Addition of a second, different allogeneic graft accelerates white cell and platelet engraftment after T-cell–depleted bone marrow transplantation

Significant delays in engraftment and lymphoid recovery are the 2 major challenges in cord blood transplantation. The cause for this delay is presumed to be the low numbers of hematopoietic precursors found in one unit of cord blood. One approach to increase the stem cell doses could be to combine cord blood units from different donors differing at the major histocompatibility complex (MHC). As a first step toward this goal, the kinetics of hematologic engraftment and immune reconstitution were compared between 1 unit (2.5 x 10(6) cells) of T-cell-depleted bone marrow cells from a single donor (C57BL/6 H2(b) or SJL/J H2(s)) and 2 units from different donors (C57BL/6 + SJL/J) after transplantation into lethally irradiated (8.5 Gy) BALB/c recipients (H2(d)). Addition of T-cell-depleted bone marrow from an MHC-mismatched allogeneic donor doubled the white blood counts compared with recipients of one single unit on days +10 and +14. Similar effects were also observed on platelets. The beneficial effect of additional cells on peripheral T-cell counts were first observed on day +14. Cells both from donors (C57BL/6 and/or SJL/J) and recipients (BALB/c) contributed to myeloid and lymphoid reconstitution. The chimeras containing cells from 3 strains of mice were able to mount a recall immune response. Our data suggest that combining stem cells from MHC-mismatched allogeneic donors is feasible, that it has beneficial effects on myeloid engraftment and T-cell phenotypic recovery, and that the long-term stable mixed chimeras are immunologically normal following T-cell-depleted bone marrow transplantation.