Background.

Previous studies showed the feasibility of inducing transplantation tolerance to cadaveric renal allografts in patients given pretransplant total lymphoid irradiation (TLI). Microchimerism has been theorized to be an important or necessary factor in long-term graft acceptance and tolerance in humans.

Methods.

A cadaveric renal transplant recipient given pretransplant total lymphoid irradiation and withdrawn from immunosuppressive drugs more than 12 years ago was tested for microchimerism using a sensitive nested polymerase chain reaction technique, and for anti-donor reactivity using the mixed leukocyte reaction and an ELISA screen for anti-HLA antibodies. Donor and recipient were mismatched for all HLA-A, B, and DR antigens.

Results.

The “tolerant” recipient had good graft function, no detectable donor-type cells in the blood by polymerase chain reaction analysis, vigorous reactivity to donor stimulator cells in the mixed leukocyte reaction, and no detectable serum anti-HLA antibodies.

Conclusions.

Operational tolerance to HLA-A, B, and DR mismatched organ allografts can be induced prospectively in humans for at least 12 years after withdrawal of immunosuppressive drugs. The allograft can be maintained in the absence of detectable donor microchimerism and in the presence of anti-donor reactivity in the mixed leukocyte reaction, suggesting that neither chimerism nor clonal deletion or anergy of recipient T cells to alloantigens presented by donor Class II HLA molecules is required for persistence of the tolerant state using this total lymphoid irradiation protocol.