NK cell development requires Tsc1-dependent negative regulation of IL-15-triggered mTORC1 activation

Abstract

Activation of metabolic signalling by IL-15 is required for natural killer (NK) cell development. Here we show that Tsc1, a repressor of mTOR, is dispensable for the terminal maturation, survival and function of NK cells but is critical to restrict exhaustive proliferation of immature NK cells and activation downstream of IL-15 during NK cell development. Tsc1 is expressed in immature NK cells and is upregulated by IL-15. Haematopoietic-specific deletion of Tsc1 causes a marked decrease in the number of NK cells and compromises rejection of ‘missing-self' haematopoietic tumours and allogeneic bone marrow. The residual Tsc1-null NK cells display activated, pro-apoptotic phenotype and elevated mTORC1 activity. Deletion of Raptor, a component of mTORC1, largely reverses these defects. Tsc1-deficient NK cells express increased levels of T-bet and downregulate Eomes and CD122, a subunit of IL-15 receptor. These results reveal a role for Tsc1-dependent inhibition of mTORC1 activation during immature NK cell development.

MenTORing Immunity: mTOR Signaling in the Development and Function of Tissue-Resident Immune Cells

“…NK cell activation by innate signals such as poly(I:C) or cytokines such as IL-2 and IL-12 promote blastogenesis and a glycolytic shift to Warburg metabolism, both of which are rapamycin sensitive (Donnelly et al, 2014). IL-15 promotes NK cell development and activation during inflammation and infection through a rapamycin-sensitive mTOR-dependent pathway of development (Marçais et al, 2014; Yang et al, 2016) that requires regulation by TSC1 in order to prevent excessive NK cell activation and cell death (Wu et al, 2014; Yang et al, 2016). iNKT cell development is also highly dependent on mTORC1, as Raptor deficiency results in iNKT cells failing to reach the periphery from the thymus (Shin et al, 2014; Wei et al, 2014; Zhang et al, 2014).…”

Transcriptional control of natural killer cell differentiation

“…41 IL-15 promotes Nfil3 expression via the 3 0 -phosphoinositide-dependent kinase 1 and mechanistic target of rapamycin (PDK1-mTOR) signaling axis, although Nfil3 itself reinforces IL-15 signaling by maintaining CD122 expression in NK cells. 46,53,54 Consistent with its key role in NK cell biology, Nfil3 expression is tightly regulated in NK cells, including through posttranscriptional modifications such as SUMOylation and phosphorylation that influence its functions in development. 55 Direct gene targets of Nfil3 include Eomes, ID2, and Notch, other TFs that critically shape NK cell development and function (discussed above and below), although one group reported normal ID2 expression in developing E4bp4 À/À NK cells.…”

“…Mechanistically, Nfil3 is thought to act at least partially downstream of IL‐15 signaling, as ectopic Nfil3 expression can rescue aspects of NK cell development in mice lacking components of the IL‐15 signaling pathway . IL‐15 promotes Nfil3 expression via the 3′‐phosphoinositide‐dependent kinase 1 and mechanistic target of rapamycin (PDK1‐mTOR) signaling axis, although Nfil3 itself reinforces IL‐15 signaling by maintaining CD122 expression in NK cells . Consistent with its key role in NK cell biology, Nfil3 expression is tightly regulated in NK cells, including through post‐transcriptional modifications such as SUMOylation and phosphorylation that influence its functions in development .…”

“…Eomes is thought to act after T‐bet in NK cell development, but Eomes‐deficient NK cells also exhibit features of impaired maturation, including failure to up‐regulate CD11b, lower levels of CD49b (DX5), and expression of a limited Ly49 receptor repertoire . Like many other NK‐defining TFs, T‐bet and Eomes are regulated by IL‐15 and mTOR signaling . In addition, Eomes has been shown to directly bind the Il2rb promoter and enhance CD122 expression, and hence IL‐15 responsiveness, in both NK cells and CD8 + T cells …”

“…80 Like many other NK-defining TFs, T-bet and Eomes are regulated by IL-15 and mTOR signaling. 53 In addition, Eomes has been shown to directly bind the Il2rb promoter and enhance CD122 expression, and hence IL-15 responsiveness, in both NK cells and CD8 + T cells. 81 Zinc Finger E-box Binding Homeobox 2 (Zeb2) is another TF that regulates NK cell maturation.…”