MAC1 AS A POTENTIAL MARKER FOR FUNCTIONAL HEMATOPOIETIC STEM CELLS DURING EARLY REGENERATION AFTER TRANSPLANTATION

Abstract

Mouse hematopoietic stem cells (HSCs) can be isolated to high purity using Lineage-, Sca1+, cKit+, CD48-, CD150+ (LSK SLAM) as markers. Cycling HSCs in the fetal liver have been shown to express the CD11b/CD18 Macrophage-1 antigen (Mac1), a marker expressed mostly on mature cells in the adult bone marrow (Weissman et al., 1995). Furthermore, Mac1 is upregulated in activated adult HSCs after treatment with the cytotoxic drug 5-fluorouracil. For this reason, Mac1 should be omitted from the lineage staining when analyzing fetal liver and 5-FU treated mice. Here we asked whether Mac1 expression is altered also during reconstitution after transplantation into irradiated recipients, and if it can be used to further define the functional HSC compartment in a state of active regeneration. To address this question, LSK cells were transplanted into lethally irradiated (900cGy) primary recipient mice. When HSCs were analysed for expression of Mac1 at several time points after transplantation (3, 4, 8 and 16 weeks), we observed an increase in the proportion of Mac1+ cells. To investigate if functional HSC properties are linked to the observed changes in Mac1 expression, we performed secondary transplantations of Mac1+ and Mac1- HSCs. At all times during regeneration, we found robust reconstituting activity in the Mac1+ HSCs. Remarkably, we observed that the reconstituting activity is higher in Mac1+ compared to Mac1- HSCs during the early phase of regeneration. In the late phase of regeneration, however, no difference in reconstitution could be seen. Taken together, our findings suggest that Mac1 expression is associated with functional HSC activity during early regeneration and we are currently investigating in more detail qualitative, as well as quantitative aspects of HSCs in relation to Mac1 expression.