Immunoglobulin G (IgG) is one of the most abundant proteins in human serum, accounting for about 15% of all plasma protein content. It is the major class of the five immunoglobulins in human beings, IgM, IgD, IgG, IgA, and IgE. Humans express four types of immunoglobulin G, known as IgG1, IgG2, IgG3, and IgG4.

It appears that IgG4 is a very specific molecule with unique characteristics. Let’s meet with this enigmatic subclass antibody, not fully understood yet.

IgG4 presents different surprising features, including:

• IgG4 is the least represented human IgG subclass in serum (4% of the total IgG), whereas IgG1 counts for 60%.

• IgG4 has the unique ability to undergo Fab-arm exchange mechanism (FAE), in which bi-specific, functionally monovalent antibodies are created. The process involves separation of the two IgG4 heavy chains to form ‘half-molecules’ comprising just one heavy and light chain. Half-molecules of any specificity can recombine to create bi-specific monovalent binding antibodies. While the IgG4 core hinge contains equivalent cysteine residues, it also contains a Pro228Ser substitution which is suggested to promote a more flexible hinge region, leading to the formation of intra- rather than inter-heavy chain disulfide bonds. Calculations led by Peter Molenaar suggest that up to 99% of IgG4 should be bispecific in the body. That means 37.5% of IgG4 would be κ/λ bispecific.

• IgG4 present conformational differences with IgG1 Cc2 domain loops. While the overall domain structure is conserved, the conformation of BC and FG loops is different, and in IgG4, the FG loop folds away from the Cc2 domain. As the differences are very slight, it’s very difficult to create anti-IgG4 antibody that doesn’t cross-react with the other IgG classes.

This is what SYnAbs has successfully achieved, creating 10 different rat hybridoma clones producing anti-human IgG4, without cross-reaction to IgG3, IgG2 or IgG1.

• IgG4 shows anti-inflammatory properties and lacks the ability to form immune complexes and activate complement. In fact, the pathological mechanism of IgG4 is blocking of essential protein-protein interaction, contrary to IgG1 & IgG3.

• IgG4 antigens are not receptors, ion channels or multi subunit proteins.

• IgG4 results from TH2-related cytokine expression, whereas other IgG result from TH1-related cytokine.

The lack of effector functions, such as antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity, is desirable for therapeutic purposes. For example, pembrolizumab and nivolumab, anti-PD-1 (programmed death-1) IgG4 cancer therapeutics, both approved in the USA in 2014, inhibit the interaction between the immunoinhibitory T-cell PD-1 receptor and its ligands, but do not elicit ADCC or CDC. While the therapeutic monoclonal antibody market is dominated by the IgG1 subclass, a number of IgG4 antibodies, with wild type or stabilized hinges, are currently in clinical trials, including:

• anti-IL-5 reslizumab for the treatment of asthma (trade name Cinqair, by Teva Phamaceuticals),
• anti-IL-17 ixekizumab for the treatment of psoriasis (trade name Taltz, by Lilly),
• anti-IL-13 tralokinumab for the treatment of asthma (by Leo Pharma, under Phase III),
• anti-CD22 inotuzumab ozogamicin, an antibody-drug conjugate (trade name Besponsa, by Pfizer/Wyeth) for the treatment of acute lymphoblastic leukemia.

The involvement of IgG4 in disease is increasingly recognized. These diseases are now known as IgG4-related disease “IgG4-RD”, formerly known as IgG4-related systemic disease.

IgG4-associated autoimmune diseases are systemic diseases affecting multiple organs of the body. This chronic inflammatory condition is characterized by tissue infiltration with lymphocytes and IgG4-secreting plasma cells resulting in diffuse organ swelling, tissue-destructive lesions and fibrosis. This can lead to organ dysfunction or organ failure.

In the 1980s, pemphigus, a skin-blistering disease, was recognized as the first autoimmune disease that is hallmarked by IgG4 autoantibody predominance. Pemphigus is a chronic blistering skin disease caused by IgG4-autoantibodies which can be detected in the patient’s skin blisters and serum. Serum immunoglobulin G4 is often elevated but this is not always the case. In case serum titer is superior to 1,35 mg/ml, diagnosis confirmation requires tissue biopsy and immunohistochemistry of the affected organ.

Elevated serum IgG4 levels, and IgG4 auto-antibodies directed against IgG and citrullinated proteins, are some features of rheumatoid arthritis.

Evidence for the IgG4-mediated autoimmune diseases has been recognized in several cases, including:

• Thrombotic thrombocytopenic purpura
• Chronic inflammatory polyneuropathy
• Limbic encephalitis, neuromyotonia and Morvan syndrome
• Chronic inflammatory polyneuroathy
• Non-REM and REM parasomnia
• Myasthenia gravis
• Chronic inflammatory polyneuropathy
• Membranous nephropathy
• Bullous pemphigoid
• Goodpasture disease
• Encephalitis

IgG4 plays a protective role in allergy by acting as a blocking antibody, and inhibiting mast cell degranulation, but a deleterious role in malignant melanoma, by impeding IgG1-mediated anti-tumor immunity. These findings highlight the importance of better understanding the interaction between IgG4 and Fcγ receptors.