TIGIT inhibition and lenalidomide synergistically promote antimyeloma immune responses after stem cell transplantation in mice.

Autologous stem cell transplantation (ASCT) with subsequent lenalidomide maintenance is standard consolidation therapy for multiple myeloma, and a subset of patients achieve durable progression-free survival that is suggestive of long-term immune control. Nonetheless, most patients ultimately relapse, suggesting immune escape. TIGIT appears to be a potent inhibitor of myeloma-specific immunity and represents a promising new checkpoint target. Here we demonstrate high expression of TIGIT on activated CD8+ T cells in mobilized peripheral blood stem cell grafts from patients with myeloma. To guide clinical application of TIGIT inhibition, we evaluated identical anti-TIGIT antibodies that do or do not engage FcγR and demonstrated that anti-TIGIT activity is dependent on FcγR binding. We subsequently used CRBN mice to investigate the efficacy of anti-TIGIT in combination with lenalidomide maintenance after transplantation. Notably, the combination of anti-TIGIT with lenalidomide provided synergistic, CD8+ T cell-dependent, antimyeloma efficacy. Analysis of bone marrow (BM) CD8+ T cells demonstrated that combination therapy suppressed T cell exhaustion, enhanced effector function, and expanded central memory subsets. Importantly, these immune phenotypes were specific to the BM tumor microenvironment. Collectively, these data provide a logical rationale for combining TIGIT inhibition with immunomodulatory drugs to prevent myeloma progression after ASCT.

Restoration of T-cell Effector Function, Depletion of Tregs, and Direct Killing of Tumor Cells: The Multiple Mechanisms of Action of a-TIGIT Antagonist Antibodies.

TIGIT is an immune checkpoint inhibitor expressed by effector CD4+ and CD8+ T cells, NK cells, and regulatory T cells (Tregs). Inhibition of TIGIT-ligand binding using antagonistic anti-TIGIT mAbs has shown in vitro potential to restore T-cell function and therapeutic efficacy in murine tumor models when combined with an anti-PD(L)-1 antibody. In the current work, we demonstrate broader TIGIT expression than previously reported in healthy donors and patients with cancer with expression on γδ T cells, particularly in CMV-seropositive donors, and on tumor cells from hematologic malignancies. Quantification of TIGIT density revealed tumor-infiltrating Tregs as the population expressing the highest receptor density. Consequently, the therapeutic potential of anti-TIGIT mAbs might be wider than the previously described anti-PD(L)-1-like restoration of αß T-cell function. CD155 also mediated inhibition of γδ T cells, an immune population not previously described to be sensitive to TIGIT inhibition, which could be fully prevented via use of an antagonistic anti-TIGIT mAb (EOS-448). In PBMCs from patients with cancer, as well as in tumor-infiltrating lymphocytes from mice, the higher TIGIT expression in Tregs correlated with strong antibody-dependent killing and preferential depletion of this highly immunosuppressive population. Accordingly, the ADCC/ADCP-enabling format of the anti-TIGIT mAb had superior antitumor activity, which was dependent upon Fcγ receptor engagement. In addition, the anti-TIGIT mAb was able to induce direct killing of TIGIT-expressing tumor cells both in human patient material and in animal models, providing strong rationale for therapeutic intervention in hematologic malignancies. These findings reveal multiple therapeutic opportunities for anti-TIGIT mAbs in cancer therapeutics.

Inhibition of Tryptophan-Dioxygenase Activity Increases the Antitumor Efficacy of Immune Checkpoint Inhibitors.

Tryptophan 2,3-dioxygenase (TDO) is an enzyme that degrades tryptophan into kynurenine and thereby induces immunosuppression. Like indoleamine 2,3-dioxygenase (IDO1), TDO is considered as a relevant drug target to improve the efficacy of cancer immunotherapy. However, its role in various immunotherapy settings has not been fully characterized. Here, we described a new small-molecule inhibitor of TDO that can modulate kynurenine and tryptophan in plasma, liver, and tumor tissue upon oral administration. We showed that this compound improved the ability of anti-CTLA4 to induce rejection of CT26 tumors expressing TDO. To better characterize TDO as a therapeutic target, we used TDO-KO mice and found that anti-CTLA4 or anti-PD1 induced rejection of MC38 tumors in TDO-KO, but not in wild-type mice. As MC38 tumors did not express TDO, we related this result to the high systemic tryptophan levels in TDO-KO mice, which lack the hepatic TDO needed to contain blood tryptophan. The antitumor effectiveness of anti-PD1 was abolished in TDO-KO mice fed on a tryptophan-low diet that normalized their blood tryptophan level. MC38 tumors expressed IDO1, which could have limited the efficacy of anti-PD1 in wild-type mice and could have been overcome in TDO-KO mice due to the high levels of tryptophan. Accordingly, treatment of mice with an IDO1 inhibitor improved the efficacy of anti-PD1 in wild-type, but not in TDO-KO, mice. These results support the clinical development of TDO inhibitors to increase the efficacy of immunotherapy of TDO-expressing tumors and suggest their effectiveness even in the absence of tumoral TDO expression.See article by Hoffmann et al., p. 19.

