The overall goal of our laboratory is to enhance the number of patients that respond to checkpoint inhibitor immunotherapy. The recently published Keynote-012 trial demonstrates a 20% response rate to programmed death receptor-1 (PD-1) checkpoint blocking antibodies. These responses tend to be long-lasting, consistent with the activation of anti-tumor immunity.
We aim to enhance these responses through combination treatment, and explore different combinations of checkpoint blocking antibodies with standard-of-care, targeted and other immunotherapies. One of the major barriers to effective immunotherapy seems to be the presence of immunosuppressive immune cells alongside the anti-tumor immune cells within the tumor. In head and neck cancer, immature myeloid cells and regulatory T-cells are potent suppressors of anti-tumor immunity. Previously our laboratory had fully characterized different models of oral cavity cancer that represent head and neck cancer that have evidence of an activated anti-tumor immune response (T-cell inflamed tumors) and those that do not (non-T-cell inflamed tumors). These models allow our team to explore treatments designed to enhance an existing anti-tumor immune response, as well as treatment designed to activate an anti-tumor immune response that does not already exist. Using laboratory techniques to measure the amount of immune cells infiltrating a tumor, we have shown that granulocytic myeloid cells (gMDSC), but not regulatory T-cells, massively infiltrate tumors with tumor progression in our syngeneic models. This gMDSC infiltration is inversely related with the infiltration of anti-tumor immune cells, such as CD8 T-cells, natural killer cells, mature macrophages and dendritic cells. We previously demonstrated that within both types of tumors, the presence of immunosuppressive tumor infiltrating myeloid cells was not effectively altered by using drugs that block the oncogenic signaling pathways within the tumor cells. This led us to consider other approaches to eliminating gMDSC from tumors. Using techniques to selectively eliminate these gMDSC at different times, we have demonstrated that eliminating gMDSCs from T-cell inflamed tumors completely reverses suppression of T-cell function that normally occurs with tumor progression. This was not true in non-T-cell inflamed tumors. This led us to hypothesize that functional inhibition or elimination of gMDSC could make T-cell inflamed tumors respond to checkpoint inhibition better. We first used techniques only possible in mice to eliminate gMDSC, and found that we could consistency induce complete rejection of T-cell inflamed tumors in mice with combination gMDSC elimination and checkpoint inhibition. As we explored how to make this feasible in patients with head and neck cancer, we used a small molecule inhibitor that selectively inhibits subunits of the signaling protein PI3K that are primarily expressed in immune cells. We found that this indeed blocked suppressive capacity of gMDSC; however, this same treatment also blocked T-cell function to a degree, likely limiting the efficacy of this treatment approach (work submitted for publication). We are now exploring other means of inhibiting gMDSC infiltration or function in ways that are possible in both mice and patients with head and neck cancer. One other important project in our laboratory this year has been the use of combination immunotherapies. The STING receptor is an innate immune receptor responsible in part for activating the first immune cells involved in the development of an anti-tumor immune response. Synthetic molecules that activate STING were used to induce a robust anti-tumor immune response in T-cell inflamed tumors. We found that upregulation of immune checkpoints within tumors was one mechanisms of incomplete responses to STING activation, and showed that combination STING activation and checkpoint inhibition induced complete regression of nearly all tumors treated. STING-based treatment are now moving into clinical trials for patients with advanced head and neck cancer. We are excited to continue our work investigating different combinations of standard-of-care, targeted and immune treatments with checkpoint inhibition, with the ultimate goal of moving each promising combination into early phase clinical trials at the NIH and other academic medical centers.
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