Positive selection of autologous peripheral blood stem cells

The development of monoclonal antibodies against differentiation antigens o n human haematopoietic cells has led to a new concept in stem cell purifica tion: the positive selection. In terms of autologous PBSC transplantation, the immature stem cells are identified by their expression of a specific an tigen, the CD34. The CD34 antigen is expressed on early lymphohaematopoieti c stem cells and progenitor cells, but not on mature blood cells or on tumo ur cells of several diseases. CD34(+) cells are found in low numbers in bon e 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 gro wth factors. Several techniques have been set up to enrich PBSC grafts in CD34(+) stem c ells. The quality of each system is here analysed in terms of CD34 purity o f the selected cell fraction, the CD34 cell recovery, the tumour cell deple tion efficiency and the functional capacity ex vivo and in vivo of the sele cted cells. The final CD34(+) cell purity of the selected fractions is corr elated to the concentration of CD34(+) cells before selection. The optimal recoveries and the highest purities were generally obtained when the initia l CD34 content was roughly over 1%. Below this figure, the final purity see ms to be less predictable. Besides the better tolerance resulting from the reduction in the number of autologous cells, and consequently the total vol ume of DMSO reinfused to the patient, the selective enrichment of the CD34 cell population offers a new approach to tumour purging. The procedure by i tself results in elimination of about: 99% in the total number of initial c ells, thus allowing reduction of the overall tumour cell number in the fina l autograft. However, its major interest is that, in diseases where tumour cells do not express the CD34 antigen, it is theoretically able to complete ly eliminate the tumour contamination of the graft. Based on previous data showing that lymphoma, myeloma, neuroblastoma and br east cancer cells are not CD34(+), pilot clinical trials for the separation and transplantation of CD34(+) cells selected from PBSC of patients with t hese 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 CD3 4(+) cells are capable of supporting haematopoietic reconstitution in autol ogous transplantation. However, until now no study has demonstrated clear e vidence that the reduction of tumour cells from PBSC of patients by CD34(+) cell selection resulted in a lower relapse rate post-transplant, as compar ed to unselected PBSC infusion.