Side population cells in highly enriched CD34-positive cells from peripheral blood progenitor cells identify an immature subtype of hematopoietic progenitor cells but do not predict time to engraftment in patients treated with high-dose therapy.

Objective: A Hoechst 33342 dye efflux assay can be used to define a population of immature hematopoietic progenitor cells (HPC) that are called side population (SP) cells. Previously, SP cells examined from bone marrow (BM) and peripheral blood progenitor cells (PBPC) were found to be predominantly CD34 negative.

Methods And Results: In this study, we show that the level of CD34+ cells within the SP fraction increases from 2% in BM to 15% in mobilized PBPC. Furthermore, SP cells are found in highly enriched CD34+ cells from both BM and PBPC, and these cells define an immature phenotype of HPC. We also observed a higher level of CD133+ cells within the SPCD34+ cell population. Moreover, the frequency of long-term culture-initiating cells (LTC-IC) was markedly increased in SPCD34+ cells. To further investigate whether variations in the level of SP cells in the CD34+ cell fraction influenced short-term engraftment, we studied 20 patients with Hodgkin lymphoma that were autotransplanted with highly enriched CD34+ cells from PB. The percentage of SP cells in the PBCD34+ cell fraction was highly variable, ranging from 0.3 to 22%. No correlation was found between the content of SP cells in the autotransplanted CD34+ cells and time to short-term engraftment.

Conclusion: SPCD34+ cells in PBPC define an immature phenotype of HPC with increased numbers of LTC-IC, and they are more frequently found in PBPC than in BM. The number of SP cells does not predict time to engraftment.