Homing and Engraftment of Mobilized Hematopoietic Stem Cells: Involvement of VLA-5.

VLA-5 has been implicated in the adhesive interactions of stem and progenitor cells with the bone marrow extracellular matrix and stromal cells and is therefore considered to play an important role in the hematopoietic reconstitution after stem cell transplantation. In normal bone marrow (BM) from CBA/H mice 79±3% of the cells in the lineage negative fraction express VLA-5. After mobilization with cyclophosphamide/G-GSF, the number of VLA-5 expressing cells in mobilized peripheral blood cells (MPB) decreases to 38±3%. Despite this low frequency of VLA-5⁺ cells, however, even when equal numbers of progenitor cells are transplanted MPB cells provide a much faster hematopoietic recovery compared to BM cells. To shed more light on the role of VLA-5 in the process of homing and engraftment, we investigated whether differences in homing potential of the stem cell subsets might be responsible for this enhanced reconstitution. At 3 hours post-transplant, however, no differences in homing efficiency of progenitor and stem cells from MPB and BM grafts in both bone marrow and spleen could be detected. It should be realized that MPB and BM grafts demonstrate different ratios of stem/progenitor cells which might be another explanation for the observed differences in repopulation potential. Furthermore, MPB cells migrating in vitro towards SDF-1α showed potent reconstitution while VLA-5 expression was reduced on these cells. In fact, in vitro treatment with SDF-1α showed further decrease in VLA-5 expressing cells (from 38% to 4%) in the lin- fraction. When equal numbers of MPB were transplanted with and without SDF-1α pretreatment, no difference in hematopoietic reconstitution was observed suggesting a minor role of VLA-5 in homing and engraftment. On the other hand, after VLA-5 blocking an inhibition of 59±7% in the homing of MPB progenitor cells in the bone marrow could be found, whereas homing in the spleen of the the recipients is only inhibited by 11±4%. To elucidate whether the observed enhanced reconstitution could be explained by a selective homing of VLA-5⁺ cells or a rapid upregulation of VLA-5 expression, cells were labelled with PKH67-GL and transplanted in lethally irradiated recipients. It could be demonstrated that at 3 hours post-transplant cells from MPB grafts showed a rapid increase from 38±3% up to 66±9% of VLA-5⁺ cells in the bone marrow of the recipient. In the spleen no significant increase in VLA-5⁺ cells was observed. When MPB cells were transplanted after pretreatment with SDF-1α an increase from 2±1% up to 33±5% of VLA-5⁺ cells in the bone marrow was detected. When calculating the number of cells recovered from bone marrow, a selective homing of VLA-5⁺ cells cannot be excluded. Therefore, we also assessed the number of VLA-5⁺ cells in the PKH⁺ fraction in peripheral blood from the recipient immediately (½-1 hour) after transplantation but found no increase during that time period. So far it can be concluded that MPB cells show low number of VLA-5⁺ cells but these cells possess an enhanced hematopoietic reconstitution potential. Homing of progenitor cells to the spleen seems to be less dependent on VLA-5 expression than homing to the bone marrow. A rapid upregulation of VLA-5 expression on engrafting MPB cells early after transplantation does not occur and hence our data are suggestive for the preferential homing of VLA-5⁺ cells in the bone marrow after transplantation.