Differential Contribution of Three Immune Checkpoint (VISTA, CTLA-4, PD-1) Pathways to Antitumor Responses Against Squamous Cell Carcinoma

V domain-containing Ig suppressor of T-cell activation (VISTA)/PD-1H is a novel immune checkpoint molecule for regulating T-cell activation. We examined the effects of anti-VISTA mAb monotherapy and combination therapy with CTLA-4 or PD-1 blockade in a squamous cell carcinoma (SCCVII) model. VISTA monotherapy did not show clear tumor growth regression, but efficiently induced CD8(+) T cell activation by converting resting and exhausted cells into functional effector cells. VISTA monotherapy did not inhibit recruitment of regulatory T cells (Tregs) in the tumor microenvironment (TME). As an additional treatment to VISTA, CTLA-4 blockade, but not PD-1 blockade, elicited further tumor regression. The CTLA-4 and VISTA combination efficiently inhibited Treg recruitment and increased the ratios of both CD8 T/Treg and CD4 conventional T (Tcon)/Treg in the TME, whereas the PD-1 and VISTA combination dramatically increased tumor-recruiting CD8(+) T cells, but markedly reduced the Tcon/Treg ratio. Our results demonstrate that VISTA blockade efficiently converts CD8(+) T cells into functional effector T cells, but is not sufficient to regress tumor growth due to weak Treg suppression in the TME. Our results suggest that combined CTLA-4 and VISTA blockade is more efficacious than combined PD-1 and VISTA blockade for tumors like head and neck squamous cell carcinoma in which Treg-mediated immune regulation is dominant.