Expression of CD74 by AML blasts and cell lines, and enhanced in vitro cytotoxicity of anti-CD74 antibody after interferon-gamma (IFN-γ) treatment.

Abstract

6576 Background: CD74 (DR-associated invariant chain) is expressed alone or with DR in a wide range of hematologic cancers and solid tumors. Humanized anti-CD74 mAb, milatuzumab (Immunomedics, Morris Plains, NJ), exhibits direct cytotoxicity for NHL, CLL and MM cell lines, and is in clinical evaluation for therapy of NHL, MM and CLL. Also, CD74 is induced by interferons in multiple cancer cell lines. Here we report an evaluation of CD74 expression and function in AML, and the effect of CD74 upregulation by treatment with IFN-γ on the cytotoxicity of milatuzumab for AML cell lines. Methods: CD74 expression in bone marrow biopsy (BMB) specimens from non-M3 AML patients was evaluated by immunohistochemistry and, for the 3 human AML cell lines, by flow cytometry, with/without permeabilization and with/without IFN-γ (40 and 200 U/mL). These cell lines were also tested in proliferation assays for responses to milatuzumab, with/without IFN-γ. Results: In 13/14 BMB specimens, there was moderate to strong CD74 expression by leukemic blasts, which was mostly intracellular, usually with a perinuclear distribution. Three AML cell lines also showed moderate to strong expression of CD74, which was mostly intracellular. WithoutIFN-g, surface expression of CD74 was present, but IFN-γ treatment of these 3 lines resulted in upregulation of surface CD74 by 69-117%. Much higher levels of intracellular CD74 were observed in all 3 lines, with and without IFN-γ. IFN- γ induced intracellular CD74 in all 3 lines (from 85%-868%). In 2/3 lines, IFN-γ increased milatuzumab-mediated growth inhibition (23.7 to 44.8% and -3.9 to 30.9%, respectively). Conclusions: CD74 is expressed in AML patient specimens and in AML cell lines, with the majority of CD74 expression found intracellularly. Cell surface and cytoplasmic expression of CD74 were upregulated in AML lines after IFN-γ exposure. This increased expression resulted in increased cytotoxicity of the anti-CD74 mAb, milatuzumab, in 2/3 AML lines. Thus, AML is another cancer type where combined IFN-γ and milatuzumab treatment may be useful. Supported in part by NIH grant PO1- CA103985. Author Disclosure Employment or Leadership Position Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Immunomedics, Inc. Immunomedics, Inc.