Implications of O-glycan modifications in the hinge region of a plant-produced SARS-CoV-2-IgA antibody on functionality

Introduction: Prolyl-4-hydroxylases (P4H) catalyse the irreversible conversion of proline to hydroxyproline, constituting a common posttranslational modification of proteins found in humans, plants, and microbes. Hydroxyproline residues can be further modified in plants to yield glycoproteins containing characteristic O-glycans. It is currently unknown how these plant endogenous modifications impact protein functionality and they cause considerable concerns for the recombinant production of therapeutic proteins in plants. In this study, we carried out host engineering to generate a therapeutic glycoprotein largely devoid of plant-endogenous O-glycans for functional characterization. Methods: Genome editing was used to inactivate two genes coding for enzymes of the P4H10 subfamily in the widely used expression host Nicotiana benthamiana. Using glycoengineering in plants and expression in human HEK293 cells we generated four variants of a potent, SARS-CoV-2 neutralizing antibody, COVA2-15 IgA1. The variants that differed in the number of modified proline residues and O-glycan compositions of their hinge region were assessed regarding their physicochemical properties and functionality. Results: We found that plant endogenous O-glycan formation was strongly reduced on IgA1 when transiently expressed in the P4H10 double mutant N. benthamiana plant line. The IgA1 glycoforms displayed differences in proteolytic stability and minor differences in receptor binding thus highlighting the importance of O-glycosylation in the hinge region of human IgA1. Discussion: This work reports the successful protein O-glycan engineering of an important plant host for recombinant protein expression. While the complete removal of endogenous hydroxyproline residues from the hinge region of plant-produced IgA1 is yet to be achieved, our engineered line is suitable for structure-function studies of O-glycosylated recombinant glycoproteins produced in plants.

The Role of IgA in the Manifestation and Prevention of Allergic Immune Responses

PURPOSE OF REVIEW

Immunoglobulin A (IgA) mediates immune exclusion of antigens in the gut. Notably, IgA plays also a role in the prevention of IgE-mediated allergies and induction of immune tolerance. The present review addresses the role of IgA in the manifestation of IgE-mediated allergies, including allergen-specific immunotherapy (AIT), the regulation of IgA production, and the mechanism of IgA in immune cell activation.

RECENT FINDINGS

The majority of studies report an association of IgA with the induction of immune tolerance in IgE-mediated allergies. However, reports on the involvement of humoral and mucosal IgA, IgA subtypes, monomeric and polymeric IgA, and the mechanism of IgA-mediated immune cell activation are confounding. Effects by IgA are likely mediated by alteration of microbiota, IgE-blocking capacity, or activation of inhibitory signaling pathways. However, the precise mechanism of IgA-regulation, the contribution of serum and/or mucosal IgA, and IgA1/2 subtypes, on the manifestation of IgE-mediated allergies, and the underlying immune modulatory mechanism are still elusive.

Fc receptors act as innate immune receptors during infection?

Innate immunity constitutes the first nonspecific immunological line of defense against infection. In this response, a variety of mechanisms are activated: the complement system, phagocytosis, and the inflammatory response. Then, adaptive immunity is activated. Major opsonization mediators during infections are immunoglobulins (Igs), the function of which is mediated through Fc receptors (FcRs). However, in addition to their role in adaptive immunity, FcRs have been shown to play a role in innate immunity by interacting directly with bacteria in the absence of their natural ligands (Igs). Additionally, it has been hypothesized that during the early phase of bacterial infection, FcRs play a protective role via innate immune functions mediated through direct recognition of bacteria, and as the infection progresses to later phases, FcRs exhibit their established function as receptors in adaptive immunity. This review provides detailed insight into the potential role of FcRs as innate immune mediators of the host defense against bacterial infection independent of opsonins.

