Induction of species-specific host accommodation in the hamster-to-rat xenotransplantation model.

The combination of two immunosuppressants, leflunomide and cyclosporin A (CsA), completely inhibits immune xenoreactions in the hamster-to-Lewis rat xenotransplantation model. In addition, the control of acute xenograft rejection with this combination of immunosuppressants subdues early T-independent xenoreactivity and uncovers a late immune response that can be controlled by CsA alone. We attribute this acquired responsiveness to CsA to a modification in the recipient's humoral response to the xenograft, and refer to this change as host accommodation. Host accommodation can be induced in Lewis rats receiving hamster hearts by the combination of leflunomide and CsA. A 7-day treatment with leflunomide and CsA was able to convert xenoreactivity from one that was resistant to CsA treatment into one that was controlled by CsA. The presence of the hamster xenograft was critical for the induction of host accommodation since the immunosuppressive regimen, either alone or in combination with a transfusion with donor-specific spleen cells, was unable to modify the anti-hamster reactivity in Lewis rats. When accommodation was induced in the presence of hamster hearts, these accommodated rats were able to acutely reject third-party mouse hearts while under CsA therapy, thus indicating that the host accommodation is species specific. Finally, we demonstrate that host accommodation is associated with a loss in the ability to produce species-specific, T-independent xenoantibodies. These novel observations suggest that xenoreactive T-independent humoral responses can be deleted selectively without significant loss of other innate, Ag-specific T-independent humoral responses.