*Purpose: Simultaneous transplantation of a solid organ and bone marrow from an identical donor is a possible means of achieving transplant tolerance. Here, we attempted to identify biomarkers that indicate transplant tolerance for discontinuation of immunosuppressants in combined kidney and bone marrow transplantation (CKBMT).

*Methods: We compared various immunological parameters that evaluate the transplant tolerance status between conventional kidney transplanted (KT) recipients (n = 20) and CKBMT recipients (n = 6) by using flow cytometry. To measure the anti-donor responses, mixed lymphocyte reaction (MLR) assays were performed with responder peripheral blood mononuclear cells (PBMCs) of CKBMT recipients and stimulator PBMCs of autologous, donor, and third party.

*Results: Among six CKBMT recipients, three successfully discontinued immunosuppressants (tolerant group) and three could not (non-tolerant group). The CD45RA^–FoxP3^hi activated regulatory T (Treg) cell subpopulation was expanded in CKBMT recipients compared to conventional kidney transplant patients, and this was more obvious in the tolerant group than the non-tolerant group. The ratio of CD45RA^–FoxP3^hi activated Treg cells to CD45RA^–FoxP3^lo non-suppressive cells showed good discrimination between the tolerant and non-tolerant groups.

*Conclusions: Thus, our findings propose a biomarker that can distinguish CKBMT patients who achieve transplant tolerance and are eligible for discontinuation of immunosuppressants and may provide insight into tolerance mechanisms in CKBMT.

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