To determine the percentage of naïve and memory IL-18Rα+ CD4+ T cells that were capable of producing IFN-γ upon IL-12 plus IL-18 stimulation, purified IL-18Rα+ CD4+ T cells from naïve (RA+18+) and memory (RO+18+) populations along with the corresponding IL-18Rα− cells (RA+18−/RO+18−) were activated with IL-12 plus IL-18 in the presence or absence of SB203580 for 48 hours, and the percentage of IFN-γ+ CD4+ T cells was determined by intracellular staining. Here, IL18R1 is linked to rheumatoid arthritis.