CD28 and skin squamous cell carcinoma: The significant upregulation of PD-1, PD-L1, and CD28, as well as the calculated ratio of increased PD-L1 inhibitory ligand to T-cell activating ligand CD86, in cSCC as compared to BCC, provides a possible explanation for the better objective response rates to anti-PD-1 therapy in cSCC as compared to BCC.