PF537 THE STAT SIGNALING BIOSIGNATURE OF CD4+ T CELL SUBSETS REFLECT THE IMMUNOGENICITY OF HIGH‐RISK MDS AND PREDICT AZACYTIDINE TREATMENT OUTCOME

Abstract

Background:Azacytidine (AZA) is the mainstream treatment in high‐risk Myelodysplastic Syndromes (MDS), but the exact mechanism of action and resistance is largely unknown. AZA potentially acts, at least partially, by upregulating the antitumor immune response, but literature reports are still contradictory. Similarly, CD4+ T cells appear to play a critical role in the antitumor immune response in MDS, even though the exact mechanism remains unclear.Aims:As STAT proteins are key regulators of both physiological and pathological polarization and differentiation of CD4+cells, we investigated the STAT signaling profile of high‐risk MDS patients during AZA treatment and its correlations with clinical and biological parameters.Methods:Peripheral blood mononuclear cells were isolated before the initiation of AZA from 67 high‐risk MDS patients and 13 age‐matched healthy donors. We applied phosphospecific flow cytometry to measure both basal and potentiated levels of pSTAT1, pSTAT3 and pSTAT5. A signaling profile (SP) was constracted for T conventional (Tconv) and T regulatory cells (Tregs) and their functional subsets: PD1+ cells, central memory (TCM), naïve (TN), effector memory (TEM) and terminal effector memory (TEMRA) cells. We further characterized the functional and polarization state of T cells by staining with Perforin, IFNg, IL4, IL17 and Helios.Results:pSTAT signaling profile of Tconv cells revealed three signaling clusters (SC). SC#2 was characterized by higher potentiated responses of pSTAT3 after IL‐6 stimulation (IL‐6/pSTAT3) and was enriched in patients with longer median‐overall survival (mOS, p = 0.047, figure 1A). The SP of both PD1+ Tconv and TEM revealed three SCs. In both cases patients with shorter median event free survival (mEFS) segregated in SCs with downregulated potentiated responses (p = 0.055 and p = 0.04 respectively). The pretreatment SP of Tregs was also associated with mEFS though without reaching significance (p = 0.068), whereas the SP of PD1+ Tregs separated patients with intermediate potentiated responses which showed longer OS (p = 0.023).Summary/Conclusion:Collectively, are results reveal for the first time the clinically significant heterogeneity of CD4+ T cell subsets and their diverse role in the antitumor immunity in MDS. Moreover, the beneficial effect of the activation of IL‐6/pSTAT3 pathway in CD4conv cells comes in line with current data in CAR‐T cells and further suggests that the STAT signaling biosignature on CD4+ cells may represent a response biomarker to AZA and a therapeutic target for high‐risk MDS.image