Efficient Binding, but Moderate Modulation, of Human Dendritic Cell Functions by Milatuzumab, a Humanized Anti-CD74 Monoclonal Antibody

Milatuzumab (hLL1, Immunomedics, Inc.), a humanized anti-CD74 immunoglobulin-G monoclonal antibody (MAb), has been shown to have therapeutic activity against CD74-expressing B-cell malignancies in vitro and in xenografts models, and is in clinical evaluation as a therapeutic MAb for non-Hodgkin lymphoma, chronic lymphocytic leukemia, and multiple myeloma. Since it is unclear whether this MAb has any effects on human antigen-presenting cells that express CD74, we investigated the binding efficiency, viability, and functional modulation of human dendritic cells (DC), the professional and most potent antigen-presenting cells, exposed to milatuzumab. We found that milatuzumab bound efficiently with B cells, monocytes, and different subsets of blood DCs including myeloid DC1 (BDCA-1+), myeloid DC2 (BDCA-3+) and plasmacytoid DC (BDCA-2+) in human PBMC, as well as with monocyte-derived immature DCs, but not LPS-matured DCs. The side-by-side comparative cytotoxicity assay showed that milatuzumab, in the presence of a second antibody for cross-linking (GAH, the F(ab′)₂ of goat anti-human IgG Fcgamma-specific), dramatically reduced the cell viability of Daudi B-lymphoma cells, but did not influence the cell viability or induce apoptosis in monocyte-derived DCs, even at high concentations up to 50 μg/ml. At the concentrations ranging from 0.05 to 5 μg/ml, milatuzumab upregulated the expression of the antigen-presenting molecule, HLA-DR, and costimulatory molecules, CD54 and CD86, in human monocyte-derived DCs in a moderate, but dose-dependent manner, suggesting that milatuzumab could enhance DC constitutive maturation. Although this effect was not reflected by an enhanced T-cell expansion, as shown by unaltered CFSE-low, -medium, and –high peaks in total and CD4+ and CD4− T cells, milatuzumab-treated DCs could moderately promote the differentiation of CD4+ naïve T cells toward more Th1 effector cells, suggesting that milatuzumab can modulate DC functions, inducing the polarization and differentiation of functional Th cells. These data highlight the prospects of milatuzumab as a novel immunotherapeutic agent that possesses not only direct anti-proliferative effects against CD74+ hematological malignancies, but also some regulatory effects on DC-mediated immune functions, and no cytotoxic effect on DCs.