Regulation of LFA-1 Expression by CD34 Positive Cells and Inducible Growth Factor Production by Stroma Enable Formation of Bone Marrow Compartments; Subject Heading

Leukocyte function associated antigen 1 (LFA-1) is an adhesion molecule indispensable in immune and inflammatory reactions, but its role in hematopoiesis is poorly understood. LFA-1 is considered as a marker of late stage stem cell maturation when expressed on CD34(+) bone marrow cells. We observed that CD34(+) bone marrow cells express LFA-1, that based on LFA-1 expression several subpopulations can be distinguished, and that the level of expression appeared highly variable among different donors. Unanticipated, in time course experiments we observed that CD34(+) LFA-1(-) cells expressed LFA-1 within 24 hours upon culture. These in vitro findings support the hypothesis that once contacts with bone marrow stroma are lost, LFA-1 is upregulated by default, due to the lack of negative regulating signals from stromal cells. This might also explain the widely variable expression 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, thus equipping cells with adequate adhesion receptors to migrate throughout the bone marrow. Interestingly, the expression of the LFA-1 specific activation epitope L16 on these cells is low, even after culture. This demonstrated that LFA-1 is not activated, as was confirmed by low adhesion to ICAM-1. Activation of adhesion molecules is induced by several growth factors. Indeed, we show here that an osteoblastic cell line under normal conditions does hardly produce hematopoietic growth factors but these are rapidly induced after stimulation. Such rapid induction endows the bone marrow stroma with the property to modulate the adhesive strength and enabling migration through the different environments within the stroma. Prove for such compartments within the bone marrow is provided by histological data.