Recombinant protein expression: Challenges in production and folding related matters

Protein folding is a biophysical process by which proteins reach a specific three-dimensional structure. The amino acid sequence of a polypeptide chain contains all the information needed to determine the final three-dimensional structure of a protein. When producing a recombinant protein, several problems can occur, including proteolysis, incorrect folding, formation of inclusion bodies, or protein aggregation, whereby the protein loses its natural structure. To overcome such limitations, several strategies have been developed to address each specific issue. Identification of proper protein refolding conditions can be challenging, and to tackle this high throughput screening for different recombinant protein folding conditions can prove a sound solution. Different approaches have emerged to tackle refolding issues. One particular approach to address folding issues involves molecular chaperones, highly conserved proteins that contribute to proper folding by shielding folding proteins from other proteins that could hinder the process. Proper protein folding is one of the main prerequisites for post-translational modifications. Incorrect folding, if not dealt with, can lead to a buildup of protein misfoldings that damage cells and cause widespread abnormalities. Said post-translational modifications, widespread in eukaryotes, are critical for protein structure, function and biological activity. Incorrect post-translational protein modifications may lead to individual consequences or aggregation of therapeutic proteins. In this review article, we have tried to examine some key aspects of recombinant protein expression. Accordingly, the relevance of these proteins is highlighted, major problems related to the production of recombinant protein and to refolding issues are pinpointed and suggested solutions are presented. An overview of post-translational modification, their biological significance and methods of identification are also provided. Overall, the work is expected to illustrate challenges in recombinant protein expression.

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 and FcαRI: Versatile Players in Homeostasis, Infection, and Autoimmunity

Mucosal surfaces constitute the frontiers of the body and are the biggest barriers of our body for the outside world. Immunoglobulin A (IgA) is the most abundant antibody class present at these sites. It passively contributes to mucosal homeostasis via immune exclusion maintaining a tight balance between tolerating commensals and providing protection against pathogens. Once pathogens have succeeded in invading the epithelial barriers, IgA has an active role in host-pathogen defense by activating myeloid cells through divers receptors, including its Fc receptor, FcαRI (CD89). To evade elimination, several pathogens secrete proteins that interfere with either IgA neutralization or FcαRI-mediated immune responses, emphasizing the importance of IgA-FcαRI interactions in preventing infection. Depending on the IgA form, either anti- or pro-inflammatory responses can be induced. Moreover, the presence of excessive IgA immune complexes can result in continuous FcαRI-mediated activation of myeloid cells, potentially leading to severe tissue damage. On the one hand, enhancing pathogen-specific mucosal and systemic IgA by vaccination may increase protective immunity against infectious diseases. On the other hand, interfering with the IgA-FcαRI axis by monovalent targeting or blocking FcαRI may resolve IgA-induced inflammation and tissue damage. This review describes the multifaceted role of FcαRI as immune regulator between anti- and pro-inflammatory responses of IgA, and addresses potential novel therapeutic strategies that target FcαRI in disease.

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N-glycosylation of CREBH improves lipid metabolism and attenuates lipotoxicity in NAFLD by modulating PPARα and SCD-1

INTRODUCTION

Cyclic adenosine monophosphate (AMP)-responsive element-binding protein H (CREBH), an endoplasmic reticulum-anchored transcription factor essential for lipid metabolism and inflammation in nonalcoholic fatty liver disease (NAFLD), is covalently modified by N-acetylglucosamine. Glycosylation is a ubiquitous type of protein involved in posttranslational modifications, and plays a critical role in various biological processes. However, the mechanism of glycosylated CREBH remains poorly understood in NAFLD.

METHODS

CREBH glycosylation mutants were obtained by site-mutation methods. After transfection with plasmids, AML-12, LO2, or HepG2 cells were treated with palmitic acid (PA) proteolysis, tunicamycin (Tm), or their combination. Glycosyltransferase V (GnT-V) was used induce hyperglycosylation to further understand the effect of CREBH. In addition, glycosylation mutant mice and hyperglycosylated mice were generated by lentivirus injection to construct two kinds of NAFLD animal models. The expression of NAFLD-related factors was detected to further verify the role of N-linked glycosylation of CREBH in lipid and sterol metabolism, inflammation, and lipotoxicity.

