Deficiency of host CD96 and PD-1 or TIGIT enhances tumor immunity without significantly compromising immune homeostasis.

Multiple non-redundant immunosuppressive pathways co-exist in the tumor microenvironment and their co-targeting can increase clinical responses. Indeed, concurrent blockade of CTLA-4 and PD-1 in patients with advanced melanoma increased clinical responses over monotherapy alone although the frequency and severity of immune related adverse events (irAEs) also increased. Nevertheless, a substantial number of patients still display an innate resistance phenotype and are unresponsive to current approved immunotherapies even when utilized in combination. In this study, we generated Pdcd1 ^{- / -} CD96 ^{- / -} and Tigit ^{- / -} CD96 ^{- /-} mice to investigate how loss of CD96 in combination with PD-1 or TIGIT impacts on immune homeostasis and hence the potential of inducing immune related toxicities following co-targeting of these pairs of receptors. The ability of Pdcd1 ^{- / -} CD96 ^{- / -} and Tigit ^{- / -} CD96 ^{- /-} mice to suppress primary tumor growth was also assessed using the MC38 colon carcinoma and SM1WT1 BRAF-mutated melanoma tumor models. Both Pdcd1 ^{- / -} CD96 ^{- / -} or Tigit ^{- / -} CD96 ^{- /-} mice displayed no overt perturbations in immune homeostasis over what was previously reported with Pdcd1 ^{- / -} or Tigit ^{- / -} mice even when aged for 22 months. Interestingly, increased suppression of subcutaneous tumor growth and complete responses was seen in Pdcd1 ^{- / -} CD96 ^{- / -} mice compared to Pdcd1 ^{- / -} or CD96 ^{- / -} mice depending upon the tumor model. In contrast, in these models, growth suppression in Tigit ^{- / -} CD96 ^{- / -} were similar to Tigit ^{- / -} or CD96 ^{- / -} . This enhanced anti-tumor efficacy of Pdcd1 ^{- / -} CD96 ^{- / -} appeared to be due to favorable changes in the ratio of CD8 ⁺ T cells to T regulatory cells or CD11b ⁺ GR-1 ^hi myeloid cells in the tumor microenvironment. Co-targeting CD96 and PD-1 may increase anti-tumor immunity over targeting PD-1 alone and potentially not induce serious immune-related toxicities and thus appears a promising strategy for clinical development.