Specific Immune Responses for Leukemia-Associated Antigens Against Myeloid Leukemic Cells Are Increased By Immune Checkpoint Inhibition

Immunotherapy in cancer treatment has gained importance in the last few years. The efficacy of immunotherapeutic approaches such as immune-checkpoint inhibitors, chimeric antigen receptor T cells (CARs) or bi-specific T cell activating antibodies becomes more and more obvious. However, mechanisms of these immune responses and responsible antigen structures have to be further elucidated. Leukemia-associated antigens (LAA) represent immunogenic antigens that are candidates for specific immunotherapy since they are target structures relevant for elimination of malignant cells by cytotoxic T cells (CTL).

In this work, we investigated the influence of PD-1 antibody Nivolumab and CTLA-4 antibody Ipilimumab on the antigen-specific immune responses by specific T cells against leukemic myeloid blasts in functional T cell assays using ELISpot Assays, tetramer-analysis and colony-forming immunoassays. We investigated T cell responses stimulated against known LAAs like RHAMM, PRAME, WT-1, SSXIP2, Proteinase 3, Survivin, Aurorakinase A and NPM1 against several AML cell lines and samples of AML patients. Expression of different LAAs were measured and correlated to functional T cell assays.

Colony forming unit immunoassays displayed a significant inhibition of CFU (colony forming units) in AML patient samples 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. The LAAs PRAME, RHAMM and WT1 showed highest frequency and intensity of immunogenic reactions: PRAME stimulated CTL induced an immune response in 83% of tested samples in CFU, stimulated with WT1 in 75% and with RHAMM in 58% of colonies from AML samples. Specific immune responses of cytotoxic T cells were additionally detected by ELISpot assays and correlated to results detected in CFU. Immune effects increased adding nivolumab to the CTL for several days before starting CFU immunoassays whereas no effect was measured when CTL were incubated with Ipilimumab. The combination of Nivolumab and Ipilimumab showed no additional effect of immune responses compared to Nivolumab alone.

Taken together, the immune checkpoint inhibitor Nivolumab increases specific T cell responses of LAA-stimulated cytotoxic T cells and the cytotoxic effect of T cells against blasts of AML patients. No additional effect was detected with Ipilimumab. These data suggest that PD-1 antibodies could be an immunotherapeutic approach in AML and combination with LAA-directed vaccination strategies might open interesting application possibilities.