Introduction Transplantation of autologous or allogeneic hem

Introduction Transplantation of autologous or allogeneic hematopoietic stem growth hormone secretagogue (HSCs) dramatically improved the disease free survival of numerous patients with hematological diseases and presently remains the only widely used cellular transplantation procedure (Deeg and Bartenstein, 2011; Gladstone and Fuchs, 2012). Bone marrow (BM) HSCs are characterized by their life-long capacities to either remain quiescent or proliferate and to either self-renew or differentiate (Lerner and Harrison, 1990; Bradford et al., 1997). These tightly regulated balances allow the life-long homeostatic production of mature functional blood cells, as well as the adaptation/reconstitution of hematopoiesis after infection, hemorrhage or myeloablative stress. During the last 40years the continuous improvement of our knowledge about hematopoiesis and the development of suitable in vitro techniques for the large-scale isolation, maintenance and expansion of HSCs have increased the numbers and indications of hematopoietic transplantations. They have reduced in parallel the duration of the post-transplantation cytopenia and its related morbidity/mortality consequences. In 1984, the identification of the CD34 glycoprotein as a marker of HSCs and of hematopoietic progenitors (HPs) (Civin et al., 1984) opened the way to their isolation from BM, umbilical cord blood (CB) and blood mobilized CD34+ cells and to the development of transplantation with in vitro expanded cells. However medical, economical and/or technical constraints still limit their therapeutic use in some patients and/or countries. The main purpose of this brief review is to discuss the potential interest of steady state peripheral blood (SSPB) CD34+ cells trapped in leukoreduction filters (LRFs), which are discarded after the preparation of therapeutic red blood cell (RBC) concentrates, as a new alternative source of HSCs for research and hematopoietic transplantation. We will also mention and discuss briefly some other possible uses for the CD34+ cells and other cell types eluted from these LRFs.
Current therapeutic HSC sources: interests and limitations
Steady state peripheral blood as a source of HSCs? The limitations described above and some results published years ago (briefly mentioned below and reviewed in Körbling and Freireich, 2011) encouraged us to explore LRFs as an alternative new source of CD34+ cells for basic research, development and future cell therapy/engineering. Indeed, in the 80\'s, it was shown by three groups that autologous HSCs from SSPB collected by repeated cytapheresis allowed a complete hematological reconstitution in delays comparable to those observed with BM (Kessinger et al., 1986; Reiffers et al., 1986; To et al., 1984). Some years later, it was evidenced that SSPB contains 1 to 4×103 CD34+cells/mL (Bender et al., 1991; Herbein et al., 1994) and that their counting by flow cytometry was a reliable marker of their engrafting capacity (Siena et al., 1991).
Most of the potential therapeutic uses of engineered cells and tissues did not yet reach the bedside level due to limitations such as: i) technical/ethical difficulties to obtain some types of stem cells; ii) insufficient knowledge of the mechanisms controlling their self-renewal/differentiation balance and their homing to one specific tissue; and iii) still suboptimal in vitro cellular reprogrammation procedures. Adult stem cells are promising sources for regenerative medicine (Zuba-Surma et al., 2012; Sánchez et al., 2012). Those trapped in LRFs from healthy blood donors are thus an interesting potential material since discarded filters are easily available in large quantities. We suggest below some possible applications for these cells. LRF CD34+ cells are good candidates for cell/tissue engineering such as cardiac cell therapy. Indeed, according to the recent procedure set-up by Losordo et al. (2007)) and latter confirmed by a phase II study (Losordo et al., 2011), an intramyocardial injection of a low dose of G-CSF-mobilized autologous CD34+ cells (1×105cells/kg) led to a significant functional cardiac improvement. Autologous SSPB CD34+ cells collected by apheresis could be thus compatible with this application and could be easily tested. Industrial production of RBCs could also be one important opening for LRF CD34+ cells since the procedure developed by the group of Douay (Douay, 2010) with CB cells will probably reach the therapeutic level in the next few years. We recently demonstrated that LRF CD34+ cells could also give rise to RBC production ex vivo (Vlaski et al., 2009). Using SSPB instead of CB would facilitate the recruitment of numerous samples from a large panel of donors.