PERIPHERAL-BLOOD STEM-CELL TRANSPLANTATION - CRITICAL-REVIEW

Autologous blood stem cell transplantations have been increasingly per formed worldwide for almost ten years in place of autologous bone marr ow transplantation and even of allogenic bone marrow transplantation. Several crucial issues were the subjects of impassioned controversies. Some of them are now satisfactorily answered while others still remai n unresolved. First, it is now possible to conclude today that periphe ral blood stem cells (PBSC) are undoubtedly capable of restoring short term hematopoiesis when reinfused after myeloablative therapy as well and even more rapidly than bone marrow stem cells, provided that they have been previously collected in sufficient amounts. On the opposite , it is still impossible to firmly prove that their very immature CD34 + cell subset, although in vitro functionally and phenotypically almos t identical to their marrow counterpart, is actually responsible for s ustained long term hematopoietic recovery, even if it is likely that t hese cells play a key role. Most of the time, using chemotherapy alone or a combination of chemotherapy and cytokine(s), mobilizing regimens allow collection of appropriate yields of PBSC with only a small numb er of apheresis cycles, provided that a sufficient number of residual stem cells remains to be stimulated, when, on the contrary, collection in steady-state is time-consuming and does not provide further accele rated post transplant hematopoietic recovery. It was initially hypothe sized that PBSC could have a lower likelihood of tumoral contamination compared with bone marrow. In fact, biological as well as clinical da ta are discordant and probably depend largely on the type of disease, its evolutive history and its way of dissemination. Furthermore, the r espective impact on the development of further relapse of graft contam ination and of residual tumor cells into patient remains to be determi ned. Finally, although it has often been claimed that the cost of mobi lization, collection and cryopreservation of PBSC would be much higher than the cost of bone marrow harvesting, it is now possible to assert that the whole ABSCT procedure, including this preliminary phase, as well as the post-transplant period, allows an indisputable saving comp ared with ABMT. These advantages are already sufficient reasons ''per se'' to propose ABSCT in place of ABMT or allo-BMT in many indications even if their clinical benefit, in terms of disease-outcome, remains to be prospectively explored.