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751 PMC-309, a highly selective anti-VISTA antibody reverses immunosuppressive TME to immune-supportive TME
  1. Cheon Ho Park,
  2. Sang Soon Byun,
  3. Ki Dae Kim,
  4. Hye Rim Han and
  5. Weon Sup Lee
  1. PharAbcine Inc., Daejeon, Korea, Republic of


Background The tumor microenvironment (TME) consists of blood & lymphatic vessels, stromal cells, and immune cells such as lymphocytes, macrophages, and APC cells. V-domain immunoglobulin suppressor of T cell activation (VISTA) is highly expressed in myeloid-derived cells and its blockade enhances antitumor immunity in multiple tumor models. Therefore, anti-VISTA agents may provide additional cancer immunotherapy and may work synergistically with PD1/PDL1 targeting drugs.

Methods Target cell analysis: human peripheral blood mononuclear cells (PBMC) containing lymphocyte and myeloid lineage cells were used for target cell analysis with flow cytometry.

T cell activity (ex vivo): human PBMC was employed for the evaluation of T cell activity (IRB #1041107-201703-BR-002-02) and CD3+ T cells and CD14+ monocytes were isolated from human PBMC and co-cultured in a 2.5 mg/ml anti-CD3 antibody-coated plate for 6 days in the presence of PMC-309 or other drugs. Culture media were harvested and evaluated the secreted levels of IFN-γ by ELISA.

MDSC suppression activity (ex vivo): MDSCs cells (5x10^4 cells) were cultured with CD3+ T cells (2x10^5 cells) in the presence of anti-CD3/28 bead with 10, 30, 100 μg/ml of PMC-309, or 100 μg/ml of anti-PD1, anti-PDL1, and anti-CTLA4 and for 3 days. IFN-g production was measured by ELISA.

In vivo study: MC38 bearing human VISTA knock-in (KI) mice were employed for the assessment of anti-tumor activity of PMC-309.

The tumor infiltrated immune cells: Immune cells in the TME were evaluated by immunohistochemistry (IHC) or flow cytometry (FACS) analysis.

Results PMC-309 binding to VISTA expressing cells is highly selective and the selectivity is maintained even in the low pH conditions that mimic TME. PMC-309 enhances the secretion of IFN-gamma, TNF-alpha, and IL-2 in T cell and monocyte co-culture settings. In addition, PMC-309 promoted monocyte differentiation into M1 macrophage that stimulates proinflammatory cytokine secretion of T cells. For the in vivo study, PMC-309 was intravenously administrated in VISTA-KI mice. The tumor growth rate was suppressed accompanied by a synergistic effect with an anti-PD1 antibody. The anti-tumor activity was associated with enhanced T cell activation, increased secretion of pro-inflammatory cytokines, and increased penetration of cytotoxic T cells, but lowing immune-suppressive MDSC cells into TME as demonstrated with IHC analysis.

Conclusions PMC-309 increased the number of T cell infiltration while a decrease of MDSCs in the TME. PMC-309 in combination with chemotherapy or other IO drugs could address highly medical unmet needs from patients with drug resistance to currently available IO treatment options.

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