Frequency and Stimulation of NPM1-Specific Immune Responses By Anti-PD1 Antibodies in NPM1-Mutated AML

Nucelophosmin1 (NPM1) is one of the most commonly mutated genes in AML, represents a distinct entity according to the WHO and is commonly associated with a favorable prognosis. We described specific immune responses against immunogenic epitopes derived from the mutational region of NPM1 in AML and specific immune responses against leukemic progenitor and stem cells (LPC/LSC). Immune responses play an increasing role in AML treatment options. However, the role of Immuncheckpoint inhibition is still controversially discussed in AML.

In this work, we investigated NPM1 specific immune responses but also responses against other leukemia associated antigens (LAA) like PRAME (P300), Wilms' Tumor 1 (WT1) and RHAMM (R3) in an extended cohort. We investigated these immune responses in LSC/LPC using colony-forming immunoassays (CFI) and ELISpots. We examine whether immuncheckpoint inhibition using the anti-programmed death 1 (anti-PD-1) antibody might increase immune responses against stem cell like cells, comparing NPM1-mutated (NPM1^mut) to NPM1 wildtype (NPM1^wt) patients.

In AML NPM1^mut patients, specific immune responses of cytotoxic T cells against the epitope NPM1 showed a mean reduction of colonies in CFI of 27%, for P300 of 38%, for WT1 of 32% and for R3 of 42%. NPM1^wt patients showed a mean reduction of colonies in CFI of for P300 of 28%, for WT1 of 28% and for R3 of 41%. Results are comparable, for NPM1^mut and NPM1^wt patients, there were immune responses seen in all epitopes.

When adding the anti-PD-1 antibody to CFI, in NPM1^mut patients the mean additional inhibition using NPM1 as target was 47%, and for the other epitopes: P300 23%, WT1 27% and R3 26%. For NPM1^WT patients, the additional inhibition with the addition of anti-PD-1 to CFI was for P300 25%, for WT1 34% and for R3 23%.

For the NPM1 epitope, 8 of 10 NPM1^mut patients showed an immune response and 3/10 showed reduction in CFI of > 50%. The NPM1 epitope displayed an additional reduction with anit-PD-1 in all 10 NPM1^mut Patients and in 6 of 10 a reduction of > 50%. Thus, especially the NPM1 specific immune responses are strong in NPM1^mut patients by adding anti-PD-1.

Taken together, the anti-PD-1 antibody increases specific T cell responses of LAA-stimulated CTL and the cytotoxic effect of T cells against LPC/LSC. The effect was strongest in NPM1^mut patients against the immunogenic epitope derived from the mutational region of NPM1 and even stronger when adding anti-PD-1. These data suggest there could be a NPM1 mutation directed immunotherapeutic approach using LAA-directed vaccination strategies in NPM1-mutated AML and combination with anti-PD-1 antibodies might open new application possibilities.