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Abstract: EP386

Type: E-Poster Presentation

Session title: Acute myeloid leukemia - Biology & Translational Research

Background

In AML Nucelophosmin1 (NPM1) is one of the most frequently mutated genes and, according to the WHO classification, represents a separate entity that is often associated with a favorable prognosis. Other groups and ours described specific immune responses against immunogenic epitopes derived from the mutational region of NPM1 in AML patients (pts), as well as specific immune responses against leukemic progenitor and stem cells (LPC/LSC).

In a further step, we added the immune checkpoint inhibitor anti programmed death 1 (anti-PD-1), an inhibitory receptor expressed on T cells, to colony forming immunoassays (CFI), to discover whether the immune responses intensify against LSC/LPC comparing NPM1^mut and NPM1^wt pts.

Aims

In a larger cohort of 15 NPM1^mut and 15 NPM1^wt AML pts, we investigated immune responses against the mutational epitope NPM1 but also other LAA like PRAME (P300), Wilms’ Tumor 1 (WT1) and RHAMM (R3), and compared NPM1^mut to NPM1^wt pts in CFI to detect CTL mediated immune responses against LSC/LPC.

Methods

We analysed the influence of anti-PD-1 on antigen-specific immune responses by allogeneic cytotoxic T lymphocytes (CTL) against LPC/LSC in functional T cell assays using ELISpot, FACS analysis and colony forming immunoassays (CFI) in NPM1^mut vs. NPM1^wt pts.

Results

We set out by detecting the reduction of colonies in CFI with the epitope NPM1 and several LAA. In NPM1^mut AML pts 9/15 (60%) showed immune responses against all four epitopes. Mean colony reduction for LAA were 32% for P300, 45% for WT-1 and 43% for R3. In NPM1^WT AML patient’s inhibitory potential of all LAA was seen in 8/15 (53%) samples. Mean colony reduction for LAA was 33% for P300, 34% for WT-1 and 46% for R3. Against the NPM1 epitope NPM1^mut patients showed 35% reduction and, as expected, in NPM1^WT patients no responses were found, the responses against the other antigens were comparable to the NPM1 mutated patients.

With the addition of anti-PD-1 to CTL before starting CFI, for AML NPM1^mut pts the inhibition in CFI using the LAA P300 as target ranged from 0-81% (mean: 32%), for WT1 from 0-74% (mean: 22%), and for R3 from 0-75% (mean: 21%). For AML NPM1^WT pts the reduction with P300 with anti-PD-1 as target ranged from 0-59% (mean: 25%), for WT1 from 0-81% (mean: 34%), and for R3 from 0-60% (mean: 23%). Results for LAA were again comparable in both cohorts.

Remarkably, the immune effect in NPM1^mut patients increased markedly with the epitope NPM1 and even more when anti-PD-1 was added to CFI, with a mean reduction of 47%. All 15 showed a reduction with anti-PD1, even those that did not show any reduction with the epitope NPM1 alone. A particularly strong additional reduction with the anti-PD1 antibody in CFI of 50% or more was observed in 7/15 NPM1^mut pts. 4 of 15 pts showed a slightly lower additional reduction with anti-PD1 in the range of 44-28%.

Conclusion

We investigated the immune responses of AML pts to LPC/LSC with NPM1/LAA-specific T cells in a larger cohort. With the addition of anti-PD-1 to CFI, the immune resistance could be overcome.

In conclusion, the immune checkpoint inhibitor anti-PD-1 increases the cytotoxic effect of T cells against LPC/LSC. The effect was strongest in NPM1^mut pts with the epitope NPM1 and even stronger when adding anti-PD-1 to CFI. These data suggest that immunotherapy with a PD-1 antibody, for instance in combination with a NPM1-specific vaccine, possibly in the context of minimal residual disease or maintenance therapy, could be an interesting option for NPM1-mutated patients.

Keyword(s): Acute myeloid leukemia, Cytotoxic T lymphocyte, Leukemia-associated antigen, Leukemic stem cell