CD28 and hepatocellular carcinoma: Thus, under the same anti-CD3 and anti-CD28 stimulation system, three subpopulations of CD8+ T cells were sorted by Aria II (BD) with purity > 90% (Additional file 7: Figure S5) and co-cultured in U-bottom plate with HCCLM3 HCC tumor cell line pre-labeled with CFSE according to the manufacture’s instruction.