Characterization of the Selective Indoleamine 2,3-Dioxygenase-1 (IDO1) Catalytic Inhibitor EOS200271/PF-06840003 Supports IDO1 as a Critical Resistance Mechanism to PD-(L)1 Blockade Therapy.

Tumors use indoleamine 2,3-dioxygenase-1 (IDO1) as a major mechanism to induce an immunosuppressive microenvironment. IDO1 expression is upregulated in many cancers and considered to be a resistance mechanism to immune checkpoint therapies. IDO1 is induced in response to inflammatory stimuli such as IFNγ and promotes immune tolerance by depleting tryptophan and producing tryptophan catabolites, including kynurenine, in the tumor microenvironment. This leads to effector T-cell anergy and enhanced Treg function through upregulation of FoxP3. As a nexus for the induction of key immunosuppressive mechanisms, IDO1 represents an important immunotherapeutic target in oncology. Here, we report the identification and characterization of the novel selective, orally bioavailable IDO1 inhibitor EOS200271/PF-06840003. It reversed IDO1-induced T-cell anergy in vitro In mice carrying syngeneic tumor grafts, PF-06840003 reduced intratumoral kynurenine levels by over 80% and inhibited tumor growth both in monotherapy and, with an increased efficacy, in combination with antibodies blocking the immune checkpoint ligand PD-L1. We demonstrate that anti-PD-L1 therapy results in increased IDO1 metabolic activity thereby providing additional mechanistic rationale for combining PD-(L)1 blockade with IDO1 inhibition in cancer immunotherapies. Supported by these preclinical data and favorable predicted human pharmacokinetic properties of PF-06840003, a phase I open-label, multicenter clinical study (NCT02764151) has been initiated.

Deciphering tumor growth by clonal analysis.

One of the key goals of cancer research is to understand more precisely the mechanisms of tumor growth. It has been hypothesized for several decades that some tumors may be hierarchically organized as normal tissues. Many studies have suggested the existence of cancer stem cells (CSCs) at the top of the cellular hierarchy in tumors. The proof of concept for the existence of the CSC model has been demonstrated by the ability of particular tumor cells to reform tumor growth upon transplantation into severely immunodeficient mice. CSCs have been described in many different human cancers and have been hypothesized to sustain tumor growth, to resist chemo- and radiotherapy, and to be responsible for tumor relapse. While these studies clearly show the differential potential of cancer cells in these experimental conditions and demonstrate functional intratumoral heterogeneity, they do not necessarily reflect the actual fate of tumor cells in their native environment; the existence of CSCs during unperturbed tumor growth remained unproven for a long time. Different recent reports used clonal analysis and lineage tracing tools to demonstrate the hierarchical organization of tumors in vivo in unperturbed colon and skin cancer models. This review discusses the unique opportunities opened through the use of tracing methods to further characterize CSCs in different tumor models as well as their utility in dissecting the mechanism of action of new therapeutics targeting specifically CSCs.

SOX2 controls tumour initiation and cancer stem-cell functions in squamous-cell carcinoma.

