The role of accessory cells in allogeneic peripheral blood stem cell transplantation

Granulocyte colony-stimulating factor (G-CSF)-mobilized peripheral blood mo nonuclear cells (G-PBMC) have been used increasingly to reconstitute hemato poiesis after myeloablative therapy in allogeneic transplantation. Compared with conventional bone marrow, faster engraftment is consistently observed with G-PBMC, with differences more pronounced in platelet than in neutroph il recovery. G-PBMC contain not only severalfold more CD34+ cells than bone marrow but also, on average, 50-fold more monocytes, which may stimulate s tromal cell function and facilitate engraftment. Although G-PBMC also conta in 10-fold more T cells, the incidence and severity of acute graft-vs.-host disease (GVHD) is no higher than that observed in allogeneic bone marrow t ransplantation. Hypothetically, these clinical observations can be explaine d by the direct effect of G-CSF on T cell function as demonstrated by polar ization of T cells expressing the T helper type 2 (Th 2) cytokine interleuk in (IL)-4 in the murine model. Alternatively, G-PBMC may contain cells that actively suppress donor T cell responsiveness. Recent reports indicate tha t the large number of CD14+ monocytes in G-PBMC can suppress donor T cell p roliferation in vitro. This effect may be attributable to both the increase d ratio of CD14+:CD3+ cells in G-PBMC and the evidence that CD14+ cells in G-PBMC have decreased expression of both B7.2 and HLA-DR. There is some ind ication that natural killer (NK) cell number and function may be augmented in G-PBMC, which could have a favorable impact on the graft-vs.-leukemia (G VL) effect. Therefore, both the CD34+ and accessory cell content of G-PBMC may be important in early engraftment by controlling acute GVHD and facilit ating GVL. (C) 1999 The Japanese Society of Hematology.