Differential binding to human FcγRIIa and FcγRIIb receptors by human IgG wildtype and mutant antibodies

When binding is enough: nonactivating antibody formats

Most therapeutic antibodies currently used in the clinic are based on the human IgG1 format, which is a bivalent molecule that efficiently interacts with the immune system's effector functions. In clinical applications where binding to the target alone is sufficient for therapeutic efficacy; however, engagement of the immune system is not required and may even cause unwanted side-effects. Likewise, bivalent binding to the target may negatively influence the therapeutic efficacy of an antibody. Here we discuss the state of the art for antibody-based therapeutics, designed to be nonactivating (i.e. do not engage the innate immune system's effector functions), in both monovalent and bivalent formats.

Disulfide connectivity of human immunoglobulin G2 structural isoforms

In this communication we present the detailed disulfide structure of IgG2 molecules. The consensus structural model of human IgGs represents the hinge region positioned as a flexible linker connecting structurally isolated Fc and Fab domains. IgG2 molecules are organized differently from that model and exhibit multiple structural isoforms composed of (heavy chain-light chain-hinge) covalent complexes. We describe the precise connection of all the disulfide bridges and show that the IgG2 C H1 and C-terminal C L cysteine residues are either linked to each other or to the two upper hinge cysteine residues specific to the IgG2 subclass. A defined arrangement of these disulfide bridges is unique to each isoform. Mutation of a single cysteine residue in the hinge region eliminates these natural complexes. These results show that IgG2 structure is significantly different from the conventionally accepted immunoglobulin structural model and may help to explain some of the unique biological activity attributed only to this subclass.

Structural characterization of a human Fc fragment engineered for lack of effector functions

The first three-dimensional structure of a human Fc fragment genetically engineered for the elimination of its ability to mediate antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity is reported. When introduced into the lower hinge and CH2 domain of human IgG1 molecules, the triple mutation L234F/L235E/P331S ('TM') causes a profound decrease in their binding to human CD64, CD32A, CD16 and C1q. Enzymatically produced Fc/TM fragment was crystallized and its structure was solved at a resolution of 2.3 A using molecular replacement. This study revealed that the three-dimensional structure of Fc/TM is very similar to those of other human Fc fragments in the experimentally visible region spanning residues 236-445. Thus, the dramatic broad-ranging effects of TM on IgG binding to several effector molecules cannot be explained in terms of major structural rearrangements in this portion of the Fc.

Immunologic and structural analysis of eight novel domain-deletion beta3 integrin peptides designed for detection of HPA-1 antibodies

BACKGROUND

The single-nucleotide polymorphism (SNP) rs5918 in the ITGB3 gene defines the human platelet antigen-1 (HPA-1) system encoding a Leu (HPA-1a) or Pro (HPA-1b) at position 33. HPA-1 antibodies are clinically the most relevant in the Caucasoid population, but detection currently requires alpha(IIb)beta3 integrin from the platelets of HPA-genotyped donors.

OBJECTIVES

We set out to define the beta3 integrin domains required for HPA-1a antibody binding and produce recombinant soluble beta3 peptides for HPA-1 antibody detection.

METHODS

We designed two sets (1a and 1b) of four soluble beta3 domain-deletion peptides (deltaSDL, deltabetaA, PSIHybrid, PSI), informed by crystallography studies and computer modeling. The footprints of three human HPA-1a-specific phage antibodies were defined by analyzing binding patterns to the beta3 peptides and canine platelets, and models of antibody-antigen interfaces were derived. Specificity and sensitivity for HPA-1a detection were assessed using sera from 140 cases of fetomaternal alloimmune thrombocytopenia (FMAIT).

RESULTS

Fusion of recombinant proteins to calmodulin resulted in high-level expression in Drosophila S2 cells of all eight beta3 peptides. Testing of FMAIT samples indicated that deltabetaA-Leu33 is the superior peptide for HPA-1a antibody detection, with 96% sensitivity and 95% specificity. The existence of type I and II categories of HPA-1a antibodies was confirmed by the study of HPA-1a phage antibody footprints and the reactivity pattern of clinical samples with the four beta3-Leu33 peptides, but there was no correlation between antibody category and clinical severity of FMAIT.

CONCLUSIONS

Soluble recombinant beta3 peptides can be used for detection of clinical HPA-1a antibodies.