Cancer stem cells (CSCs) have been reported in various cancers, including in skin squamous-cell carcinoma (SCC). The molecular mechanisms regulating tumour initiation and stemness are still poorly characterized. Here we find that Sox2, a transcription factor expressed in various types of embryonic and adult stem cells, was the most upregulated transcription factor in the CSCs of squamous skin tumours in mice. SOX2 is absent in normal epidermis but begins to be expressed in the vast majority of mouse and human pre-neoplastic skin tumours, and continues to be expressed in a heterogeneous manner in invasive mouse and human SCCs. In contrast to other SCCs, in which SOX2 is frequently genetically amplified, the expression of SOX2 in mouse and human skin SCCs is transcriptionally regulated. Conditional deletion of Sox2 in the mouse epidermis markedly decreases skin tumour formation after chemical-induced carcinogenesis. Using green fluorescent protein (GFP) as a reporter of Sox2 transcriptional expression (SOX2-GFP knock-in mice), we showed that SOX2-expressing cells in invasive SCC are greatly enriched in tumour-propagating cells, which further increase upon serial transplantations. Lineage ablation of SOX2-expressing cells within primary benign and malignant SCCs leads to tumour regression, consistent with the critical role of SOX2-expressing cells in tumour maintenance. Conditional Sox2 deletion in pre-existing skin papilloma and SCC leads to tumour regression and decreases the ability of cancer cells to be propagated upon transplantation into immunodeficient mice, supporting the essential role of SOX2 in regulating CSC functions. Transcriptional profiling of SOX2-GFP-expressing CSCs and of tumour epithelial cells upon Sox2 deletion uncovered a gene network regulated by SOX2 in primary tumour cells in vivo. Chromatin immunoprecipitation identified several direct SOX2 target genes controlling tumour stemness, survival, proliferation, adhesion, invasion and paraneoplastic syndrome. We demonstrate that SOX2, by marking and regulating the functions of skin tumour-initiating cells and CSCs, establishes a continuum between tumour initiation and progression in primary skin tumours.

Conditional deletion of PTEN in peripheral T cells augments TCR-mediated activation but does not abrogate CD28 dependency or prevent anergy induction.

PTEN is thought to play a critical role in T cell activation by negatively regulating the PI3K signaling pathway important for cellular activation, growth, and proliferation. To directly eliminate PTEN in postthymic T cells for studies of functional effects, we used CAR transgenic × PTEN(flox/flox) mice, which enabled gene deletion using a Cre adenovirus in vitro. These mice were also immunized to generate stable Th1 clones that could have PTEN deleted when desired. PTEN-deleted T cells exhibited enhanced IL-2 production, proliferation, and Akt phosphorylation upon TCR/CD28 engagement, whereas T cell survival was not potentiated. Gene expression profiling revealed a small subset of induced genes that were augmented upon PTEN deletion. However, PTEN-deficient T cells still required CD28 costimulation for IL-2 production and remained susceptible to anti-CD3-induced anergy. The absence of PTEN within the CD8 T cell compartment led to markedly increased cytolytic activity following an allogeneic MLR in vitro, without increasing autologous MLR activity. Our results indicate that deletion of PTEN can augment the activation of postthymic T cells but does not mediate CD28 independence or anergy resistance. Nonetheless, PTEN inhibition may be a viable target for immune potentiation owing to increased cytokine production by activated CD4(+) cells and increased cytotoxicity by CD8(+) T cells.

Transcriptional regulator early growth response gene 2 (Egr2) is required for T cell anergy in vitro and in vivo.

T cell receptor engagement in the absence of costimulation results in a hyporesponsive state termed anergy. Understanding the transcriptional regulation of other T cell differentiation states has provided critical information regarding the biology of T cell regulation in vivo. However, the transcriptional regulation of T cell anergy has been poorly understood. Using the key anergy target gene diacylglycerol kinase (DGK) α as a focal point, we identified early growth response gene 2 (Egr2) as a central transcription factor that regulates the anergic state. Conditional Egr2 deletion in peripheral T cells abolishes induced expression of DGK-α and other anergy genes and restores Ras/MAPK signaling, IL-2 production, and proliferation upon attempted anergy induction. Using superantigen- and tumor-induced anergy models, we found that Egr2 is necessary for anergy induction in vivo. Collectively, our results implicate Egr2 as an essential transcriptional regulator of the T cell anergy program.

Defining the mode of tumour growth by clonal analysis.

Recent studies using the isolation of a subpopulation of tumour cells followed by their transplantation into immunodeficient mice provide evidence that certain tumours, including squamous skin tumours, contain cells with high clonogenic potential that have been referred to as cancer stem cells (CSCs). Until now, CSC properties have only been investigated by transplantation assays, and their existence in unperturbed tumour growth is unproven. Here we make use of clonal analysis of squamous skin tumours using genetic lineage tracing to unravel the mode of tumour growth in vivo in its native environment. To this end, we used a genetic labelling strategy that allows individual tumour cells to be marked and traced over time at different stages of tumour progression. Surprisingly, we found that the majority of labelled tumour cells in benign papilloma have only limited proliferative potential, whereas a fraction has the capacity to persist long term, giving rise to progeny that occupy a significant part of the tumour. As well as confirming the presence of two distinct proliferative cell compartments within the papilloma, mirroring the composition, hierarchy and fate behaviour of normal tissue, quantitative analysis of clonal fate data indicates that the more persistent population has stem-cell-like characteristics and cycles twice per day, whereas the second represents a slower cycling transient population that gives rise to terminally differentiated tumour cells. Such behaviour is shown to be consistent with double-labelling experiments and detailed clonal fate characteristics. By contrast, measurements of clone size and proliferative potential in invasive squamous cell carcinoma show a different pattern of behaviour, consistent with geometric expansion of a single CSC population with limited potential for terminal differentiation. This study presents the first experimental evidence for the existence of CSCs during unperturbed solid tumour growth.

