Background: CD97 is a member of the epidermal growth factor-seven transmembrane (EGF-TM7) family of adhesion receptors and is broadly expressed on hematopoietic cells. The aim of this study was to investigate the expression of CD97 on hematopoietic stem- and progenitor cells (HSC/HPC).
Design and methods: CD97 expression on hematopoietic stem- and progenitor cells was studied in BALB/c, C57BL/6 and DBA/1 mice using flow cytometry. Functional hematopoietic stem- and progenitor cell characteristics were investigated in vitro and in vivo by progenitor cell assays, cobblestone area forming cell assays and bone marrow cell transplantation.
Results: Analysis of CD97 expression on murine bone marrow cells showed three major populations i.e. CD97(HI), CD97(INT) and CD97(NEG) cells. Functional studies revealed that radioprotective capacity and cobblestone area forming cell day 28-35 activity resides in the CD97(INT) bone marrow cell fraction while CFU-GM colony-forming capacity mainly resides in the CD97(NEG) population in all strains. In C57BL/6 and DBA/1 mice CD97(NEG) and CD97(HI) bone marrow cells show hematopoietic stem cell characteristics as well. Further functional analysis of BALB/c CD97(INT) bone marrow cells revealed that c-Kit(HI)CD97(INT) bone marrow cells exhibit HSC activity and are 1.5-fold enriched for cobblestone area forming cell-day 35 activity compared to c-Kit(HI) bone marrow cells. Moreover, phenotypical analysis showed that BALB/c and C57BL/6 HSC are CD97(INT), while DBA/1 HSC are CD97(HI).
Conclusions: CD97 is differentially expressed on hematopoietic stem cells and hematopoietic progenitor cells. Committed progenitor cell activity is largely comprised in the CD97(NEG) fraction, while the CD97(INT) population contains hematopoietic stem cell activity. In BALB/c mice, CD97 expression can be applied to almost completely separate colony-forming cells and cells exhibiting radioprotective capacity. In addition we propose that the CD97(INT)c-Kit(HI) phenotype allows simple and rapid purification of murine hematopoietic stem cells.