CLL-1, a Receptor Expressed on Myeloid Leukemic Stem Cells, Is a Potential Target for Immunotherapy of AML

CLL-1 (C-type Lectin-Like Molecule-1) is an inhibitory receptor expressed on myeloid cells which was previously shown to be expressed on AML cancer stem cells. To further validate the potential therapeutic against CLL-1, we generated a series of monoclonal antibodies (mAbs) against CLL-1 and assessed their cytotoxic and anti-cancer activities. While expression of CLL-1 was restricted to myeloid cells, it was expressed in 80% (37/46) of AML blasts but not in ALL blasts (n=5). Expression on AML CD34+/CD38- stem cells was detected in 71% (12/17) of cases. Selected anti-CLL-1 mAbs mediated dose-dependent. Complement Dependent Cytotoxicity (CDC) against various AML-derived cell lines with no detectable cytotoxic activity against lymphoid derived cell lines. Moreover, human embryonic kidney 293 cells became susceptible to anti-CLL-1 mAb mediated killing only after transfection with CLL-1, demonstrating that cytotoxic activity is mediated through a specific interaction with CLL-1. Furthermore, anti-CLL-1 mAbs showed CDC activity against all AML blasts tested in ex vivo assays (n=13), while no activity was observed against ALL blasts. CLL-1 efficiently internalizes upon antibody binding in both 293 transfected cells as well as AML cell lines, demonstrating the potential therapeutic opportunity for toxin-conjugation of anti-CLL-1 mAbs. In vivo anti-cancer activity of the lead chimeric mAb against CLL-1 was tested in an HL-60 xenograft model. In this model, growth of established tumors was reduced by 38% at 5 mg/kg twice weekly dosing. Our results demonstrate CLL-1 as an attractive target for AML with restricted expression on cells from myeloid origin, AML blasts and leukemic stem cells, as well as specific cytotoxic activity in in vitro, ex vivo and in vivo assays. We are currently undertaking additional xenograft models to evaluate the therapeutic potential of these mAbs against primary AML engrafted cells and in combination with chemotherapy in vivo.