Antibody blockade of the IL-6R significantly attenuates the severity of GVHD. (A) Lethally irradiated (900 cGy) Balb/c mice received a transplant of T cell–depleted (TCD) B6 BM alone (10 × 10⁶; ○, n = 9) or together with B6 spleen cells adjusted to yield a T-cell dose of 0.7 × 10⁶ αβ T cells. Cohorts of mice that received adjunctive spleen cells were then treated with either rat IgG isotype control (n = 15) or anti–IL-6R (MR-16-1) antibody (n = 12) once weekly for 4 weeks beginning on the day of transplantation. Overall survival is depicted. Data are cumulative results from 3 independent experiments. (B) Lethally irradiated Balb/c mice received a transplant of TCD Foxp3^EGFP BM (10 × 10⁶) and 0.4 to 0.5 × 10⁶ Foxp3^EGFP spleen cells. Cohorts of mice were then treated with either rat IgG isotype control (n = 12) or anti–IL-6R (MR-16-1) antibody (n = 12) once weekly for 4 weeks beginning on the day of transplantation. Mice from both groups were killed 34 to 37 days after BMT. (B) The percentage of original body weight over time in mice from both groups is depicted. (C) Pathological damage in the colon, liver, and lung using a semiquantitative scoring system as detailed in “Histologic analysis.” (D) Histology of colon, liver, and lung from representative recipients treated with either isotype or anti–IL-6R antibody. In isotype control animals, colon shows extensive inflammation in the lamina propria, goblet cell depletion, and crypt cell destruction; liver reveals portal triad inflammation with mononuclear cells and endothelialitis; and lung demonstrates perivascular and peribronchial cuffing with mononuclear cells. In anti–IL-6R antibody–treated mice, colon has normal-appearing mucosa with no attendant inflammation, liver has reduced portal triad inflammation, and lung demonstrates a similar reduction in perivascular and peribronchial cuffing. (E) Total spleen cellularity and (F) absolute number of splenic Tregs (CD4⁺ EGFP⁺) are shown. (G) Lethally irradiated (900 cGy) Balb/c mice received a transplant of TCD B6 BM plus 0.4 × 10⁶ B6 spleen cells. Cohorts of animals that underwent transplantation were then treated with either isotype control (n = 12) or anti–IL-6R antibody (n = 12) on the day of transplantation. Mice were bled 6 days after transplantation and serum was assayed for proinflammatory cytokines. Data are presented as the mean (± SEM) and are the cumulative results from 3 independent experiments. (Statistics: *P ≤ .05, **P < .01.)