Low-dose FK506 reduces RV fibrosis by increasing BMP signaling, whereas high-dose FK506 is less effective, and cyclosporine worsens RV fibrosis in response to chronic RV pressure overload. (A–D) Histological analysis of wheat germ agglutinin (A), isolectin B4 (B), and Masson’s trichrome (C) staining in RV tissue sections from male C57Bl6 mice after sham surgery or PAB and treatment with either placebo, low-dose FK506 (0.05 mg/kd/d) (gray bar), high-dose FK506 (1 mg/kd/d), low-dose FK506+ LDN-193189 (2.5 mg/kd/d), or cyclosporine (25 mg/kg/d) demonstrated that PAB-induced reductions in capillarization are attenuated by low-dose FK506 therapy independent from LDN-193189 coadministration and high-dose FK506 but not cyclosporine treatment (D). (E) PAB-induced RV fibrosis assessed as RV collagen amount is reduced by low-dose (gray bar) but not high-dose FK506 administration and lost after concomitant LDN-193189 treatment, whereas immunosuppression with cyclosporine worsens RV fibrosis. n = 3 sham-operated animals; n = 4–6 PAB mice per group. One-way ANOVA followed by Tukey’s multiple comparison post hoc analysis. #P < 0.05 versus sham. §P < 0.05 versus placebo. $P < 0.05 versus low-dose FK506. Scale bars, 100 μm.