Costimulatory Expression Profiling of AML Blasts Identifies Surface Markers with High Correlation to Isolated NPM1 Mutation

Abstract 2511

Acute myeloid leukemia (AML) is a heterogeneous disease. In recent years, numerous cytogenetic and molecular markers providing prognostic information have been established. The success of allogeneic stem cell transplantation and some autologous immunotherapeutic strategies has proven that immunological effects play an important role for treatment of AML, especially for the eradication of minimal residual disease. However, the effect is dampened by immunosuppressive factors provided by AML blasts. One of the possible immunomodulatory mechanisms is the expression pattern of costimulatory and coinhibitory molecules on AML cells, which influences their interaction with specific T cells.

In order to elucidate the potential significance of this mechanism in AML, we analyzed the surface expression of a broad panel of costimulatory and coinhibitory molecules (CD80, CD86, CD273, CD274, CD275, CD276, B7-H4, HVEM) on blast cells of 102 AML patients at initial diagnosis by flow cytometry. The mean fluorescence intensity of these markers on the CD33 positive blast cell population was correlated with morphologic, cytogenetic and molecular characteristics of the disease. Statistical significance of differences in expression levels was tested with the Mann-Whitney-U test for independent samples.

First, we could show that the AMLs morphologically classified as M4 or M5 according to FAB (n = 35) expressed higher levels of costimulatory markers, especially CD86 (p < 0.001) and CD276 (p = 0.016), compared to the other morphologic subgroups. For CD86, this is in concordance with the literature (e.g. Maeda et al., Br J Haematol 1998). Correlation with CD276 has not been published before, but is well in line with the monocytic lineage of these AML cases, inclining them to good antigen presentation capacities.

In contrast to morphologic characteristics, correlations of the AML costimulatory profile with cytogenetic and molecular markers of the disease have never been studied before. AML patients were classified according to ELN prognostic groups, which are based on cytogenetic and molecular markers (Döhner et al., Blood 2010). Interestingly, we found that the AML blast cells of patients allocated to the favorable risk group (n = 23) showed higher expression of CD86 (p = 0.049), CD274 (p = 0.010), CD276 (p = 0.003), and B7-H4 (p = 0.001), compared to patients of the non-favorable risk groups. In particular, the cases of leukemia with normal karyotype and an isolated NPM1 mutation (without accompanying FLT3-ITD mutation), which constituted about half of the favorable risk group (n = 11), were responsible for this pattern. In these cases, the balance of costimulatory and coinhibitory molecules was clearly shifted toward expression of positive costimulatory molecules, reflected by a significantly higher ratio of the positive costimulatory molecule CD86 to various coinhibitory molecules (e.g. p = 0.003 for the ratio CD86/CD274). To further dissect the influence of NPM1 mutation on the costimulatory expression profile, all patients with an NPM1 mutation irrespective of FLT3 mutational status and karyotype were analyzed. In this cohort of 33 patients, a similar trend to higher expression of these molecules was observed, but correlations were less pronounced (p = 0.012 for CD86) or not significant (CD274, CD276, B7-H4). The karyotype and the FLT3 mutational status by itself, however, showed no significant correlation with the costimulatory profile.

We hypothesize that the expression pattern of costimulatory molecules contributes to prognosis of blast clearance and especially relapse. If this is the case, we are interested to analyze whether the marker profile constitutes a surrogate for molecular markers or an independent prognostic marker. Potentially, this could be highly relevant for application in immunotherapy, e.g. allogeneic stem cell transplantation. A preliminary analysis of the clinical data will be presented.