Resistance to proteoglycan-induced arthritis (PGIA) in CD28-deficient mice. (a) The expression of CD4+CD25+ T cells in naïve wild-type (WT) and CD28-deficient BALB/c mice was determined. (b) Wild-type and CD28-deficient BALB/c mice were immunized with proteoglycan (PG) in complete Freund's adjuvant, and boosted in incomplete Freund's adjuvant at days 21 and day 42. The mice developed arthritis after the third immunization. The incidence of arthritis in WT and CD28-deficient BALB/c mice was monitored. (c) CD4+ T cells (1 × 106/ml) from PG-immunized WT and CD28-deficient mice were cultured for 5 days with antigen-presenting cells (2500 rad-irradiated syngenic splenocytes, 2 × 106/ml) in the presence of proteoglycan (10, 25 and 50 μg/ml). The cell proliferation was determined by [3H]thymidine incorporation.