CD44+ Cells in Head and Neck Squamous Cell Carcinoma Suppress T-Cell-Mediated Immunity by Selective Constitutive and Inducible Expression of PD-L1.

Human tumors consist of heterogeneous populations of cells with distinct marker expression and functional properties. In squamous cell carcinoma of the head and neck (SCCHN), CD44 is a well-characterized marker of a resilient subpopulation of cells associated with increased tumorigenesis, radioresistance, and chemoresistance. Evidence indicates that these cells have an immunosuppressive phenotype; however, mechanisms have been elusive. Using primary human SCCHN tumor samples and patient-derived xenografts, we examined the phenotypes of subsets of tumor cells and investigated mechanisms regulating their immunogenicity. CD44(+) cells in primary human SCCHN were found to have an epithelial-to-mesenchymal (EMT) phenotype and were less immunogenic than CD44(-) cells when cultured with autologous CD8(+) tumor-infiltrating T cells. Selective expression of the programmed death-ligand 1 (PD-L1) was observed on CD44(+) cells compared with CD44(-) cells and was associated with constitutive phosphorylation of STAT3 on CD44(+) cells. Importantly, inhibition of STAT3 decreased expression of PD-L1 on CD44(+) cells. IFNγ treatment preferentially induced even further PD-L1 expression on CD44(+) cells and was associated with enhanced IFNγ receptor expression and phosphorylation of STAT1. Finally, the decreased immunogenicity of CD44(+) cells was partially reversed by antibody blockade of the programmed death 1 (PD-1) receptor, indicating that the differences in PD-L1 expression between CD44(+) and CD44(-) cells are biologically and clinically relevant. Our findings provide a mechanism by which long-lived CD44(+) tumor-initiating cells can selectively evade host immune responses and provide rationale for targeting the PD-1 pathway in the adjuvant therapy setting of SCCHN. Clin Cancer Res; 22(14); 3571-81. ©2016 AACR.