P429: THERAPY-INDUCED SENESCENCE TRIGGERS UPREGULATION OF ANTIGEN PRESENTATION VIA PRC2 INHIBITION AND PROMOTES ANTI-TUMOUR IMMUNITY IN ACUTE MYELOID LEUKAEMIA

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Background: Acute myeloid leukemia (AML) is an aggressive hematological malignancy currently treated with high-dose chemotherapy. Therapy-induced senescence (TIS) is a cell-intrinsic fail-safe mechanism controlling tumor initiation and progression but, if persistent, may even contribute to cancer relapse. Yet, the contribution of TIS to early response to therapy in AML remains ill-defined. Aims: Our work aims to delve into the crosstalk between TIS AML blast and adaptive immune cells with the final long-term goal of unveiling mechanistic insights into the factors dictating AML response to chemotherapy and immunotherapy. Methods: We integrated multiparametric flow cytometry analysis with quantitative imaging and gene expression profiles to measure several senescent markers (cell cycle, cytokine release and DNA damage) and human leukocyte antigen (HLA) expression. A mixed lymphocyte reaction assay was adopted to evaluate T cell activation. Results: We conducted flow cytometry and RNA-seq analysis on ex-vivo cultured primary AML samples at diagnosis and upon chemotherapy and stratified patients in Senescence High (SH) and Senescence Low (SL). RNA-seq analysis also revealed enrichment in IFNγ, IFNα and immune-related categories in the group SH compared to SL. We then measured human leukocyte antigen (HLA) class I and class II on the surface of untreated or TIS AML blasts and observed an upregulation of them only in SH patients after chemotherapy treatment suggesting a causal link between TIS and immunogenicity. To evaluate the contribution of DNA damage and senescence program activation, we used Palbociclib (CDK4/6 inhibitor) to promote senescence without inducing physical DNA damage. We were able to recapitulate HLA class I and II upregulation upon this treatment suggesting the senescence program is essential to promote increased antigen presentation. Evaluating the functional consequences of HLA upregulation on anti-tumor immunity, we found that TIS AML samples promote autologous CD4+ and CD8+ T-cell activation in a mixed lymphocyte reaction (MLR) experiment resulting in increased leukemia recognition and eradication; conversely, in SL patients no increase in T-cell activation upon chemotherapy treatment was reported. The use of immune checkpoint blockade (ICB) is an emerging treatment largely tested in solid tumours while their efficacy in AML is still suboptimal. We tested the combination of ICB with senescence-inducing therapies and reported an increased CD8+ T-cell activation in SH patients compared to monotherapy while SL did not benefit from this combinatorial approach. To delve in deep the molecular mechanisms involved in HLA upregulation, we identify from RNA-seq the downregulation of polycomb repressor complex 2 (PRC2) core genes in SH patients compared to the SL group after chemotherapy treatment with consequent upregulation of SUZ12 and EZH2 target genes, suggesting a chromatin remodeling downstream of senescence induction able to re-activate HLA class I and II molecules. Finally, we pharmacologically inhibited the EZH2 subunit with Tazemetostat in SL patients and found first upregulation of HLA molecules and second an increased in autologous CD4+ and CD8+ T-cell activation compared to untreated controls. Summary/Conclusion: Overall, our results highlight senescence as a potent immune-related anti-leukemic mechanism that may rapidly translate into innovative senescence-based strategies to cure or prevent AML post-therapy relapse. Keywords: Cancer immunotherapy, AML, HLA, Senescence

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