RESULTS

N-glycosylation enhanced the ability of CREBH to activate transcription and modulated the production of peroxisome proliferator-activated receptor alpha (PPARα) and stearoyl-CoA desaturase-1 (SCD-1) activity by affecting their promoter-driven transcription activity and protein interactions, leading to reduce lipid deposition and attenuate lipotoxicity. Deglycosylation of CREBH induced by Tm could inhibit the proteolysis of CREBH induced by PA. The addition of unglycosylated CREBH to cells upregulates gene and protein expression of lipogenesis, lipotoxicity, and inflammation, and aggravates liver damage by preventing glycosylation in cells, as well as in mouse models of NAFLD. Furthermore, increased N-glycosylation of CREBH, as achieved by overexpressing GnT-V could significantly improve liver lesion caused by unglycosylation of CREBH.

CONCLUSION

These findings have important implications for the role of CREBH N-glycosylation in proteolytic activation, and they provide the first link between N-glycosylation of CREBH, lipid metabolism, and lipotoxicity processes in the liver by modulating PPARα and SCD-1. These results provide novel insights into the N-glycosylation of CREBH as a therapeutic target for NAFLD.

When Therapeutic IgA Antibodies Might Come of Age

BACKGROUND

Extensive efforts have been made in optimizing monoclonal immunoglobulin (Ig)G antibodies for use in clinical practice. Accumulating evidence suggests that IgA or anti-FcαRI could also represent an exciting avenue toward novel therapeutic strategies.

SUMMARY

Here, we underline that IgA is more effective in recruiting neutrophils for tumor cell killing and is potently active against several pathogens, including rotavirus, poliovirus, influenza virus, and SARS-CoV-2. IgA could also be used to modulate excessive immune responses in inflammatory diseases. Furthermore, secretory IgA is emerging as a major regulator of gut microbiota, which impacts intestinal homeostasis and global health as well. As such, IgA could be used to promote a healthy microbiota in a therapeutic setting. Key messages: IgA combines multifaceted functions that can be desirable for immunotherapy.

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.

Distinct Fcα receptor N-glycans modulate the binding affinity to immunoglobulin A (IgA) antibodies

Human immunoglobulin A (IgA) is the most prevalent antibody class at mucosal sites with an important role in mucosal defense. Little is known about the impact of N-glycan modifications of IgA1 and IgA2 on binding to the Fcα receptor (FcαRI), which is also heavily glycosylated at its extracellular domain. Here, we transiently expressed human epidermal growth factor receptor 2 (HER2)-binding monomeric IgA1, IgA2m(1), and IgA2m(2) variants in Nicotiana benthamiana ΔXT/FT plants lacking the enzymes responsible for generating nonhuman N-glycan structures. By coinfiltrating IgA with the respective glycan-modifying enzymes, we generated IgA carrying distinct homogenous N-glycans. We demonstrate that distinctly different N-glycan profiles did not influence antigen binding or the overall structure and integrity of the IgA antibodies but did affect their thermal stability. Using size-exclusion chromatography, differential scanning and isothermal titration calorimetry, surface plasmon resonance spectroscopy, and molecular modeling, we probed distinct IgA1 and IgA2 glycoforms for binding to four different FcαRI glycoforms and investigated the thermodynamics and kinetics of complex formation. Our results suggest that different N-glycans on the receptor significantly contribute to binding affinities for its cognate ligand. We also noted that full-length IgA and FcαRI form a mixture of 1:1 and 1:2 complexes tending toward a 1:1 stoichiometry due to different IgA tailpiece conformations that make it less likely that both binding sites are simultaneously occupied. In conclusion, N-glycans of human IgA do not affect its structure and integrity but its thermal stability, and FcαRI N-glycans significantly modulate binding affinity to IgA.

