Myeloid Derived Suppressor Cells in Acute Leukemia and Its Association with Conventional Cytogenetic and Molecular Risk Factors

Abstract 1446

Myeloid derived suppressor cells (MDSC) are a relatively novel population of myeloid derived immunomodulatory cells which have been reported to be associated with poor prognosis in solid tumors. Preliminary data suggest that this population is also increased in high risk newly diagnosed patients with chronic myeloid leukemia. There is no reported data evaluating the proportion of these cells in newly diagnosed acute leukemia and their association with conventional risk factors in acute leukemia. We undertook a prospective study and evaluated peripheral blood (PB) samples of healthy controls (HC; n=10) and newly diagnosed leukemia patients (B-ALL=49, T-ALL=17, APL= 47 and AML=137) for presence of MDSC. To detect MDSC, PB mononuclear cells were isolated and labeled with a panel of monoclonal antibodies to CD14, CD33, HLADR and CD11b directly conjugated with FITC, PE, PerCP and APC respectively. Cells were then washed and analyzed using FACSCalibur and CellQuestPro software. MDSC subset was defined as previously reported by the phenotype of CD14-HLADR-CD33+ CD11b+ (Nature Reviews Immunol, 2009; Figure 1A summarizes the gating strategy). The median percentage of MDSC in PB samples of HC and newly diagnosed B-ALL, T-ALL, APL and AML was 0.7% (range: 0.11–0.97), 0.14% (range: 0.01–2.76 ), 0.05% (range: 0.01–0.9 ), 0.58% (range: 0.04–5.94) and 1.07 % (range:0.01–31.1% ) respectively (Figure 1B). The median values were significantly lower in ALL and APL when compared with AML (P=0.001 and 0.005 respectively). There was no significant association among cases with ALL and APL with conventional risk factors. Among newly diagnosed cases of AML there is significant heterogeneity in the proportion of MDSC with 77 (56%) having values above the median of HC (Figure 1B). There was weak but significant positive correlation of MDSC with WBC counts at diagnosis (Figure 1C) and a significant negative correlation with CD34 percentage at diagnosis (Figure 1D). Comparison of AML cases in the highest quartile (MDSC>3.74%; n=34) with the remaining patients (MDSC≤3.74%; n=103) is summarized in Table 1. AML patients in the highest quartile of MDSC had significantly higher WBC count and a significantly lower CD34 count at diagnosis; there were also a significantly higher proportion of cases that were NPM1 and FLT3 mutation positive in this quartile as was the high risk NPM1+and FLT3 mutation+ subset (Table 1). The clinical follow up in this series of AML patients was short with too few events to analyze the impact of this population on clinical outcomes.

In summary MDSC at diagnosis in PB samples is low in ALL and APL. There is significant heterogeneity among cases with AML and a significant association with WBC count at diagnosis, NPM1 and FLT3 mutations and a significant negative correlation with CD34 percentage at diagnosis. The biological relevance and impact of this population on clinical outcomes warrants a more detailed study.

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