NK Cells Efficiently Prevent Engraftment of Donor Stem Cells after Tolerigenic Bone Marrow Transplantation.

Introduction: Immunologic tolerance remains an elusive goal of transplantation. In mice, mixed-chimerism and donor-specific tolerance can be induced by blocking the CD28/CD40L T-cell costimulatory pathways after bone marrow transplant (BMT). However, large doses of marrow (~1x10⁹ cells/kg) are required, and these regimens have not yet been successfully translated to clinical practice. There is a growing body of evidence that NK cells may play a central role in the failure of low doses of donor bone marrow to engraft, but the mechanisms underlying NK alloreactivity remain to be determined.

Methods: (1) BMT in the presence of CD28/CD40L T cell costimulation blockade was performed using C57BL/6 (B6) recipients and Balb/C donor bone marrow. The role of host-anti-donor NK alloreactivity in preventing engraftment was determined by specifically depleting B6 NK cells. The contribution of the NK cell-surface receptor, LFA1 to NK alloreactivity was determined with the anti-LFA1 blocking antibody M17/5.2. (2) An in vivo NK alloreactivity assay was developed that should allow the investigation of the mechanism of NK alloreactivity and the molecular mediators of this process. In this assay, CFSE-labeled B6 splenocytes were adoptively transferred into B6xBalbC F1 progeny. As such, alloreactivity was specifically mediated by NK cells. NK alloreactivity was measured flow-cytometrically by the disappearance of the CFSE-labeled B6 population.

Results: Transient depletion of recipient NK cells resulted in increased donor stem cell survival and the induction of stable mixed-chimerism and tolerance despite BMT with low doses (≤2x10⁶ cells) of donor bone marrow. This effect was specific to allogeneic donor cells: depletion of NK cells did not increase engraftment of syngeneic bone marrow. Blocking the adhesion molecule, LFA-1 recapitulated the effects of whole-scale NK depletion. Newly emergent NK cells exhibited significantly lower expression of the donor-specific activating receptor, Ly49D, and these NK cells did not exhibit in vivo alloreactivity. These results suggest that the NK repertoire in the mixed-chimeric setting exhibited donor-specific tolerance.

Using the in vivo hybrid resistance NK alloreactivity assay, we measured 80% NK-specific target killing 8 days after adoptive transfer. Significantly less killing occurred at 2, 4, and 6 days. Pre-sensitizing the recipient for 4 days increased the efficiency of killing—from 50% to 80%, suggesting a potent activation phenomenon required for efficient NK allorecognition and/or cytotoxicity.

Implications: These results reveal the importance of NK alloreactivity in the acquisition of mixed-chimerism after BMT at limiting stem cell doses, and suggest that clinical approaches to tolerance-induction transplantation may require mechanisms to control NK alloreactivity.