Abstract B012: Understanding the role of VISTA in regulating T cell function and the therapeutic potential of blocking VISTA in triple negative breast cancer
Authors: Abudula Maidinaimu , Astuti Yuliana , Raymant Meirion Raymant , Sharma Vijay , Schmid Michael , Mielgo Ainhoa 
Published: November 2024

Abstract
Triple-negative breast cancer (TNBC) is the most aggressive and highly metastatic breast cancer subtype with limited effective treatment options. Immunotherapy with immune checkpoint inhibitors (ICIs) targeting checkpoint molecules have been widely used and have shown therapeutic efficacy in treating multiple malignancies in the last decade. In TNBC, ICI targeting PD-1, alone or in combination with chemotherapy has been approved for the treatment of patients with both early and advanced stage of the disease. Although, there has been significant clinical benefit achieved from anti-PD-1 and chemotherapy, treatment efficacy is often limited to a subgroup of patients. The limited response to ICIs has been attributed to the increased infiltration of immunosuppressive cells (including MDSCs, neutrophils and tumor-associated macrophages), more importantly, the upregulation of alternative immune checkpoint receptors/ligands that may further suppress cytotoxic T cells (CD8+ T cells) in the tumor microenvironment (TME). Therefore, it has been suggested that targeting other immune checkpoint molecules may be more effective in bolstering anti-tumor immune response. V- domain Ig suppressor of T-cell activation (VISTA) is one of the immune checkpoint molecules attracting increasing attention in the context of anti-tumor immunity due to its broad expression across various cancer types and increased expression upon development of immune checkpoint resistance. However, the role of VISTA in regulating the anti-tumor immune response and its therapeutic potential in TNBC remains unclear. In our study, we observed that VISTA is expressed in TNBC patients, predominantly in neutrophils and macrophages. In the Py230 murine breast cancer model, VISTA expression increased upon paclitaxel (PTX), anti-PD-L1 and paclitaxel combined with anti-PD-L1 treatment compared to the control group, suggesting its potential role as a resistance mechanism in sustaining or further inhibiting T cells. In the Py230 model, VISTA blockade increased the infiltration and activation of CD8+ T cells, as evidenced by the higher number of CD8+ T cells, increased CD69+CD8+ T cells, and Granzyme B production, indicating that VISTA blockade improved CD8+ T cells mediated anti-tumor immunity. To functionally test the role of VISTA, we stimulated CD8+ T cells and co-cultured them with naïve macrophages or tumor educated macrophages in the presence or absence of VISTA blocking antibody. Tumor educated macrophages showed increased immunosuppressive activity which was abrogated with VISTA blockade. Moreover, we found that the immunosuppressive effect of macrophages on T cell activation needs direct cell-cell interaction between VISTA on macrophages and T cells. In conclusion, this study shows the mechanism of action of VISTA in the regulation of T cells mediated anti-tumor immune response and provides the rationale for targeting VISTA in TNBC.
Citation Format: Maidinaimu Abudula, Yuliana Astuti, Meirion Raymant Raymant, Vijay Sharma, Michael Schmid, Ainhoa Mielgo. Understanding the role of VISTA in regulating T cell function and the therapeutic potential of blocking VISTA in triple negative breast cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Tumor-body Interactions: The Roles of Micro- and Macroenvironment in Cancer; 2024 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2024;84(22_Suppl):Abstract nr B012.