Cysteine residues required for the attachment of the light chain in human IgA2

Engineering Dimeric EGFR-directed IgA Antibodies Reveals a Central Role of CD147 during Neutrophil-mediated Tumor Cell Killing of Head and Neck Squamous Cancer Cells

Human IgA Abs engage neutrophils for cancer immunotherapy more effectively than IgG Abs. Previous studies demonstrated that engineering approaches improved biochemical and functional properties. In this study, we report a novel, to our knowledge, IgA2 Ab against the epidermal growth factor receptor generated by protein engineering and polymerization. The resulting molecule demonstrated a covalent linkage of L and H chains and an effective polymerization by the joining chain. The engineered dimer outperformed its monomeric variant in functional experiments on Fab-mediated modes of action and binding to the Fc receptor. The capacity to engage neutrophils for Ab-dependent cell-mediated cytotoxicity (ADCC) of adherent growing target cancer cells was cell line dependent. Although the engineered dimer displayed a long-term efficacy against the vulva carcinoma cell line A431, there was a notable in-efficacy against human papillomavirus (HPV)- head and neck squamous cell carcinoma (HNSCC) cell lines. However, the highly engineered IgA Abs triggered a neutrophil-mediated cytotoxicity against HPV+ HNSCC cell lines. Short-term ADCC efficacy correlated with the target cells' epidermal growth factor receptor expression and the ability of cancer cell-conditioned media to enhance the CD147 surface level on neutrophils. Notably, the HPV+ HNSCC cell lines demonstrated a significant increment in releasing soluble CD147 and a reduced induction of membranous CD147 on neutrophils compared with HPV- cells. Although membranous CD147 on neutrophils may impair proper IgA-Fc receptor binding, soluble CD147 enhanced the IgA-neutrophil-mediated ADCC in a dose-dependent manner. Thus, engineering IgA Abs and impedance-based ADCC assays provided valuable information regarding the target-effector cell interaction and identified CD147 as a putative critical parameter for neutrophil-mediated cytotoxicity.

Stability Engineering of Recombinant Secretory IgA

Secretory IgA (SIgA) presents a promising avenue for mucosal immunotherapy yet faces challenges in expression, purification, and stability. IgA exists in two primary isotypes, IgA1 and IgA2, with IgA2 further subdivided into two common allotypes: IgA2m(1) and IgA2m(2). The major differences between IgA1 and IgA2 are located in the hinge region, with IgA1 featuring a 13-amino acid elongation that includes up to six O-glycosylation sites. Furthermore, the IgA2m(1) allotype lacks a covalent disulfide bond between heavy and light chains, which is present in IgA1 and IgA2m(2). While IgA1 demonstrates superior epitope binding and pathogen neutralization, IgA2 exhibits enhanced effector functions and stability against mucosal bacterial degradation. However, the noncovalent linkage in the IgA2m(1) allotype raises production and stability challenges. The introduction of distinct single mutations aims to facilitate an alternate disulfide bond formation to mitigate these challenges. We compare four different IgA2 versions with IgA1 to further develop secretory IgA antibodies against SARS-CoV-2 for topical delivery to mucosal surfaces. Our results indicate significantly improved expression levels and assembly efficacy of SIgA2 (P221R) in Nicotiana benthamiana. Moreover, engineered SIgA2 displays heightened thermal stability under physiological as well as acidic conditions and can be aerosolized using a mesh nebulizer. In summary, our study elucidates the benefits of stability-enhancing mutations in overcoming hurdles associated with SIgA expression and stability.

