Review Role of IgG3 in Infectious Diseases 1 2 1 1, Timon Damelang, Stephen J. Rogerson, Stephen J. Kent, and Amy W. Chung * IgG3 comprises only a minor fraction of IgG and has remained relatively under- Highlights studied until recent years. Key physiochemical characteristics of IgG3 include IgG3 has been associated with enhanced control or protection against an elongated hinge region, greater molecular flexibility, extensive polymor- a range of intracellular bacteria, para- phisms, and additional glycosylation sites not present on other IgG subclasses. sites, and viruses. These characteristics make IgG3 a uniquely potent immunoglobulin, with the IgG3 Abs are potent mediators of potential for triggering effector functions including complement activation, effector functions, including enhanced antibody (Ab)-mediated phagocytosis, or Ab-mediated cellular cytotoxicity ADCC, opsonophagocytosis, comple- ment activation, and neutralization, (ADCC). Recent studies underscore the importance of IgG3 effector functions compared with other IgG subclasses. against a range of pathogens and have provided approaches to overcome IgG3-associated limitations, such as allotype-dependent short Ab half-life, and Future Ab-based therapeutics and vaccines should consider utilizing excessive proinflammatory activation. Understanding the molecular and func- IgG3, based on features of enhanced tional properties of IgG3 may facilitate the development of improved Ab-based functional capacity. immunotherapies and vaccines against infectious diseases. Investigating the impact of glycosyla- tion patterns and allotypes on IgG3 Human IgG3 an Understudied but Highly Potent Immunoglobulin function may expand our understand- Antibodies (Abs) play a major role in protection against infections by binding to and inactivating ing of IgG3 responses and their ther- apeutic potential. invading pathogens. Although IgG3 constitutes a minor proportion of total human immuno- globulins, a growing number of recent studies have highlighted IgG3 as critical for the control and/or protection of a range of pathogens, including viruses (e.g., HIV [1–5]), bacteria (e.g., Neisseria spp. [6–8]), and parasites (e.g., Plasmodium spp. [9–13]). This review describes the unique characteristics of human IgG3 that contribute to its potent anti-pathogenic functions, including enhanced neutralization, Fcg receptor (FcgR) (see Glossary) affinity, and Fc- mediated activity. Of relevance, the structure and enhanced functionality of human IgG3 is unique compared with most mammals, including macaque species, which do not have an equivalent analog among their IgG subclasses [14–16]. This is particularly significant for non- human primate models, where macaque species are commonly used for pre-human clinical trials and may historically have contributed to why human IgG3 has been understudied in the control of infectious diseases, until recently. Furthermore, this review discusses how the unique properties of human IgG3 can be harnessed for future preventative and therapeutic manip- ulations, highlighting the major obstacles and outstanding questions that are pertinent. 1 Human IgG3: a Unique Antibody Subclass Department of Microbiology and Immunology, Peter Doherty Institute The glycoprotein IgG is the most abundant isotype in healthy human plasma, and can be for Infection and Immunity, University separated into four subclasses: IgG1 (60–70% in plasma), IgG2 (20–30%), IgG3 (5–8%), and of Melbourne, Melbourne, VIC, IgG4 (1–3%) [17]. The amino acid sequences of the human IgG subclasses are >95% Australia 2 Department of Medicine, Peter homologous in the constant domains of their heavy chains (Figure 1). IgG3 has a distinct Doherty Institute for Infection and amino acid composition and structure in the hinge region between CH1 and CH2 [18]. The Immunity, University of Melbourne, fl > > > exibility of the hinge region decreases in the order IgG3 IgG1 IgG4 IgG2 [19]. Further- Melbourne, VIC, Australia more, IgG3 has an extended hinge region (Figure 1 and Table 1), consisting of up to 62 amino acids, forming a polyproline double helix consisting of 11 disulfide bridges (compared with two *Correspondence: in the hinge regions of IgG1 and IgG4, and four in the hinge region of IgG2) [20]. This is the result [email protected] of duplications of a hinge exon, encoded by one single exon in IgG1, IgG2, and IgG4, but up to (A.W. Chung). Trends in Immunology, March 2019, Vol. 40, No. 3 https://doi.org/10.1016/j.it.2019.01.005 197 © 2019 Elsevier Ltd. All rights reserved. Downloaded for Anonymous User (n/a) at University of Melbourne from ClinicalKey.com.au by Elsevier on March 19, 2019. For personal use only. No other uses without permission. Copyright ©2019. Elsevier Inc. All rights reserved. four exons