Treg-cell suppressor function is reduced when CD153 (CD30L) is blocked in the early phase after adoptive transfer. Expansion of luc⁺ donor Tconv cells in animals receiving Tconv cells alone or Tconv cells and Treg cells alone or in conjunction with isotype control Ab (rat IgG_2b) or with anti-CD153 blocking Ab administered on days −2, 0, 2, and 4 as shown for 3 representative animals at days 3, 4, 6, 8, and 10 after BMT (A) and as quantified in emitted photons over total body area at serial time points after BMT (B). (A) Proliferation of luc⁺ Tconv cells (first column) is reduced by cotransfer of Treg cells (second column). While isotype IgG Ab does not interfere with Treg-mediated suppression (third column), addition of anti-CD153 blocking Ab reduces the suppressive effect of Treg cells (fourth column). The anti-CD153 blocking Ab has no impact on expansion of luc⁺ CD4/CD8 Tconv cells when given as indicated in the early phase after BMT (fifth column). (B) TCD-BM (▵, n = 15), with Tconv cells (□, n = 15), with Tconv cells and Treg cells (▪, n = 10), with Tconv cells and Treg cells and isotype IgG (○, n = 10), with Tconv cells and Treg cells and anti-CD153 blocking Ab (•, n = 10), and with Tconv cells and anti-CD153 blocking Ab (⋄, n = 10). Signal intensity is significantly higher in animals receiving Tconv cells and Treg cells and anti-CD153 blocking Ab compared with isotype IgG (• versus ○, P < .01, days 6-14). Error bars indicate SD from the mean. (C) Survival of Balb/c recipients is significantly lower when combining Treg cells with anti-CD153 blocking Ab compared with isotype IgG (• versus ○, P < .001). Anti-CD153 blocking Ab does not improve survival compared with PBS injection when administered on day −2 to day +4 (⋄ versus □, NS). Survival data from 3 independent experiments are combined.