CD28 and head and neck squamous cell carcinoma: Next, to confirm that Stat4−/− T cells from HNSCC tumor bearing mice were deficient in their ability to produce IFN-γ, we re-stimulated splenocytes from tumor bearing WT and Stat4−/− mice with anti-CD3 and anti-CD28 antibodies, then determined IFN-γ production by ELISA.