3D Structures of IgA, IgM, and Components

Immunoglobulin G (IgG) is currently the most studied immunoglobin class and is frequently used in antibody therapeutics in which its beneficial effector functions are exploited. IgG is composed of two heavy chains and two light chains, forming the basic antibody monomeric unit. In contrast, immunoglobulin A (IgA) and immunoglobulin M (IgM) are usually assembled into dimers or pentamers with the contribution of joining (J)-chains, which bind to the secretory component (SC) of the polymeric Ig receptor (pIgR) and are transported to the mucosal surface. IgA and IgM play a pivotal role in various immune responses, especially in mucosal immunity. Due to their structural complexity, 3D structural study of these molecules at atomic scale has been slow. With the emergence of cryo-EM and X-ray crystallographic techniques and the growing interest in the structure-function relationships of IgA and IgM, atomic-scale structural information on IgA-Fc and IgM-Fc has been accumulating. Here, we examine the 3D structures of IgA and IgM, including the J-chain and SC. Disulfide bridging and N-glycosylation on these molecules are also summarized. With the increasing information of structure–function relationships, IgA- and IgM-based monoclonal antibodies will be an effective option in the therapeutic field.

IgA: Structure, Function, and Developability

Immunoglobulin A (IgA) plays a key role in defending mucosal surfaces against attack by infectious microorganisms. Such sites present a major site of susceptibility due to their vast surface area and their constant exposure to ingested and inhaled material. The importance of IgA to effective immune defence is signalled by the fact that more IgA is produced than all the other immunoglobulin classes combined. Indeed, IgA is not just the most prevalent antibody class at mucosal sites, but is also present at significant concentrations in serum. The unique structural features of the IgA heavy chain allow IgA to polymerise, resulting in mainly dimeric forms, along with some higher polymers, in secretions. Both serum IgA, which is principally monomeric, and secretory forms of IgA are capable of neutralising and removing pathogens through a range of mechanisms, including triggering the IgA Fc receptor known as FcαRI or CD89 on phagocytes. The effectiveness of these elimination processes is highlighted by the fact that various pathogens have evolved mechanisms to thwart such IgA-mediated clearance. As the structure–function relationships governing the varied capabilities of this immunoglobulin class come into increasingly clear focus, and means to circumvent any inherent limitations are developed, IgA-based monoclonal antibodies are set to emerge as new and potent options in the therapeutic arena.

An Anti-EGFR IgA That Displays Improved Pharmacokinetics and Myeloid Effector Cell Engagement In Vivo

Antibodies of IgA isotype effectively engage myeloid effector cells for cancer immunotherapy. Here, we describe preclinical studies with an Fc engineered IgA2m(1) antibody containing the variable regions of the EGFR antibody cetuximab. Compared with wild-type IgA2m(1), the engineered molecule lacked two N-glycosylation sites (N166 and N337), two free cysteines (C311 and C472), and contained a stabilized heavy and light chain linkage (P221R mutation). This novel molecule displayed improved production rates and biochemical properties compared with wild-type IgA. In vitro, Fab- and Fc-mediated effector functions, such as inhibition of ligand binding, receptor modulation, and engagement of myeloid effector cells for antibody-dependent cell-mediated cytotoxicity, were similar between wild-type and engineered IgA2. The engineered antibody displayed lower levels of terminal galactosylation leading to reduced asialoglycoprotein-receptor binding and to improved pharmacokinetic properties. In a long-term in vivo model against EGFR-positive cancer cells, improved serum half-life translated into higher efficacy of the engineered molecule, which required myeloid cells expressing human FcαRI for its full efficacy. However, Fab-mediated effector functions contributed to the in vivo efficacy because the novel IgA antibody demonstrated therapeutic activity also in non-FcαRI transgenic mice. Together, these results demonstrate that engineering of an IgA antibody can significantly improve its pharmacokinetics and its therapeutic efficacy to inhibit tumor growth in vivo.

