Identification and Characterisation of The Immune Response to Salmonella-mediated Cancer Therapy and Its Use in Developing Combination Cancer Therapy

Colorectal cancers (CRC) are the third most commonly diagnosed and second most deadly cancers worldwide. Conventional treatment options are associated with resistance, off-target effects and toxicity, while current targeted therapies are difficult to deliver and only affect a fraction of CRC patients, thus highlighting the need for the development of novel remedies. Disturbing the functionality of genes involved in cancer development and bacterial cancer therapy are two emerging methods for targeted tumour treatment. Previous work conducted by our group, combining these two methods, demonstrated successful Salmonella-mediated delivery of RNA interference for the oncogene c-MYC to tumour cells in vitro and in vivo. The objectives of this project were to develop Salmonella typhimurium SL7207 strains targeting the immune checkpoint PD-L1, overexpressed on cancer cells thus inhibiting immune cell function, or targeting both c-MYC and PD-L1 genes to simultaneously enhance tumour cell death and the anti-tumour immune response. Additionally, this project aimed to address the key differences in the immune response resulting from Salmonella therapy alone and in combination with knockdown of the two target genes. Following successful generation of the strains, in vitro cell analysis confirmed that both c-MYC and PD-L1 can be silenced on a gene and at a protein level. CRC cell Salmonella treatment and silencing of the targets were also found to have a positive effect on natural killer and cytotoxic T cell activity, where the silencing strains promoted tumour cell death and further increased the cytolytic functions of these immune subsets. Changes in the systemic immune response to Salmonella therapy and in the immune subsets present in the intestines and the tumour microenvironment, in response to the bacteria and c-MYC silencing, were also recorded in two CRC mouse models. These findings could aid the progression of developing a novel, more efficient treatment option for CRC patients.