PDCD1 and head and neck squamous cell carcinoma: While Dowell and colleagues propose a mechanism by which PD-L2 is cleaved from the cell surface in MIBCs, Yearley and colleagues found that the clinical response to pembrolizumab in patients with head and neck squamous cell carcinoma (HNSCC) with recurrent or metastatic disease may be related partly to the blockade of PD-1/PD-L2 interactions [38].