Background V-domain Ig suppressor of T cell activation (VISTA) is the immune checkpoint protein which could display both receptor and ligand properties. It is also engaged in intracellular biochemical networks, where its role remains to be identified. We have recently reported [1] that galectin-9 could act as a ligand for VISTA, when it is located on the T cell surface where it displays receptor properties. This suppresses granzyme B-dependent cytotoxic activities of T cells [1]. However, VISTA could also display T cell suppressive activity when acting as a ligand derived from cancer cells. But, its receptor and downstream biochemical activities are yet to be discovered. The aim of this work is to investigate T cell suppressive activities of VISTA as a ligand and its contribution to intracellular biochemical networks.

Materials and Methods We used human cancer and non-cancerous cell lines including LN-18 glioblastoma cells, BEAS-2B bronchial epithelial cells, THP-1 human acute myeloid leukaemia monocytes, Jurkat T cells, TALL-104 cytotoxic T cells and primary human CD3-positive T cells. Western blot analysis, ELISA set-ups, flow cytometry, on-cell Western analysis, fluorescent microscopy, quantitative RT-PCR, a wide range of biochemical assays and synchrotron radiation circular dichroism spectroscopy were used to conduct the studies.

Results In this work we identified the T-cell associated signalling receptor which recognises VISTA as a ligand. We found that VISTA downregulates PI-3-kinase/Akt [2, 3] and suppresses IL-2 production by T helpers. This effect is taking place due to VISTA-dependent activation of Src homology 2 (SH2) domain containing non-transmembrane protein tyrosine phosphatase (SHP2). In cytotoxic T cells (CTCs), VISTA-dependent impact on these signalling events leads to downregulation of BCL-XL family proteins thus preventing anti-apoptotic activities and allowing CTCs to die when they suffer from leakage of granzyme B from the granules inside them. This process of granzyme B leakage in CTCs can be induced by other components of immune evasion machinery operated by cancer cells (e. g . galectin-9 [1, 2]). We found that on the intracellular level VISTA can be involved in activation of AMP-dependent kinase (AMPK) and thus in control of mTOR activity in partnership with transforming growth factor-β-activated kinase 1 and galectin-9.

Conclusions Our results suggest that malignant tumours escape immune surveillance by operating complex biochemical machinery, where immune evasion pathways are cross-linked with each other forming complementary network. The immune checkpoint protein VISTA is a multifunctional component of cancer immune evasion machinery and T cell suppression displaying both ligand and receptor properties on the extracellular/inter-cellular levels. In addition, VISTA can be involved in intracellular signalling networks.

References

1. Yasinska IM, Meyer NH, Schlichtner S, et al. Front Immunol 2020;13:837097.

2. Schlichtner S, Yasinska IM, Lall GS, et al. J Immunother Cancer 2023;11:e005714.

3. Schlichtner S, Yasinska IM, Klenova E, et al. OncoImmunology 2023;12:2244330

References V.V. Sumbayev: None. I.M. Yasinska: None. M. Abooali: None. S. Schlichtner: None. E. Fasler-Kan: None.