Blockade of CD153 reverses Treg-mediated protection from accumulation of proinflammatory cytokines, apoptosis in T effector cells, and GvHD histopathology of the GIT. (A-B) Serum was collected on day 7 after transplantation from Balb/c recipients of TCD-BM plus Tconv cells (n = 17), TCD-BM plus Tconv cells and CD153 blocking Ab (n = 12), TCD-BM plus Tconv cells and Treg cells and CD153 blocking Ab (n = 18), TCD-BM plus Tconv cells and Treg cells and isotype Ab (n = 18), or TCD-BM alone (n = 17); *P < .05. Blockade of CD153 in the presence of Treg cells leads to increased IFN-γ (A) and TNF-α (B) levels that reach significance for IFN-γ when compared with Treg cells plus isotype IgG. Data are pooled from 3 independent experiments. (C) The diagrams present results from TUNEL staining of splenic donor-type (H-2k^q) CD4 or CD8 cells isolated on day 3 or day 5 after BMT from Balb/c mice receiving TCD-BM plus Tconv cells, TCD-BM plus Tconv cells and CD153 blocking Ab (aCD153), TCD-BM plus Tconv cells and Treg cells and isotype IgG, and TCD-BM plus Tconv cells and Treg cells and CD153 blocking Ab, as indicated for the respective bar. Data are pooled from 3 independent experiments with a total of 9 mice per group (*P < .05). Error bars indicate SD from the mean. (D) The left panel (i-iv) depicts expansion of luc⁺ Tconv cells on day 8 after BMT in the GIT and in the mLNs as assessed by ex vivo BLI. (i) Background signal in the GIT of animals having received only TCD-BM without any luc⁺ cells. Proliferation of Tconv cells in the mLN and the GIT (ii) is reduced by cotransfer of Treg cells in the presence of isotype IgG (iii). Addition of anti-CD153 blocking Ab reduces the protective effect of Treg cells (iv). The right panel shows representative histologic sections of the bowel corresponding to the groups in the left panel. Significant T-cell infiltration (arrow) is found in animals receiving Tconv cells only (vi) or Tconv cells and Treg cells and anti-CD153 blocking Ab (viii).