Therapeutic targeting of ST2 ameliorates HA. (A) Graphical illustration of treatment of F8^em1−/− mice with HpB to block IL-33 binding to ST2 during needle-induced HA. (B) Increase in the knee joint diameter (percentage) after needle injury of F8^em1−/− mice treated with HpB or PBS. Crosses indicate deaths of F8^em1−/− mice treated with PBS (red) or HpB (blue). (C) Hct in untreated mice and F8^em1−/− mice 3 days after needle injury that were treated with PBS or HpB. (D). Representative hematoxylin and eosin staining of the injured knee joint of F8^em1−/− mice treated with HpB or PBS on day 28 after injury, with synovium indicated x----x, and synovitis scores. (E) Images of Prussian blue–stained sections and hemosiderin, indicated by arrow, and deposition scores. (F) Images of safranin O–stained sections and cartilage degradation scores. The scale bar represents 500 μm. Data are from 2 separate experiments and are mean ± SEM from 9 to 12 mice per group. Statistical analysis by ANOVA and the Tukey multiple comparison test or Student t test; ∗P ≤ .05; ∗∗∗P ≤ .001. ns, nonsignificant.