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. Here, ENTPD1 is linked to head and neck squamous cell carcinoma.