Amelioration of arthritis and reduction of osteoclast numbers in the affected joints by blockade of Dll1. Mice with K/BxN serum-induced arthritis were treated with control hamster IgG (Ham) or HMD1-5 (D1). (a) Severity of arthritis was assessed by an arthritis score. Mean score ± SEM is shown (n = 7 mice per group). *, P < 0.05; **, P < 0.01 versus control hamster IgG. Representative hematoxylin and eosin staining of joints on Day 13 is shown. Original magnification ×100. Similar results were obtained in two independent experiments. (b) TRAP immunohistochemistry of joints. Number of TRAP+ MNCs in joints on Day 13 was measured. Shown are the mean number of TRAP+ MNCs relative to bone surface ± SEM of 10 joints from 5 mice per group. ***, P < 0.001 versus control hamster IgG. Representative staining is shown. Original magnification; ×100, ×400.