Arthritis scores of wild-type C57BL/6 and mice deficient for MMP-13 after transfer of K/BxN serum. (A) Inflammation (infiltration of the synovial tissue by polymorphonuclear mononuclear leukocytes and exudation of granulocytes, hyperplasia of lining cells) and (B) cartilage damage in mice was induced with K/BxN sera in wild-type mice and mice deficient for matrix metalloproteinase-13 ( MMP-13–/–). The histology scores for synovial inflammation, cartilage and bone destruction were significantly lower in MMP-13–/– mice compared with the C57BL/6 mice (P <0.05). Bars show the mean ± standard deviation. P values determined using the Mann–Whitney test (n = 3 per genotype per time point). KO, knockout.