Long live sox2: sox2 lasts a lifetime.

Sox2 is a key transcription factor required for the maintenance of pluripotency in embryonic cells and morphogenesis of several epithelia. In this issue of Cell Stem Cell, Arnold et al. (2011) demonstrate that Sox2 marks long-lived adult stem cells to ensure homeostasis in a broad range of adult tissues.

A vascular niche and a VEGF-Nrp1 loop regulate the initiation and stemness of skin tumours.

Angiogenesis is critical during tumour initiation and malignant progression. Different strategies aimed at blocking vascular endothelial growth factor (VEGF) and its receptors have been developed to inhibit angiogenesis in cancer patients. It has become increasingly clear that in addition to its effect on angiogenesis, other mechanisms including a direct effect of VEGF on tumour cells may account for the efficiency of VEGF-blockade therapies. Cancer stem cells (CSCs) have been described in various cancers including squamous tumours of the skin. Here we use a mouse model of skin tumours to investigate the impact of the vascular niche and VEGF signalling on controlling the stemness (the ability to self renew and differentiate) of squamous skin tumours during the early stages of tumour progression. We show that CSCs of skin papillomas are localized in a perivascular niche, in the immediate vicinity of endothelial cells. Furthermore, blocking VEGFR2 caused tumour regression not only by decreasing the microvascular density, but also by reducing CSC pool size and impairing CSC renewal properties. Conditional deletion of Vegfa in tumour epithelial cells caused tumours to regress, whereas VEGF overexpression by tumour epithelial cells accelerated tumour growth. In addition to its well-known effect on angiogenesis, VEGF affected skin tumour growth by promoting cancer stemness and symmetric CSC division, leading to CSC expansion. Moreover, deletion of neuropilin-1 (Nrp1), a VEGF co-receptor expressed in cutaneous CSCs, blocked VEGF's ability to promote cancer stemness and renewal. Our results identify a dual role for tumour-cell-derived VEGF in promoting cancer stemness: by stimulating angiogenesis in a paracrine manner, VEGF creates a perivascular niche for CSCs, and by directly affecting CSCs through Nrp1 in an autocrine loop, VEGF stimulates cancer stemness and renewal. Finally, deletion of Nrp1 in normal epidermis prevents skin tumour initiation. These results may have important implications for the prevention and treatment of skin cancers.

Beta-catenin inhibits T cell activation by selective interference with linker for activation of T cells-phospholipase C-γ1 phosphorylation.

Despite the defined function of the ß-catenin pathway in thymocytes, its functional role in peripheral T cells is poorly understood. We report that in a mouse model, ß-catenin protein is constitutively degraded in peripheral T cells. Introduction of stabilized ß-catenin into primary T cells inhibited proliferation and cytokine secretion after TCR stimulation and blunted effector cell differentiation. Functional and biochemical studies revealed that ß-catenin selectively inhibited linker for activation of T cells phosphorylation on tyrosine 136, which was associated with defective phospholipase C-γ1 phosphorylation and calcium signaling but normal ERK activation. Our findings indicate that ß-catenin negatively regulates T cell activation by a previously undescribed mechanism and suggest that conditions under which ß-catenin might be inducibly stabilized in vivo would be inhibitory for T cell-based immunity.

Development of a successful antitumor therapeutic model combining in vivo dendritic cell vaccination with tumor irradiation and intratumoral GM-CSF delivery.

