HUMAN PERIPHERAL-BLOOD STEM-CELLS

''Peripheral blood stem cells'' (PBSC) usually refer to apheresis prod ucts collected in patients primed with chemotherapy and/or recombinant hematopoietic growth factors such as granulocyte-colony stimulating f actor (G-CSF) or granulocyte-macrophage-colony stimulating factor (GM- CSF). The proportion of true ''stem cells'' in these cell populations is very small, much smaller than the frequency of other, more mature p rogenitors, or of differentiated cells. However, the use of apheresis products has greatly changed the clinical practice of high-dose chemot herapy over the last few years. Harvesting is easy and does not requir e general anesthesia, as does bone marrow collection. Time to hematopo ietic recovery is usually shorter in patients receiving PBSC as compar ed to patients receiving bone marrow. In association with the use of h ematopoietic growth factors during chemotherapy-induced cytopenia, PBS C help reduce morbidity associated with high-dose chemotherapy, and th us allow more patients with poor risk cancer to benefit from this tech nique. Many scientific questions remain unsolved. Identification of st em cells is still hampered by the lack of a simple and reproducible in vitro assay. These difficulties do not allow to accurately appreciate the contribution of different populations of progenitors to hematopoi etic recovery, and thus to predict the quality of a hematopoietic graf t. The ability of PBSC to repopulate lymphoid lineages is poorly defin ed. Despite these uncertainties, PBSC have numerous potentialities. As sociated with the clinical use of techniques such as selection of norm al progenitors (based on the expression of the CD34 antigen for exampl e), or liquid cultures supplemented with recombinant cytokines (the so -called ''ex-vivo expansion''), they offer the opportunity to change t he balance between normal hematopoietic progenitors on one hand, and r esidual tumor cells or accessory cells such as T lymphocytes on the ot her hand. As a consequence, purging or prevention of graft-versus-host disease with T cell depletion can be envisioned from a new standpoint . Finally, PBSC represent a very interesting target for genetic manipu lations. These multiple potentialities explain why the use of apheresi s products contributes to broaden the application of hematopoietic ste m cell transplantation, a practice that in many cases comes closer to transfusion. It also explains why PBSC already appear as a matter of e conomic and ethical controversies, being one of the first widely used ''cellular therapies''.