Blocking Immunosuppressive Factors and Taking Advantage of the Nutrient Supply Within the Tumor Microenvironment: Pathways to Achieve Improved Cancer Immunotherapeutic Efficacy for Patients With Metastatic Melanoma
The incidence of melanoma is increasing. Immunotherapy commonly fails due to the immunosuppressive tumor microenvironment (TME). The aim of my dissertation is to develop strategies that dampen the TME’s immunosuppressive capacity and improve the antitumor performance of vaccine-induced melanoma-associated antigen (MAA)-specific CD8+T cells. I pursued this goal using three approaches. First, interactions between co-inhibitors on CD8+ tumor-infiltrating lymphocytes (TILs) with immunoinhibitory ligands within the TME impair T cells’ effector functions. I assessed whether blocking immunoinhibitory signaling during T cell priming augments their antitumor activity. I designed a melanoma vaccine expressing MAAs within herpes simplex virus (HSV) glycoprotein D (gD), which blocks the inhibitory BTLA/CD160-HVEM pathway. Compared to a non-gD vaccine, the gD-adjuvanted vaccine enhances CD8+T cells to low avidity epitopes and prolongs survival of tumor-bearing mice. gD renders MAA-specific CD8+TILs more resistant to functional impairment within TME, which increases their ability to limit tumor progression. Second, the stroma of solid tumors is crucial for tumorigenesis and suppresses the CD8+TILs’ effector functions. To determine whether destroying tumor stroma could improve the MAA-specific CD8+T cells’ tumoricidal capacity, I designed a vaccine targeting the fibroblast activation protein (FAP), which is expressed at high levels on tumor-stromal fibroblasts. Combining the vaccines to FAP and MAAs significantly improves the survival of tumor-bearing mice. This is caused by destruction of FAP+ cells, which reduces frequencies and inhibitory functions of immunosuppressive cells. It also decreases the MAA-specific CD8+TILs’ metabolic stress and delays their progression towards functional exhaustion. Finally, the TME commonly lacks nutrients and oxygen needed for the CD8+TILs’ energy production. My data demonstrate that these metabolic challenges profoundly contribute to the CD8+TILs' functional impairment. Using 13C-stable isotope tracing in vivo, I show that metabolically stressed CD8+TILs in late stage tumors increasingly depend on fatty acids (FAs) catabolism for energy production. Promoting FA catabolism by CD8+TILs improves their effector functions and capacity to delay tumor growth. Overall, my studies show that blocking inhibitory factors while taking advantage of the available nutrients within the TME could improve the performance of vaccine- or adoptive transfer-induced CD8+TILs. These strategies provide new avenues for cancer immunotherapy that may benefit cancer patients.
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