PDCD1 and head and neck squamous cell carcinoma: Li et al. (2015) analyzed tumor-infiltrating and peripheral blood lymphocytes in head and neck squamous cell carcinoma patients and concluded on the inhibitory effect of SHP2-mediated PD-1 on tumor Th1 cell immunity and that the PD-1 or SHP2 blockade was sufficient to restore Th1 immune activity and to activate T cells, thus reversing immunosuppression in tumor microenvironment.