Signaling Pathways Leading to mTOR Activation Downstream Cytokine Receptors in Lymphocytes in Health and Disease.

CD8+ T cells and Natural Killer (NK) cells are cytotoxic lymphocytes important in the response to intracellular pathogens and cancer. Their activity depends on the integration of a large set of intracellular and environmental cues, including antigenic signals, cytokine stimulation and nutrient availability. This integration is achieved by signaling hubs, such as the mechanistic target of rapamycin (mTOR). mTOR is a conserved protein kinase that controls cellular growth and metabolism in eukaryotic cells and, therefore, is essential for lymphocyte development and maturation. However, our current understanding of mTOR signaling comes mostly from studies performed in transformed cell lines, which constitute a poor model for comprehending metabolic pathway regulation. Therefore, it is only quite recently that the regulation of mTOR in primary cells has been assessed. Here, we review the signaling pathways leading to mTOR activation in CD8+ T and NK cells, focusing on activation by cytokines. We also discuss how this knowledge can contribute to immunotherapy development, particularly for cancer treatment.

mTOR Activation Underlies Enhanced B Cell Proliferation and Autoimmunity in PrkcdG510S/G510S Mice.

Autosomal recessive PRKCD deficiency has previously been associated with the development of systemic lupus erythematosus in human patients, but the mechanisms underlying autoimmunity remain poorly understood. We introduced the Prkcd G510S mutation that we previously associated to a Mendelian cause of systemic lupus erythematosus in the mouse genome, using CRISPR-Cas9 gene editing. PrkcdG510S/G510S mice recapitulated the human phenotype and had reduced lifespan. We demonstrate that this phenotype is linked to a B cell-autonomous role of Prkcd. A detailed analysis of B cell activation in PrkcdG510S/G510S mice shows an upregulation of the PI3K/mTOR pathway after the engagement of the BCR in these cells, leading to lymphoproliferation. Treatment of mice with rapamycin, an mTORC1 inhibitor, significantly improves autoimmune symptoms, demonstrating in vivo the deleterious effect of mTOR pathway activation in PrkcdG510S/G510S mice. Additional defects in PrkcdG510S/G510S mice include a decrease in peripheral mature NK cells that might contribute to the known susceptibility to viral infections of patients with PRKCD mutations.

Sequential actions of EOMES and T-BET promote stepwise maturation of natural killer cells.

EOMES and T-BET are related T-box transcription factors that control natural killer (NK) cell development. Here we demonstrate that EOMES and T-BET regulate largely distinct gene sets during this process. EOMES is dominantly expressed in immature NK cells and drives early lineage specification by inducing hallmark receptors and functions. By contrast, T-BET is dominant in mature NK cells, where it induces responsiveness to IL-12 and represses the cell cycle, likely through transcriptional repressors. Regardless, many genes with distinct functions are co-regulated by the two transcription factors. By generating two gene-modified mice facilitating chromatin immunoprecipitation of endogenous EOMES and T-BET, we show a strong overlap in their DNA binding targets, as well as extensive epigenetic changes during NK cell differentiation. Our data thus suggest that EOMES and T-BET may distinctly govern, via differential expression and co-factors recruitment, NK cell maturation by inserting partially overlapping epigenetic regulations.