Understanding Fc Receptor Involvement in Inflammatory Diseases: From Mechanisms to New Therapeutic Tools

Fc receptors (FcRs) belong to the ITAM-associated receptor family. FcRs control the humoral and innate immunity which are essential for appropriate responses to infections and prevention of chronic inflammation or auto-immune diseases. Following their crosslinking by immune complexes, FcRs play various roles such as modulation of the immune response by released cytokines or of phagocytosis. Here, we review FcR involvement in pathologies leading notably to altered intracellular signaling with functionally relevant consequences to the host, and targeting of Fc receptors as therapeutic approaches. Special emphasis will be given to some FcRs, such as the FcαRI, the FcγRIIA and the FcγRIIIA, which behave like the ancient god Janus depending on the ITAM motif to inhibit or activate immune responses depending on their targeting by monomeric/dimeric immunoglobulins or by immune complexes. This ITAM duality has been recently defined as inhibitory or activating ITAM (ITAMi or ITAMa) which are controlled by Src family kinases. Involvement of various ITAM-bearing FcRs observed during infectious or autoimmune diseases is associated with allelic variants, changes in ligand binding ability responsible for host defense perturbation. During auto-immune diseases such as rheumatoid arthritis, lupus or immune thrombocytopenia, the autoantibodies and immune complexes lead to inflammation through FcR aggregation. We will discuss the role of FcRs in autoimmune diseases, and focus on novel approaches to target FcRs for resolution of antibody-mediated autoimmunity. We will finally also discuss the down-regulation of FcR functionality as a therapeutic approach for autoimmune diseases.

IgA and FcαRI: Pathological Roles and Therapeutic Opportunities

Immunoglobulin A (IgA) is the most abundant antibody class present at mucosal surfaces. The production of IgA exceeds the production of all other antibodies combined, supporting its prominent role in host-pathogen defense. IgA closely interacts with the intestinal microbiota to enhance its diversity, and IgA has a passive protective role via immune exclusion. Additionally, inhibitory ITAMi signaling via the IgA Fc receptor (FcαRI; CD89) by monomeric IgA may play a role in maintaining homeostatic conditions. By contrast, IgA immune complexes (e.g., opsonized pathogens) potently activate immune cells via cross-linking FcαRI, thereby inducing pro-inflammatory responses resulting in elimination of pathogens. The importance of IgA in removal of pathogens is emphasized by the fact that several pathogens developed mechanisms to break down IgA or evade FcαRI-mediated activation of immune cells. Augmented or aberrant presence of IgA immune complexes can result in excessive neutrophil activation, potentially leading to severe tissue damage in multiple inflammatory, or autoimmune diseases. Influencing IgA or FcαRI-mediated functions therefore provides several therapeutic possibilities. On the one hand (passive) IgA vaccination strategies can be developed for protection against infections. Furthermore, IgA monoclonal antibodies that are directed against tumor antigens may be effective as cancer treatment. On the other hand, induction of ITAMi signaling via FcαRI may reduce allergy or inflammation, whereas blocking FcαRI with monoclonal antibodies, or peptides may resolve IgA-induced tissue damage. In this review both (patho)physiological roles as well as therapeutic possibilities of the IgA-FcαRI axis are addressed.

Galectin-3 promotes caspase-independent cell death of HIV-1-infected macrophages

HIV-1-infected macrophages are a key contributor to the formation of a viral reservoir and new treatment strategies focus on eliminating this pool of virus. Galectin-3 is a potent apoptosis-inducing protein that regulates diverse cellular activities. In the present study, we investigated whether galectin-3 could induce cell death in HIV-1-infected macrophages using HIV-1-infected THP1 monocytes (THP1-MNs) and THP1-derived macrophages (THP1-MΦs) as in vitro cellular models. We found that THP1-MΦs were more resistant than the THP1-MNs to HIV-1 infection-induced death, and that HIV-1 infection of the THP1-MΦs increased expression of the anti-apoptotic proteins Mcl-1, Bcl-2 and Bcl-xL. Additionally, galectin-3 but not FasL, tumor necrosis factor (TNF)-related apoptosis-inducing ligand or TNF-α, could induce cell death in HIV-1-infected THP1-MΦs. A similar result was shown for primary human monocyte-derived macrophages. Galectin-3-induced cell death was also significantly increased in macrophages obtained from SIVmac251-infected macaques compared to that of macrophages from healthy macaques. Furthermore, galectin-3-induced cell death in HIV-1-infected THP1-MΦs was caspase independent. Interestingly, endonuclease G (Endo G) was increased in the nucleus and decreased in the cytoplasm of galectin-3-treated cells; thus, galectin-3-induced cell death in HIV-1-infected THP1-MΦs is most likely related to the translocation of Endo G from the cytoplasm to the nucleus. These findings suggest that galectin-3 may potentially aid in the eradication of HIV-1/SIV-infected macrophages.

