CD28 and neoplasm: For a mitogen-driven assay, CFSE-labeled splenocytes from heathy C57BL/6 mice were incubated with anti-CD3 and anti-CD28 antibodies and cultured with rPAUF-treated or untreated MDSCs from EL4 tumor-bearing mice at different effector: target ratios (0:1, 0.1:1, 0.5:1, and 1:1).