Intertumoral Differences Dictate the Outcome of TGF-beta Blockade on the Efficacy of Viro-Immunotherapy

The absence of T cells in the tumor microenvironment of solid tumors is a major barrier to cancer immunotherapy efficacy. Oncolytic viruses, including reovirus type 3 Dearing (Reo), can recruit CD8(+) T cells to the tumor and thereby enhance the efficacy of immunotherapeutic strategies that depend on high T-cell density, such as CD3-bispecific antibody (bsAb) therapy. TGF-beta signaling might represent another barrier to effective Reo&CD3-bsAb therapy due to its immunoinhibitory characteristics. Here, we investigated the effect of TGF-beta blockade on the antitumor efficacy of Reo&CD3-bsAb therapy in the preclinical pancreatic KPC3 and colonMC38 tumor models, where TGF-beta signaling is active. TGF-beta blockade impaired tumor growth in both KPC3 andMC38 tumors. Furthermore, TGF-beta blockade did not affect reovirus replication in both models and significantly enhanced the Reo-induced T-cell influx in MC38 colon tumors. Reo administration decreased TGF-beta signaling in MC38 tumors but instead increased TGF-beta activity in KPC3 tumors, resulting in the accumulation of a-smooth muscle actin (aSMA(+)) fibroblasts. In KPC3 tumors, TGF-beta blockade antagonized the antitumor effect of Reo&CD3-bsAb therapy, even though T-cell influx and activity were not impaired. Moreover, genetic loss of TGF-beta signaling in CD8(+) T cells had no effect on therapeutic responses. In contrast, TGF-beta blockade significantly improved therapeutic efficacy of Reo&CD3-bsAb in mice bearing MC38 colon tumors, resulting in a 100% complete response. Further understanding of the factors that determine this inter tumor dichotomy is required before TGF-beta inhibition can be exploited as part of viroimmunotherapeutic combination strategies to improve their clinical benefit. Significance: Blockade of the pleiotropic molecule TGF-beta can both improve and impair the efficacy of viro-immunotherapy, depending on the tumor model. While TGF-beta blockade antagonized Reo&CD3-bsAb combination therapy in the KPC3 model for pancreatic cancer, it resulted in 100% complete responses in the MC38 colon model. Understanding factors underlying this contrast is required to guide therapeutic application.colonmodel