Agonist anti-GITR antibody significantly enhances the therapeutic efficacy of Listeria monocytogenes-based immunotherapy

Background

We previously demonstrated that in addition to generating an antigen-specific immune response, Listeria monocytogenes (Lm)-based immunotherapy significantly reduces the ratio of regulatory T cells (Tregs)/CD4⁺ and myeloid-derived suppressor cells (MDSCs) in the tumor microenvironment. Since Lm-based immunotherapy is able to inhibit the immune suppressive environment, we hypothesized that combining this treatment with agonist antibody to a co-stimulatory receptor that would further boost the effector arm of immunity will result in significant improvement of anti-tumor efficacy of treatment.

Methods

Here we tested the immune and therapeutic efficacy of Listeria-based immunotherapy combination with agonist antibody to glucocorticoid-induced tumor necrosis factor receptor-related protein (GITR) in TC-1 mouse tumor model. We evaluated the potency of combination on tumor growth and survival of treated animals and profiled tumor microenvironment for effector and suppressor cell populations.

Results

We demonstrate that combination of Listeria-based immunotherapy with agonist antibody to GITR synergizes to improve immune and therapeutic efficacy of treatment in a mouse tumor model. We show that this combinational treatment leads to significant inhibition of tumor-growth, prolongs survival and leads to complete regression of established tumors in 60% of treated animals. We determined that this therapeutic benefit of combinational treatment is due to a significant increase in tumor infiltrating effector CD4⁺ and CD8⁺ T cells along with a decrease of inhibitory cells.

Conclusion

To our knowledge, this is the first study that exploits Lm-based immunotherapy combined with agonist anti-GITR antibody as a potent treatment strategy that simultaneously targets both the effector and suppressor arms of the immune system, leading to significantly improved anti-tumor efficacy. We believe that our findings depicted in this manuscript provide a promising and translatable strategy that can enhance the overall efficacy of cancer immunotherapy.

Listeria monocytogenes mutants defective in gallbladder replication represent safety-enhanced vaccine delivery platforms

The Gram positive intracellular pathogen Listeria monocytogenes represents a promising vaccine or therapeutic DNA delivery vector that has been successfully administered to humans in clinical trials. However in generating Listeria mutants with therapeutic potential it is important to balance safety attenuation with efficacy. Here we show that L. monocytogenes mutants with a reduced capacity for murine gallbladder replication are capable of stimulating T cell responses in mice and protecting vaccinated animals from secondary challenge. Mutation of L. monocytogenes genes lmo2566 or lmo0598 resulted in significant attenuation in the murine model yet mutants retained a capacity for intracellular growth and stimulation of T cell responses against key Listeria epitopes (LLO_91-99 and P60_217-225). Importantly the mutants showed a reduced capacity for growth in the gallbladders of vaccinated mice as well as significantly reduced faecal shedding indicating that this approach generates live Listeria-based vector delivery systems with a reduced capacity for the spread of live genetically modified microorganisms into the natural environment.