A comparison of anti-HER2 IgA and IgG1 in vivo efficacy is facilitated by high N-glycan sialylation of the IgA

Monomeric IgA has been proposed as an alternative antibody format for cancer therapy. Here, we present our studies on the production, purification and functional evaluation of anti-HER2 IgA antibodies as anti-cancer agents in comparison to the anti-HER2 IgG1 trastuzumab. MALDI-TOF MS analysis showed profound differences in glycosylation traits across the IgA isotypes and cell lines used for production, including sialylation and linkage thereof, fucosylation (both core and antennary) and the abundance of high-mannose type species. Increases in sialylation proved to positively correlate with in vivo plasma half-lives. The polymerization propensity of anti-HER2 IgA2m2 could be suppressed by an 18-aa deletion of the heavy chain tailpiece - coinciding with the loss of high-mannose type N-glycan species - as well as by 2 cysteine to serine mutations at positions 320 and 480. The HER2 F(ab')2-mediated anti-proliferative effect of the IgA2m1 and IgA2m2 subtypes was similar to IgG1, whereas the IgA1 isotype displayed considerably lower potency and efficacy. The Fc-mediated induction of antibody-dependent cell-mediated cytotoxicity (ADCC) using human whole blood ADCC assays did not demonstrate such clear differences between the IgA isotypes. However, the potency of the anti-HER2 IgA antibodies in these ADCC assays was found to be significantly lower than that of trastuzumab. In vivo anti-tumor activity of the anti-HER2 IgA antibodies was compared to that of trastuzumab in a BT-474 breast cancer xenograft model. Multiple dosing and sialylation of the IgA antibodies compensated for the short in vivo half-life of native IgA antibodies in mice compared to a single dose of IgG1. In the case of the IgA2m2 antibody, the resulting high plasma exposure levels were sufficient to cause clear tumor stasis comparable to that observed for trastuzumab at much lower plasma exposure levels.

Effect of a tail piece cysteine deletion on biochemical and functional properties of an epidermal growth factor receptor-directed IgA2m(1) antibody

Antibodies of human IgA isotype are critical components of the mucosal immune system, but little is known about their immunotherapeutic potential. Compared with IgG antibodies, IgA molecules carry a C-terminal tail piece extension of 18 amino acids with a free cysteine at position 471. This cysteine is required for the formation of dimeric IgA antibodies, but may impair molecular characteristics of monomeric IgA antibodies as therapeutic reagents. Thus, we generated and characterized a d471-mutated antibody against the epidermal growth factor receptor (EGFR) and compared it to its respective IgA2m(1) wild type antibody. Both wild type and mutated IgA antibodies demonstrated similar EGFR binding and were similarly efficient in inhibiting EGF binding and in blocking EGF-mediated cell proliferation. Recruitment of Fc-mediated effector functions like antibody-dependent cell-mediated cytotoxicity by monocytes, macrophages or PMN was similar, but the d471-mutated IgA exhibited different biochemical properties compared with wild type antibody. As expected, mutated IgA did not form stable dimers in the presence of human joining (J)-chain, but we also observed reduced levels of dimeric aggregates in the absence of J-chain. Furthermore, glycoprofiling revealed different glycosylation patterns for both antibodies, including considerably less mannosylation of d471-mutated antibodies. Overall, our results demonstrate that the deletion of the C-terminal cysteine of IgA2 did not affect the investigated effector functions compared with wild type antibody, but it improved biochemical properties of an IgA2m(1) antibody against EGFR, and may thereby assist in exploring the immunotherapeutic potential of recombinant IgA antibodies.

Structure of a human IgA1 Fab fragment at 1.55 Å resolution: potential effect of the constant domains on antigen-affinity modulation

Despite being the most abundant class of immunoglobulins in humans and playing central roles in the adaptive immune response, high-resolution structural data are still lacking for the antigen-binding region of human isotype A antibodies (IgAs). The crystal structures of a human Fab fragment of IgA1 in three different crystal forms are now reported. The three-dimensional organization is similar to those of other Fab classes, but FabA1 seems to be more rigid, being constrained by a hydrophobic core in the interface between the variable and constant domains of the heavy chain (VH-CH1) as well as by a disulfide bridge that connects the light and heavy chains, influencing the relative heavy/light-chain orientation. The crystal structure of the same antibody but with a G-isotype CH1 which is reported to display different antigen affinity has also been solved. The differential structural features reveal plausible mechanisms for constant/variable-domain long-distance effects whereby antibody class switching could alter antigen affinity.

