Topical imiquimod induces recruitment of donor CD8 cells and GVHD in the skin of allogeneic chimeras. MCs were treated with 3 mg topical imiquimod (Imq) or nil in an area of approximately 2.25 cm² above the base of the tail at day 0, day 5, and day 10 after donor splenocyte transfer. (A) Left: Absolute numbers of Thy1.1⁺ CD4 or CD8 cells in the skin draining LN at day 14 (n = 13-21 mice per group); data pooled from 8 independent experiments. The mean is indicated by the horizontal bar. Right: Percentage of gated Thy1.1⁺ CD4 or CD8 cells on day 14 that were CD62L^low (n = 7-11 mice per group); data pooled from 3 independent experiments. *P < .05. The mean is indicated by the horizontal bar. (B) Representative contour plots (top row) showing accumulation of Thy1.1⁺ cells in the epidermis at day 14. Plot in the bottom row represents CD4 and CD8 expression in gated Thy1.1⁺ cells of imiquimod-treated MCs. (C) Summary data showing accumulation of total Thy1.1⁺ cells (left) or Thy1.1⁺ CD8 cells (right) in the epidermis of control or imiquimod-treated MCs (n = 12-19 mice per group); data pooled from 8 independent experiments. *P < .05. The mean is indicated by the horizontal bar. (D) Representative histologic sections showing skin histology on day 14 in imiquimod-treated and control mice (original magnification ×200). (E) Summary data of histologic GVHD score as evaluated single blind (n = 9-13 per group); data pooled from 4 inde-pendent experiments. ***P < .001. The median is indicated by the horizontal bar.