Immune checkpoint inhibition improves antimyeloma activity of bortezomib and STING agonist combination in Vk*MYC preclinical model

Multiple myeloma (MM), a hematological malignancy of plasma cells, has remained incurable despite the development of novel therapies that improve patients' outcome. Recent evidence indicates that the stimulator of interferon genes (STING) pathway may represent a novel target for induction of antitumor immune response in multiple myeloma. Here, we investigated antitumor effects of STING agonist with bortezomib with or without checkpoint inhibitor in the treatment of MM.

METHODS

STING expression in bone marrow plasma cells of 58 MM patients was examined by immunohistochemical staining. The effectiveness of the proposed therapy was evaluated in vivo in a syngeneic transplantable mouse model of MM (Vĸ*MYC) in immunocompetent mice. Flow cytometry was used to assess tumor burden and investigate activation of immune response against MM. ELISA was performed to measure serum inflammatory cytokines concentrations upon treatment.

RESULTS

Combining a STING agonist [2'3'-cGAM(PS)2] with bortezomib significantly decreased tumor burden and improved the survival of treated mice compared to either of the compounds used alone. The combination treatment led to secretion of pro-inflammatory cytokines and increased the percentage of neutrophils, activated dendritic cells and T cells in the tumor microenvironment. However, it resulted also in increased expression of PD-L1 on the surface of the immune cells. Addition of anti-PD1 antibody further potentiated the therapeutic effects.

CONCLUSIONS

Our findings indicate high antimyeloma efficacy of the three-drug regimen comprising bortezomib, STING agonist, and a checkpoint inhibitor.

Potent but transient immunosuppression of T-cells is a general feature of CD71+ erythroid cells

CD71+ erythroid cells (CECs) have been recently recognized in both neonates and cancer patients as potent immunoregulatory cells. Here, we show that in mice early-stage CECs expand in anemia, have high levels of arginase 2 (ARG2) and reactive oxygen species (ROS). In the spleens of anemic mice, CECs expansion-induced L-arginine depletion suppresses T-cell responses. In humans with anemia, CECs expand and express ARG1 and ARG2 that suppress T-cells IFN-γ production. Moreover, bone marrow CECs from healthy human donors suppress T-cells proliferation. CECs differentiated from peripheral blood mononuclear cells potently suppress T-cell activation, proliferation, and IFN-γ production in an ARG- and ROS-dependent manner. These effects are the most prominent for early-stage CECs (CD71highCD235adim cells). The suppressive properties disappear during erythroid differentiation as more differentiated CECs and mature erythrocytes lack significant immunoregulatory properties. Our studies provide a novel insight into the role of CECs in the immune response regulation.

Inhibition of protein disulfide isomerase induces differentiation of acute myeloid leukemia cells

A cute myeloid leukemia is a malignant disease of immature myeloid cells. Despite significant therapeutic effects of differentiation-inducing agents in some acute myeloid leukemia subtypes, the disease remains incurable in a large fraction of patients. Here we show that SK053, a thioredoxin inhibitor, induces differentiation and cell death of acute myeloid leukemia cells. Considering that thioredoxin knock-down with short hairpin RNA failed to exert antiproliferative effects in one of the acute myeloid leukemia cell lines, we used a biotin affinity probe-labeling approach to identify potential molecular targets for the effects of SK053. Mass spectrometry of proteins precipitated from acute myeloid leukemia cells incubated with biotinylated SK053 used as a bait revealed protein disulfide isomerase as a potential binding partner for the compound. Biochemical, enzymatic and functional assays using fluorescence lifetime imaging confirmed that SK053 binds to and inhibits the activity of protein disulfide isomerase. Protein disulfide isomerase knockdown with short hairpin RNA was associated with inhibition of cell growth, increased CCAAT enhancer-binding protein α levels, and induction of differentiation of HL-60 cells. Molecular dynamics simulation followed by the covalent docking indicated that SK053 binds to the fourth thioredoxin-like domain of protein disulfide isomerase. Differentiation of myeloid precursor cells requires the activity of CCAAT enhancer-binding protein α, the function of which is impaired in acute myeloid leukemia cells through various mechanisms, including translational block by protein disulfide isomerase. SK053 increased the levels of CCAAT enhancer-binding protein α and upregulated mRNA levels for differentiation-associated genes. Finally, SK053 decreased the survival of blasts and increased the percentage of cells expressing the maturation-associated CD11b marker in primary cells isolated from bone marrow or peripheral blood of patients with acute myeloid leukemia. Collectively, these results provide a proof-of-concept that protein disulfide isomerase inhibition has potential as a therapeutic strategy for the treatment of acute myeloid leukemia and for the development of small-molecule inhibitors of protein disulfide isomerase.