A novel tritherapy overcoming the immune suppressive tumor microenvironment elicits robust antitumor activity

Immune checkpoint inhibitors (ICIs), such as those that engage PD-1 on cytotoxic CD8 + T cells, can improve survival outcomes across multiple solid cancer types. However, in several cancers, such as triple-negative breast cancer (TNBC), the use of these agents is effective in limited subsets of patients. It is thought that this limitation is tied to numerous barriers which inhibit their efficacy, including the immune suppressive tumor microenvironment (TME) and a low infiltration of antigen (Ag)-specific CD8 + T cells. Myeloid-derived suppressor cells (MDSCs) constitute a prominent immune suppressive component of the TME and are produced in response to tumor-derived factors. MDSCs consist of immature myeloid subpopulations that inhibit the proliferation or effector functions of cytotoxic CD8 + T cells. To overcome this obstacle of immune suppression, our laboratory has developed a novel approach to target MDSC ‘biogenesis' in the bone marrow to mitigate their production and bolster novel immunotherapies. We identified a metabolic susceptibility in MDSCs, which mitigates MDSC function using agents known as dihydroorotate dehydrogenase (DHODH) inhibitors. DHODH inhibitors block de novo pyrimidine metabolism and are being used as an anti-AML therapy to promote the maturation of myeloid progenitors common to both leukemic cells and MDSCs. This anti-MDSC approach boosted ICI activity, which significantly diminished tumor growth and metastasis; however, tumors persisted. To achieve more effective tumor response, we turned to the concept of epitope-specific immunization (ESI). We hypothesized that not only does DHODH blockade ‘reprogram' MDSCs, but a peptide-based ESI approach targeting an endogenous tumor Ag in combination with our anti-PD-1/anti-MDSC regimen would result in greater tumor reduction by activating and expanding low frequencies of Ag-specific CD8 + T cells. We optimized a tri-therapy regimen consisting of our anti-PD-1/anti-MDSC regimen with an ESI to boost intra-tumoral T cells. We showed that such a novel tri-therapy regimen caused significant antitumor responses, substantially more so compared with the single- and double-agent controls, which correlated with an increase in intra-tumoral Ag-specific CD8 + T cells. To further demonstrate that this tri-therapy enhanced T cell function, we integrated an ‘add-back' approach. We isolated CD8 + T cells from tri-therapy-treated mice and showed following adoptive transfer that they significantly reduced tumor growth compared to CD8 + T cells derived from vehicle or the dual-agent combination controls. Altogether, our results suggest that a novel multi-modal strategy that concurrently reduces MDSCs, overcomes T cell exhaustion, and expands tumor-reactive CD8 + T cells, has important therapeutic implications to improve outcomes against ICI-refractory tumors.