THE PROBLEM
This project aims to dissect the microRNA (miRNA) networks that control the differentiation of effector and regulatory T cell subsets in vivo, in various experimental models of infection and autoimmunity. We are focusing on three critical mediators of T cell functions: interferon- (IFN-) and interleukin-17A (IL-17), highly pro-inflammatory cytokines; and Foxp3, the transcription factor that confers suppressive properties to regulatory T cells. We envisage the identification of specific miRNAs that can modulate the balance between effector and regulatory T cell subsets, and thus impact on protective immune responses and/ or immune-mediated pathology.
THE IMPORTANCE
This project will provide major conceptual and experimental advances towards manipulating miRNAs either to boost immunity or to treat autoimmunity. This is particularly relevant given the increase in incidence of cancer and autoinflammatory diseases (such as multiple sclerosis or Crohn’s disease) in our society. MiRNAs are an exciting prospect, especially by being easily manipulated, to address this unmet medical need via manipulation of pro-/ anti-inflammatory cytokines.
OBJECTIVES
This project has 5 main objectives:
1. Characterize the miRNA repertoires of in vivo-generated effector and regulatory T cell subsets, isolated from infection or autoimmune models established in a reporter mouse for Ifng, Il17 and Foxp3.
2. Define the individual miRNAs that impact selectively on effector or regulatory T cell differentiation, based on loss- and gain-of-function experiments.
3. Determine the impact of miRNA expression modulation on effector or regulatory T cell subsets in vivo, using infection and autoimmune models, thus attesting the physiological relevance of the miRNA-mediated mechanisms.
4. Dissect the external cues and intracellular mechanisms that regulate candidate miRNA expression in specific effector or regulatory T cell subsets.
5. Identify the mRNA networks controlled by candidate miRNAs using a combination of bioinformatics and biochemical assays, and couple the effects of miRNA and mRNA manipulation on effector or regulatory T cell subsets in vivo.
CONCLUSIONS
This project provided major advances to our understanding of miRNA-mediated regulation of T cell differentiation by identifying novel miRNA regulators of effector T cell functions in vivo:
1. In the murine model of multiple sclerosis, experimental autoimmune encephalomyelitis (EAE), miR-122 is a negative regulator of the pathogenic phenotype of Th17 cells, acting as a brake of pathogenicity (as seen in peripheral lymphoid organs) that is lost in the central nervous system. These data demonstrate the therapeutic potential of miRNAs that modulate CD4+ T cell differentiation in autoimmunity.
2. In CD8+ T cells, we found that miR-181a limits IFN- production by suppressing the expression of the transcription factor Id2, which in turn promotes the Ifng expression program. Importantly, upon murid herpes virus 4 (MuHV-4) challenge, miR-181a-deficient mice showed a more vigorous IFN-+ CD8+ T cell response, and were able to control viral infection significantly more efficiently than control mice. Collectively, these data established a novel role for miR-181a in regulating IFN-–mediated effector CD8+ T cell responses, with important implications for anti-viral immunity.
3. In gamma-delta (gd) T cells, we identified various miRNAs involved in the differentiation of the their main effector subsets producing IFN-γ and/ or IL-17. This included the first miRNA shown to play a non-redundant role in gd T cell differentiation: miR-146a. We found miR-146a to limit the functional plasticity of IL-17-producing gd T cells, namely their acquisition of IFN-γ expression. This impacted on immunity to intracellular bacteria, but may also be relevant to other diseases where IFN-γ plays pivotal roles, such as cancer, viral infection or severe malaria.