Effects of anchor structure and glycosylation of Fcγ receptor III on ligand binding affinity

The anchor structure of CD16 affects its binding affinity in a ligand-specific manner. The ligand binding affinity decreases for human IgG1 but increases for murine IgG2a when the anchor is changed from full to partial to none. Removing N-glycosylation from CD16 also increases the ligand binding affinity.

Isoforms of the Fcγ receptor III (FcγRIII or CD16) are cell surface receptors for the Fc portion of IgG and important regulators of humoral immune responses. Different ligand binding kinetics of FcγRIII isoforms are obtained in three dimensions by surface plasmon resonance and in two dimensions by a micropipette adhesion frequency assay. We show that the anchor structure of CD16 isoforms isolated from the cell membrane affects their binding affinities in a ligand-specific manner. Changing the receptor anchor structure from full to partial to none decreases the ligand binding affinity for human IgG1 (hIgG1) but increases it for murine IgG2a (mIgG2a). Removing N-glycosylation from the CD16 protein core by tunicamycin also increases the ligand binding affinity. Molecular dynamics simulations indicate that deglycosylation at Asn-163 of CD16 removes the steric hindrance for the CD16-hIgG1 Fc binding and thus increases the binding affinity. These results highlight an unexpected sensitivity of ligand binding to the receptor anchor structure and glycosylation and suggest their respective roles in controlling allosterically the conformation of the ligand binding pocket of CD16.

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.

Repressive effect of primary virus replication on superinfection correlated with gut-derived central memory CD4(+) T cells in SHIV-infected Chinese rhesus macaques

A possible mechanism of susceptibility to superinfection with simian-human immunodeficiency virus (SHIV)-1157ipd3N4 was explored in twelve SHIV_SF162P3-infected Chinese rhesus macaques. Based on the kinetics of viral replication for the second infecting virus following SHIV-1157ipd3N4 inoculation, the monkeys were divided into two groups: those relatively resistant to superinfection (SIR) and those relatively sensitive to superinfection (SIS). We found that superinfection-resistant macaques had high primary viremia, whereas superinfection-sensitive macaques had low primary viremia, suggesting that primary SHIV_SF162P3 infection with a high viral-replication level would repress superinfection with a heterologous SHIV-1157ipd3N4. Although no correlation of protection against superinfection with virus-specific CD4⁺ T cell or CD8⁺ T cell immune responses from gut was observed prior to superinfection, superinfection susceptibility was strongly correlated with CD4⁺ Tcm cells from gut both prior to the second infecting virus inoculation and on day 7 after superinfection, but not with CD4⁺ Tem cells from gut or with CD4⁺ Tcm cells from peripheral blood and lymph node. These results point to the important roles of gut-derived CD4⁺ Tcm cells for the study of the mechanisms of protection against superinfection and the evaluation of the safety and efficacy of vaccines and therapies against acquired immune deficiency syndrome (AIDS).

IgG-Fc N-glycosylation at Asn297 and IgA O-glycosylation in the hinge region in health and disease

Immunoglobulins (Igs) are the major molecules secreted by B lymphocytes during an adaptive immune response. They are glycoproteins with distinctive glycosylation patterns, resulting in wide variations in the number, type and location of their oligosaccharides in each isotype and subclass. The sugars play specific structural roles, maintaining and modulating effector functions of Igs. Aberrant glycosylation might contribute to disease pathogenesis. This review will focus on the glycosylation of IgG and IgA because they have been studied more extensively than other immunoglobulins. Rheumatoid arthritis and IgA nephritis are used to describe the association of glycosylation aberration and disease pathogenesis.