in IgG3. The extended hinge region of IgG3 expands the Fab arms further away from Glossary the Fc. This distance enables high rotational freedom, which provides the molecule with greater Antibody-dependent cell flexibility and reach [21]. The longer hinge region and the greater reach of the Fab arms together cytotoxicity (ADCC): killing of an Ab-coated target cell by a cytotoxic might explain why this subclass can probe into less exposed antigens and could contribute to effector cell through a nonphagocytic the high potential of IgG3 to activate effector functions [20,21]. process. Antibody-dependent cellular IgG3 Allotypes inhibition (ADCI): a process by which Abs can inhibit Plasmodium Allotypes are polymorphisms in the constant regions of immunoglobulin heavy and light chains growth in the presence of (Figure 1B). IgG3 is the most polymorphic subclass with 13 Gm allotypes termed G3m; thought to monocytes. fi be due to its higher rapid evolutionary diversi cation compared with other subclasses [20]. G3 m Antibody-dependent cellular viral allotypes are inherited in different combinations or G3 m alleles (encoded by one or several IGHG3 inhibition (ADCVI): Fcg-receptor- mediated antiviral activity, occurring alleles) shared among individuals within populations (Table 2). Several G3 m allotypes consist of a when Ab bound to virus-infected combination of multiple amino acids, which lead to changes of their tertiary protein structures, with target cells engages FcgR-bearing all known G3 m allotypes located within the CH2 and CH3 domains [22]. IgG3 allotypic variations effector cells, (e.g., NK cells, can have structural and functional consequences, such as shorter hinge regions and extended monocytes, or macrophages). Antibody-dependent half-life compared with other allotypes [23]. In addition, polymorphisms in the CH3 domain affect phagocytosis (ADP): process by the CH3–CH3 interdomain interactions [24], with potential consequences for C1q binding in which innate immune phagocytic fi fi complement activation [24,25]. Speci c allotypes also contribute to underappreciated dif cul- effector cells (e.g., neutrophils, macrophages, or monocytes) ingest ties in purifying human IgG3 from serum samples, where only allotypes containing a histidine or engulf other cells, organisms, or residue at position 435 can be purified by protein A [26]. particles. Allotype: amino acid differences in Glycosylation of IgG3 the constant region of either the heavy or light chains of an Ab within Glycosylation is a post-translational modification of Abs, which can be regulated by a range of a subclass of Abs. B cell stimuli, including environmental factors, such as stress or disease, cytokine activity, and Avidity: the overall sum of binding innate immune signaling receptors, such as Toll-like receptors. Hence, exposure to specific strength of multiple affinities, often pathogens, antigens, or vaccination has the potential to skew Ab glycan profiles [27,28]. used to describe the accumulated strength of two Ab Fab arms with its Glycosylation is an inherent mechanism of Ab diversification, on top of V(D)J recombination, antigens. somatic hypermutation (SHM), and class switch recombination (CSR), thereby contrib- Bacteriolysis: destruction of uting to the extent of the Ab repertoire of B cells [29]. bacterial cells, often be mediated by Abs. Bispecific antibody: recombinant IgG3 Abs can include up to three potential glycosylation sites. The most well-described protein that can simultaneously bind glycosylation site is found in all human IgG subclasses, where carbohydrate groups are two different types of antigens. attached to asparagine 297 in the CH2 domain (Figure 1C). The glycans at this N-glycosylation Broadly neutralizing antibody site can influence Ab stability [30], binding to FcgRs and complement [31], consequently (bNAb): special type of Abs that can recognize and block many strains of modulating effector functions, such as complement-dependent cytotoxicity (CDC) and a particular pathogen (e.g., virus) Ab-dependent cell cytotoxicity (ADCC) [32–35]. For instance, IgG3 monoclonal Abs from entering healthy cells. (mAbs) expressed from a1,6-fucosyltransferase gene knockout Chinese hamster ovary C-reactive protein: protein produced by the liver in response to (CHO) cell lines (lacking the ability to attach fucose to asparagine 297 in Abs), have demon- inflammation or infection. strated increased binding to FcgRIIIa relative to wild-type controls [25,34]. Furthermore, a high Class switch recombination: degree of agalactosylation has been associated with disease progression for various infections, process by which proliferating B cells including chronic HIV, hepatitis B, leishmaniasis, and active tuberculosis (TB) [28,36,50], while rearrange constant