CD274 and head and neck squamous cell carcinoma: To determine whether this mechanism may be preventing recognition of MHC-II+ tumor cells by CD4+ T cells, we cultured CD4+ T cells expressing our KRASG12V-specific TCR or the E6-specific F12 TCR with NCI-H2444 CIITA and HNSCC-56 CIITA, respectively, with or without anti–PD-L1–blocking Abs.