Transductional Targeting of oncolytic adenoviruses by adapter molecules improves therapy of pSia-expressing tumors by triggering adaptive immune responses

Polysialic acid (polySia) represents an excellent tumor-selective target for oncolytic adenovirus-based therapies since it is selectively expressed on several malignant tumors including small cell lung cancer (SCLC). In this study, we developed a strategy to retarget oncolytic Adenoviruses (OAV) to polySia-expressing tumors using a recombinant bispecific adapter protein. To generate the adapter molecule CARsc-pSia, the soluble CAR ectodomain was fused to a polySia-binding scFv via a fibritin-derived trimerization motif. By infection experiments in cells of defined polySia-status using CARsc-pSia-coated Ad5 particles, we could show viral cell entry in a highly polySia-selective manner. Furthermore, only CARsc-pSia treatment allowed efficient adenoviral infection of normally refractory polySia-expressing human tumor cells. After systemic administration of CARsc-pSia-complexed adenoviruses we could show improved adenoviral infection of s.c. grown polySia-expressing human tumor xenografts. Masking of the fiberknob by the molecular adapter led to significant reduction of the natural tropism since we could observe a significantly reduced adenoviral liver load and hepatotoxicity in mice. To study the involvement of antitumoral immune responses, therapeutic investigations were therefore performed in an orthotopic SCLC model in both immunocompetent C57/Bl6 and immunodeficient nude mice. In these models, only CARsc-polySia-complexing of viruses allowed for significant transduction. Interestingly, improved transduction led to improved local tumor regression and prolonged survival in immunocompetent mice but not in T-cell deficient nude mice. This observation revealed that adaptive T-cell responses play a crucial role in the clearance of infected lung colonies rather than replication-associated oncolysis. In conclusion, our results show that polySia-specific retargeting of OAV represents a promising hepatotoxicity-reduced viroimmunotherapeutic strategy of clinically relevant pSia-expressing tumor species.