Live Attenuated Oral Salmonella Platform for Effective T- and B-cell Targeting of PD-L1.

74 Background: Significant progresses have been recently achieved in cancer vaccines, yet novel immunization solutions to deliver efficiently tumor-associated antigens to professional antigen-presenting cells, and to overcome the peripheral tolerance and the immunosuppressive tumor microenvironment that prevent the eradication of cancer in most of patients, are urgently needed. VAXIMM is developing a unique and versatile oral T-cell vaccination platform based on the FDA-approved live-attenuated Salmonella Typhi strain Ty21a vaccine Vivotif, capable of delivering tumor-associated antigens encoded in DNA expression construct to the gut-associated lymphoid tissue, breaking immune tolerance and inducing effective anti-tumor immunity. Methods: This study summarizes the immunogenicity and antileukemia efficacy of VXM10 vaccines based on the live-attenuated Salmonella Typhimurium strain SL7207, transformed with a eukaryotic expression plasmid encoding different variants of the murine PD-L1 protein. Results: The antileukemia activity of VXM10 was evaluated in the FBL-3 disseminated model of leukemia, in which the tumor cells strongly express PD-L1. Multiple oral administrations of VXM10 vaccines produced a strong anti-tumor effect, with 100% of surviving animals 80 days after challenge with FBL3 leukemia in the highest dose groups. Moreover, 100% of long-term surviving mice resisted re-challenge with FBL-3 cells, demonstrating that vaccination with VXM10 generated a potent memory T cell response against the leukemia. Importantly, full leukemia control was achieved in both prophylactic and therapeutic settings. Upon immunization with VXM10 vaccines, T cell response was raised against PD-L1 epitopes after in vitro restimulation of the splenocytes, and anti-PD-L1 antibodies were detected in the serum. The precise mechanism of action of VXM10 vaccines is currently being investigated. Conclusions: This study provides further evidence that VAXIMM’s oral T-cell vaccination platform can be employed to stimulate anti-tumor immunity against antigens of the immune checkpoint regulatory protein PD-L1. These data paved the way for advancing the development of VXM10 into clinical development.