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BACKGROUND: Mesenchymal stem cells (MSCs) can reduce the incidence of graft-versus-host disease because of their ability to inhibit T-lymphocyte proliferation. There are no publications on the effect that MSCs have on cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells, effector cells vital for the graft-versus-leukemia effect. METHODS: Cytotoxic T cells were primed in mixed lymphocyte culture (MLC) against irradiated stimulator lymphocytes, and irradiated third-party MSCs were added at different time points. The CTLs were collected, and their cytotoxic potential was analyzed in a chromium-release assay against the same stimulator cells as in the MLC. Purified NK cells were mixed with irradiated MSCs, and the lysis was measured in chromium-release assay against K562 target cells. RESULTS: We found that MSCs inhibited CTL-mediated lysis by 70% if added at the beginning of the 6-day MLC. The lysis was not affected on day 3 or in the cytotoxic phase. Furthermore, MSCs inhibited the formation of cytotoxic lymphocytes when the cells were separated in a transwell system, which indicates that the effect is mediated by a soluble factor. NK cell-mediated lysis of K562 cells was not inhibited by MSCs. MSCs did not induce proliferation of allogeneic lymphocytes, and they were not lysed by allogeneic CTLs or NK cells. CONCLUSION: Our findings indicate that MSCs escape recognition by CTLs and alloreactive NK cells, and inhibit the formation of cytotoxic T cells by secreting a soluble factor, but that they do not interfere with CTLs and NK cell lysis.
Rasmusson et al. (Mon,) studied this question.