An immunoglobulin E-reactive chimeric human immunoglobulin G1 anti-idiotype inhibits basophil degranulation through cross-linking of FcepsilonRI with FcgammaRIIb

BACKGROUND

IgE binds to mast cells and basophils via its high-affinity receptor, FcepsilonRI, and cross-linking of FcepsilonRI-bound IgE molecules by allergen leads to the release of allergic mediators characteristic of type I hypersensitivity reactions. Previous work has shown that cross-linking of FcepsilonRI with FcgammaRIIb, an ITIM-containing IgG receptor, leads to inhibition of basophil triggering. 2G10, a chimeric human IgG1 anti-idiotype, has broad reactivity with human IgE and as such has the potential to bind simultaneously to FcepsilonRI-bound IgE, via its Fab regions, and the negative regulatory receptor, FcgammaRIIb, via its Fc region.

OBJECTIVE

To assess the ability of human 2G10 to inhibit anti-IgE and allergen-driven basophil degranulation through cross-linking of FcepsilonRI-bound IgE with FcgammaRIIb.

METHODS

2G10 was assessed for its ability to bind to FcgammaRIIb on transfected cells and on purified basophils. In the basophil degranulation assay, basophils were purified from peripheral blood of atopic individuals and activated with either anti-IgE or the house dust mite allergen Der p 1, in the presence or absence of human 2G10. Basophil activation was quantified by analysis of CD63 and CD203c expression on the cell surface, and IL-4 expression intracellularly, using flow cytometery.

RESULTS

Human 2G10 was able to bind to FcgammaRIIb on transfected cells and on purified basophils, and induce a dose-dependent inhibition of both anti-IgE and Der p 1-driven degranulation of basophils.

CONCLUSION

The inhibition of basophil degranulation by the human IgG1 anti-idiotype 2G10 highlights the therapeutic potential of IgE-reactive IgG antibodies in restoring basophil integrity through recruitment of the inhibitory receptor FcgammaRIIb.

Quantitative in vivo comparisons of the Fcγ receptor-dependent agonist activities of different fucosylation variants of an immunoglobulin G antibody

Although it has been shown that functions of immunoglobulin G (IgG) antibodies (Abs) that depend on binding to certain Fc gamma receptors (Fc gamma R) can be influenced by Fc glycan fucosylation, quantitative in vivo analyses comparing the effects of different levels of fucose are still lacking. We used a simple mouse model to compare Fc gamma R-dependent T cell activation induced by different fucosylation variants of a hamster/human IgG1 chimeric version of anti-mouse CD3 monoclonal Ab, 145-2C11 (2C11). Initial studies supported the expectation that this agonist activity by 2C11 was a reflection of Fc gamma R binding, including comparisons of human IgG1 and IgG4 variants of 2C11 that showed the IgG4 to be dramatically less active at inducing T cell activation. Dose-response analyses in mice then showed that a sample of the human IgG1 version of 2C11 Ab in which 40% of the Fc glycans in the population of Ab molecules were fucosylated was 3-5 times more potent than a sample with 90% of its Fc glycans fucosylated. A sample with 10% fucosylation showed the same activity as the 40% fucosylated sample, revealing that complete absence of fucose was not necessary to achieve maximal Fc function in this model. In vitro binding to recombinant mouse Fc gamma Rs by the 2C11 variants revealed interesting relationships between fucose content and receptor affinity, and suggested the involvement of Fc gamma RIV in mediating 2C11 activity in vivo. These analyses showed that low-fucose human IgG1 Abs indeed show greater Fc gamma R-dependent activities in mice, but that Abs with moderate levels of fucose may be just as potent as Abs with very low or no fucose.

Function-blocking antibodies to human vascular adhesion protein-1: a potential anti-inflammatory therapy

Human vascular adhesion protein-1 (VAP-1) is a homodimeric 170-kDa sialoglycoprotein that is expressed on the surface of endothelial cells and functions as a semicarbazide-sensitive amine oxidase and as an adhesion molecule. Blockade of VAP-1 has been shown to reduce leukocyte adhesion and transmigration in in vivo and in vitro models, suggesting that VAP-1 is a potential target for anti-inflammatory therapy. In this study we have constructed mouse-human chimeric antibodies by genetic engineering in order to circumvent the potential problems involved in using murine antibodies in man. Our chimeric anti-VAP-1 antibodies, which were designed to lack Fc-dependent effector functions, bound specifically to cell surface-expressed recombinant human VAP-1 and recognized VAP-1 in different cell types in tonsil. Furthermore, the chimeric antibodies prevented leukocyte adhesion and transmigration in vitro and in vivo. Hence, these chimeric antibodies have the potential to be used as a new anti-inflammatory therapy.

