antibodies Review The Ligands for Human IgG and Their Effector Functions Steven W. de Taeye 1,2,*, Theo Rispens 1 and Gestur Vidarsson 2 1 Sanquin Research, Dept Immunopathology and Landsteiner Laboratory, Amsterdam UMC, University of Amsterdam, 1066 CX Amsterdam, The Netherlands; [email protected] 2 Sanquin Research, Dept Experimental Immunohematology and Landsteiner Laboratory, Amsterdam UMC, University of Amsterdam, 1066 CX Amsterdam, The Netherlands; [email protected] * Correspondence: [email protected] Received: 26 March 2019; Accepted: 18 April 2019; Published: 25 April 2019 Abstract: Activation of the humoral immune system is initiated when antibodies recognize an antigen and trigger effector functions through the interaction with Fc engaging molecules. The most abundant immunoglobulin isotype in serum is Immunoglobulin G (IgG), which is involved in many humoral immune responses, strongly interacting with effector molecules. The IgG subclass, allotype, and glycosylation pattern, among other factors, determine the interaction strength of the IgG-Fc domain with these Fc engaging molecules, and thereby the potential strength of their effector potential. The molecules responsible for the effector phase include the classical IgG-Fc receptors (FcγR), the neonatal Fc-receptor (FcRn), the Tripartite motif-containing protein 21 (TRIM21), the first component of the classical complement cascade (C1), and possibly, the Fc-receptor-like receptors (FcRL4/5). Here we provide an overview of the interactions of IgG with effector molecules and discuss how natural variation on the antibody and effector molecule side shapes the biological activities of antibodies. The increasing knowledge on the Fc-mediated effector functions of antibodies drives the development of better therapeutic antibodies for cancer immunotherapy or treatment of autoimmune diseases. Keywords: Antibodies; IgG; Fc effector molecules; allotypes; glycosylation 1. Introduction The human adaptive humoral immune system is dependent on antigen recognition via the B cell receptor on naïve B cells, which initiates B cell maturation and eventually production of antibodies by plasmablasts and plasma cells. IgM is the initial antibody class that is made when naïve B cells are activated and can be found as a membrane-bound B cell receptor (BCR) on naïve B cells together with IgD. Like all immunoglobulins, the basic secreted unit is a dimer of two identical heavy chains, each coupled to identical light chains. For IgM, five such units associate together with a Joining (J) chain forming a pentamer, which is a strong activator of the classical complement pathway [1]. Class switching from the initial IgM isotype allows the humoral immune system to engage with each antigen in a specific manner, with unique effector mechanisms being imprinted by each class (IgM, IgG, IgA, IgE, and IgD). Additionally, IgA and IgG are further subdivided in two and four subclasses, respectively (IgA1-2 and IgG1-4). Although the IgA subclasses seem to have similar if not identical effector functions, the abundance at different locations (serum/mucosa) is very different. The effector functions of IgG subclasses are very different and will be a major topic of this review. During the onset of initial class switching in a given B cell any class switching event is theoretically possible from IgM to any other isotype. However, further sequential class switching events are dependent on the order of the Ig heavy chain constant genes on chromosome 14 (IgM, IgD, IgG3, IgG1, IgA1, IgG2, IgG4, IgE, and IgA2) [2]. This is because of genetic excisions of constant regions, e.g., the Antibodies 2019, 8, 30; doi:10.3390/antib8020030 www.mdpi.com/journal/antibodies Antibodies 2019, 8, 30 2 of 18 exons encoding for IgM, IgD, and IgG3 constant regions are deleted after a class switch event from IgM to IgG1, preventing descendants of the proliferating B cell from generating IgG3. These class switching events of naïve B cells in the germinal center during clonal expansion are not completely random, but are regulated through signals received from T-helper cells and antigen presenting cells (APC). Cytokines produced by T-helper cells and signaling via toll-like receptors (TLR) on B cells initiate class switching of antigen specific B cells via activation-induced deaminase (AID) activity [3]. All the immunoglobulin isotypes have their own biodistribution, function and are often elicited upon specific triggers. IgD, for example, may be found in a secreted form, mostly in the tonsils, but its function remains enigmatic [4,5]. IgE is known to interact with mast cells to trigger the release of histamine mostly through the high affinity IgE-Fc Receptor I (Fc"RI), but it also interacts with the atypical Fc"RII (CD23), a c-type lectin. IgA