ABSTRACT
CD19, a B cell-restricted receptor critical for B-cell development, is expressed in most B-cell malignancies. The Fc-engineered anti-CD19 antibody, XmAb5574, has enhanced Fcgamma receptor (FcgammaR) binding affinity, leading to improved FcgammaR-dependent effector cell functions and antitumor activity in murine xenografts compared with the non-Fc-engineered anti-CD19 IgG1 analog. Here, we use XmAb5574 and anti-CD19 IgG1 to further dissect effector cell functions in an immune system closely homologous to that of humans, the cynomolgus monkey. XmAb5574 infusion caused an immediate and dose-related B-cell depletion in the blood (to <10% of baseline levels) concomitant with a sustained reduction of natural killer (NK) cells. NK cells had fully recovered by day 15, whereas B-cell recovery was underway by day 57. B cells in secondary lymphoid tissues were depleted (to 34%-61% of vehicle), with involuted germinal centers apparent in the spleen. Anti-CD19 IgG1 had comparable serum exposure to XmAb5574 but demonstrated no B-cell depletion and no sustained NK-cell reduction. Thus, increasing FcgammaR binding affinity dramatically increased B-cell clearing. We propose that effector cell functions, possibly those involving NK cells, mediate XmAb5574 potency in cynomolgus monkeys, and that enhancing these mechanisms should advance the treatment of B-cell malignancies in humans.
Subject(s)
Antibodies, Monoclonal/immunology , Antigens, CD19/immunology , B-Lymphocytes/immunology , Immunoglobulin Fc Fragments/immunology , Immunoglobulin G/immunology , Protein Engineering , Receptors, IgG/immunology , Animals , Antibodies, Monoclonal/genetics , Antibodies, Monoclonal/pharmacology , Antigens, CD19/genetics , Hematologic Neoplasms/drug therapy , Hematologic Neoplasms/genetics , Hematologic Neoplasms/immunology , Humans , Immunoglobulin Fc Fragments/genetics , Immunoglobulin Fc Fragments/pharmacology , Immunoglobulin G/genetics , Immunoglobulin G/pharmacology , Killer Cells, Natural/immunology , Lymphocyte Depletion/methods , Macaca fascicularis , Receptors, IgG/geneticsABSTRACT
Antibody-dependent cell-mediated cytotoxicity, a key effector function for the clinical efficacy of monoclonal antibodies, is mediated primarily through a set of closely related Fcgamma receptors with both activating and inhibitory activities. By using computational design algorithms and high-throughput screening, we have engineered a series of Fc variants with optimized Fcgamma receptor affinity and specificity. The designed variants display >2 orders of magnitude enhancement of in vitro effector function, enable efficacy against cells expressing low levels of target antigen, and result in increased cytotoxicity in an in vivo preclinical model. Our engineered Fc regions offer a means for improving the next generation of therapeutic antibodies and have the potential to broaden the diversity of antigens that can be targeted for antibody-based tumor therapy.
