LKB1 isoform expression modulates T cell plasticity downstream of PKCθ and IL-6.

Following activation, CD4 T cells undergo metabolic and transcriptional changes as they respond to external cues and differentiate into T helper (Th) cells. T cells exhibit plasticity between Th phenotypes in highly inflammatory environments, such as colitis, in which high levels of IL-6 promote plasticity between regulatory T (Treg) cells and Th17 cells. Protein Kinase C theta (PKCθ) is a T cell-specific serine/threonine kinase that promotes Th17 differentiation while negatively regulating Treg differentiation. Liver kinase B1 (LKB1), also a serine/threonine kinase and encoded by Stk11, is necessary for Treg survival and function. Stk11 can be alternatively spliced to produce a short variant (Stk11S) by transcribing a cryptic exon. However, the contribution of Stk11 splice variants to Th cell differentiation has not been previously explored. Here we show that in Th17 cells, the heterogeneous ribonucleoprotein, hnRNPLL, mediates Stk11 splicing into its short splice variant, and that Stk11S expression is diminished when Hnrnpll is depleted using siRNA knock-down approaches. We further show that PKCθ regulates hnRNPLL and, thus, Stk11S expression in Th17 cells. We provide additional evidence that exposing induced (i)Tregs to IL-6 culminates in Stk11 splicing downstream of PKCθAltogether our data reveal a yet undescribed outside-in signaling pathway initiated by IL-6, that acts through PKCθ and hnRNPLL to regulate Stk11 splice variants and facilitate Th17 cell differentiation. Furthermore, we show for the first time, that this pathway can also be initiated in developing iTregs exposed to IL-6, providing mechanistic insight into iTreg phenotypic stability and iTreg to Th17 cell plasticity.

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

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.