Interleukin 12 (IL-12) and IL-18 synergistically induce interferon-gamma (IFN-gamma) production by T cell infiltrates in rheumatoid arthritis (RA). To investigate this synergism, we examined the expression and regulation of IL-12 receptor (IL-12R) and IL-18R on peripheral blood (PB) and synovial tissue (ST) CD4+ T cells from patients with RA.
The mRNA and cell surface expression of IL-12R and IL-18R in CD4+ T cells were determined by reverse transcriptase-polymerase chain reaction and flow cytometry, respectively. IFN-gamma and IL-4 production by CD4+ T cells stimulated with phorbol myristate acetate (PMA) and calcium ionophore A23187 was measured by intracellular cytokine staining and flow cytometry.
Despite the negligible expression of IL-12R on fresh cells, PB CD4+ T cells from RA patients expressed higher levels of both IL-12Rbeta1 and beta2 subunits after stimulation with anti-CD3 antibody (Ab) than the cells of healthy controls. ST CD4+ T cells contained mRNA transcripts encoding IL-12Rbeta1 and beta2, and expressed detectable levels of these 2 subunits on the cell surface. Their IL-12R expression was markedly augmented by costimulation with anti-CD3 Ab and IL-18. In contrast, IL-18Ralpha was expressed on freshly isolated PB CD4+ T cells from RA patients and controls, and the level of expression was higher in RA. IL-18Ralpha+ CD4+ T cells were further increased in the ST lesion, where IL-18Rbeta mRNA was constitutively detected. IL-12Rbeta1 and beta2 were induced mainly on IL-18Ralpha+ CD4+ T cells after anti-CD3 Ab stimulation. PMA and A23187-activated ST CD4+ T cells mostly expressed IL-18Ralpha and produced high levels of IFN-gamma.
These results indicate that IL-18R-expressing CD4+ T cells are accumulated in the ST of patients with RA, where the functional IL-12R is locally induced by stimuli such as CD3 activation and IL-18. Activation of both cytokine receptors may be necessary for the IFN-gamma-dominant cytokine response.