IDO1 and head and neck squamous cell carcinoma: Increased expression of IDO by the HNSCC cells, or by MDSCs and dendritic cells has been shown to reduce tryptophan levels needed for T-cell growth and the production of Granzyme B, thereby enhancing immune evasion.155 Moreover, increased expression of HLA-E by HNSCC cells that are known to inhibit NK and CD8+ T cells via NKG2A represents another mode for immunosuppression.149 Ectonucleotidases like CD39 and CD73 expressed by Tregs convert ATP into immunosuppressive adenosine.