Maternal-fetal relationship, natural chimerism and bilateral transplantation tolerance as the basis for non-myeloablative stem cell transplantation

B7.2-/- mature dendritic cells generate T-helper 2 and regulatory T donor cells in fetal mice after in utero allogeneic bone marrow transplantation

In utero hematopoietic stem cell transplantation (IUT) results in limited chimerism and tolerance to alloantigens. We studied the relative role of B7.1 and B7.2 expression by dendritic cells (DCs) in engraftment and in generating donor-specific tolerance in fetal mice. Mature dendritic cells (mDCs) from B7.1(-/-) or B7.2(-/-) donors and wild-type (WT) lineage-depleted (lin(-)) C57BL/6 (B6) bone marrow (BM) were injected into BALB/c fetuses. Six weeks after IUT, B7.1(-/-) recipients had multilineage engraftment (4.7% +/- 0.8% T cells and 5.7% +/- 1.1% granulocytes) associated with graft-versus-host disease (GVHD) and decreased survival, but by 12 weeks only donor CD3(+) cells (2.1% +/- 1.3%) were present. Recipients of B7.2(-/-) mDCs and lin(-) WT B6 BM had exclusively CD3(+)CD4(+) T cells (11.8% +/- 8.5% at 6 weeks and 6.5% +/- 2.5% at 12 weeks). Most of the cells were T-helper 2, although 10.4% +/- 1.4% were of the T-regulatory (T(reg)) phenotype, ie, CD4(+)CD25(+). Donor T(reg) cells were detected both in the thymus and spleen, thus suggesting an effect on both central and peripheral immunity. The animals with T(reg) cells had better survival (82.3% versus 47.4%; P < .01) and no GVHD (0% versus 65%; P < .001). This group alone demonstrated multilineage engraftment of donor hematopoietic cells after postnatal transplantation with megadoses of donor lin(-) BM. Both the engrafted donor CD4(+)CD25(-) and CD4(+)CD25(+) cells induced comparable in vitro suppression of T-cell proliferation, thus suggesting their role in the persistence of the donor T cells in vivo. The CD4(+)CD25(-) cells produced interleukin 10 or interleukin 4 and were inhibited by anti-T-helper 2 cytokine-neutralizing antibodies, whereas the CD4(+)CD25(+) cells showed no evidence of any involvement of a cytokine-like soluble mediator and expressed cytotoxic T-lymphocyte antigen 4 (CTLA-4) and foxp3 constitutively. Donor mDCs and donor CD4 T cells were detected among the thymocytes of the recipients of B7.2(-/-) mDCs and lin(-) WT B6 BM. Thus, it seems that costimulatory molecule expression of donor DCs can play a significant immunomodulatory role in survival, GVHD, engraftment, and homing of allogeneic BM cells after IUT through the generation of T(reg) cells.

Nonmyeloablative stem cell transplantation and cell therapy for malignant and non-malignant diseases

The conditioning prior to allogeneic stem cell transplantation was originally designed as a myeloablative conditioning, designed to eliminate malignant or genetically abnormal cells and then use the transplant procedure for rescue of the patients or to replace missing bone marrow products. However, allografts can induce effective graft vs. malignancy effects and can also eliminate undesirable hematopoietic stem cells in patients with genetic disorders and autoimmune diseases, thus documenting that alloreactive effects mediated by donor lymphocytes post-grafting can play a major role in eliminating hematopoietic cell of host origin, as well as provide effective immunotherapy for the treatment of disease recurrence. The efficacy of donor lymphocyte infusion (DLI) could be improved by activation with rIL-2 or by donor immunization. The cumulative experience over the years suggesting that alloreactive donor lymphocytes were most effective in eliminating tumor cells of host origin resulted in an attempt to reduce the intensity of the conditioning in preparation for the transplant procedure used for the treatment of hematological and other malignancies as well as life-threatening non-malignant disorders for which allogeneic stem cell transplantation may be indicated. Our working hypothesis proposed that the myeloablative conditioning which is hazardous and may be associated with early and late side effects, may not be required for treatment of patients with any indication for allogeneic stem cell transplantation. Instead, nonmyeloablative conditioning based on the use of reduced intensive preparatory regimen, also known as nonmyeloablative stem cell transplantation, may be sufficient for engraftment of donor stem cells while avoiding procedure-related toxicity and mortality, followed by elimination of undesirable cells of host origin by post-transplant effects mediated by alloreactive donor lymphocytes infused along with donor stem cells or administered subsequently as DLI. Improvement of the immediate outcome of stem cell transplantation using NST due to a significant decrease in transplant related mortality has broadened the spectrum of patients eligible for allogeneic stem cell transplantation, including elderly patients and other patients with less than optimal performance status. Likewise, the safer use of stem cell transplantation prompted expanding the scope of potential indications for allogeneic stem cell transplantation, such as metastatic solid tumors and autoimmune disorders, which now are slowly becoming much more acceptable. Current strategies focus on the need to improve the capacity of donor lymphocytes to eliminate undesirable malignant and non-malignant hematopoietic cells of host origin, replacing abnormal or malignant stem cells or their products with normal hematopoietic stem cells of donor origin, while minimizing procedure-related toxicity and mortality and improving the quality of life by reducing the incidence and severity of hazardous acute and chronic GVHD.