Vaccination of dendritic cells (DC) combined with GM-CSF secreting tumor cells has shown good therapeutic efficacy in several tumor models. Nevertheless, the engineering of GM-CSF secreting tumor cell line could represent a tedious step limiting its application for treatment in patients. We therefore developed in rats, an "all in vivo" strategy of combined vaccination using an in vivo local irradiation of the tumor as a source of tumor antigens for DC vaccines and an exogenous source of GM-CSF. We report here that supplying recombinant mGM-CSF by local injections or surgical implantation of osmotic pumps did not allow reproducing the therapeutic efficacy observed with in vitro prepared combined vaccines. To bypass this limitation possibly due to the short half-life of recombinant GM-CSF, we have generated adeno-associated virus coding for mGM-CSF and tested their efficacy to transduce tumor cells in vitro and in vivo. The in vivo vaccines combining local irradiation and AAV2/1-mGM-CSF vectors showed high therapeutic efficacy allowing to cure 60% of the rats with pre-implanted tumors, as previously observed with in vitro prepared vaccines. Same efficacy has been observed with a second generation of vaccines combining DC, local tumor irradiation, and the controlled supply of recombinant mGM-CSF in poloxamer 407, a biocompatible thermoreversible hydrogel. By generating a successful "all in vivo" vaccination protocol combining tumor radiotherapy with DC vaccines and a straightforward supply of GM-CSF, we have developed a therapeutic strategy easily translatable to clinic that could become accessible to a much bigger number of cancer patients.

A designer ligand specific for Kv1.3 channels from a scorpion neurotoxin-based library.

Venomous animals immobilize prey using protein toxins that act on ion channels and other targets of biological importance. Broad use of toxins for biomedical research, diagnosis, and therapy has been limited by inadequate target discrimination, for example, among ion channel subtypes. Here, a synthetic toxin is produced by a new strategy to be specific for human Kv1.3 channels, critical regulators of immune T cells. A phage display library of 11,200 de novo proteins is designed using the alpha-KTx scaffold of 31 scorpion toxin sequences known or predicted to bind to potassium channels. Mokatoxin-1 (moka1) is isolated by affinity selection on purified target. Moka1 blocks Kv1.3 at nanomolar levels that do not inhibit Kv1.1, Kv1.2, or KCa1.1. As a result, moka1 suppresses CD3/28-induced cytokine secretion by T cells without cross-reactive gastrointestinal hyperactivity. The 3D structure of moka1 rationalizes its specificity and validates the engineering approach, revealing a unique interaction surface supported on an alpha-KTx scaffold. This scaffold-based/target-biased strategy overcomes many obstacles to production of selective toxins.

Costimulatory and coinhibitory receptors in anti-tumor immunity.

SUMMARY: Despite the expression of antigens by tumor cells, spontaneous immune-mediated rejection of cancer seems to be a rare event. T-cell receptor engagement by peptide/major histocompatibility complexes constitutes the main signal for the activation of naive T cells but is not sufficient to initiate a productive generation and maintenance of effector cells. Full activation of T cells requires additional signals driven by costimulatory molecules present on activated antigen-presenting cells but rarely on tumors. Following the discovery of B7-1 (CD80), several other costimulatory molecules have been shown to contribute to T-cell activation and have relevance for improving anti-tumor immunity. Moreover, increasing the understanding of coinhibitory receptors has highlighted key additional pathways that can dominantly inhibit anti-tumor T-cell function. Improving positive costimulation, and interfering with negative regulation, continues to represent an attractive immunotherapeutic approach for the treatment of cancer. This review focuses upon those pathways with the highest potential for clinical application in human cancer patients.

Synergy between dendritic cells and GM-CSF-secreting tumor cells for the treatment of a murine renal cell carcinoma.

Dendritic cell (DC) immunotherapy for cancer certainly holds promises but definitely needs improvements, especially for enhancing tumor-specific responses able to eradicate preexisting tumors. To this end, we investigated here, for the treatment of a preimplanted murine renal cell carcinoma Renca, a new vaccination approach combining injection of DC and granulocyte macrophage colony-stimulating factor (GM-CSF) gene-transduced tumor cells. When treatment by either DC or Renca-mGM-CSF cells alone had no therapeutic effect at all, combined vaccines induced therapeutic response in 50% of the tumor-bearing mice, in a GM-CSF dose-dependent manner. Importantly, all these cured mice were protected against a rechallenge with parental Renca cells, indicating the generation of memory immune response. The combined vaccines induced elevated cytotoxic responses in all the cured mice and half of the uncured ones and a stronger systemic CD4+ T-cell-mediated interferon-gamma production in the cured vaccinated mice as compared with uncured ones. In conclusion, vaccines associating DC and GM-CSF-secreting tumor cells induce high therapeutic effect in mice with preexisting renal cell carcinoma that are correlated to the induction of specific CD8 and CD4+ T-cell responses. This original vaccination approach should be further evaluated in a clinical trial for the treatment of metastatic human renal cell carcinoma.

