Background: VISTA (V-domain Ig suppressor of T cell Activation) is a transmembrane B7 family protein that is described as a negative checkpoint of T cell response. K01401-020 is a novel anti-VISTA antibody entering clinical trials (Loukili et al, AACR 2019). Unlike PD1 protein for which PDL1 and PDL2 are clearly demonstrated as its main ligands, several VISTA candidate receptors have been suggested with various degrees of evidence. The prominent candidate receptors published for VISTA are VSIG3/IGSF11 (Wang et al, Immunology, 2018; Mehta et al, Cell Rep, 2019), VSIG8 (WO2016090347), LRIG1 (WO2015187395), and VISTA itself (WO2015179799). PSGL1 recently joined the list based on in vitro biochemical experiments describing VISTA as an acidic pH selective ligand for PSGL1 (Johnston et al, 2019).

Methods: Here we report a series of in vitro protein/protein interactions results aimed at testing the VISTA candidate receptors above. mRNA co-expression was analyzed via bioinformatics in a range of human cancer samples. The mRNA and protein of each candidate receptor were also precisely localized in tumor slices using amplified RNA-ISH or protein-IHC staining.

Results: We found specific physicochemical conditions, in vitro, for each candidate, to show specific protein/protein interactions with VISTA, mostly occurring at acidic pH and being effectively disrupted by K01401-020 anti-VISTA antibody. In addition, K01401-020 was also confirmed to bind to VISTA whatever the pH conditions tested [6 to 7.4].

Despite the potential for biochemical interaction for all candidates, only VISTA and PSGL1 mRNAs were found strongly correlated across most cancer indications with PSGL1 being one of the best correlated genes to VISTA when compared to the whole transcriptome. In addition, when compared to the other proteins explored (VSIG3, LRIG1 and VSIG8), PSGL1 expression showed much greater correlation of expression with a myeloid-infiltrate identified by a transcriptomic signature. Dual amplified RNA-ISH on human tumors showed co-occurrence for VISTA and PSGL1, and only rare co-occurrence for VISTA and the other receptors.

At the protein level, using dual IHC and proximity ligation staining, the proximity of VISTA and PSGL1 was confirmed in tumors, with metrics comparable to PD1 and PDL1 proximity.

Potential interactions may still happen between circulating cells outside of tumors.

Conclusions: VISTA interactions with other proteins (PSGL1, VISTA, VISG3, VSIG8 and LRIG1) were evidenced in vitro. However, based on RNA correlations and protein spatial distribution in a series of patient tumors, VISTA/PSGL1 may be a biologically relevant interaction in cancer. The likelihood that interactions with VISG3, VSIG8 and LRIG1play a meaningful biological role in tumors is relatively lower.

Other potential candidate receptors to VISTA, including VISTA itself, may still need to be revealed.

Citation Format: Francisco Cruzalegui, Noureddine Loukili, Olivier Delfour, Nicolas Boute, Céline Thuilliez, Thierry Champion, Martine Malissard, Pierre Launay, Eric Chetaille, Alexandre Passioukov, Nathalie Corvaïa, Pierre Ferré. VISTA interaction with PSGL1, a likely VISTA receptor in tumors, is effectively disrupted by K0401-020 anti-VISTA antibody [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3372.