Purpose: Recently, we induced donor specific tolerance to rat hind limb allografts across major histocompatibility complex (MHC) barrier under 35 days course of a b T-cell receptor (a b TCR) mAb and Cyclosporine A (CsA) therapy. In this report, we investigated the role of shorter course of immunomodulating protocol on the development of donor specific, lymphoid chimerism and tolerance induction.

Methods: Fifty-two hind limb transplantations across MHC barrier, between Lewis-Brown-Norway donors (LBN, RT1^l+n) and Lewis recipients (LEW, RT1^l) were performed to test the impact of 21, 7 and 5 day protocols of combined a b -TCR mAb/CsA treatment on tolerance induction. The mono-therapies of CsA and TCR served as controls. Donor specific tolerance and immunocompetence were tested by mixed lymphocyte reaction (MLR) in vitro and by standard skin grafting in vivo. The efficacy of immunosuppressive protocol and donor specific chimerism was assessed by flow cytometry.

Results: Without immunosuppression allografts were rejected at day 7. Mono-therapy with CsA and a b -TCR, prolonged survival up to 21 and 13 days, respectively. All transplants under 5, 7, and 21 days of combined a b -TCR mAb/CsA therapy survived over 350 days without rejection. At day 100^th post-transplant clinical tolerance and immunocompetence were confirmed by acceptance of skin grafts from the donor and rejection of the third-party alloantigen (ACI). MLR in vitro revealed unresponsiveness to the host and donor antigens but strong reactivity against third-party alloantigens. Donor chimerism ranged between 10%-12% for CD4⁺/RT1ⁿ⁺ and 6%-9% for CD8⁺/RT1ⁿ⁺ cells.