The Th1 cytokines IL-2 and IFN-γ, which inhibit T cell proliferation and promote activation induced cell death, may be required to diminish alloreactive T cell numbers and to foster tolerance across full allogeneic barriers. However, we hypothesized that these cytokines might be dispensable when the alloreactive T cell clone size is relatively small, as is seen in recipients of minor-mismatched grafts. We show that alloreactive T cell clone size of C57BL/6 mice against multiple minor-mismatched 129X1/sv mice was ∼4–9-fold smaller than that against MHC-mismatched BALB/c mice. In the MHC-mismatched combination, CD28-B7 blockade by CTLA4Ig induced long-term graft survival in wild-type recipients, but this treatment was ineffective in IFNγ–/– or IL-2–/– recipients. In contrast, in the minor-mismatched combination, CTLA4Ig induced long-term allograft survival in wild-type, IFNγ–/–, and IL-2–/– recipients. Bcl-xL transgenic animals, which are defective in "passive" T cell death, are likewise sensitive to the effects of CTLA4Ig only in the setting of the minor-mismatch grafts. Therefore, the alloreactive T cell clone size is an important determinant affecting the need for Th1 cytokines and T cell death in tolerance induction. These data have implications for the design of tolerance strategies in transplant recipients with varying degrees of MHC mismatching.
Koji Kishimoto, Sigrid Sandner, Jaime Imitola, Masayuki Sho, Yongsheng Li, Peter B. Langmuir, David M. Rothstein, Terry B. Strom, Laurence A. Turka, Mohamed H. Sayegh