Effects of anti-FRβ immunotoxin on knee joint swelling in rat AIA. (A) FRβ-expressing macrophages in synovium of the arthritis rat were examined on the indicated days after AIA onset as described in Methods. Values are the mean ± SEM of knee joints from four rats per group. (B) AIA was induced using intra-articular injection of methylated BSA as described in Methods. Arthritic rats were injected in the left knee joint with 50 μg of VH-PE38 (filled circle) or with 2 µg (open circle), 10 µg (filled square) or 50 μg (filled triangle) of dsFv anti-FRβ-PE38 on days one, three, five and seven. Knee joint swelling was measured for each rat by measuring the difference in diameter between the arthritic left knee and the normal right knee. Values are the mean ± SEM of knee joint swelling from eight rats per group. *P <0.05 and **P <0.01 compared to the VH-PE38-treated group. AIA, antigen-induced arthritis; BSA, bovine serum albumin; FRβ, folate receptor β; SEM, standard error of the mean.