Correlation between CD16a binding and immuno effector functionality of an antigen specific immunoglobulin Fc fragment (Fcab)

Development of bispecific antibodies in China: overview and prospects

A bispecific antibody (bsAb) can simultaneously bind two different epitopes or antigens, allowing for multiple mechanistic functions with synergistic effects. BsAbs have attracted significant scientific attentions and efforts towards their development as drugs for cancers. There are 21 bsAbs currently undergoing clinical trials in China. Here, we review their platform technologies, expression and production, and biological activities and bioassay of these bsAbs, and summarize their structural formats and mechanisms of actions. T-cell redirection and checkpoint inhibition are two main mechanisms of the bsAbs that we discuss in detail. Furthermore, we provide our perspective on the future of bsAb development in China, including CD3-bsAbs for solid tumors and related cytokine release syndromes, expression and chemistry, manufacturing and controls, clinical development, and immunogenicity.

Fcab-HER2 Interaction: a Ménage à Trois. Lessons from X-Ray and Solution Studies

The crystallizable fragment (Fc) of the immunoglobulin class G (IgG) is an attractive scaffold for the design of novel therapeutics. Upon engineering the C-terminal loops in the CH3 domains, Fcabs (Fc domain with antigen-binding sites) can be designed. We present the first crystal structures of Fcabs, i.e., of the HER2-binding clone H10-03-6 having the AB and EF loop engineered and the stabilized version STAB19 derived by directed evolution. Comparison with the crystal structure of the Fc wild-type protein reveals conservation of the overall domain structures but significant differences in EF-loop conformations. Furthermore, we present the first Fcab-antigen complex structures demonstrating the interaction between the engineered Fcab loops with domain IV of HER2. The crystal structures of the STAB19-HER2 and H10-03-6-HER2 complexes together with analyses by isothermal titration calorimetry, SEC-MALS, and fluorescence correlation spectroscopy show that one homodimeric Fcab binds two HER2 molecules following a negative cooperative binding behavior.

Antibody-Based Cancer Therapy: Successful Agents and Novel Approaches

Since their discovery, antibodies have been viewed as ideal candidates or "magic bullets" for use in targeted therapy in the fields of cancer, autoimmunity, and chronic inflammatory disorders. A wave of antibody-dedicated research followed, which resulted in the clinical approval of a first generation of monoclonal antibodies for cancer therapy such as rituximab (1997) and cetuximab (2004), and infliximab (2002) for the treatment of autoimmune diseases. More recently, the development of antibodies that prevent checkpoint-mediated inhibition of T cell responses invigorated the field of cancer immunotherapy. Such antibodies induced unprecedented long-term remissions in patients with advanced stage malignancies, most notably melanoma and lung cancer, that do not respond to conventional therapies. In this review, we will recapitulate the development of antibody-based therapy, and detail recent advances and new functions, particularly in the field of cancer immunotherapy. With the advent of recombinant DNA engineering, a number of rationally designed molecular formats of antibodies and antibody-derived agents have become available, and we will discuss various molecular formats including antibodies with improved effector functions, bispecific antibodies, antibody-drug conjugates, antibody-cytokine fusion proteins, and T cells genetically modified with chimeric antigen receptors. With these exciting advances, new antibody-based treatment options will likely enter clinical practice and pave the way toward more successful control of malignant diseases.

Fc Engineering for Developing Therapeutic Bispecific Antibodies and Novel Scaffolds

Therapeutic monoclonal antibodies have become molecules of choice to treat autoimmune disorders, inflammatory diseases, and cancer. Moreover, bispecific/multispecific antibodies that target more than one antigen or epitope on a target cell or recruit effector cells (T cell, natural killer cell, or macrophage cell) toward target cells have shown great potential to maximize the benefits of antibody therapy. In the past decade, many novel concepts to generate bispecific and multispecific antibodies have evolved successfully into a range of formats from full bispecific immunoglobulin gammas to antibody fragments. Impressively, antibody fragments such as bispecific T-cell engager, bispecific killer cell engager, trispecific killer cell engager, tandem diabody, and dual-affinity-retargeting are showing exciting results in terms of recruiting and activating self-immune effector cells to target and lyse tumor cells. Promisingly, crystallizable fragment (Fc) antigen-binding fragment and monomeric antibody or half antibody may be particularly advantageous to target solid tumors owing to their small size and thus good tissue penetration potential while, on the other hand, keeping Fc-related effector functions such as antibody-dependent cellular cytotoxicity, complement-dependent cytotoxicity, antibody-dependent cell-mediated phagocytosis, and extended serum half-life via interaction with neonatal Fc receptor. This review, therefore, focuses on the progress of Fc engineering in generating bispecific molecules and on the use of small antibody fragment as scaffolds for therapeutic development.

Engineered IgG1-Fc--one fragment to bind them all

The crystallizable fragment (Fc) of the immunoglobulin class G (IgG) is a very attractive scaffold for the design of novel therapeutics due to its quality of uniting all essential antibody functions. This article reviews the functionalization of this homodimeric glycoprotein by diversification of structural loops of CH3 domains for the design of Fcabs, i.e. antigen-binding Fc proteins. It reports the design of libraries for the selection of nanomolar binders with wildtype-like in vivo half-life and correlation of Fc receptor binding and ADCC. The in vitro and preclinical biological activity of selected Fcabs is compared with that of clinically approved antibodies. Recently, the great potential of the scaffold for the development of therapeutics for clinical use has been shown when the HER2-binding Fcab FS102 entered clinical phase I. Furthermore, methods for the engineering of biophysical properties of Fcabs applicable to proteins in general are presented as well as the different approaches in the design of heterodimeric Fc-based scaffolds used in the generation of bispecific monoclonal antibodies. Finally, this work critically analyzes and compares the various efforts in the design of highly diverse and functional libraries that have been made in the engineering of IgG1-Fc and structurally similar scaffolds.

