INDUCTION OF LFA-1 ON PLURIPOTENT CD34(-MARROW CELLS DOES NOT AFFECT LINEAGE COMMITMENT() BONE)

Leukocyte function associated antigen 1 (LFA-1) is an adhesion molecul e indispensable in immune and inflammatory reactions, but its role in hematopoiesis remains obscure. Since LFA-1 is predominantly expressed by leukocytes, it is considered as a marker of late stage stem cell ma turation when expressed on CD34(+) bone marrow cells, and represents m ore mature hematopoietic progenitor cells. We observed that freshly is olated CD34(+) bone marrow cells express LFA-1, and that the level of expression is highly variable. Interestingly, the expression of the LF A-1 specific activation epitope L16 on these cells is low, even after culture. This demonstrates that LFA-1 is not activated, as was confirm ed by low adhesion to ICAM-1. Culturing sorted CD34(+)LFA-1(+) cells i n single cell per well assays in medium supplemented with SCF, Epo, IL -3, IL-6, GM-CSF, and G-CSF revealed that they gave rise to dispersed macrophage-like colonies, supporting the notion that CD34(+)LFA-1(+) c ells indeed consist of a mature committed cell population. In contrast , sorted CD34(+)LFA-1(-) cells had high proliferative potential and de veloped into large multilineage colonies within 14 days of culture. Un anticipated, in time course experiments we observed that these CD34(+) LFA-1(-) cells expressed LFA-1 within 24 hours upon culture. This indu ction was neither caused by the monoclonal antibody used to tag CD34 c ells, nor dependent on growth factors present in the medium. These fin dings demonstrate that two populations of CD34(+)LFA-1(+) cells can be discriminated: leukocyte lineage committed CD34(+) cells in freshly i solated bone marrow cells, and multipotent CD34(+) cells that acquired LFA-1 upon in vitro culture. These in vitro findings support the hypo thesis that once contacts with bone marrow stroma are lost, LFA-1 is u pregulated by default, due to the lack of negative regulating signals from stromal cells. This might also explain the widely variable expres sion of LFA-1 as a result of crowding of cells in the bone marrow with subsequent loss of contact with stroma and upregulation of LFA-1, pro viding those cells with adhesion receptors enabling migration in the p eriphery. (C) 1996 by The American Society of Hematology.