Characterization of a mutated IgA2 antibody of the m(1) allotype against the epidermal growth factor receptor for the recruitment of monocytes and macrophages

IgA antibodies constitute an important part of the mucosal immune system, but their immunotherapeutic potential remains rather unexplored, in part due to biotechnological issues. For example, the IgA2m(1) allotype carries an unusual heavy and light chain pairing, which may confer production and stability concerns. Here, we report the generation and the biochemical and functional characterization of a P221R-mutated IgA2m(1) antibody against the epidermal growth factor receptor (EGFR). Compared with wild type, the mutated antibody demonstrated heavy chains covalently linked to light chains in monomeric as well as in joining (J)-chain containing dimeric IgA. Functional studies with wild type and mutated IgA2m(1) revealed similar binding to EGFR and direct effector functions such as EGFR down-modulation and growth inhibition. Furthermore, both IgA molecules triggered similar levels of indirect tumor cell killing such as antibody-dependent cell-mediated cytotoxicity (ADCC) by isolated monocytes, activated polymorphonuclear cells, and human whole blood. Interestingly, the dimeric IgA antibodies demonstrated higher efficiency in direct as well as in indirect effector mechanisms compared with their respective monomeric forms. Both wild type and mutated antibody triggered effective FcαRI-mediated tumor cell killing by macrophages already at low effector to target cell ratios. Interestingly, also polarized macrophages mediated significant IgA2-mediated ADCC. M2 macrophages, which have been described as promoting tumor growth and progression, may convert to ADCC-mediating effector cells in the presence of EGFR-directed antibodies. In conclusion, these results provide further insight into the immunotherapeutic potential of recombinant IgA antibodies for tumor immunotherapy and suggest macrophages as an additional effector cell population.

Structural correlates of mouse IgA allotypes

A set of mouse IgAs containing amino acids differing amongst the six alpha-chain allotypes was constructed by mutating an S107-IgA plasmid and transfecting it into a non-producer myeloma cell line along with a kappa-chain plasmid. The secreted IgAs were examined for their possession of a covalent bond between alpha- and light (L)-chains and for their ability to bind to three anti-allotypic monoclonal antibodies, HIS-M2, HY-15, and HY-16. IgA of the Igh-2(a) allotype was found to be unique in its total lack of a covalent bond between alpha and L: -chains, formation of which apparently depends on the presence of an "extra" Cys in the hinges of all of the other five allotypes. The allotypic epitopes are associated with identifiable amino acids in the Calpha1 region of the molecule. Binding to HIS-M2 requires Ala 216 whereas binding to HY-15 requires Pro 216 and Asp 222. Binding to Hy-16 requires Arg 183 and either Pro 216 or Ser 216 but not Ala 216. However, binding to HY-16 by all of the IgAs produced by transfectants is impaired by defective glycosylation in the transfected myeloma and is only revealed after deglycosylation.

Alternative pathways of disulfide bond formation yield secretion-competent, stable and functional immunoglobulins

Disulfide bonds within and between proteins are responsible for stabilizing folding and covalent assembly. They are thought to form by an obligatory pathway that leads to a single native structure compatible with secretion. We have previously demonstrated that the intradomain disulfide in the C(H)1 domain of the Ig gamma2b heavy chains was dispensable for secretion [Elkabetz, Y., Argon, Y., Bar-Nun, S., 2005. Cysteines in C(H)1 underlie retention of unassembled Ig heavy chains. J. Biol. Chem. 280, 14402-14412]. Here we show that the heavy chain-light chain interchain disulfide is also dispensable. gamma2b with mutated Cys128, which normally disulfide bonds with the light chain, still assembled with lambdaI light chain into a secretion-competent, tetrameric IgG2b. This assembly comprised of a covalent homo-dimer of mutant heavy chains (C128S(2)) accompanied non-covalently by a covalent homo-dimer of light chains (lambda(2)). The lambda(2) homo-dimer formed only upon association with C128S(2), through disulfide bonding of the two "orphan" heavy chain-interacting Cys214 in lambdaI. The unique Ig tetramer was secreted efficiently as a functional antibody whose antigen-binding capacity resembled that of normal IgG2b. Therefore, disulfide bonding of Ig manifests considerable plasticity and can generate more than one functional structure that is considered native by the cellular quality control system.

