MAC-1 marks a quiescent and functionally superior HSC subset during regeneration

Mouse hematopoietic stem cells (HSCs) have been extensively defined both molecularly and functionally at steady state, while regenerative stress induces immunophenotypical changes that limit high purity isolation and analysis. It is therefore important to identify markers that specifically label activated HSCs to gain further knowledge about their molecular and functional properties. Here, we assessed the expression of macrophage-1 antigen (MAC-1) on HSCs during regeneration following transplantation and observed a transient increase in MAC-1 expression during the early reconstitution phase. Serial transplantation experiments demonstrated that reconstitution potential was highly enriched in the MAC-1(+) portion of the HSC pool. Moreover, in contrast to previous reports, we found that MAC-1 expression inversely correlates with cell cycling, and global transcriptome analysis showed that regenerating MAC-1(+) HSCs share molecular features with stem cells with low mitotic history. Taken together, our results suggest that MAC-1 expression marks predominantly quiescent and functionally superior HSCs during early regeneration.