IREM-1 Is a Cell Surface Receptor Expressed in AML Blasts: Potential for Targeted Immunotherapy in AML.

Background: Acute myeloid leukemia (AML) in adults is associated with a poor prognosis, with many patients suffering from treatment-resistant relapse. Novel targeted therapies are needed. Immune Receptor Expressed by Myeloid Cells (IREM-1), a member of the CD300L gene family, is an inhibitory receptor expressed by myeloid cells. We have generated monoclonal antibodies (mAbs) specific for IREM-1, characterized IREM-1 expression in AML blasts and explored the therapeutic potential of these antibodies in AML.

Methods: mAbs were generated against the extracellular domain of IREM-1. Fresh peripheral blood or bone marrow cells from AML patients, healthy donors, or cell lines OCI-AML-2 (IREM-1⁺) and K562 (IREM-1⁻) were used for expression studies by flow cytometry (FC) with FITC-conjugated mAbs. Immunohistochemistry (IHC) was carried out on Tissue Microarrays (TMAs) of normal human tissues. mAbs were evaluated for their ability to induce lysis in Complement-dependent cytotoxicity (CDC) assays against cell lines in vitro and AML patient cells ex vivo.

Results: By IHC, normal tissues lacked expression of IREM-1, with the exception of spleen and tonsil. However, using FC, expression of IREM-1 was demonstrated on monocytes, granulocytes and dendritic cells, but not on lymphocytes and platelets. FC of AML blasts showed that 18/24 (75%) cases expressed IREM-1 (range: 23.8–93.2%). The 6 negative cases of IREM-1 include 2 AML cases of inv (16) (p13q22). IREM-1 was not detectable in acute lymphocytic leukemia (ALL) blasts (n=5). CD34+ hematopoietic stem cells of control bone marrows were largely negative (n=7), only one case shown low level expression. Functionally, anti-IREM-1 antibodies mediated CDC in OCI-AML-2, while no effect was seen in IREM-1 negative K562 cells. Human embryonic kidney 293 cells only became susceptible to anti-IREM-1 antibody mediated killing after transfection with IREM-1. Furthermore, an anti-IREM-1 mAb showed dose-dependent CDC activity in all IREM-1⁺ AML cases tested (n=4), while no CDC activity was observed against IREM-1⁻ AML cases (n=3) and ALL blasts (n=3).

Conclusion: Our study identifies IREM-1 as a novel cell surface antigen expressed in majority of AML blasts. We have generated mAbs that are capable of mediating CDC activity against AML blasts. Currently we are testing the killing activity of anti-IREM-1 chimeric Ab and undertaking mouse xenograft model to evaluate the therapeutic impact of this target in vivo, which would offer opportunities for therapeutic intervention.