has differential function depending on whether it is secretory IgA (SIgA) or serum IgA. SIgA is a dimeric form containing the J-chain (also found in IgM) that is associated with the extracellular domain of the polymeric Ig-Receptor (pIgR), which cleaves off after the transcytosis of dimeric IgA by the pIgR on epithelial cell of the mucosa [6]. Only serum IgA, which is monomeric and not associated with the J-chain, can bind and activate the myeloid IgA-receptor FcαRI efficiently and trigger a strong cellular response [7–9]. These isotypes—IgA, IgE, and IgD—generally do not activate complement, and therefore rely on other mechanisms to carry out their function [5,10,11]. Thus detailed discussion of these isotypes is beyond the scope of this chapter where we will focus on the biology of IgG subclasses. 2. Immunoglobulin G (IgG) In the majority of humoral antibody responses, whether it is the protection against viral or cellular pathogens, IgG-mediated effector functions are involved. This includes humoral responses in allo- or autoimmune diseases. IgG1 is the most abundant antibody in human sera, followed by IgG2, IgG3, and IgG4 respectively [12]. Although the IgG subclasses are more than 90% identical on the amino acid level, each IgG subclass has a unique profile with respect to structure, antigen binding, immune complex formation, complement activation, triggering of FcγR, half-life, and placental transport [12] (Figure1). IgG1, IgG3, and to some extent, IgG4 are generally formed against protein antigens, while IgG2 is the major subclass formed against repetitive T cell-independent polysaccharide structures found on encapsulated bacteria [13]. IgG3 is often the first subclass to form, which is followed by IgG1 responses that later dominate. The development of IgG4 responses is often the outcome of repeated or prolonged antigen exposure, although class switching from IgM expressing naïve B cells to IgG4 is possible [14]. The unusually weak CH3–CH3 interactions in the Fc domain of IgG4 and the redox sensitive disulfide bonds in the hinge of IgG4 facilitate exchange of two half-molecules (each consisting of one heavy and one light chain from a single IgG4 molecule), which enables the formation of bispecific IgG4 molecules [14–16]. For IgG2, two isoforms (IgG2 A/B) exist as a result of different disulfide bonding in the Fab and hinge domain, which determines the rigidity of the Fab domains when engaging antigen [17]. Functionally, IgG1 and IgG3 are strong inducers of Fc-mediated effector mechanisms, such as antibody-dependent cellular cytotoxicity (ADCC), complement dependent cytotoxicity (CDC), and antibody-dependent cellular phagocytosis (ADCP). IgG2 and IgG4, on the other hand, generally induce more subtle responses, although IgG2 has been shown to be quite capable of inducing good complement and Fc-receptor-mediated responses against epitopes of high-density such as polysaccharides [18,19]. This capacity of IgG2 may be related also to the peculiar rigidity of the hinge, shown to result in super agonistic antibodies and triggering strong signaling when targeting immune costimulatory receptors such as CD40 [20]. Below, the interaction of different ligands with human IgG subclasses is discussed as well as the effector functions triggered via these interactions. Antibodies 2019, 8, 30 3 of 18 Antibodies 2019, 8, x FOR PEER REVIEW 3 of 18 Figure 1.1. Structural variation of immunoglobulin G subclasses. A A structural representation of the immunoglobulin G (IgG) subclasses and thethe variationvariation withinwithin thesethese subclasses,subclasses, includingincluding allotypeallotype variation, hinge variation, and glycosylation.glycosylation. The variation originat originatinging from allotypic polymorphisms in thethe immunoglobulinimmunoglobulin heavy heavy gamma gamma (IGHG) (IGHG) constant constant domain domain is indicatedis indicated by by blue blue stars. stars. Except Except for thefor starthe star representing representing the variationthe variation in hinge in hinge length length between between IgG3 allotypes,IgG3 allotypes, each blue each star blue indicates star indicates amino acidamino variation acid variation at one particularat one partic residueular residue in the constantin the constant domain. doma Fabin. glycosylation Fab glycosylation is indicated is indicated and is presentand is present in 5–30% in of5–30% antibodies of antibodies in serum, in depending serum, de onpending subclass on andsubclass antigen and specificity. antigen specificity. The glycoform The ofglycoform the N297 of Fc the glycan N297 is Fc highly glycan variable, is highly for whichvariable, the for frequency which the of each frequency glycan of moiety each glycan on IgG antibodiesmoiety on inIgG human antibodies serum in is human