S131 IL7R EXPRESSION PREDICTS T‐ALL SENSITIVITY TO JAK INHIBITORS REGARDLESS OF IL7R/JAK/STAT MUTATIONAL STATUS

Background: Normal T‐cell development is supported by cytokine signaling with IL7 as key player. This is highlighted by the absence of T‐cells in gamma‐chain SCID‐patients. T‐ALL represents a heterogeneous disease characterized by expansion of immature T‐cells blocked in their differentiation. Despite therapeutic improvements, it remains associated with a poor prognosis mainly due to relapses. This results in a medical need for new therapeutic approaches. Several studies reported the deregulation of IL7R signaling notably via genomic alterations accounting for ∼30% of T‐ALL. In keeping with this, preclinical models supported the possibility to target the IL7R‐pathway (IL7Rp) in IL7Rp‐mutated T‐ALL using pharmacological JAK inhibitors such as ruxolitinib. Moreover, ex vivo culture of primary T‐ALL samples requires addition of IL7 cytokine in culture medium to support leukemic cell survival. Altogether, this data supports a strong dependency of T‐ALL on IL7R signalling. However, the question of which T‐ALL patients may benefit from JAK inhibitors remains unclear. Aims: We aimed to investigate surface IL7R (sIL7R) expression and mutational status of IL7Rp in T‐ALL, and to identify a readout for patient JAK inhibitor sensitivity. Methods: 159 T‐ALL samples were prospectively analyzed by FACS for IL7R expression. Genetic data of IL7R pathway genes (IL7R, JAK1/2, STAT5) were assessed on 87 adult T‐ALL enrolled in the ongoing GRAALL‐2014 trial by targeted‐sequencing. 21 patient derived xenograft (PDX) T‐ALL were generated for ex vivo studies including i) phosphoprotein flow analysis of pSTAT5 upon IL7 stimulation and JAK inhibition, ii) apoptosis assay by Annexin V‐PI staining, and iii) proliferation assay using Celltrace. Results: sIL7R was expressed at early stages of normal T‐cell development, downregulated during the cortical stage, and up‐regulated at the mature stage. In T‐ALL, sIL7R was expressed in 84/159 cases (53%). We found IL7Rp mutations in 29% of cases and all IL7R mut cases expressed sIL7R defining 3 classes of T‐ALL depending on sIL7R expression and mutational status of IL7Rp: IL7R ‐/WT , IL7R +/WT , IL7R +/mut . IL7R + T‐ALLs were mainly of immature non‐ETP, cortical and TCRgd phenotype. These data support frequent and abnormal expression of sIL7R in T‐ALL as regard to normal expression pattern in thymocytes. sIL7R expression was always associated with functional signaling of the receptor: pSTAT5 was activated upon IL7 exposure and abrogated by the addition of ruxolitinib in both normal thymocytes and PDX T‐ALL. Constitutive STAT5 phosphorylation was observed in IL7R +/mut but not in IL7R +/WT PDX T‐ALL. However, pSTAT5 activation reached similar levels in both IL7R +/mut and IL7R +/WT PDX T‐ALL upon IL7 exposure regardless of their mutational status, which was completely reversed by ruxolitinib addition. Ex vivo treatment of PDX T‐ALL with increasing doses of ruxolitinib induced apoptosis and cytostatic effect in both IL7R +/WT and IL7R +/mut PDX T‐ALL. Summary/Conclusion: IL7Rp genes are mutated in 29% in T‐ALL and sIL7R is expressed in 53% of cases. Ruxolitinib induces apoptosis and impairs cell proliferation in sIL7R + T‐ALL irrespective of IL7Rp mutational status. Our data shows that sensitivity to JAK inhibitors is determined by the expression of IL7R regardless of the IL7Rp genomic status. IL7R expression is a readout for JAK inhibitors sensitivity, and provides a fast and accurate companion theranostic tool using flow‐cytometry.