Abstract 2885: Somatic mutation of STAT3 leads to the preferential Th17 differentiation in human naïve CD4-positive cells and favor TCR-mediated proliferation

Intro: Mature Th17 lymphocytes play a key role in the host defenses against bacteria and fungi although unchecked Th17 activation can lead to autoimmune disorders, inflammation and cancer. The engagement of CD4+ naïve T-cells via IL6 and TGFβ favors a Th17 differentiation program, which requires the presence of IL21 and IL23 to reach a complete maturation and the maintenance of Th17 phenotype. The Th17 differentiation requires multiple lineage defining transcription factors (i.e. RORα, RORγt, and STAT3) and the activation of STAT3 signaling, which regulates a plethora of factors (i.e. RORγt, IL23R), is required for the full execution of the Th17 program. We have recently shown that JAK1/STAT3 activating mutations in Anaplastic Large cell Lymphomas (ALCL) are oncogenic, but it is unknown their protumorigenic contribution in modulating/derailing the host microenviroment and the host responses. Methods: 50 FFPE ALCLs were investigated with pSTAT3, GATA3, RORγ and T-Bet antibodies using a dual color approach. CD4+ CD62L+ CD25- CD45RAhi CD4+ naïve cells of healthy donor were sorted and cultured (7 days) in KBM (IL23 (25ng/ml), IL6 (25ng/ml), IL21 (25ng/ml), IL1β (12,5ng/ml), TGFβ (5ng/ml) + 1% FBS and anti-CD3/CD28 beads (1:50). Forced expressions of WT or mutated Y640F (YF) STAT3 were achieved via lentiviral transduction. STAT3 qRTPCR, luciferase assay, ATPlite, and ELISA were used as readouts on samples collected at day 7 post-transduction. Results: IHC stains showed that pSTAT3+ ALCL preferentially co-expressed nuclear RORγ (p