A folate receptor beta-specific human monoclonal antibody recognizes activated macrophage of rheumatoid patients and mediates antibody-dependent cell-mediated cytotoxicity

Introduction: Folate receptor beta (FR beta) is only detectable in placenta and limited to some hematopoietic cells of myeloid lineage in healthy people. Studies have indicated that FR beta is over-expressed in activated macrophages in autoimmune diseases and some cancer cells. In this study we aimed to develop an FR beta-specific human monoclonal antibody (mAb) that could be used as a therapeutic agent to treat rheumatoid arthritis and other autoimmune diseases, as well as FR beta positive cancers. Methods: Functional recombinant FR beta protein was produced in insect cells and used as antigen to isolate a mAb, m909, from a human naive Fab phage display library. Binding of Fab and IgG1 m909 to FR beta was measured by ELISA, surface plasmon resonance, immune fluorescence staining, and flow cytometry. Antibody-dependent cell-mediated cytotoxicity (ADCC) was evaluated with FR beta positive CHO cells as target cells and isolated peripheral blood monocytes as effector cells in an in vitro assay. Results: Fab m909 bound with relatively high affinity (equilibrium dissociation constant 57 nM) to FR beta. The IgG1 m909 showed much higher (femtomolar) avidity as measured by ELISA, and it bound to FR beta positive cells in a dose-dependent manner, but not to parental FR beta negative cells. m909 did not compete with folate for the binding to FR beta on cells. m909 was not only able to select FR beta positive, activated macrophages from synovial fluid cells of arthritis patients as efficiently as folate, but also able to mediate ADCC in FR beta positive cells. Conclusions: Unlike folate-drug conjugates, m909 selectively binds to FR beta, does not recognize FR alpha, and has at least one effector function. m909 alone has potential to eliminate FR beta positive cells. Because m909 does not compete with folate for receptor binding, it can be used with folate-drug conjugates in a combination therapy. m909 can also be a valuable research reagent.