Differential Expression of α2 Integrin Separates Long-Term and Short-Term Reconstituting Lin−/loThy1.1loc-kit+ Sca-1+ Hematopoietic Stem Cells

Abstract

Self-renewing, multipotent hematopoietic stem cells are highly enriched within the Lin− Thy1.1loc-kit+ Sca-1+ subset of mouse bone marrow. However, heterogeneous expression within this population of certain cell surface markers raises the possibility that it may be further fractionated phenotypically and perhaps functionally. We previously identified α2-integrin (CD49b) as a surface marker with heterogeneous expression on Lin− /loThy1.1loc-kit+ Sca-1+ stem cells. To determine whether differences in α2 expression were indicative of differences in stem cell function, we purified α2− and α2hi stem cells by fluorescence-activated cell sorting and analyzed their function in long- and short-term hematopoietic reconstitution assays. Both α2− and α2hi cells could give rise to mature lymphoid and myeloid cells after transplantation into lethally irradiated congenic recipients. However, α2hi cells supported hematopoiesis for only a short time (<4 weeks), whereas α2− cells reproducibly yielded robust, long-term (>20 weeks) reconstitution, suggesting that α2− cells represent a more primitive population than do α2hi cells. Consistent with this idea, α2− Lin− /loThy1.1loc-kit+ Sca-1+ cells exhibited an approximately sixfold decreased frequency of spleen colony-forming units (day 12) versus α2hi cells. Furthermore, bone marrow cells isolated from animals transplanted >20 weeks previously with 20 α2− Lin− /loThy1.1loc-kit+ Sca-1+ cells included both α2− and α2hi stem cells of donor origin, indicating that α2hi cells are likely lineal descendents of α2− cells. Interestingly, α2 integrin expression is significantly reduced on lineage-restricted oligopotent progenitors in the marrow, suggesting that high level expression of α2 selectively marks a subset of primitive hematopoietic cells which retains multilineage reconstitution potential but exhibits reduced self-renewal capacity.