Sirtuin 5 is Dispensable for CD8+ T Cell Effector and Memory Differentiation

CD8+ T cell effector and memory differentiation is tightly controlled at multiple levels including transcriptional, metabolic, and epigenetic regulation. Sirtuin 5 (SIRT5) is a protein deacetylase mainly located at mitochondria, but it remains unclear whether SIRT5 plays key roles in regulating CD8+ T cell effector or memory formation. Herein, with adoptive transfer of Sirt5+/+ or Sirt5−/− OT-1 cells and acute Listeria monocytogenes infection model, we demonstrate that SIRT5 deficiency does not affect CD8+ T cell effector function and that SIRT5 is not required for CD8+ T cell memory formation. Moreover, the recall response of SIRT5 deficient memory CD8+ T cells is comparable with Sirt5+/+ memory CD8+ T cells. Together, these observations suggest that SIRT5 is dispensable for the effector function and memory differentiation of CD8+ T cells.

P01.13 MERTK signaling is critical for T cell proliferation and memory

BackgroundOverexpression of TAM receptors, including MERTK, in some cancers are integral for chemoresistance, proliferation and metastasis.1 Our group has previously demonstrated that T cells also express MERTK and engagement of MERTK signaling is responsible for increased proliferation, functional capacity and metabolic fitness.2 It is therefore important to further study the effect of MERTK inhibition on T cell function in the context of cancer treatments where MERTK inhibitors may play a role. Here we provide evidence that MERTK inhibition impacts greatly on T cell proliferation, specifically reducing phosphorylated mTOR. We have also demonstrated that MERTK expression is increased on CD8 central memory subsets during longterm expansion providing evidence that this signaling pathway may be important for sustaining T memory responses.Materials and MethodsFlow cytometric analysis was used to investigate the effect of titration of MERTK small molecule inhibitor UNC2025 on healthy donor T cells activated with CD3/CD28 dynabeads. Cell trace dye was used to track proliferation of CD4 and CD8 T cells along with markers of memory differentiation (CCR7 and CD45RO), activation (CD137) and function (IFNy, Tnfa and IL-2). MERTK signaling was assessed using phospho flow cytometric methodology of phosphorylated mTOR, AKT, ERK1/2, p38-MAPK and STAT5. Long term cultures of donor T cells of up to 28 days were investigated for MERTK expression alongside memory differentiation.ResultsWe demonstrated that moderate concentrations of MERTK inhibitor reduced proliferation of activated T cells. Despite inhibition of cell division, cell size still increased 2 fold compared to resting cells and cell viability remained unchanged. Additionally, the proportion of central memory to effector memory populations and intracellular cytokine production was not impacted. Analysis of molecules involved in MERTK signaling revealed that phosphorylated mTOR was significantly modulated following the addition of MERTK inhibitor. Long term culture of CD8 T cells demonstrated MERTK was significantly increased following early and late re-stimulation, and expression of MERTK was strongly associated with central memory subsets.ConclusionsOur results demonstrate that inhibition of MERTK signaling on T cells reduces cell division where mTOR is significantly impacted. Despite this, other functional aspects, such as intracellular cytokine production remain unchanged. Therefore, interruption of MERTK signaling on T cells has a specific effect on cell division rather than cytotoxic function on a cell by cell basis. This has potential ramifications on the use of MERTK inhibitors to treat tumors where the ability to form substantial cytotoxic T cell populations might be reduced. In addition, increased MERTK expression on central memory subsets during long term culture suggests this signaling pathway could be critical for generating memory pools of T cells and provide new avenues for the improvement of adoptive T cell therapy protocols.ReferencesCummings CT, Deryckere D, Earp HS, Graham DK. Molecular pathways: MERTK signaling in cancer. Clin Cancer Res 2013;19(19):5275–5280.Peeters MJW, Dulkeviciute D, Draghi A, et al. MERTK Acts as a Costimulatory Receptor on Human CD8+T Cells. Cancer Immunol Res 2019;7(9):1472–1484.Disclosure InformationR.M. Powell: None. M.J.W. Peeters: None. A. Rachbech: None. P.T. Straten: None.

CD4+ T cell help creates memory CD8+ T cells with innate and help-independent recall capacities

CD4+ T cell help is required for the generation of CD8+ cytotoxic T lymphocyte (CTL) memory. Here, we use genome-wide analyses to show how CD4+ T cell help delivered during priming promotes memory differentiation of CTLs. Help signals enhance IL-15-dependent maintenance of central memory T (TCM) cells. More importantly, help signals regulate the size and function of the effector memory T (TEM) cell pool. Helped TEM cells produce Granzyme B and IFNγ upon antigen-independent, innate-like recall by IL-12 and IL-18. In addition, helped memory CTLs express the effector program characteristic of helped primary CTLs upon recall with MHC class I-restricted antigens, likely due to epigenetic imprinting and sustained mRNA expression of effector genes. Our data thus indicate that during priming, CD4+ T cell help optimizes CTL memory by creating TEM cells with innate and help-independent antigen-specific recall capacities.