ligands, oxidized fatty acids and prostanoids can bind to and activate the receptor. Natural and synthetic PPARgamma activators have been studied extensively in many inflammatory settings and in most instances they have been shown to be anti-inflammatory. In this review we give an overview of the different molecular mechanisms how PPARgamma and its agonists exert their anti-inflammatory effects both at the cellular level and the level of the organism. The action of PPARgamma in acute and chronic inflammatory diseases and disease models will be presented.

Measuring biomarkers to assess the therapeutic effects of PPAR agonists?

The metabolic syndrome is defined as a clustering of cardiovascular risk factors with insulin resistance, including dyslipidemia, coagulation disturbances and hypertension. Activators of the peroxisome proliferator-activated receptors (PPARs) modulate several of the metabolic risk factors predisposing to atherosclerosis. Fibrates are hypolipidemic drugs acting through activation of PPARα, whereas glitazones are insulin sensitizers activating PPARγ. In addition, these drugs exert pleiotropic anti-inflammatory actions. In this review, we will focus on the effects of fibrates and glitazones on biomarker modulation and their usefulness in the treatment of cardiovascular disease.

Rosiglitazone attenuates transplant arteriosclerosis after allogeneic aorta transplantation in rats

BACKGROUND

Transplant arteriosclerosis is a leading cause of chronic transplant dysfunction and is characterized by occlusive neointima formation in intragraft arteries. Development of transplant arteriosclerosis is refractory to conventional immunosuppressive drugs and adequate therapy is not available. In this study, we determined the efficacy of the synthetic peroxisome proliferator-activated receptor (PPAR)-gamma agonist rosiglitazone to attenuate the development of transplant arteriosclerosis in rat aortic allografts.

METHODS

Lewis aortic allografts were transplanted into Brown Norway recipient rats. Recipient rats received either approximately 5 mg rosiglitazone/day (starting 1 week before transplantation until the end of the experiment) or were left untreated. Transplant arteriosclerosis was quantified using morphometric analysis. Alloreactivity was measured in vitro using mixed lymphocyte reactions. Regulatory T cell frequency and function were analyzed using flow cytometry and in vitro suppression assays, respectively. Intragraft gene expression was analyzed using real-time polymerase chain reaction. Finally, medial and neointimal vascular smooth muscle cell proliferation was analyzed in vitro.

RESULTS

Rosiglitazone significantly reduced transplant arteriosclerosis development 8 weeks after transplantation (P<0.01 vs. nontreated). Rosiglitazone reduced T cell alloreactivity which was not mediated through modulation of CD4+CD25+FoxP3+ regulatory T cells. Reduced development of transplant arteriosclerosis coincided with reduced intragraft expression of stromal-derived factor-1alpha and platelet-derived growth factor receptor-beta. Finally, rosiglitazone reduced growth-factor-driven proliferation of both medial and neointimal vascular smooth muscle cells in vitro, which was not mediated through PPARgamma.

CONCLUSION

PPARgamma agonists may offer a new therapeutic strategy in clinical transplantation to attenuate the development of transplant arteriosclerosis and thereby chronic transplant dysfunction.