Host sialoglycans and bacterial sialidases: a mucosal perspective

Sialic acids are nine-carbon-backbone sugars that occupy outermost positions on vertebrate cells and secreted sialoglycoproteins. These negatively charged hydrophilic carbohydrates have a variety of biological, biophysical and immunological functions. Mucosal surfaces and secretions of the mouth, airway, gut and vagina are especially sialoglycan-rich. Given their prominent positions and important functions, a variety of microbial strategies have targeted host sialic acids for adherence, mimicry and/or degradation. Here we review the roles of bacterial sialidases (neuraminidases) during colonization and pathogenesis of mammalian mucosal surfaces. Evidence is presented to support the myriad roles of mucosal sialoglycans in protecting the host from bacterial infection. In opposition, many bacteria hydrolyse sialic acids during associations with the gastrointestinal, oral, respiratory and reproductive tracts. Sialidases promote bacterial survival in mucosal niche environments in several ways, including: (i) nutritional benefits of sialic acid catabolism, (ii) unmasking of cryptic host ligands used for adherence, (iii) participation in biofilm formation and (iv) modulation of immune function. Bacterial sialidases are among the best-studied enzymes involved in pathogenesis and may also drive commensal and/or symbiotic host associations. Future studies should continue to define host substrates of bacterial sialidases and the mechanisms of their pathologic, commensal and symbiotic interactions with the mammalian host.

The human immunoglobulin A Fc receptor FcαRI: a multifaceted regulator of mucosal immunity

Immunoglobulin A (IgA) is commonly recognized as the most prevalent antibody (Ab) at mucosal sites with an important role in defense by shielding mucosal surfaces from invasion by pathogens. However, its potential to both actively dampen excessive immune responses or to initiate potent proinflammatory cellular processes is less well known. Interestingly, either functional outcome is mediated through interaction with the myeloid IgA Fc receptor FcαRI (CD89). Monomeric interaction of IgA with FcαRI triggers inhibitory signals that block activation via other receptors, whereas multimeric FcαRI crosslinking induces phagocytosis, reactive oxygen species production, antigen presentation, Ab-dependent cellular cytotoxicity, and cytokine release. Thus, FcαRI acts as a regulator between anti- and proinflammatory responses of IgA. As such, the biology of FcαRI, and its multifaceted role in immunity will be the focus of this review.

Small changes huge impact: the role of protein posttranslational modifications in cellular homeostasis and disease

Posttranslational modifications (PTMs) modulate protein function in most eukaryotes and have a ubiquitous role in diverse range of cellular functions. Identification, characterization, and mapping of these modifications to specific amino acid residues on proteins are critical towards understanding their functional significance in a biological context. The interpretation of proteome data obtained from the high-throughput methods cannot be deciphered unambiguously without a priori knowledge of protein modifications. An in-depth understanding of protein PTMs is important not only for gaining a perception of a wide array of cellular functions but also towards developing drug therapies for many life-threatening diseases like cancer and neurodegenerative disorders. Many of the protein modifications like ubiquitination play a decisive role in various drug response(s) and eventually in disease prognosis. Thus, many commonly observed PTMs are routinely tracked as disease markers while many others are used as molecular targets for developing target-specific therapies. In this paper, we summarize some of the major, well-studied protein alterations and highlight their importance in various chronic diseases and normal development. In addition, other promising minor modifications such as SUMOylation, observed to impact cellular dynamics as well as disease pathology, are mentioned briefly.

Immunoglobulin A: A next generation of therapeutic antibodies?

Although immunoglobulin (Ig) A is commonly recognized as the most prevalent antibody subclass at mucosal sites with an important role in mucosal defense, its potential as a therapeutic monoclonal antibody is less well known. However, IgA has multifaceted anti-, non-, and pro-inflammatory functions that can be exploited for different immunotherapeutical strategies, which will be the focus of this review.