1142 mRNA LNP vaccination with the tumor-specific antigen EGFRvIII bolsters CAR T cell activity in vivo

Background CAR T cell persistence is the only known clinical correlate of activity. Often in solid tumors, CAR T cells fail to persist in patients shortly after infusion. Modulation and maintenance of CAR T cell in patients is crucial for this type of therapy to work. We have completed two CAR T cell clinical trials targeting the tumor specific antigen EGFR variant III (EGFRvIII) in glioblastoma (GBM). Clinical trial data indicated that CAR T cell persistent suffered shortly after infusion. One mechanism to maintain and modulate CAR T cell levels is through mRNA vaccination with the CAR T cell antigen. Methods We designed a lipid nanoparticle (LNP) carrying the mRNA for the EGFRvIII antigen to boost CAR T cell levels in vivo. We used a combination of in vitro and in vivo assays to test the ability of a truncated EGFRvIII or the corresponding negative control truncated CD19 to activate and boost CAR T cells. Two CARs directed to EGFR were used to evaluate the efficacy of boosting CAR T cell via mRNA LNP vaccination. Results In vitro gene transfer assays were used to determine the ability of mRNA LNPs to transiently transfect target cells. The truncated EGFRvIII and truncated CD19 expressed in transfected cells. In coculture, CAR T cells were able to recognize and be activated by target cells transfected with their cognate antigen. Cytokine production in CAR T cell demonstrated antigen specific activation in vitro. We next evaluated the ability of mRNA LNPs to boost CAR T cells in a murine tumor model. Tumor bearing mice were treated with sub-therapeutic doses of CAR T cells and subsequently boosted twice using mRNA vaccines to EGFRvIII or CD19. Only mice boosted with the EGFRvIII saw a delay in tumor growth and a conferred survival advantage. Conclusions Taken together, these data demonstrate that mRNA LNP vaccination with CAR T cell antigens can be an effect way to bolster CAR T cells activity in tumor bearing mice. The antigen specific activation of CAR T cell both in vivo and in vitro highlight the utility of this approach in modulating CAR T cell activity. Furthermore, this technology could be used to rescue poor engraftment of CAR T cell in patients.