In Response:

Abstract

In our study, we classify CD34+Lin−Thy-1+ cells as human stem cells because the Thy-1+ subpopulation of CD34+ cells have self-renewal characteristics of human stem cells and are capable to generate all hematopoietic lineages. Effectively, primitive hematopoietic stem cells in normal human peripheral blood and bone marrow have been characterized [1Udomsakdi C. Lansdorp P.M. Hogge D.E. Reid D.S. Eaves A.C. Eaves C.J. Characterization of primitive hematopoietic cells in normal human peripheral blood.Blood. 1992; 80: 2513PubMed Google Scholar, 2Srour E.F. Brandt J.E. Briddell R.A. Leemhuis T. van Besien K. Hoffman R. Human CD34+ HLA-DR-bone marrow cells contain progenitors capable of self-renewal, multilineage differentiation, and long-term in vitro hematopoiesis.Blood Cells. 1991; 17: 287PubMed Google Scholar, 3Baum C.M. Weissman I.L. Tsukamoto A.S. Buckle A.M. Peault B. Isolation of a candidate human hematopoietic stem cell population.Proc Natl Acad Sci U S A. 1992; 89: 2804Crossref PubMed Scopus (888) Google Scholar]. These cells are CD34+Lin−Thy-1+, constitute less than 0.05% of whole bone marrow, are capable of multilineage differentiation, including myeloid, B-, and T-cell lineages, and have extensive self-renewal capacity [3Baum C.M. Weissman I.L. Tsukamoto A.S. Buckle A.M. Peault B. Isolation of a candidate human hematopoietic stem cell population.Proc Natl Acad Sci U S A. 1992; 89: 2804Crossref PubMed Scopus (888) Google Scholar, 4Kyoizumi S. Baum C.M. Kaneshima H. McCune J.M. Yee E.J. Namikawa R. Implantation and maintenance of functional human bone marrow in SCID-hu mice.Blood. 1992; 79: 1704PubMed Google Scholar]. CD34+Lin−Thy-1+ cells resemble hematopoietic stem cells based on in vitro assays for long-term culture-initiating cells and in vivo assays for hematopoietic stem cells utilizing the SCID-hu bone assay system [5Craig W. Kay R. Cutler R.L. Lansdorp P.M. Expression of Thy-1 on human hematopoietic progenitor cells.J Exp Med. 1993; 177: 1331Crossref PubMed Scopus (421) Google Scholar] and SCID-hu thymus assay system [6Peault B. Weisman I.L. Baum C. McCune J.M. Tsukamoto A. Lymphoid reconstitution of the human fetal thymus in SCID mice with CD34+ Precursor cells.J Exp Med. 1991; 174: 1283Crossref PubMed Scopus (130) Google Scholar]. CD34+Lin−Thy-1+ cells have been shown to be able to fully reconstitute hematopoiesis in laboratory animals including non-human primates [7Donahue R.E. Kessler S.W. Kirby M.R. et al.Autologous bone marrow transplantation in non-human primates using a subpopulation of CD34+ cells expressing Thy-1.Blood. 1993; 82: 1175PubMed Google Scholar]. In humans, CD34+Lin−Thy-1+ cells can be mobilized in the peripheral blood using a high dose of cyclophosphamide and growth factor regimens and isolation in sufficient numbers to potentially permit hematologic reconstitution following myeloablative regimen. It was observed that the percentage of Thy-1+ in the CD 34+ population of successfully mobilized PBSC was regularly increased well above that seen in resting bone marrow, and it is far from clear what that means. The mobilization phenomenon may cause selective release of ontologically more primitive cells. Our study aimed at finding the minimal effective dose, which at that time was thought to be very low, and the median dose finally used in our study reflected the cell-dose-de-escalation design of the study with an actual median dose of 0.69. Therefore it was decided to base the analysis on a 0.8 × 106 CD+Lin−Thy-1+/kg threshold dose reported by Systemix in a more extensive analysis of a multidisease population. CD34+Lin−Thy-1+ cells are capable of producing fast and durable hematopoietic engraftment at cell doses greater than 0.8 × 106 CD34+ Thy−1+/kg, and the two patients (8 and 30) who experienced delayed engraftment developed sepsis, followed (for patient 30) by an hemolytic and uremic syndrome. Considering the number of CD34+ cells required for rapid engraftment, our results are comparable to those obtained after the infusion of 2.5 × 106 to 15 × 106 CD34+cells. Moreover, it has been demonstrated that the rapidity of hematopietic reconstitution was related to the number of CD34+ cells infused [8Ketterer N. Salles G. Raba M. et al.High CD 34+ cell count decrease hematologic toxicity of autologous peripheral blood progenitor cell transplantation.Blood. 1998; 91: 3148PubMed Google Scholar].