PD-1/PD-L1 Blocking Enhances CD33/CD3-Bispecific BiTE® Antibody (AMG 330) Mediated Lysis of Primary AML Cells
Authors: Krupka Christina , Kufer Peter , Kischel Roman , Zugmaier Gerhard , Köhnke Thomas , Lichtenegger Felix S , Schnorfeil Frauke M , Altmann Torben , Schneider Stephanie , Fiegl Michael , Spiekermann Karsten , Sinclair Angus M , Newhall Kathryn J , Frankel Stanley R , Baeuerle Patrick , Hiddemann Wolfgang , Riethmüller Gert , Subklewe Marion 
Published: October 2019

Abstract
Antibody-based immunotherapy represents a promising strategy to target and eliminate chemoresistant leukemic cells in acute myeloid leukemia (AML). In our previous work, the potency of the CD33/CD3-bispecific BiTE® antibody AMG 330 to activate and redirect T-cells to AML cells was evaluated in a long-term culture system using unmanipulated peripheral blood (PB) or bone marrow (BM) samples from AML patients. We were able to show effective elimination of AML cells by AMG 330-activated and -expanded residual autologous T-cells (n=13, Krupka et. al, Blood 2014 123(3):356-65). The goal of the present study was to identify and manipulate factors that interfere with AMG 330-mediated lysis of primary AML cells.
In our ex-vivo long-term culture system (n= 32 primary diagnosis, 3 relapse), we observed that successful elimination of primary AML cells was predominantly influenced by the initial effector:target (E:T) ratio. Reduced short-term lysis efficacy was observed for patient samples with low E:T ratios (median lysis: E:T ratio <1:10 23.5% vs >1:10 99.3%) after 4 days of culture. However, after 12 days of culture this effect was less prominent as effector T-cell numbers increased and lysis was observed even in cultures with very low initial E:T ratios (up to an E:T ratio of 1:80).
Considering the ubiquitous expression pattern of CD33 within the myeloid compartment, the number of AMG 330 treatment days will be limited due to expected hematotoxicity. Therefore, AMG 330 efficacy might benefit by increasing the E:T ratio within a short time period. Programmed cell death-1 (PD-1) and its primary ligand PD-L1 are immune checkpoints that limit T-cell responses and may contribute to slower lysis kinetics during AMG 330 treatment. Therefore, we assessed PD-1 expression on T-cells and PD-L1 expression on primary AML cells. No constitutive expression of PD-1/PD-L1 on T-cells and corresponding AML cells was found at primary diagnosis (PD-1: n=23; PD-L1: n=193). Upon the addition of AMG 330 to our primary ex-vivo AML cultures, we observed a strong upregulation of PD-1 on activated T-cells, which correlated with the extent of T-cell proliferation (10/10). This was most prominent within the subpopulation of CD4+/CD45RA-/CCR7- effector memory T-cells. Furthermore, in response to AMG 330-mediated T-cell activation, we observed an upregulation of PD-L1 on primary AML cells (16/19). Data obtained from AML cell lines confirmed an upregulation of PD-L1 after co-cultivation with T-cells and AMG 330. This phenomenon was cytokine-mediated as the addition of IFNγ and TNFα also induced PD-L1 expression (n=6). Interestingly, we also observed a PD-L1 upregulation on T-cells upon activation with AMG 330, but to a much lower extent compared to primary AML cells (n=17; mean MFI ratio: T-cells: 4.7; AML cells: 12.1). Blockade of the PD-1/PD-L1 interaction through the addition of an inhibitory antibody induced an increase in T-cell proliferation in ex-vivocultures resulting in enhanced cytotoxicity against primary AML cells (lysis on day 18: with/without PD-1 blocking antibody: 75 vs 44%; fold change T-cell expansion: 6 vs 3). This was accompanied by a significant increase in IFNγ production (with/without PD-1 blocking antibody: 280 pg/ml vs 81 pg/ml). Complimentary experiments using primary AML cells and allogeneic T-cells at defined E:T ratios demonstrated that this effect is most prominent in co-cultures with a low percentage of T-cells within the primary sample (lysis on day 7 with/without PD-L1 blocking antibody: E:T ratio 1:1: 100% vs 94.5%; E:T ratio 1:5: 96.6% vs 82.7%; fold change T-cell expansion: E:T ratio 1:1: 3.1 vs 3.1; E:T ratio 1:5: 23.4 vs 9.8). Intriguingly, upon addition of lenalidomide to our primary AML cultures, we were also able to circumvent the immune inhibitory effect of the PD-1/PD-L1 interaction by a decrease in PD-L1 upregulation on primary AML cells.
Our data show that AMG 330-mediated lysis of primary AML cells can be enhanced by inhibiting the PD-1/PD-L1 axis on T-cells and corresponding AML cells. The modulation of the PD-1/PD-L1 axis increased T-cell proliferation and accelerated attaining a beneficial E:T ratio. We hypothesize that PD-L1 upregulation on primary AML cells is a relevant immune escape mechanism employed by AML cells to escape cytokine-mediated immune responses. Prospective clinical trials will be needed to assess the relevance of our finding in AMG 330-treated AML patients.

Disclosures
Krupka: AMGEN Inc.: Research Funding. Kufer:AMGEN Inc.: Equity Ownership; AMGEN Research (Munich): Employment. Kischel:AMGEN Inc.: Equity Ownership; AMGEN Research (Munich): Employment. Zugmaier:AMGEN Research (Munich): Employment; AMGEN Inc.: Equity Ownership. Sinclair:AMGEN Inc.: Employment, Equity Ownership. Newhall:AMGEN Inc.: Employment, Equity Ownership. Frankel:AMGEN Inc.: Employment, Equity Ownership. Baeuerle:AMGEN Research (Munich): Employment; AMGEN Inc.: Equity Ownership. Riethmüller:AMGEN Inc.: Equity Ownership. Subklewe:AMGEN Inc.: Research Funding.