Intravascular survival of red cells coated with a mutated human anti-D antibody engineered to lack destructive activity

Alloimmune feto-maternal destruction of blood cells is thought to be mediated by binding of alloantibodies to Fc receptors on effector cells. Blocking the antigen using inert antibodies might prolong cell survival. We have performed a "proof of principle" study in volunteers to measure the intravascular survival of autologous red cells coated with human recombinant IgG antibody containing a novel constant region, G1Deltanab, devoid of in vitro cytotoxic activity. RhD-positive red blood cells (RBCs), labeled with chromium-51 or technetium-99m, were separately coated to equal levels with wild-type IgG1 or G1Deltanab anti-D antibody (Fog-1). After re-injection, there was complete, irreversible clearance of IgG1-coated RBCs by 200 minutes, concomitant with appearance of radiolabel in plasma. Gamma camera imaging revealed accumulation in spleen and, at higher coating levels, in liver. In contrast, clearance of G1Deltanab-coated cells was slower, incomplete, and transient, with whole blood counts falling to 7% to 38% injected dose by about 200 minutes before increasing to 12% to 67% thereafter. There was no appearance of plasma radiolabel and no hepatic accumulation. These findings suggest that G1Deltanab-coated RBCs were not hemolysed but temporarily sequestered in the spleen and that our approach merits investigation in larger studies.

Activating and inhibitory IgG Fc receptors on human DCs mediate opposing functions

Human monocyte-derived DCs (moDCs) and circulating conventional DCs coexpress activating (CD32a) and inhibitory (CD32b) isoforms of IgG Fcgamma receptor (FcgammaR) II (CD32). The balance between these divergent receptors establishes a threshold of DC activation and enables immune complexes to mediate opposing effects on DC maturation and function. IFN-gamma most potently favors CD32a expression on immature DCs, whereas soluble antiinflammatory concentrations of monomeric IgG have the opposite effect. Ligation of CD32a leads to DC maturation, increased stimulation of allogeneic T cells, and enhanced secretion of inflammatory cytokines, with the exception of IL-12p70. Coligation of CD32b limits activation through CD32a and hence reduces the immunogenicity of moDCs even for a strong stimulus like alloantigen. Targeting CD32b alone does not mature or activate DCs but rather maintains an immature state. Coexpression of activating and inhibitory FcgammaRs by DCs reveals a homeostatic checkpoint for inducing tolerance or immunity by immune complexes. These findings have important implications for understanding the pathophysiology of immune complex diseases and for optimizing the efficacy of therapeutic mAbs. The data also suggest novel strategies for targeting antigens to the activating or inhibitory FcgammaRs on human DCs to generate either antigen-specific immunity or tolerance.

Engineered vascular-targeting antibody-interferon-gamma fusion protein for cancer therapy

A number of cytokines are either approved drugs or are in advanced clinical trials, yet these biopharmaceuticals do not typically localize efficiently in solid tumors and manifest their therapeutic potential at the expense of severe side effects. The targeted delivery of cytokines to solid tumors is a promising avenue for increasing the therapeutic index of these biopharmaceuticals. We engineered a fusion protein between scFv(L19), a human antibody fragment specific to the EDB domain of fibronectin, and a cysteine-free mutant of murine interferon-gamma. The resulting fusion protein was capable of targeting new blood vessels in solid tumors, and the targeting efficiency was strikingly increased in tumor-bearing knockout mice lacking the interferon-gamma receptor. ScFv(L19)-interferon-gamma displayed a strong antitumor effect in both subcutaneous and metastatic murine F9 teratocarcinomas, but was not efficacious as single agent when used to treat C51 and CT26 tumors. The potency of this fusion protein could be substantially enhanced by combination with doxorubicin and other immunocytokines. These findings are of clinical relevance, as the EDB domain is a marker of angiogenesis, with identical sequence in mouse and man, which is abundantly expressed in a variety of aggressive solid tumors but is undetectable in most normal tissues.