CD73 expression in cancer-associated fibroblasts exacerbates immune suppression and promotes tumor progression via augmenting adenosine accumulation in the tumor microenvironment

Abstract CD73, an ecto-5′-nucleotidase, is the major extracellular adenosine-generating enzyme that converts AMP to immunosuppressive adenosine (ADO). Previous studies demonstrated that CD73-mediated ADO production in the tumor microenvironment (TME) is an immune evasion mechanism explored by tumors, myeloid-derived suppressor cell (MDSCs), and regulatory T cells. Targeted suppression of CD73, either via CD73 blocking antibody or inhibitors, suppresses tumor progression and metastasis. Given the complexity of the TME and crucial involvement of cancer-associated fibroblasts (CAFs) in immunosuppression and tumor progression, we explored the contribution of CD73 in CAFs to tumor progression. Using a CD73-negatvie EL4 tumor in C57BL/6 WT and CD73KO mice, we observed a high level of CD73 expression in CAFs, associated with their higher capacity of converting extracellular AMP to ADO than that of lymphocytes and myeloid cells in culture. Interestingly, as tumor progresses, CAFs in the TME of WT mice expended extensively to form an interconnected network and the percentage of CD73+ CAFs increases, while that of tumor infiltrating lymphocytes, especially CD8 T cells, decreases. However, in CD73KO mice, EL4 tumor progression was greatly delayed, associated with their significantly higher level of tumor-infiltrating CD8+ cytotoxic T cells. Functional analysis of fibroblastic stromal cells confirmed that CD73 inactivation greatly diminished their ability to generate extracellular ADO and subsequent suppression of T cell activation. Our results suggest that CD73 also represents one of the important mechanisms of CAF-mediated immunosuppression. We will discuss the potential environmental factors that enforce CD73 expression in CAFs.