A Monoclonal Antibody Specifically Targeting CD44 Inhibits B-Cell Chronic Lymphocytic Leukemia Cell Survival In Vitro and In Vivo

Abstract 927

B-cell chronic lymphocytic leukemia (CLL) with a highly variable clinical course is characterized by the clonal expansion of CD5+ B cells in blood, secondary lymphoid tissues, and marrow. Survival of CLL cells are supported by cells within the tissue microenvironment and by signals from the extracellular matrix, which in part are mediated via interactions with CD44, a surface glycoprotein that is expressed at high levels by CLL B cells. While monoclonal antibody (mAb) therapy targeting CD20 has improved the survival of patients with CLL, a large number of CLL patients do not achieve durable responses or become refractory altogether to therapy with anti-CD20 mAb. In this study we evaluated the cytotoxic activity against CLL cells of a newly developed, humanized anti-CD44 mAb (RO5429083, Roche). We found that sub-microgram concentrations of this anti-CD44 mAb were directly cytotoxic for CLL cells of different patients (n = 32), but had little or no effect on the viability of lymphocytes isolated from the blood of healthy donors (n = 4). The levels of cytotoxicity induced by this anti-CD44 mAb were significantly associated with the levels of CD44 expressed by each of the CLL samples (Spearman R-0.5785, p<0.001). Furthermore, survival and downstream signaling events of CLL cells induced by hyaluronic acid (HA), the principal ligand of CD44, were inhibited by this CD44 mAb. Of note, this CD44 mAb also was equally cytotoxic for CLL cells co-cultured with mesenchymal stromal cells (MSC), which otherwise can support CLL-cell survival in vitro. We also examined whether this anti-CD44 mAb could induce clearance of human CLL cells engrafted into immune deficient RAG-2^−/−/γ_c^−/− mice. Treatment of such xenografted animals with as little as 1 mg/kg of this mAb resulted in the complete clearance of engrafted CLL cells, an effect not observed in control treated animals. Based on these pre-clinical studies, we consider this anti-CD44 mAb has high potential for providing effective treatment for patients with this disease.