Adhesion of B-Cell Chronic Lymphocytic Leukemia Cells to Marrow Stromal Cells is Mediated by α4ß1 but not ß2αL Integrin: MSC also Prevent Apoptosis of B-CLL Cells.

ABSTRACT

Interactions between MSC and B-CLL cells were investigated to better understand the role of adhesion proteins in the biology of B-CLL. The role of ß1 and ß2 integrins and CD44 in adherence of B-chronic lymphocytic leukemia (CLL) cells to bone marrow stromal cell (MSC) monolayers and the ability of MSC to prevent apoptosis of CLL cells was investigated. Peripheral blood mononuclear cells, from 8 patients with CD5-positive B-CLL, were effectively depleted of CD3-positive cells with an immunomagnetic column. Purity of B-CLL cells, as judged by coexpression of CD19 and CD5 on two-color fluor-ocytometry, was 92±4% (mean±SD) (n = 8). (51)Cr-labelled B-CLL cells, were incubated with isotype murine monoclonal antibodies or blocking MoAb's against the following adhesion proteins integrins ß1, α4, and αL (chain of LFA-1), CD44 and CD106 (VCAM-1). The B-CLL cell adherence to marrow stromal cell (MSC) monolayers at 2hrs was 29±12% (mean±1SD). MoAb's against CD106, α4, and ß1 caused a significant inhibition of heterotypic adherence in 2/8, 3/8 and 4/8 experiments. Despite universal expression of αL on B-CLL cells, MoAb against αL did not influence adhesion of B-CLL cells in any of the eight experiments. MoAb's against CD44 caused an increase in B-CLL cell adherence to MSC in 1/8 experiments. No correlation between basal adhesion and intensity of α4 expression was noted. The absence of this correlation can be explained by the highly variable expression of α4 on the B-CLL cells from a limited number of patients. Notably, the intensity of α4 and ß1 expression, on the B-CLL cells correlated with the degree of inhibition by anti-α4 and anti-ß1 MoAb. A significant positive correlation was noted between baseline adhesion and intensity of ß1 expression. Thus, α4ß1 and its ligand VCAM-1 are important for adhesion of B-CLL cells to MSC. However, other ligands of α4 and other as yet undescribed adhesion proteins may also play a role in B-CLL cell adhesion to MSC. In addition, when B-CLL cells were cocultured in direct contact with MSC monolayers, the proportion of B-CLL cells undergoing apoptosis decreased significantly.