Mutated Nucleophosmin 1 (NPM1) Is an Immunogenic Target and Patients with NPM1^mut Acute Myeloid Leukemia (AML) Showed High Expression of Different Leukemia-Associated Antigens (LAAs)

Abstract 3592

Nucleophosmin gene 1 mutations (NPM1^mut) are one of the most frequent molecular alterations in AML and distinct immune responses might contribute to the favorable prognosis of AML patients with NPM1^mut. Recently, we showed specific T cell responses of CD4+ and CD8+ T cells against epitopes derived from mutated regions of NPM1 (Greiner et al., Blood. 2012 May 16, Epub). In the present study, we investigated clinical parameters and the clinical outcome of NPM1^mut AML patients in accordance to their immune responses against different NPM1 epitopes. Moreover, we examined the quantitative expression of different leukemia-associated antigens (LAAs) in NPM1^mutAML patients.

In ELISpot analysis of 33 healthy volunteers and 27 AML patients, we detected T cell responses of CD4+ and CD8+ T cells against epitopes derived from the mutated region of NPM1. We performed further tetramer assays against the most interesting epitopes and chromium release assays to show the cytotoxicity of peptide-specific T cells. Microarray analysis was performed to analyze the expression of different LAAs in NPM1^mut and NPM1^wtAML patients.

Two epitopes (peptide #1 and #3) derived from NPM1^mut induced CD8+ T cell responses. 33% of the NPM1^mut AML patients showed immune responses against peptide #1 and 44% against peptide #3. NPM1^mut AML patients showed a significantly higher frequency of T cell responses against peptide #3 in contrast to HVs (p=0.046), whereas for peptide #1 the frequency of T cell responses of AML NPM1^mut patients and HVs was not significantly different. Specific lysis of pulsed T2 cells but also NPM1^mut leukemic blasts was detected in chromium release assays. Therefore, overlapping peptides (OL) were analyzed in ELISpot assays and the peptide called OL8 showed favorable results to activate both CD8+ and CD4+ T cells.

We performed survival analysis for these 33 NPM1^mut patients analyzed by ELISpot comparing cases with or without specific T cell responses. Our data suggest a trend to a better overall survival (OS) for patients with specific T cell responses against peptide #1 or #3. However, the patient numbers are small and the data have to be interpreted carefully. Analyses with material from larger controlled clinical trials with a high number of patients with NPM1^mut AML have to be performed. Our microarray analysis of 30 AML patients showed a high expression of different LAAs like RHAMM, WT-1 and BCL-2 in all subtypes of cells of NPM1^mutAML patients, also in leukemic progenitor cells. This demonstrates that NPM1 is an AML subtype suitable for poly-targeted immunotherapeutic trials.

Taken together, NPM1^mut might constitute an interesting target structure for individualized immunotherapeutic approaches in NPM1^mut AML patients. We hypothesize that immune responses to NPM1 mutation may contribute to the favorable prognosis.