Expression of PD-L1 in Leukemic Progenitor Cells Defines NPM1 Mutated AML As a Potential Subgroup for PD1/PD-L1 Directed Immunotherapy
Authors: Greiner Jochen , Schneider Vanessa , Schrezenmeier Hubert , Wiesneth Markus , Bullinger Lars , Döhner Hartmut , Hofmann Susanne , Götz Marlies 
Published: October 2019

Abstract
Clinical and preclinical data suggest that acute myeloid leukemia (AML) with mutated nucleophosmin 1(NPM1mut) may constitute an immunogenic leukemia subtype. NPM1mut AML generally correlates with a better prognosis, however the underlying mechanisms still need to be clarified. Checkpoint inhibition targeting Programmed cell death protein 1 (PD-1)/Programmed cell death 1 ligand 1 (PD-L1) has been proven to be an effective novel immunotherapeutic approach in cancer treatment including the treatment of hematological malignancies.
Expression of CD34/CD38/CD274 was evaluated in 20 NPM1mut versus 20 wild-type (NPM1wt) AML patient samples via flow cytometry analyses to assess PD-L1 (CD274) expression in leukemic cells, including leukemic progenitor and stem cells (LSC). We also investigated the influence of the anti-PD-1 antibody Nivolumab® on the antigen-specific immune responses in ELISpot assays. Additionally, we assessed the effect of Nivolumab in colony forming unit (CFU) immunoassays.
Many AML cases showed relevant expression of PD-L1. Bulk cells of NPM1mut AML showed a significantly higher PD-L1 expression in comparison to NPM1wtAML patients (median of 1.5%, range 0.0-8.5%, versus 0.3%, range 0.1-1.1%). Importantly, PD-L1 expression was detected at a higher level in leukemic progenitor cells (CD34+CD38-) of NPM1mut than of NPM1wtAML (median of 3.3%, range 0.0-17.2%, versus 0.3%, range 0.0-3.0%). In general, the LSC fraction showed a higher PD-L1 expression than the non-LSC fraction.
CFU immunoassays showed a significant inhibition of CFU when adding T cells stimulated against various LAA. In all patient samples, effectors activated against at least one LAA were successful to decrease the colony number significantly. Immune effects increased adding Nivolumab to the CTL for several days before starting CFU immunoassays.
In summary, we detected higher PD-L1 expression in NPM1mut patients, especially in the leukemic progenitor compartment. This observation further supports the hypothesis that NPM1-directed immune responses might play an important role in tumor cell rejection, which tumor cells try to escape via expression of PD-L1. Immunogenicity of neoantigens derived from NPM1mut with higher PD-L1 expression constitute promising target structures for individualized immunotherapeutic approaches.

Disclosures
Schrezenmeier: Alexion Pharmaceuticals, Inc.: Honoraria, Research Funding. Bullinger:Pfizer: Speakers Bureau; Bristol-Myers Squibb: Speakers Bureau; Janssen: Speakers Bureau; Amgen: Honoraria, Speakers Bureau; Sanofi: Research Funding, Speakers Bureau; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Bayer Oncology: Research Funding. Döhner:Novartis: Consultancy, Honoraria, Research Funding; Astellas: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Celator: Consultancy, Honoraria; Jazz: Consultancy, Honoraria; AbbVie: Consultancy, Honoraria; Celgene: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria; Celator: Consultancy, Honoraria; AROG Pharmaceuticals: Research Funding; Janssen: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding; Astex Pharmaceuticals: Consultancy, Honoraria; Seattle Genetics: Consultancy, Honoraria; Pfizer: Research Funding; Seattle Genetics: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Agios: Consultancy, Honoraria; Sunesis: Consultancy, Honoraria, Research Funding; Jazz: Consultancy, Honoraria; Agios: Consultancy, Honoraria; Astellas: Consultancy, Honoraria; Pfizer: Research Funding; Celgene: Consultancy, Honoraria, Research Funding; AbbVie: Consultancy, Honoraria; AROG Pharmaceuticals: Research Funding; Sunesis: Consultancy, Honoraria, Research Funding; Bristol Myers Squibb: Research Funding; Bristol Myers Squibb: Research Funding; Astex Pharmaceuticals: Consultancy, Honoraria.