CD44 and adhesion of normal and leukemic CD34+ cells to bone marrow stroma

CD44 has long been implicated in the interaction between hematopoietic progenitors and bone marrow stroma. More recently it has become apparent that CD44 antibodies cannot only inhibit CD44 mediated adhesion to hyaluronic acid and cellular ligands but can stimulate adhesion to these ligands. The mechanism involved in CD44 antibody stimulated adhesion to cellular layers is still not known. While adhesion of T cells to keratinocytes is integrin mediated it appears that adhesion of hematopoietic progenitors to bone marrow stromal layers is the result of an antibody induced conformational change in the CD44 molecule similar to that seem for the augmentation of hyaluronic acid binding by some CD44 antibodies. The ligand for CD44 involved in this binding has not been identified but it does not appear to be hyaluronic acid.