Engrafted mice showed specific tolerance to donor T cells.

(A) CD4⁺ T cells from engrafted animals (■) ceased to utilize Vβ5.1/2, similarly to the donor (BALB/c, ░). Nonengrafted mice (▪) utilized Vβ5.1/2 throughout the observation period. Data show the average of 7 representative mice from each group at 150 days after transplantation. (B) T cells were examined for their proliferative capacity against donor, recipient, and third party using an in vivo alloproliferation model with CFSE-labeled T cells from engrafted and nonengrafted animals. The concentration of CFSE within the cell decreases by 50% after each division. Labeled T cells from engrafted and nonengrafted animals were adoptively transferred into lethally irradiated (1.8 Gy [1800 rads]) BALB/c (donor), sickle (recipient), and C3H (third-party) mice. Histograms of representative animals demonstrate that CD4⁺ and CD8⁺ T cells from engrafted recipients are essentially unresponsive to BALB/c (donor) and sickle (recipient) hosts. Both subsets of T cells, however, when transferred into C3H mice (third party) undergo maximal division (up to 8 divisions). Tolerant animals, therefore, show no proliferation to donor or recipient but a normal proliferative response to third party (C3H, H-2^k).