Abstract
Ovarian cancer is known for its poor neoantigen expression and strong immunosuppression. Here, we utilized an attenuated non-pathogenic bacterium Listeria monocytogenes to deliver a highly immunogenic Tetanus Toxoid protein (Listeria-TT), as a neoantigen surrogate, into tumor cells through infection in a metastatic mouse ovarian cancer model (Id8p53-/-Luc). Gemcitabine (GEM) was added to reduce immune suppression. Listeria-TT+GEM treatments resulted in tumors expressing TT and reactivation of pre-existing CD4 and CD8 memory T cells to TT (generated early in life). These T cells were then attracted to the TT-expressing tumors now producing perforin and granzyme B. This correlated with a strong reduction in tumor burden, and significant improvement of the survival time compared to all control groups. Checkpoint inhibitors have little effect on ovarian cancer partly because of low neoantigen expression. Here we demonstrated that Listeria-TT+GEM+anti-PD1 was significantly more effective (efficacy and survival) than anti-PD1 or Listeria-TT+GEM alone. Of clinical interest, high doses of anti-PD1 (PD1H) (when added to Listeria-TT+GEM) were less effective than the low doses (PD1L). IHC and ELISPOT demonstrated that high doses of anti-PD1 inhibited T cell function in the TME. Using RNAseq, Differentially Expressed Genes (DEG) analysis and Genes Set Enrichment Analysis (GSEA) showed that gene expression levels and biological pathways were predominantly upregulated in the PD1H compared to the PD1L group, in correlation with low immune infiltration in tumors, more immune suppression, and more aggressive ovarian cancer. In summary, this study suggests that our approach may benefit ovarian cancer patients.