A little help from mTOR's friends

The effects of mammalian target of rapamycin (mTOR) signalling on the T helper (T_H) cell response depend on the partners with which mTOR associates, according to new research published in Nature Immunology.

The kinase mTOR regulates cellular metabolism in response to environmental factors, and many recent studies have shown that this pathway controls the differentiation and function of immune cells in response to the immune microenvironment. However, mTOR can form two signalling complexes — mTORC1 and mTORC2 — and this study is the first to show that these complexes have distinct physiological effects on T cells.

RHEB proteins are crucial regulators of mTORC1 signalling, so to investigate the role of mTORC1 in T_H cell differentiation the authors created mice with a conditional deletion of Rheb in CD4⁺ T cells (T-Rheb^−/− mice). This deletion resulted in the elimination of all mTORC1 activity, but mTORC2 activity was preserved. Rheb^−/− T cells from these mice failed to differentiate into T_H1 or T_H17 cells under appropriate skewing conditions in vitro, but could still differentiate into T_H2 cells. To investigate the effects in vivo, Rheb^−/− T cells specific for ovalbumin (OVA) were transferred into recipient mice infected with an OVA-expressing vaccinia virus. In this model, the Rheb^−/− T cells failed to differentiate into interferon-γ (IFNγ)-producing T_H1 cells and instead produced interleukin-4 (IL-4). Similarly, Rheb^−/− T cells could not differentiate into T_H17 cells in the gut Peyer's patches. Finally, T-Rheb^−/− mice had lower clinical disease scores than wild-type mice in a model of experimental autoimmune encephalomyelitis (EAE), which is mediated by T_H1 and T_H17 cells, and developed a non-classical response mediated by T_H2 cells. So, mTORC1 signalling is required for T_H1 and T_H17 cell differentiation and its absence results in increased T_H2 cell differentiation.

By contrast, CD4⁺ T cells with a conditional deletion of rapamycin-insensitive companion of mTOR (Rictor) lacked mTORC2 activity but had normal mTORC1 activity. These cells could differentiate into T_H1 and T_H17 cells, but not T_H2 cells, under appropriate in vitro conditions. Moreover, Rictor^−/− T cells showed normal production of IFNγ in response to vaccinia virus infection and normal susceptibility to EAE, but they failed to produce a T_H2 cell response to immunization with OVA plus alum.

The authors next investigated the mechanisms behind these responses. Rheb^−/− T cells had decreased phosphorylation of T_H1 cell-promoting signal transducer and activator of transcription 4 (STAT4) and of T_H17 cell-promoting STAT3 in response to IL-12 and IL-6, respectively. However, the phosphorylation of T_H2 cell-promoting STAT6 in response to IL-4 was increased in these cells. Conversely, Rictor^−/− T cells had normal phosphorylation of STAT4 and STAT3 but decreased phosphorylation of STAT6 in response to the relevant cytokines. This differential STAT activation between Rheb^−/− and Rictor^−/− T cells was shown to result from increased expression of suppressor of cytokine signalling 3 (SOCS3) or SOCS5, respectively. Knockdown of SOCS3 expression in Rheb^−/− T cells increased their ability to produce IFNγ, whereas knockdown of SOCS5 expression in Rictor^−/− T cells increased their ability to produce IL-4. Furthermore, SOCS3 knockdown correlated with increased expression of the T_H1 cell-associated transcription factor T-bet, and knockdown of SOCS5 resulted in increased expression of the T_H2 cell transcription factor GATA3.

Therefore, the mTORC1 and mTORC2 signalling complexes selectively regulate the ability of CD4⁺ T cells to respond to cytokines that promote T_H1 and T_H17 cells or T_H2 cells, respectively. This control mechanism involves, in part, SOCS-mediated regulation of the STAT signalling pathways that upregulate lineage-specific transcription factors in response to the cytokine environment.