Collagen-induced arthritis development in C57BL/6 and u-PA-/- chimeric mice. Urokinase-type plasminogen activator (u-PA) production by bone marrow-derived cells is required for full expression of collagen-induced arthritis (CIA). (a) Severity (mean clinical score for all mice ± standard error of the mean) for C57 (n = 10), C57 → C57 (n = 12), C57 → u-PA-/- (n = 16) and u-PA-/- (n = 8) mice. P < 0.05, C57 → u-PA-/- vs. u-PA-/- mice. (b) Severity for C57 (n = 10), u-PA-/- → C57 (n = 11), u-PA-/- →u-PA-/- (n = 9), u-PA-/- (n = 8) mice. P < 0.01, u-PA-/- → C57 vs. C57 mice.