Introduction: The expression of co-stimulatory (costim) receptors/ligands in the bone marrow (BM) and peripheral blood (PB) in patients (pts) with AML has not been defined. Identification of immune-checkpoint pathways that dominate in AML will guide the rational selection of specific antibodies for clinical trials.

Methods: Between March, 2015 and October 2015 we performed 17-color multi-parameter flow-cytometry (MFC) on 15 untreated AML and 25 relapsed AML to assess expression of costim ligands (4-1BBL, B7-1, B7-2, ICOSL, PDL-1, PDL-2, OX40L) on leukemic blasts and costim receptors (4-1BB, CTLA-4, ICOS, PD-1, OX40, GITR, LAG-3, TIM-3) on T cell subsets: CD4 T effector cells [Teff]: CD3+CD4+CD127lo/+Foxp3, CD4 T regulatory cells [Treg]: CD3+CD4+CD127Foxp3+, and CD8 T cells: CD3+CD8+. Four healthy human BMs were used as control. Expression is denoted by percentage of specific T-cell subset or gated AML blasts positive for the marker indicated. PB mononuclear cells (PBMCs) and blasts were evaluated at the same time-point.

Results: OX40+ Teffs and OX40+ CD8 cells were higher in untreated AML BM as compared to healthy donor BM (median [med]: 7.2% versus [vs] 0.26%; P = 0.0005 and med: 2.03% vs 0.06%; P = 0.07, respectively). PD1+ Tregs and OX40+ Tregs were also higher in untreated AML BM as compared to healthy donor BM (med: 19.7% vs 7.5%; P = 0.03 and med: 10.3% vs 0.5%; P = 0.02, respectively). PD1+ Teffs, OX40+ Teffs, and ICOS+ Teffs were higher in relapsed AML BM as compared to healthy donor BM (med: 17.7% vs 6.7%; P = 0.047, med: 12% vs 0.27%, P = 0.002, and med: 13.3% vs 1.1%; P = 0.07, respectively). OX40+ CD8 cells, ICOS+ CD8 cells, TIM3+ CD8 cells (med: 5.0% vs 0.07%; P = 0.04, med: 18.5% vs 2.6%; P = 0.09, and med: 2.7% vs 0.7%; P = 0.01, respectively) and OX40+ Tregs (med: 15.5% vs 0.5%; P<0.0001) were also higher in relapsed AML BM as compared to healthy donor BM. GITR+ Teffs, PD1+ CD 8 cells, and LAG3+ Tregs were higher in relapsed AML BM as compared to new AML BM (med: 10.9% vs 1.7%; P = 0.08, med: 36.2% vs 21.3%; P = 0.03, and med: 46.7% vs 16.9%; P = 0.07, respectively). No other noteworthy differences were noted for costim receptor expression. There were no noteworthy differences in ligand expression patterns between relapsed AML and new AML. There was significant variability in BM expression of costim receptors and ligands between individual pts. The expression of costim receptors and ligands differed significantly between BM and PB from the same time-point in the same pt. A larger sample size is needed to confirm these data and find additional associations and this is currently underway at our institution.

Conclusions: Clinically targetable checkpoint receptors including PD1, OX40, and ICOS appear to be overexpressed in the BM of pts with AML as compared to healthy donor BM. Relapsed AML had higher expression of costim receptors than untreated AML. Three anti-PD1 based clinical trials for pts with AML are enrolling at our institution (NCT02397720, NCT02464657, NCT02532231).

Citation Format: Naval Daver, Sreyashi Basu, Guillermo Garcia-Manero, Jorge Cortes, Farhad Ravandi, Steven Kornblau, Marina Konopleva, Michael Andreeff, Gautam Borthakur, Nitin Jain, William Wierda, Srdan Verstovsek, Peter Ruvolo, Tapan Kadia, Jairo Matthews, Wilmer Flores, Hui Yang, Carlos Bueso-Ramos, Narmeen Somani, Jorge Blando, James Allison, Hagop Kantarjian, Padmanee Sharma. Defining the immune checkpoint landscape of acute myeloid leukemia (AML). [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3205.