Defining the let-7 microRNA-mediated molecular mechanisms regulating T cell differentiation

CD4+ and CD8+ T cells are lymphocytes of the adaptive immune system that play essential roles in immunity. Both T cell subsets recognize their cognate antigen through the T cell receptor (TCR), which induces the proliferation and differentiation of these antigen-specific cells into effector T cells. CD4+ T cells have the potential to differentiate into one of multiple lineages of helper T (Th) cells and participate indirectly in antigen clearance by orchestrating the function of other cells. CD8+ T cells differentiate into cytotoxic T lymphocytes (CTL), which directly contributes to the resolution of an infection by killing cancerous or virally-infected cells. Upon antigen clearance, most effector T cells die, but some survive and generate long-lived memory T cells that will respond faster and more efficiently to subsequent encounters with the same antigen. When antigen fails to be cleared, such as in chronic infections and cancer, effector T cells are diverted into a hyporesponsive state, exhaustion, characterized by the upregulation of co-inhibitory receptors that transmits inhibitory signals resulting in the loss of effector function and memory potential. Moreover, when T cell differentiation is dysregulated, T cell responses become aberrant, causing autoimmune diseases. Therefore, understanding the molecular mechanisms controlling T cell responses is important to develop innovative treatments that can enhance T cell activity during infections and cancer, and dampen the generation of disease-causing T cells in autoimmunity. We have uncovered a novel post-transcriptional mechanism regulating T cell differentiation. Particularly, we showed that the let-7 family of miRNAs is highly expressed in naive T cells, but gets dramatically downregulated upon antigen encounter, proportionally to both the strength and duration of TCR stimulation. Specifically, let-7 downregulation was required for the differentiation of pathogenic Th17 cells in experimental autoimmune encephalomyelitis (EAE), a mouse model of the autoimmune disease multiple sclerosis (MS). In CD8+ T cells, although let-7 inhibits CTL differentiation in vitro, let-7 was demonstrated both in silico and in vivo to promote memory CD8+ T cell formation, while repressing the differentiation of terminal effectors, which are susceptible to exhaustion. Thus, let-7 constitutes a promising tool for the therapeutic manipulation of T cell responses

Modulation of let-7 miRNAs controls the differentiation of effector CD8 T cells

The differentiation of naive CD8 T cells into effector cytotoxic T lymphocytes upon antigen stimulation is necessary for successful antiviral, and antitumor immune responses. Here, using a mouse model, we describe a dual role for the let-7 microRNAs in the regulation of CD8 T cell responses, where maintenance of the naive phenotype in CD8 T cells requires high levels of let-7 expression, while generation of cytotoxic T lymphocytes depends upon T cell receptor-mediated let-7 downregulation. Decrease of let-7 expression in activated T cells enhances clonal expansion and the acquisition of effector function through derepression of the let-7 targets, including Myc and Eomesodermin. Ultimately, we have identified a novel let-7-mediated mechanism, which acts as a molecular brake controlling the magnitude of CD8 T cell responses

Survival of Naïve T Cells Requires the Expression of Let-7 miRNAs

Maintaining the diversity and constant numbers of naïve T cells throughout the organism's lifetime is necessary for efficient immune responses. Naïve T cell homeostasis, which consists of prolonged survival, occasional proliferation and enforcement of quiescence, is tightly regulated by multiple signaling pathways which are in turn controlled by various transcription factors. However, full understanding of the molecular mechanisms underlying the maintenance of the peripheral T cell pool has not been achieved. In the present study, we demonstrate that T cell-specific deficiency in let-7 miRNAs results in peripheral T cell lymphopenia resembling that of Dicer1 knockout mice. Deletion of let-7 leads to profound T cell apoptosis while overexpression prevents it. We further show that in the absence of let-7, T cells cannot sustain optimal levels of the pro-survival factor Bcl2 in spite of the intact IL-7 signaling, and re-expression of Bcl2 in let-7 deficient T cells completely rescues the survival defect. Thus, we have uncovered a novel let-7-dependent mechanism of post-transcriptional regulation of naïve T cell survival in vivo

Let-7 enhances murine anti-tumor CD8 T cell responses by promoting memory and antagonizing terminal differentiation

Abstract The success of the CD8 T cell-mediated immune response against infections and tumors depends on the formation of a long-lived memory pool, and the protection of effector cells from exhaustion. The advent of checkpoint blockade therapy has significantly improved anti-tumor therapeutic outcomes by reversing CD8 T cell exhaustion, but fails to generate effector cells with memory potential. Here, using in vivo mouse models, we show that let-7 miRNAs determine CD8 T cell fate, where maintenance of let-7 expression during early cell activation results in memory CD8 T cell formation and tumor clearance. Conversely, let-7-deficiency promotes the generation of a terminal effector population that becomes vulnerable to exhaustion and cell death in immunosuppressive environments and fails to reject tumors. Mechanistically, let-7 restrains metabolic changes that occur during T cell activation through the inhibition of the PI3K/AKT/mTOR signaling pathway and production of reactive oxygen species, potent drivers of terminal differentiation and exhaustion. Thus, our results reveal a role for let-7 in the time-sensitive support of memory formation and the protection of effector cells from exhaustion. Overall, our data suggest a strategy in developing next-generation immunotherapies by preserving the multipotency of effector cells rather than enhancing the efficacy of differentiation