Murine Hematopoietic Progenitor Cells With Colony-Forming or Radioprotective Capacity Lack Expression of the b2-Integrin LFA-1

Recently, we have demonstrated that antibodies that block the function of the b2-integrin leukocyte function-associated antigen-1 (LFA-1) completely abrogate the rapid mobilization of hematopoietic progenitor cells (HPC) with colonyforming and radioprotective capacity induced by interleukin-8 (IL-8) in mice. These findings suggested a direct inhibitory effect of these antibodies on LFA-1–mediated transmigration of stem cells through the bone marrow endothelium. Therefore, we studied the expression and functional role of LFA-1 on murine HPC in vitro and in vivo. In steady state bone marrow 6 50% of the mononuclear cells (MNC) were LFA-1neg. Cultures of sorted cells, supplemented with granulocyte colony-stimulating factor (G-CSF)/granulocyte-macrophage colony-stimulating factor (GM-CSF)/IL-1/ IL-3/IL-6/stem cell factor (SCF) and erythropoietin (EPO) indicated that the LFA-1neg fraction contained the majority of the colony-forming cells (CFCs) (LFA-1neg 183 6 62/7,500 cells v LFA-1pos 29 6 17/7,500 cells, P F .001). We found that the radioprotective capacity resided almost exclusively in the LFA-1neg cell fraction, the radioprotection rate after transplantation of 103, 3 3 103, 104, and 3 3 104 cells being 63%, 90%, 100%, and 100% respectively. Hardly any radioprotection was obtained from LFA-1pos cells. Similarly, in cytokine (IL-8 and G-CSF)–mobilized blood, the LFA-1neg fraction, which comprised 5% to 10% of the MNC, contained the majority of the colony-forming cells, as well as almost all cells with radioprotective capacity. Subsequently, primitive bone marrow-derived HPC, represented by Wheat-germ-agglutinin (WGA)1/Lineage (Lin)2/Rhodamine (Rho)2 sorted cells, were examined. More than 95% of the Rho2 cells were LFA-1neg. Cultures of sorted cells showed that the LFA-1neg fraction contained all CFU. Transplantation of 150 Rho2 LFA-1neg or up to 600 Rho2LFA-1pos cells protected 100% and 0% of lethally irradiated recipient mice, respectively. These results show that primitive murine HPC in steady-state bone marrow and of cytokine-mobilized blood do not express LFA-1. r 1999 by The American Society of Hematology.