Effect of CD16a, the surface receptor of Kupffer cells, on the growth of hepatocellular carcinoma cells

FcγRIIIa (CD16) is a low-affinity Fc receptor of IgG. As the idio-binding receptor of IgG Fc, it plays an important role in the antibody-dependent cellular cytotoxicity of natural killer cells. The aim of the present study was to investigate the distribution of Kupffer cells (KCs) and the expression of their surface receptor FcγRIIIa in hepatocellular carcinoma. Furthermore, we also aimed to observe the functional mechanism of FcγRIIIa. Immunohistochemical analysis was employed to study KCs and FcγRIIIa. In order to explore the role of FcγRIIIa in the growth of cancer cells, KCs and H22 tumor cells were co-cultured in different serum. The mRNA expression levels of tumor necrosis factor (TNF)-α and FcγRIIIa were analyzed by RT-qPCR; the TNF-α and FcγRIIIa protein expression levels were examined by enzyme-linked immunosorbent assay and western blot analysis, respectively. Our results showed that the number of Kuppfer cells in cancerous tissues (21.6±7.8) was lower than those in para-cancerous (68.8±9.1) tissues and adjacent normal hepatic tissues (62.0±1.9) (P<0.01); this decreased with the reduction in the differentiation degree of cancer (P<0.05). FcγRIIIa-positive cells were similar in morphology to KCs, and their distributive tendency was coincident (P<0.05). The increase in CD16a mRNA levels in the group treated with immune serum was 3.9-, 4.9- and 3.9-fold greater than that in the ordinary serum group at different time points, and CD16a protein expression also markedly increased (P<0.05). However, these effects were inhibited by the addition of anti-IgG Fc serum (P<0.05). The results of the present study suggested that FcγRIIIa resided in KCs, and it contributed to the inhibition of the growth of liver tumor cells.

Fc gamma receptor glycosylation modulates the binding of IgG glycoforms: a requirement for stable antibody interactions

FcγRs play a critical role in the immune response following recognition of invading particles and tumor associated antigens by circulating antibodies. In the present study we investigated the role of FcγR glycosylation in the IgG interaction and observed a stabilizing role for receptor N-glycans. We performed a complete glycan analysis of the recombinant FcγRs (FcγRIa, FcγRIIa, FcγRIIb, FcγRIIIa(Phe158/Val158), and FcγRIIIb) expressed in human cells and demonstrate that receptor glycosylation is complex and varied between receptors. We used surface plasmon resonance to establish binding patterns between rituximab and all receptors. Complex binding was observed for FcγRIa and FcγRIIIa. The IgG-FcγR interaction was further investigated using a combination of kinetic experiments and enzymatically deglycosylated FcγRIa and FcγRIIIa(Phe158/Val158) receptors in an attempt to determine the underlying binding mechanism. We observed that antibody binding levels decreased for deglycosylated receptors, and at the same time, binding kinetics were altered and showed a more rapid approach to steady state, followed by an increase in the antibody dissociation rate. Binding of rituximab to deglycosylated FcγRIIIa(Phe158) was now consistent with a 1:1 binding mechanism, while binding of rituximab to FcγRIIIa(Val158) remained heterogeneous. Kinetic data support a complex binding mechanism, involving heterogeneity in both antibody and receptor, where fucosylated and afucosylated antibody forms compete in receptor binding and in receptor molecules where heterogeneity in receptor glycosylation plays an important role. The exact nature of receptor glycans involved in IgG binding remains unclear and determination of rate and affinity constants are challenging. Here, the use of more extended competition experiments appear promising and suggest that it may be possible to determine dissociation rate constants for high affinity afucosylated antibodies without the need to purify or express such variants. The data described provide further insight into the complexity of the IgG-FcγR interaction and the influence of FcγR glycosylation.

Stability assessment on a library scale: a rapid method for the evaluation of the commutability and insertion of residues in C-terminal loops of the CH3 domains of IgG1-Fc

Antigen-binding Fc fragments (Fcab) are generated by engineering the C-terminal loop regions in the CH3 domain of human immunoglobulin G class 1-crystallizable fragment (IgG1-Fc). For an optimum library design with high percentage of well-folded clones for efficient binder selection, information about the correlation between primary structure and stability is needed. Here, we present a rapid method that allows determination of the overall stability of whole libraries of IgG1-Fc on the surface of yeast by flow cytometry. Libraries of IgG1-Fc mutants with distinct regions in AB-, CD- and EF-loops of the CH3 domains randomized or carrying therein insertions of five additional residues were constructed, incubated at increasing temperatures and probed for residual binding of generic Fc ligands. Calculated temperatures of half-maximal irreversible denaturation of the libraries gave a clear hierarchy of tolerance to randomization of distinct loop positions. Experimental data were evaluated by a computational approach and are discussed with respect to the structure of IgG1-Fc and variation in sequence and length of these loops in homologous Fc proteins. Generally, the described method allows for quick assessment of the effects of randomization of distinct regions on the foldability and stability of a yeast-displayed protein library.