CLL-1 Is a Potential Target for the Treatment of Acute Myeloid Leukemia.

CLL-1 (C-type Lectin-Like Molecule-1) is an inhibitory receptor expressed on myeloid cells. We have generated a series of monoclonal antibodies (mAbs) against CLL-1 and used these mAbs to assess expression of the receptor on normal and AML cells and their therapeutic potential in in vitro and ex vivo cytotoxicity assays. By Immunohistochemistry, normal tissues lacked expression of CLL-1, with the exception of spleen. Using flow cytometry, expression was demonstrated on monocytes, granulocytes and dendritic cells, but not on lymphocytes and platelets. Tissue micro-arrays revealed CLL-1 expression in 97.3% (37/38) of AML cases. Flow cytometry of AML blasts demonstrated expression of CLL-1 on 81% (17/21) of patient samples. No detectable expression was detected in ALL blasts (n=5). Selected anti-CLL-1 mAbs mediated dose-dependent Complement Dependent Cytotoxicity (CDC) in various AML-derived cell lines, while no cytotoxicity was observed in CLL-1 negative K562 (CML) or CA46 (B lymphoma) cells. Human embryonic kidney 293 cells only became susceptible to anti-CLL-1 mAb mediated killing after transfection with CLL-1, demonstrating specificity. Furthermore, anti-CLL-1 mAbs showed CDC activity against all AML blasts tested in ex vivo assays (n=10), while no activity was observed against ALL blasts. Our results demonstrate restricted expression of CLL-1 on cells from myeloid origin and AML blasts and specific cytotoxic activity in in vitro and ex vivo assays. We are currently undertaking xenograft models to evaluate the therapeutic potential of these mAbs in vivo.