Estrogen ameliorates antigen-induced arthritis (AIA). (A) Scheme for AIA. Total ERα-/-, Col2α1-ERα-/-, and wild-type (WT) mice were ovariectomized (OVX) at 10 weeks of age, followed by insertion of slow-release treatment pellets with placebo or estrogen (E2) (0.83 μg/day). Fourteen days after ovariectomy, the animals were immunized by an intradermal injection of methylated bovine serum (mBSA) emulsified in Freund’s complete adjuvant (0.2 mg mBSA) and 7 days later given an intra-articular (IA) injection of mBSA (0.3 mg mBSA dissolved in 30 μL of vehicle) in one knee joint. Seven days after the IA injection, the mice were bled and terminated. (B) Knee joints obtained at termination of WT mice with AIA treated with placebo or estrogen were examined with a histological scoring of synovitis and joint destruction. The histological examination was performed by using a scale from 1 to 3: 1, mild; 2, moderate; and 3, severe. *P <0.05, **P <0.01, Mann-Whitney U test. (n = 17-18). (C) Representative images of joint histology. Percentage of viable (D) neutrophils (CD11b+, Gr1+, F480-), (E) monocytes/macrophages (F480+), (F) T cells (CD3+), and (G) B cells (CD19+) in synovia tissues. *P <0.05, Student t test (n = 7-8). (H) Percentage of viable CD4+ Th (T helper) cells and CD8+ Tc (T cytotoxic) cells in the spleen. *P <0.05, Student t test. (n = 17-18). (I) Proliferation of splenocytes after stimulation with the T-cell mitogen concanavalin A (conA). Cell cultures were set in triplicates, and results are presented as proliferation index (median of count per minute in wells with conA minus the median of count per minute in control wells). *P <0.05, Student t test (n = 17-18). Results are shown as mean ± standard error of the mean.