Absence of in vitro responses to type II collagen by splenocytes from arthritic BALB/c mice is possibly caused by intrinsic CD25+ regulatory cells

Collagen-induced arthritis-resistant BALB/ c mice develop arthritis if a foreign protein is added to an emulsion of type II collagen (CII) and adjuvant. The IgG autoantibody activity to CII is increased, whereas no CII autoreactive T cells in vitro can be recorded. In this study, we have explored whether CD25⁺ cells inhibit T-cell autoreactivity to CII. We also followed the IgG anti-CII autoantibody activity and the IL-6 level in serum during the development of arthritis. BALB/ c mice were coimmunized with bovine CII (BCII) and keyhole limpet haemocyanin (KLH) in complete Freund's adjuvant and boostered 3 weeks later. Control animals were immunized with either BCII or KLH. Sera were collected prior to and during the development of arthritis and examined for IgG anti-CII antibody activity and IL-6 content. When all BCII–KLH immunized mice had developed arthritis, splenocytes were prepared, with and without CD25⁺ cells, and tested for BCII reactivity in vitro . The serum IgG, IgG1 and IgG2a anti-CII antibody activities and the IL-6 level were significantly higher in BCII–KLH immunized mice than in BCII-immunized animals that failed to develop arthritis. The BCII-specific IL-2 secretion in vitro was significantly increased in CD25-depleted splenocyte cultures prepared from arthritic BCII–KLH-immunized mice. Development of arthritis in BALB/ c mice induced by coimmunization with BCII/KLH results in increased levels of circulating IL-6 and IgG autoantibodies to CII. The arthritogenic BCII–KLH immunization potentiates BCII-specific IL-2 secretion by CD25-depleted splenocytes, but CD25⁺ cells hamper the outcome of their action, at least in vitro .