IL18 and hepatocellular carcinoma: Using an established protocol (8), PBMC were incubated with inactivated influenza H1N1 virus or, as a positive control, HCC (IL-12 5 ng/ml plus IL-18 50 ng/ml) for 16 h in the presence of a low concentration of IL-15 (1 ng/ml); IL-15 alone provided a negative control (Fig. 5A).