Enhanced VISTA Expression in a Subset of Patients with Acute Myeloid Leukemia

Introduction: Identification of tumor associated antigens (TAA) in acute myeloid leukemia (AML) will facilitate the use of targeted immunotherapies such as chimeric antigen receptor T cell therapy and bispecific antibodies. The characteristics of an ideal TAA include tumor specificity, high expression level, large percentage of positive tumor cells, and evidence for expression on putative cancer stem cells. V-domain Ig Suppressor of T cell Activation (VISTA) is a cell surface protein previously shown to be present on hematologic cells and inhibitory towards T cells. VISTA's role and presence within AML has yet to be defined. We performed detailed studies to characterize VISTA expression on tumor cells and mononuclear cell subsets in patients with AML.

Methods: Immunophenotyping was performed on mononuclear cells from primary AML patient specimens using mass cytometry (CyTOF) by staining samples with a panel containing 34 antibodies to surface markers and cytokines (Table 1). This antibody panel was used to identify blast cells (variable immunophenotype), leukemic stem cells (LSCs) (CD34+, CD38-), T cells (CD3+), monocytes (CD33+, CD14+, CD11b+, HLA-DR+), myeloid derived suppressor cells (MDSCs) (CD33+, CD11b+, HLA-DR-), and M2 macrophages (CD11b+, CD163+). Raw median VISTA expression and percentage of cells expressing VISTA were calculated for all of these cell types. Samples were split into 2 categories based on blast VISTA expression. A sample was considered VISTA high if >10% of the blasts expressed VISTA, and VISTA low if <10% of the blasts expressed VISTA. Similar data was generated from a pool of healthy donors for comparison.

Results: Fourteen bone marrow samples from patients with newly diagnosed AML and 10 from healthy donors were obtained. The average percentage of blasts expressing VISTA was 68% (range 28% - 81%) in the VISTA high group and 4.2% (range 0.3% - 6.8%) in the VISTA low group (p<0.05). VISTA expression was higher in the VISTA high group than in the VISTA low group (17.7 vs -0.1, p<0.05). The VISTA high group had a lower blast percentage in the bone marrow compared to the VISTA low group (56% vs 91%, p<0.05). The VISTA high and low groups had similar percentages of LSCs (2.5% vs 0.6%, p=0.11), but the LSCs in the VISTA high group maintained higher VISTA expression than did those in the VISTA low group (6.81 vs -0.11, p<0.05). The LSCs within the VISTA high group had similar VISTA expression to the blast population (6.81 vs 17.7, p=0.15). Within the non-blast population, VISTA expression was higher on the myeloid cell subsets than on the T cells for both the AML samples (11.5 vs 0.17, p<0.05) and the healthy donor samples (8.07 vs 0.24, p<0.05). VISTA expression was similar between the VISTA high and low groups on T cells (0.36 vs 0.02, p=0.08), monocytes (15.7 vs 19, p=0.7), MDSCs (6.3 vs 4, p=0.26), and M2 macrophages (7.4 vs 16.6, p =0.25). VISTA expression was similar between AML and healthy donor samples for T cells (0.17 vs 0.24, p=0.6), monocytes (15.7 vs 14, p=0.41), MDSCs (6.3 vs 3.6, p=0.22), and M2 macrophages (7.4 vs 6.6, p=0.19). Monocytes had the highest expression of VISTA within the healthy donor samples (p<0.05) and this difference was maintained in the AML samples.

Conclusion: A subset of patients with AML has a large percentage of blasts, including LSCs, with high VISTA expression on their surface. VISTA is expressed on normal myeloid cells in the bone marrow of patients with AML, but at low levels and to a similar degree as in the marrow of healthy donors.

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