Many different mechanisms allowing for immune escape have been described in solid tumors, but their relevance for hematological malignancies is underexplored. We analyzed possible involvement of selected processes in the acute myeloid leukemia (AML), with regard to the mutational status of NPM1 and Flt3 genes, which are important prognostic markers in AML.
The expression of HLA class I, CLIP, and of different inhibitory receptors was analyzed by flow-cytometry on fresh or cryopreserved primary mononuclear cells, which were isolated from the peripheral blood of AML patients at diagnosis (N = 38). The amount of regulatory T-cells (CD25+FoxP3+) was measured in fresh whole blood samples (N = 81). Some markers were assessed in parallel on the transcript level. AML blast lysis by NK cells was tested by flow-cytometry using donor-derived expanded NK cells.
A decrease in HLA class I expression on AML blasts to about 20 % of normal levels was detected in 26 % of cases. This decrease did not significantly enhanced AML cell lysis by NK cells. High CLIP expression was found on leukemia blasts in 61 % of cases. High PD-L1 expression, which was determined by PCR, was associated with significantly worse overall survival (OS), specifically in patients with internal tandem duplication in Flt3 (Flt3-ITD; published finding, Brodská et al. IJMS 2019). We noted no mark of lymphocyte exhaustion (Tim3, LAG3, CTLA-4) in AML samples. On the other hand, Tim-3 was frequently found on blast cells (53 % of cases), in close correlation with higher levels of Tim-3 transcript. Tim-3 negativity was associated with better OS in patients without Flt3-ITD (p = 0.07). Increased count of regulatory T-cells was detected in 17.5 % of AML patients at diagnosis.
The majority of AML patients display at least one marker of immune escape at diagnosis. These include decreased HLA class I expression, reduced antigen processing, increased PDL1 expression, secretion of Tim3 by leukemia blasts, or increased Treg counts. The mechanisms of immune suppression in AML are likely heterogenous, similarly to the disease itself. Both PD-L1 and Tim-3 might have prognostic value in selected AML subgroups.
The authors.
Ministry of Health of the Czech Republic (Grant no 16-30268A).
All authors have declared no conflicts of interest.