Positive selection of autologous peripheral blood stem cells

The development of monoclonal antibodies against differentiation antigens on human haematopoietic cells has led to a new concept in stem cell purification: the positive selection. In terms of autologous PBSC transplantation, the immature stem cells are identified by their expression of a specific antigen, the CD34. The CD34 antigen is expressed on early lymphohaematopoietic stem cells and progenitor cells, but not on mature blood cells or on tumour cells of several diseases. CD34+ cells are found in low numbers in bone marrow (<2%) and in even lower numbers in steady state blood (<0.01%) but may increase from 1 to 5% after mobilization using chemotherapy and/or growth factors. Several techniques have been set up to enrich PBSC grafts in CD34+ stem cells. The quality of each system is here analysed in terms of CD34 purity of the selected cell fraction, the CD34 cell recovery, the tumour cell depletion efficiency and the functional capacity ex vivo and in vivo of the selected cells. The final CD34+ cell purity of the selected fractions is correlated to the concentration of CD34+ cells before selection. The optimal recoveries and the highest purities were generally obtained when the initial CD34 content was roughly over 1%. Below this figure, the final purity seems to be less predictable. Besides the better tolerance resulting from the reduction in the number of autologous cells, and consequently the total volume of DMSO reinfused to the patient, the selective enrichment of the CD34 cell population offers a new approach to tumour purging. The procedure by itself results in elimination of about 99% in the total number of initial cells, thus allowing reduction of the overall tumour cell number in the final autograft. However, its major interest is that, in diseases where tumour cells do not express the CD34 antigen, it is theoretically able to completely eliminate the tumour contamination of the graft. Based on previous data showing that lymphoma, myeloma, neuroblastoma and breast cancer cells are not CD34+, pilot clinical trials for the separation and transplantation of CD34+ cells selected from PBSC of patients with these diseases have recently been conducted. The efficacy of CD34 selection in reducing the tumour load of the PBSC of patients with these diseases has been reported. However, the efficacy of purging may greatly differ between individual patients, and complete eradication of contaminating cells from PBSC grafts was not always reached. There is now evidence that purified CD34+ cells are capable of supporting haematopoietic reconstitution in autologous transplantation. However, until now no study has demonstrated clear evidence that the reduction of tumour cells from PBSC of patients by CD34+ cell selection resulted in a lower relapse rate post-transplant, as compared to unselected PBSC infusion.