Unknown functions of immunoglobulins A

Traditionally, the function of immunoglobulins A (IgA), the major type of secreted antibodies, has been thought to be restricted to binding antigens outside the epithelium basal membrane. Therefore, effector mechanisms eliminating IgA-opsonized targets have not been investigated so far. However, some indirect observations of infectious agents penetrating into tissues and blood from the environment suggest such mechanisms (analogous to IgG/IgM-dependent activation of complement and natural killers). In the present review, we examine details of IgA structure that might contribute to elucidation of IgA-dependent effector functions in human and animal immunity. Special attention is given to a putative transduction of signal about antigen binding in the active center of IgA from the Fab- to the Fc-superdomain via intramolecular conformational rearrangements. Different structure of the IgA subclasses (IgA1 and IgA2) is examined taking into account probable divergence of their functions in immune response.

Recombinant IgA Antibodies

The production of monoclonal antibodies and the development of recombinant antibody technology have made antibodies one of the largest classes of drugs in development for prophylactic, therapeutic and diagnostic purposes. Currently, all of the Food and Drug Administration (FDA)- approved antibodies are immunoglobulin Gs (IgGs). However, more than 95%of the infections are initiated at the mucosal surfaces, where IgA is the primary immune effector antibody.

Mucosal immunoglobulins

Due to their vast surface area, the mucosal surfaces of the body represent a major site of potential attack by invading pathogens. The secretions that bathe mucosal surfaces contain significant levels of immunoglobulins (Igs), which play key roles in immune defense of these surfaces. IgA is the predominant antibody class in many external secretions and has many functional attributes, both direct and indirect, that serve to prevent infective agents such as bacteria and viruses from breaching the mucosal barrier. This review details current understanding of the structural and functional characteristics of IgA, including interaction with specific receptors (such as Fc(alpha)RI, Fc(alpha)/microR, and CD71) and presents examples of the means by which certain pathogens circumvent the protective properties of this important Ig.

IgA: an immune glycoprotein

IgA is a glycoprotein containing multiple N-linked carbohydrates as well as O-linked glycans in the case of IgA1. Because of the critical role it plays in providing protection at mucosal surfaces, IgA is an ideal candidate for use as a therapeutic or prophylactic agent. The presence or absence of carbohydrates, as well as their structure, has been found to influence effector functions and binding to specific IgA receptors. In addition, changes in IgA glycosylation are associated with immune pathology. A thorough understanding of the contributions of the glycans to IgA immune protection will aid in the design of clinically suitable antibodies.

Cleavage of the human immunoglobulin A1 (IgA1) hinge region by IgA1 proteases requires structures in the Fc region of IgA

Secretory immunoglobulin A (IgA) protects the mucosal surfaces against inhaled and ingested pathogens. Many pathogenic bacteria produce IgA1 proteases that cleave in the hinge of IgA1, thus separating the Fab region from the Fc region and making IgA ineffective. Here, we show that Haemophilus influenzae type 1 and Neisseria gonorrhoeae type 2 IgA1 proteases cleave the IgA1 hinge in the context of the constant region of IgA1 or IgA2m(1) but not in the context of IgG2. Both C(alpha)2 and C(alpha)3 but not C(alpha)1 are required for the cleavage of the IgA1 hinge by H. influenzae and N. gonorrhoeae proteases. While there was no difference in the cleavage kinetics between wild-type IgA1 and IgA1 containing only the first GalNAc residue of the O-linked glycans, the absence of N-linked glycans in the Fc increased the ability of the N. gonorrhoeae protease to cleave the IgA1 hinge. Taken together, these results suggest that, in addition to the IgA1 hinge, structures in the Fc region of IgA are required for the recognition and cleavage of IgA1 by the H. influenzae and N. gonorrhoeae proteases.