Boosting of CAR-T cells with rhabdovirus is limited by type I interferon and rapid contraction

Rhabdovirus vaccines that encode tumour-associated antigens are potent boosting agents for adoptively transferred tumour-specific T cells. Employing rhabdovirus vaccines to boost adoptively transferred T cells relies on a priori knowledge of tumour epitopes, isolation of matched epitope-specific T cells, and a personalized vaccine, which limit clinical feasibility. Here, we investigated a universal strategy for boosting transferred tumour-specific T cells where boosting is provided through a chimeric antigen receptor (CAR) that is paired with a vesicular stomatitis virus (VSV) vaccine encoding the CAR-target. Boosting CAR-engineered tumour-specific T cells with paired VSV vaccines was associated with robust T cell expansion and delayed tumour progression in syngeneic models. CAR-T cell expansion and anti-tumour function was enhanced by blocking IFNAR1. However, vaccine-boosted CAR-T cells rapidly contracted and antigen-positive tumours re-emerged. In contrast, when the same T cells were boosted with VSV encoding antigen that stimulates through the TCR, the adoptively transferred T cells displayed improved persistence, tumour-specific endogenous cells expanded in parallel, and tumour cells carrying the antigen target were completely eradicated. Our findings underscore the need for further research into CAR-mediated boosting, how this differs mechanistically from TCR-mediated boosting, and the importance of engaging endogenous tumour-reactive T cells to achieve long-term tumour control.