Mesenchymal stem cells remain host-derived independent of the source of the stem-cell graft and conditioning regimen used.

BACKGROUND: Human bone marrow contains hematopoietic stem cells and stroma cells known as mesenchymal stem cells (MSC). MSC are cells with the morphological features of fibroblasts, which, in addition to their nursing function for hematopoietic stem cells, retain the ability to differentiate into cartilage, bone, fat, muscle, and tendon and have an important immunmodulatory function. To understand in more detail hematopoietic engraftment and immune modulation after hematopoietic cell transplantation, we investigated the ability of donor MSC to engraft after hematopoietic cell transplantation in dependency to the conditioning regimen (myeloablative vs. reduced intensity) and source of the graft (bone marrow vs. peripheral blood). METHODS: Bone marrow MSC of 12 patients were analyzed, a median of 23.4 (range 0.9-137.8) months after human leukocyte antigen matched but gender mismatched bone marrow transplantation after myeloablative conditioning (n=4) or peripheral blood cell transplantation after myeloablative (n=4) or reduced intensity conditioning (n=4). MSC were characterized by morphology, positivity for CD 105+, CD73+, CD 44+, and CD 90+, and by their capacity to differentiate into adipocytic and osteogenic cells. Recipient and donor origins were determined by fluorescent in situ hybridization for sex chromosomes. RESULTS: While overall blood and bone marrow chimerism was 100% donor type, MSC remained in all patients of recipient origin (>96%). There was no difference between patients receiving bone marrow and peripheral blood grafts, nor was any difference observed between patients receiving full intensity in comparison with reduced intensity conditioning. CONCLUSIONS: We conclude that MSC remain of host type irrespective of the conditioning regimen and graft source.

Spontaneous remission of acute myeloid leukemia relapse after hematopoietic cell transplantation in a high-risk patient with 11q23/MLL abnormality.

A 35-year-old female patient was diagnosed with acute myeloid leukemia with multiple genetic aberrations [48 XX, del(3)(q21), +6, t(11;15)(q23;q15), +21] including an 11q23/MLL abnormality. The patient achieved a complete remission after one induction chemotherapy cycle. After three courses of consolidation, a matched unrelated hematopoietic cell transplantation (HCT) was performed. Following an upper respiratory tract infection 7 years after transplant, her blood counts declined to leukocytes of 1 x 10(9)/l, platelets of 51 x 10(9)/l and hemoglobin of 7.5 g/dl. A bone marrow aspirate revealed 55% leukemic blasts carrying the unfavorable genetic aberrations seen at initial diagnosis (11q23/MLL). In the absence of any disease-specific treatment, the leukemic blasts cleared from the bone marrow within 6 days after diagnosis of relapse and peripheral blood counts returned to normal. Molecular analysis of the 11q23/MLL rearrangement was used to evaluate minimal residual disease, which became undetectable in repetitive FISH analyses. This is the first report of spontaneous remission in a patient with initially a multiaberrant leukemic cell clone and a proven 11q23/MLL abnormality at relapse after HCT.

Allogeneic hematopoietic cell transplantation (HCT) following reduced-intensity conditioning in patients with acute leukemias.

The majority of patients with acute leukemia enter complete remission following induction therapy, but relapse despite consolidation and maintenance chemotherapy. Allogeneic hematopoietic cell transplantation (HCT) is the most effective consolidation therapy but unfortunately associated with high transplant-related mortality (TRM). In order to decrease TRM but still apply a graft-versus-tumor effect, allogeneic HCT protocols with reduced-intensity conditioning were developed and more than 5000 HCT, of which 1500 for acute leukemia, performed. Detailed information is available on more than 400 patients with acute leukemia. The results, summarized in this article, confirm that reduced-intensity preparative regimens lead to full donor chimerism and to generation of graft-versus-leukemia (GvL) effects with curative potential in older patients (>60 years). Prospective-controlled clinical trials are needed in younger patients to compare results of HCT after reduced-intensity conditioning to those of HCT with conventional conditioning.