To examine the importance of interleukin‐12 (IL‐12) as a factor in the interferon‐γ (IFNγ)‐dominant T cell cytokine response in the synovial tissue of patients with rheumatoid arthritis (RA).

The expression of IL‐12 in synovial tissue samples from patients with chronic RA (≥2 years) was compared with that in samples from osteoarthritis (OA) patients by detection of IL‐12 p40 messenger RNA (mRNA) using reverse transcriptase‐polymerase chain reaction, measurement of IL‐12 p70 protein in culture supernatants of tissue cells by immunoassay, and immunostaining of tissue sections with anti‐IL‐12 p70. The production of IFNγ by RA synovial tissue cells cultured with or without IL‐12 was determined. In addition, T cells were obtained 14 days after culturing RA synovial tissue cells with IL‐2 alone or with IL‐2 plus IL‐12, neutralizing anti‐IL‐12, or IL‐4, and cytokine patterns (i.e., IFNγ and IL‐4 levels) were determined by stimulating cells for 24 hours with anti‐CD3 plus phorbol myristate acetate.

Synovial tissues from RA patients more strongly expressed IL‐12 p40 mRNA than did OA tissues. Dissociated tissue cells from 21 of 37 RA patients spontaneously released detectable amounts of IL‐12 p70 (≥12.5 pg/ml) in culture, whereas production of IL‐12 by OA tissues was limited. By immunohistochemical analysis, IL‐12‐producing cells were localized mainly in the sublining layer of RA synovium, and mostly expressed the CD68 antigen. Levels of IFNγ production by RA synovial tissue cells were potently and selectively enhanced by IL‐12. The ability of IL‐2‐expanding synovial T cells to produce IFNγ was augmented by costimulation with IL‐12 and diminished by anti‐IL‐12, while it was not affected by IL‐4.

These data suggest that IL‐12, produced mainly by macrophage‐lineage cells, may be involved in IFNγ‐dominant cytokine production by infiltrating T cells in joints with chronic RA.