CD97 Is Differentially Expressed on Murine Hematopoietic Stem Cells and Progenitor Cells.

CD97 is a member of the EGF-TM7 family of class II seven-span transmembrane receptors and is broadly expressed on hematopoietic cells including lymphocytes, granulocytes, and monocytes. We have recently demonstrated that CD97 is involved in IL-8-induced hematopoietic stem cell (HSC) mobilization (

We first studied expression levels on murine bone marrow (BM) cells using a CD97 specific monoclonal antibody (clone 1B2) and FACS analysis. Based on CD97 expression levels, BM cells were then sorted into different fractions and further characterized in colony-forming assays, CAFC assays and in an in-vivo transplantation model.

FACS analysis of BM cells showed three major populations i.e. CD97^HI, CD97^INT and CD97^NEG cells (71.5%, 24.4% and 4.4% of total BM cells respectively). Analysis of CFU-GM colony forming capacity of these BM subsets revealed that the majority of colony-forming cells were present in the CD97^NEG (7142 ± 5057 CFU-GM) population compared to CD97^HI (178 ± 170CFU-GM) and CD97^INT (3047 ± 2902 CFU-GM per 10⁶ BM cells). Analysis of CAFC frequencies of CD97^HI, CD97^INT and CD97^NEG BM cells showed that CAFC-day 35 activity resided in the CD97^INT BM population (3.0 ± 1.2 vs. 0.9 ± 0.1 CAFC-day 35 per 10⁵ cells for CD97^INT and total BM respectively), whereas no CAFC-day 35 activity was found in CD97^HI and CD97^NEG BM fractions. In addition, FACS analysis revealed that the majority (82.6 %) of c-kit^POSthy-1^LOLin^NEG/LO cells were present in the CD97^INT BM population. To investigate the in-vivo repopulating ability of the different CD97 BM subsets, lethally irradiated (9.5 Gy) BALB/c recipient mice (n=10 per group) were reconstituted with 10⁵ syngeneic BALB/c total BM or with CD97 sorted BM cells. Repopulating capacity entirely resided in the CD97^INT subset (repopulation rate 90% versus 0% in the CD97^HI and CD97^NEG subset).

These data indicate that 1) CD97 is differentially expressed on HPC and HSC and that 2) CD97 expression can be used to separate colony forming cells from repopulating hematopoietic stem cells.