Therapeutic administration of anti-CypA single-domain antibody A1 decreases arthritis severity in the collagen-induced arthritis model. Mice with collagen-induced arthritis (CIA) were intraperitoneally injected with phosphate-buffered saline (PBS), isotype control antibody, or single-domain antibody (sdAb) A1 once a day from 27 to 37 days (n = 10 per group). Nonimmunized and nontreated animals were used as negative controls. (A) Representative photographs of the hind paw of CIA mice on day 42. (B) Clinical scoring was carried out as described in Materials and methods. sdAbA1 significantly reduced the clinical arthritis severity in CIA mice (*P <0.05 vs. control antibody group, for area under the curve). (C) Representative radiographs of the hind paw of CIA on day 42. (D) The degree of bone erosion on the radiographs of the hind paws of the mice was scored on a scale of 0 to 3 as described in Materials and methods. Values are the mean ± standard error of the mean of 10 mice per group. *P <0.05 versus control antibody group.