CD28 and head and neck squamous cell carcinoma: Subsequently, an in vitro suppression assay was conducted by co-culturing dLN WT or KO Treg cells isolated from the HNSCC model with naïve prelabeled CD8+ T cells in the presence of anti-CD3 and anti-CD28 monoclonal antibodies, and the results revealed that KO Treg cells exhibited inferior suppressive capacity compared with WT Treg cells (Fig. 3I).