initiating cells. 20,21 The CD34 antigen is not detectable on 
the surface of most tumor cells, including breast cancer and 
myeloma cells. 22 It is present on both normal and bcr-abl + 
progenitors in patients with CML. 23,24 The expression of the 
CD3 4 antigen on hematopoietic progenitors and its absence on 
more mature cells and on most types of tumor cells has led 
to the development of positive CD34 selection techniques for 
use in autologous transplantation. 
Possible advantages of using CD34-enriched marrow for 
autologous transplantation include depletion of tumor cells 
and a reduction in the volume of marrow being cryopreserved 
and then given back to the patient, with an associated 
reduction in complications resulting from the infusion of 
cellular debris and freezing solution. Use of CD34-enriched 
hematopoietic cells for gene transfer studies has the 
practical advantage of requiring a much lower volume for 
transduction, and the related theoretical advantage of 
allowing a higher multiplicity of infection of viral 
particle to progenitor or. stem cell target, as well as 
depletion of possible inhibitory cells. 
Berenson and coworkers have developed an immunoadsorption 
technique for CD34+ cells that relies on the high affinity 
between the protein avidin and the vitamin biotin, 
(dissociation constant=10" 15 M) Cells are incubated with 
biotinylated anti-CD34 monoclonal antibody (12.8) and then 
passed through a column of avidin-coated beads. Cells with 
antibody bound to their surface are retained on the column 
while nonlabelled cells pass through. Bound cells are then 
removed from the column by mechanical agitation of the 
beads, which disrupts the relatively weak CD3 4-antibody 
interaction. Initial human studies showed that the CD34- 
enriched fraction was 35-92% pure (as assessed by CD34 
antibody staining) and enriched for CFU-GM 22-180 fold. 25 
A pilot study using CD34-selected bone marrow cells for 
autologous transplantation in heavily pretreated breast 
cancer and neuroblastoma patients found that the CD34- 
enriched population was capable of hastening engraftment 
following myeloablative therapy. 25 A more recent Phase I/II 
clinical trial of 18 patients showed rapid engraftment with 
a mean of 2.0 X 10 6 CD34+ cells/kg, with neutrophil and 
platelet recovery at 11 and 16 days post-transplant in 9 
patients given G-CSF after transplantation, (personal 
communication. Dr. Ronald Berenson) A Phase III trial 
comparing CD34-selected to nonselected marrow in autologous 
transplantation is about to begin. Peripheral blood stem 
cells can also be CD34-enriched using this technique, and 
transplantation with CD34-enriched PBSCs are in progess . 26,27 . 
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Recombinant DNA Research, Volume 16 