Glucose deprivation inhibits multiple key gene expression events and effector functions in CD8+ T cells.

We recently reported that differentiation of CD8(+) T cells from the naïve to the effector state involves the upregulation of glucose-dependent metabolism. Glucose deprivation or inhibition of glycolysis by 2-deoxy-D-glucose (2-DG) selectively inhibited production of IFN-gamma but not of IL-2. To determine a more global role of glucose metabolism on effector T-cell function, we performed gene array analysis on CD8(+) effector T cells stimulated in the presence or absence of 2-DG. We observed that expression of only 10% of genes induced by TCR/CD28 signaling was inhibited by 2-DG. Among these were genes for key cytokines, cell cycle molecules, and cytotoxic granule proteins. Consistent with these results, production of IFN-gamma and GM-CSF, cell cycle progression, upregulation of cyclin D2 protein, cytolytic activity, and upregulation of granzyme B protein and also conjugate formation were exquisitely glucose-dependent. In contrast to glucose, oxygen was little utilized by CD8(+) effector T cells, and relative oxygen deprivation did not inhibit these CTL functional properties. Our results indicate a particularly critical role for glucose in regulating specific effector functions of CD8(+) T cells and have implications for the maintenance of the effector phase of cellular immune responses in target tissue microenvironments such as a solid tumor.

Therapeutic efficacy of antitumor dendritic cell vaccinations correlates with persistent Th1 responses, high intratumor CD8+ T cell recruitment and low relative regulatory T cell infiltration.

Despite the increasing number of immunotherapeutic strategies for the treatment of cancer, most approaches have failed to correlate the induction of an anti-tumor immune response with therapeutic efficacy. We therefore took advantage of a successful vaccination strategy-combining dendritic cells and irradiated GM-CSF secreting tumor cells-to compare the immune response induced against 9L gliosarcoma tumors in cured rats versus those with progressively growing tumors. At the systemic level, the tumor specific cytotoxic responses were quite heterogeneous in uncured vaccinated rats, and were surprisingly often high in animals with rapidly-growing tumors. IFN-gamma secretion by activated splenic T cells was more discriminative as the CD4+ T cell-mediated production was weak in uncured rats whereas high in cured ones. At the tumor level, regressing tumors were strongly infiltrated by CD8+ T cells, which demonstrated lytic capacities as high as their splenic counterparts. In contrast, progressing tumors were weakly infiltrated by T cells showing impaired cytotoxic activities. Proportionately to the T cell infiltrate, the expression of Foxp3 was increased in progressive tumors suggesting inhibition by regulatory T cells. In conclusion, the main difference between cured and uncured vaccinated animals does not depend directly upon the induction of systemic cytotoxic responses. Rather the persistence of higher CD4+ Th1 responses, a high intratumoral recruitment of functional CD8+ T cells, and a low proportion of regulatory T cells correlate with tumor rejection.

15-Deoxy-12,14-prostaglandin J2 inhibits interferon gamma induced MHC class II but not class I expression on ARPE cells through a PPAR gamma independent mechanism.

Retinal pigment epithelial (RPE) cells constitute the external part of the blood-retinal-barrier and play a pivotal role in the regulation of retinal immunity. In the present work, we investigated the effects of 15-deoxy-12,14-prostaglandin J2 (15 PGJ2), an endogenous ligand of PPARgamma, on the IFNgamma-induced expression of MHC class II on RPE cells. Indeed, pathological expression of MHC class II molecules at the surface of RPE cells is a common feature of many blinding conditions. We demonstrated that 15 PGJ2 inhibited the IFNgamma-mediated induction of MHC class II on RPE cells without affecting the level of MHC class I and CD54 expression. The other PPARgamma agonist rosiglitazone or troglitazone had no similar effects. Moreover, the inhibitory effect of 15 PGJ2 was not abrogated by co-incubation with PPARgamma antagonists and did not involve the modulation of STAT-1, AKT or ERK1/2 phosphorylation, nor CIITA, IRF1 or IRF2 transcription. In conclusion, 15 PGJ2 inhibits strongly and specifically the IFNgamma-induced MHC class II expression on RPE cells by a PPARgamma independent mechanism. Given the differential role of MHC classes I and II in the development of autoimmune uveitis and the potential toxicity of 15 PGJ2, our data's suggest that the development of novel small molecules targeting similar PPARgamma independent pathways would be useful for the future management of uveitis.