Barriers to chimerism after major histocompatibility complex-mismatched stem cell transplantation: a potential role for heterologous immunity.

The induction of mixed hematopoietic chimerism following allogeneic stem cell transplantation is a potential treatment modality for numerous nonmalignant diseases and generates a robust state of donor-specific tolerance. However, despite several promising results in murine studies, the translation to nonhuman primate models and clinical trials has proven to be more difficult. In contrast to specific pathogen-free bred laboratory mice, the immune system of humans has been in repeated contact with numerous pathogens, resulting in a broad memory-T-cell repertoire. Crossreactivity of the virus-specific memory-T-cell pool against alloantigens has been described in a phenomenon called heterologous immunity. In this study, we demonstrate, in a murine stem cell transplantation model, that heterologous immunity is likely to be an important barrier for the induction of mixed hematopoietic chimerism after immunologic conditioning in the absence of cytoreduction. Additional T-cell depletion or a brief cyclosporine treatment can be applied to overcome the barrier of heterologous immunity.

Persisting NK cell alloreactivity in the presence of long-term stable hematopoietic chimerism.

OBJECTIVE: Like other lymphocytes, natural killer (NK) cells need to develop tolerance toward self. In the last decade, major progress has been made in understanding the molecular mechanisms behind this phenomenon. Yet the cellular mechanisms regulating the education of NK cells remain poorly understood. This study aimed to unravel the cellular mechanisms regulating NK-cell tolerance in the context of alloreactivity. MATERIALS AND METHODS: Using the classical hybrid resistance model and an in vivo cytotoxicity assay, we examined host NK-cell alloreactivity against donor hematopoietic cells in the presence of long-term stable chimerism. RESULTS: We demonstrate persisting host NK-cell alloreactivity against donor hematopoietic cells, despite the presence of long-term stable chimerism. This NK-cell alloreactivity decreases with increasing levels of parental chimerism. CONCLUSION: Our results indicate that long-term mixed hematopoietic chimerism does not necessarily imply the induction of full NK-cell nonresponsiveness. NK-cell nonresponsiveness is, therefore, not an absolute phenomenon, but represents a dynamic process, balancing NK-cell alloreactivity against the size of the transplanted allogeneic stem cell pool.

Donor-derived mesenchymal stem cells are immunogenic in an allogeneic host and stimulate donor graft rejection in a nonmyeloablative setting.

Mesenchymal stem cells (MSCs) are multipotent progenitor cells that have emerged as a promising tool for clinical application. Further clinical interest has been raised by the observation that MSCs are immunoprivileged and, more important, display immunosuppressive capacities. These properties may be of therapeutic value in allogeneic transplantation to prevent graft rejection and to prevent and treat graft-versus-host disease. In the present study, we examined the in vivo immunomodulatory properties of MSCs in murine models of allogeneic bone marrow (BM) transplantation. Sublethally irradiated recipients received allogeneic BM with or without host or donor MSCs. The addition of host MSCs significantly enhanced the long-term engraftment associated with tolerance to host and donor antigens. However, the infusion of donor MSCs was associated with significantly increased rejection of allogeneic donor BM cells. Moreover, we showed that the injection of merely allogeneic donor MSCs in naive mice was sufficient to induce a memory T-cell response. Although the observed engraftment-promoting effects of host MSCs in vivo support the therapeutic potential of MSCs, our results also indicate that allogeneic MSCs are not intrinsically immunoprivileged and that under appropriate conditions, allogeneic MSCs induce a memory T-cell response resulting in rejection of an allogeneic stem cell graft.

Long-term mixed chimerism after immunologic conditioning and MHC-mismatched stem-cell transplantation is dependent on NK-cell tolerance.

T-cell tolerance is mandatory for major histocompatibility complex (MHC)-mismatched stem-cell transplantation without cytoreduction. Here, we used a cytotoxicity assay based on the infusion of differentially carboxyfluorescein succinimidyl ester (CFSE)-labeled syngeneic and donor splenocytes to determine the survival of donor cells in vivo. In vivo cytotoxicity data showed that treatment with anti-CD40 ligand monoclonal antibody in combination with a low dose of MHC-mismatched bone marrow cells was sufficient to induce T-cell tolerance. However, CFSE-labeled donor cells were still eliminated. A similar elimination pattern was observed in T-cell and natural killer T-cell (NKT-cell)-deficient mice, suggesting the involvement of natural killer (NK) cells. Indeed, in vivo NK-cell depletion resulted in a prolonged survival of CFSE-labeled donor cells, confirming the role of NK cells in this process. Transplantation of a megadose of MHC-mismatched bone marrow cells was required for a complete survival of CFSE-labeled donor cells. This NK-cell tolerance was donor specific and was associated with mixed chimerism. Additional NK-cell depletion significantly enhanced engraftment and allowed long-term chimerism after transplantation of a relatively low dose of donor bone marrow cells. These data demonstrate the importance of NK cells in the rejection of MHC-mismatched hematopoietic cells and may explain the high numbers of bone marrow cells required for transplantation over MHC barriers.