TSC1 is required for iNKT cell maturation and function

Invariant Natural Killer T (iNKT) cell development is highly regulated, starting at stage 0, where DP thermocytes become committed to the iNKT cell lineage, and ending as fully mature stage 3 iNKT cells, which are capable of illiciting an immune response. iNKT cells are a subtype of T-cells that can recognise bacterial infections, viruses, and even tumours. However, upon repeated exposure, iNKT cells can become anergic and stop responding to stimuli (reviewed by Cianferoni, 2013).

Earlier this year, it was reported that the FLCN interacting protein FNIP1 was required for iNKT cells to complete the stage 2 to 3 transition, that this phenotype was cell autonomous and partially due to mTOR dysregulation (Park et al., 2014). Two studies have found that TSC1, which when mutated causes the related kidney cancer syndrome Tuberous Sclerosis Complex, is required for both iNKT cell development, and mature iNKT cell function.

In order to investigate the role of TSC1 in iNKT cell development, Wu et al. (2014a) specifically deleted TSC1 in murine T-cells using the Cd4Cre allele. They found that although these mice had similar numbers of stage 2 iNKT cells compared with wildtype mice, they had far fewer stage 3 iNKT cells, due to a higher rate of apoptosis in these cells. Therefore, similarly to FNIP1, TSC1 is required to promote the survival of mature stage 3 iNKT cells.

While the overall number of iNKT cells was reduced, a rare subset of iNKT cells which produce IL-17 (iNKT-17 cells) were present in increased numbers in the TSC1fl/fl; Cd4-Cre mice. Expression analysis showed that TSC1 causes cells to preferentially develop into iNKT rather than iNKT-17 cells by promoting the expression of T-bet and inhibiting the expression of RORɣT and ICOS.

Bone marrow transplants into irradiated mice showed using a 1:2.5 mixture of wild-type and TSC1-null bone marrow cells showed stage 1 and 2 iNKT cells were derived from both donor cell types. However, stage 3 iNKT cells were more commonly from wild-type donor cells, indicating that TSC1’s effect on iNKT cell development is cell autonomous. Furthermore, Rapamycin treatment of TSC1fl/fl; Cd4-Cre mice nearly completely reversed the block in iNKT cell development and predominance of iNKT-17 cells, meaning that dysregulated mTOR signaling was responsible for this phenotype. These results are very similar to those seen in FNIP1-null iNKT cells, suggesting that FLCN and TSC1 may co-operate to regulate iNKT cell development.

In a second study from the same team, Wu et al. (2014b) used a Tamoxifen inducible allele to delete TSC1 in mature iNKT cells. They found that, unlike wild-type iNKT cells, TSC1-null iNKT cells did not become anergic upon secondary stimulation with ɑ-galactosylceramide. The expression of the anergy promoting genes PD-1, Egr2, Egr3, Grail and p27kip were all reduced in TSC1-null iNKT cells. This suggests that TSC1 normally promotes an anergic response by activating the expression of these genes. Given the close overlap between TSC1 and FNIP1’s roles in iNKTcell development, it would also be of interest to determine whether FNIP1 or FLCN also regulate iNKT cell anergy.

Anti-cancer vaccines, like the anti-PD1 vaccine, are currently being tested in clinical trials, and preventing T-cell anergy is a significant area of interest to ensure the continued efficacy of vaccinations (Pal et al., 2014). Mice carrying TSC1-null iNKT cells developed fewer tumour nodules than wild-type mice when injected with B16F10 melanoma cells, meaning that reducing the anergic response increased iNKT cells’ ability to target tumours. These results suggest that inhibiting TSC1 in T-cells might prevent them becoming anergic, and so increase the effectiveness of cancer vaccines.

  • Cianferoni A (2014). Invariant Natural Killer T Cells. Antibodies, 3 (1), 16-36 doi: 10.3390/antib3010016
  • Park H, Tsang M, Iritani BM, & Bevan MJ (2014). Metabolic regulator Fnip1 is crucial for iNKT lymphocyte development. Proceedings of the National Academy of Sciences of the United States of America, 111 (19), 7066-71 PMID: 24785297
  • Pal SK, Hu A, Chang M, & Figlin RA (2014). Programmed death-1 inhibition in renal cell carcinoma: clinical insights and future directions. Clinical advances in hematology & oncology : H&O, 12 (2), 90-9 PMID: 24892254
  • Park H, Tsang M, Iritani BM, & Bevan MJ (2014). Metabolic regulator Fnip1 is crucial for iNKT lymphocyte development. Proceedings of the National Academy of Sciences of the United States of America, 111 (19), 7066-71 PMID: 24785297
  • Wu J, Yang J, Yang K, Wang H, Gorentla B, Shin J, Qiu Y, Que LG, Foster WM, Xia Z, Chi H, & Zhong XP (2014). iNKT cells require TSC1 for terminal maturation and effector lineage fate decisions. The Journal of clinical investigation, 124 (4), 1685-98 PMID: 24614103
  • Wu J, Shin J, Xie D, Wang H, Gao J, & Zhong XP (2014b). Tuberous sclerosis 1 promotes invariant NKT cell anergy and inhibits invariant NKT cell-mediated antitumor immunity. Journal of immunology (Baltimore, Md. : 1950), 192 (6), 2643-50 PMID: 24532578

www.bhdsyndrome.org – the primary online resource for anyone interested in BHD Syndrome.

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