The aggregate 5-year survival for all patients with leukemia or lymphomas undergoing autologous transplantation, or allogeneic transplantation from human leukocyte antigen (HLA)-matched siblings, remains approximately 50%.[1-7] A number of factors, including patient disease status, age, conditioning regimen, and (for allogeneic recipients) type of graft-vs-host disease prophylaxis, influence the probability of survival after transplantation.[1,8,9] The most significant factor in predicting relapse-free survival is the patient’s diagnosis (eg, chronic myeloid leukemia vs acute lymphoblastic leukemia) and disease status at the time of transplant (eg, chronic phase chronic myeloid leukemia vs blast crisis). The factors amenable to manipulation include the conditioning regimen, graft-vs-host disease prophylaxis, supportive care during the peritransplant period, and the quality and quantity of donor cells in the graft.
The optimal numbers for each cell type in the bone marrow or peripheral blood progenitor cell allograft are not known. A variety of studies support the role of larger numbers of CD34-positive cells in the graft in faster hematopoietic reconstitution after autologous and allogeneic transplantation.[10-15] Transplantation with fewer than 1 × 106 CD34-positive cells/kg from T-cell-depleted allogeneic bone marrow and peripheral blood stem cell grafts was associated with increased relapse and treatment-related mortality.[11,16] Recipients of T-cell-depleted bone marrow allografts have a decreased risk of developing graft-vs-host disease,[17,18] but experience delayed immune reconstitution and have an increased risk of infections, graft failure, and relapse.[19-22]
There are numerous commercially available devices to positively select CD34-positive hematopoietic progenitors and passively remove T cells. However, the higher rates of relapse after T-cell-depleted allografts are unacceptable. Indeed, a randomized, multicenter phase III study of CD34-positive cell selection in allogeneic peripheral blood stem cell transplantation was recently amended to exclude high-risk patients because of significantly higher relapse rates among recipients of CD34-positive selected grafts.
Dendritic cells have a specialized capacity to present peptide antigens to T cells and regulate the initiation of the immune response.[23,24] There is significant heterogeneity within the dendritic cell lineage. Dendritic cells develop from hematopoietic progenitor "stem" cells under the influence of cytokines that act during early myelomonocytic differentiation.[25-32] Phenotypic and functional analyses have demonstrated two general types of dendritic cellstype 1 (DC1) and type 2 (DC2)that differ in surface marker expression as well as their functional effect on cognate T cells.
Studies of cytokines used to mobilize peripheral blood hematopoietic progenitors have indicated increased numbers of DC1 cells after administration of Flt3 and granulocyte-macrophage colony-stimulating factor (GM-CSF [Leukine]), while increased numbers of DC2 cells have been noted after treatment with granulocyte colony-stimulating factor (G-CSF [Neupogen]). DC1 cells promote Th1 immune responses in responding CD4-positive T cells characterized by enhanced interferon (INF)-gamma, tumor necrosis factor (TNF), and interleukin (IL)-12 synthesis. DC2 cells promote Th2 responses in CD4-positive T cells characterized by IL-4 and IL-10 synthesis, and inhibition of INF-gamma and TNF production in cognate T cells. The effects of GM-CSF and G-CSF on the content of different dendritic cell subtypes in the blood, and the functional differences between DC1 and DC2, offer potential therapeutic indications for these cytokines.
Dendritic Cells in Hematopoietic Progenitor Cell Transplantation
Dendritic cells play an important role in the antitumor effect of autologous and allogeneic transplantation. Antigen-primed dendritic cells have been used with promising results in adoptive vaccination against tumors.[34,35] Bone marrow contains monocytes and CD86-positive, CD34-positive progenitor cells that can differentiate into DC1 cells in the presence of TNF and GM-CSF,[36-38] as well as CD123bright dendritic cell progenitors that differentiate into DC2 cells in the presence of IL-3. Most studies involving hematopoietic progenitor cell transplantation to date have examined the role of DC1 cells as a method of adoptive immunotherapy for cancer following autologous hematopoietic progenitor cell transplantation.
One recent report, using a murine transplant model, indicated that only host CD11c-positive dendritic cells (DC1 cells) were necessary for the development of acute graft-vs-host disease. Donor dendritic cells did not appear to efficiently present host antigens via cross-priming in a way that led to the initiation of graft-vs-host disease. In this study, the possible effect of donor dendritic cells on inhibiting graft-vs-host disease was not examined. The potential role of increased numbers of donor DC2 cells in regulating graft-vs-host disease after G-CSF-mobilized allogeneic peripheral stem cell transplants has recently been recognized.
Adoptive Transfer of Immature Dendritic Cells
A recently published study in normal donors evaluated the ability of immature dendritic cells to augment T-cell activation after in vitro exposure to antigen. Immature dendritic cells were derived from peripheral blood monocytes and then exposed to the influenza peptide MP and keyhole limpet hemocyanin. After return of these dendritic cells to the host, a marked decline in the frequency of MP-specific INF-gamma-producing cellswhich persisted for 30 dayswas noted. At the same time, the frequency of influenza-specific T cells was unchanged, suggesting that the decrease in MP-specific INF-gamma-producing cells was related to dendritic cell exposure to the antigen, rather than to a global reduction in influenza response.
Also, this reduction in MP-specific INF-gamma-producing cells was paralleled by an increase in the frequency of IL-10-producing MP-specific cells, and a reduction in the ability of CD8-positive MP-specific cells to kill target cells that had been loaded with MP peptides. There was no reduction in the overall frequency of MP-specific T cells after return of the immature dendritic cells to account for this change in antigen response.