An escherichia coli vaccine co-expressing listeriolysin-O and tumour antigen in cancer immunotherapy and the mechanisms of immune regulation

Abstract

Some recombinant bacterial strains have been shown to be very efficient in experimental therapies against cancer in rodent models. Amongst them is an E. coli that expresses the Listeriolysin-O protein (LLO) and a model tumour antigen ovalbumin (OVA). I have demonstrated the efficacy of E. coli-LLO/OVA in preventive or therapeutic models against OVA-expressing tumours. This effect is mediated by specific cytotoxic T-lymphocytes (CTL) against OVA antigen and inhibition of the suppressive function of Foxp3+ T-regulatory (Treg) cells. When applying a “real” and clinically-relevant tumour antigen, Wilms’ tumour-1 antigen (WT1), this vaccine (E. coli-LLO/WT1) is capable of inducing an anti-tumour effect. Furthermore, we have characterised the immunodominant epitope involved in E. coli-LLO/WT1- immunisation (pWT130-138, NAPYLPSCL) through screening of a peptide library of the WT1 protein by cytokine ELISpot, lymphocyte stimulation effects, MHC stability, and specific cytotoxicity. Also, the effect on Treg when applying a real tumour antigen is still preserved. Co-injection of pWT130-138 with E. coli-LLO resulted in an anti-tumour effect equivalent to that obtained with E. coli-LLO/WT1, demonstrating that the adjuvant properties of the E. coli-LLO vaccine can be exploited in conjunction with peptides. Treg are recognised as playing important roles in immunotherapy. An ideal vaccine for cancer would stimulate specific cytotoxic responses and suppress Treg function. This study showed that E. coli-LLO vaccine suppresses Treg cell function and the Treg RNA microarray analysis revealed expression differences of some cytokine/chemokine genes which could be relevant to the reversal of Treg suppression. This may have important implications for developing anti-tumour vaccine strategies in humans. Overall, this study demonstrated that an E. coli-LLO vaccine is effective in cancer immunotherapy, either co-expressed with a real tumour antigen or co-injected with a peptide. The efficacy of this vaccine was due to its ability to dampen Treg suppressive function.