Donor Th2 cell abrogation of rejection restricts but does not eliminate allospecific host T cells. (A) Spleen cells from 2C TCR-transgenic mice were evaluated by flow cytometry for coexpression of the clonotypic TCR (using 1B2 antibody) and Vβ8; a representative example is shown. (B-D) BALB/c-into-B6 transplantation consisted of lethal irradiation (1100 cGy; day −2), and some combination, as indicated, of host T cells (day −1; 1:1 mix of 2C and CD45.1-congenic WT T cells [each population, 0.05 × 10⁶ cells]) and BALB/c donor BM cells with or without donor Th2 cells generated ex vivo in rapamycin (Th2R cells; day 0). The ratio of donor Th2 cells to host T cells was either 100:1 (“BM/Th2R_lo”) or 500:1 (“BM/Th2R_hi”). At day 8 after BMT, the number of allospecific host T cells in the spleen (panel B) and BM (panel C) was enumerated by flow cytometry. Allospecific host T cells were identified using H-K^b+, H-2K^d−, CD45.1⁻, CD8⁺, Vβ8⁺ (experiment number 1; panels B and C left) or H-K^b+, H-2K^d−, CD45.1⁻, CD8⁺, Vβ8⁺, 1B2⁺ (experiment number 2; panels B and C right). *P < .05; **P < .01; ***P < .001. (D) Longitudinal tracking of allospecific host T cells by Vβ8 analysis at days 11, 14, 21, and 45 after BMT in the spleen (left panel) and BM (right panel) in transplant recipients of high-dose donor Th2R cells (n = 5-8 recipients at each time point). (E) In a third experiment, B6 hosts received lethal irradiation (day −2), and some combination, as indicated, of host T cells (“HT”; 1:1 mix of 2C TCR-transgenic and CD45.1-congenic WT T cells; each population, 0.01 × 10⁶ cells; day −1), and donor BM with or without donor Th2R cells (day 0; donor Th2R cell: host T-cell ratio, 500:1). Mice were followed for survival analysis for 45 days (left panel; n = 10 per cohort) and weight loss (middle panel); donor chimerism in the spleen, BM, and blood was determined at day 45 after BMT (right panel).