Theo Rispens

Immunoglobulin G4: an odd antibody

Despite its well‐known association with IgE‐mediated allergy, IgG4 antibodies still have several poorly understood characteristics. IgG4 is a very dynamic antibody: the antibody is involved in a continuous process of half‐molecules (i.e. a heavy and attached light‐chain) exchange. This process, also referred to as ‘Fab‐arm exchange’, results usually in asymmetric antibodies with two different antigen‐combining sites. While these antibodies are hetero‐ bivalent, they will behave as monovalent antibodies in most situations. Another aspect of IgG4, still poorly understood, is its tendency to mimic IgG rheumatoid factor (RF) activity by interacting with IgG on a solid support. In contrast to conventional RF, which binds via its variable domains, the activity of IgG4 is located in its constant domains. This is potentially a source of false positives in IgG4 antibody assay results. Because regulation of IgG4 production is dependent on help by T‐helper type 2 (Th2) cells, the IgG4 response is largely restricted to non‐microbial antigens. This Th2‐dependency associates the IgG4 and IgE responses. Another typical feature in the immune regulation of IgG4 is its tendency to appear only after prolonged immunization. In the context of IgE‐mediated allergy, the appearance of IgG4 antibodies is usually associated with a decrease in symptoms. This is likely to be due, at least in part, to an allergen‐blocking effect at the mast cell level and/or at the level of the antigen‐presenting cell (preventing IgE‐facilitated activation of T cells). In addition, the favourable association reflects the enhanced production of IL‐10 and other anti‐inflammatory cytokines, which drive the production of IgG4. While in general, IgG4 is being associated with non‐activating characteristics, in some situations IgG4 antibodies have an association with pathology. Two striking examples are pemphigoid diseases and sclerosing diseases such as autoimmune pancreatitis. The mechanistic basis for the association of IgG4 with these diseases is still enigmatic. However, the association with sclerosing diseases may reflect an excessive production of anti‐inflammatory cytokines triggering an overwhelming expansion of IgG4‐producing plasma cells. The bottom line for allergy diagnosis: IgG4 by itself is unlikely to be a cause of allergic symptoms. In general, the presence of allergen‐specific IgG4 indicates that anti‐inflammatory, tolerance‐inducing mechanisms have been activated. The existence of the IgG4 subclass, its up‐regulation by anti‐inflammatory factors and its own anti‐inflammatory characteristics may help the immune system to dampen inappropriate inflammatory reactions.

IgG subclasses and allotypes: from structure to effector functions

Of the five immunoglobulin isotypes, immunoglobulin G (IgG) is most abundant in human serum. The four subclasses, IgG1, IgG2, IgG3, and IgG4, which are highly conserved, differ in their constant region, particularly in their hinges and upper CH2 domains. These regions are involved in binding to both IgG-Fc receptors (FcγR) and C1q. As a result, the different subclasses have different effector functions, both in terms of triggering FcγR-expressing cells, resulting in phagocytosis or antibody-dependent cell-mediated cytotoxicity, and activating complement. The Fc-regions also contain a binding epitope for the neonatal Fc receptor (FcRn), responsible for the extended half-life, placental transport, and bidirectional transport of IgG to mucosal surfaces. However, FcRn is also expressed in myeloid cells, where it participates in both phagocytosis and antigen presentation together with classical FcγR and complement. How these properties, IgG-polymorphisms and post-translational modification of the antibodies in the form of glycosylation, affect IgG-function will be the focus of the current review.

Immunogenicity of anti-TNF biologic therapies for rheumatoid arthritis

Currently, five anti-TNF biologic agents are approved for the treatment of rheumatoid arthritis (RA): adalimumab, infliximab, etanercept, golimumab and certolizumab pegol. Formation of anti-drug antibodies (ADA) has been associated with all five agents. In the case of adalimumab and infliximab, immunogenicity is strongly linked to subtherapeutic serum drug levels and a lack of clinical response, but for the other three agents, data on immunogenicity are scarce, suggesting that further research would be valuable. Low ADA levels might not influence the efficacy of anti-TNF therapy, whereas high ADA levels impair treatment efficacy by considerably reducing unbound drug levels. Immunogenicity is not only an issue in patients treated with anti-TNF biologic agents; the immunogenicity of other therapeutic proteins, such as factor VIII and interferons, is well known and has been investigated for many years. The results of such studies suggest that investigations to determine the optimal treatment regimen (drug dosing, treatment schedule and co-medication) required to minimize the likelihood of ADA formation might be an effective and practical way to deal with the immunogenicity of anti-TNF biologic agents for RA.

Detection of infliximab levels and anti-infliximab antibodies: a comparison of three different assays

Formation of antibodies to infliximab (ATI) inversely correlates with functional drug levels and clinical outcome. Comparison of drug levels and anti‐drug antibody monitoring is hampered by lack of standardisation.

Adalimumab elicits a restricted anti-idiotypic antibody response in autoimmune patients resulting in functional neutralisation

Objectives Millions of patients worldwide are treated with therapeutic monoclonal antibodies. These biological therapeutics can be immunogenic, resulting in anti-drug antibody formation which leads to loss of response. Fully human biological agents, such as the anti-tumour necrosis factor α (anti-TNFα) antibody adalimumab, are considered to be weakly immunogenic, but anti-adalimumab antibodies (AAA) were recently detected in more than half of treated patients with rheumatoid arthritis (RA) within 28 weeks of treatment. A study was undertaken to determine the mechanism by which AAA lead to loss of response. Methods The specificity of the repertoire of AAA was investigated in a cohort of 50 AAA-positive RA patients. Inhibition experiments using TNFα and patient-derived anti-adalimumab monoclonal antibodies were performed. Results The antibody response against adalimumab is highly restricted: Fab fragments of a single monoclonal antibody specific for the idiotype of adalimumab inhibited 98.65% (25th–75th percentiles: 98.25–99.90) of the total anti-adalimumab reactivity in serum from 50 AAA-positive patients. The anti-adalimumab response was confined to the TNFα binding region of adalimumab, thereby neutralising its therapeutic efficacy. In line with this restricted specificity, small immune complexes were found in the circulation of AAA-forming patients. Conclusions The humoral immune response against adalimumab is highly restricted and limited to the idiotype of the therapeutic antibody. All antibodies result in functional neutralisation of the drug, thereby providing a mechanism by which AAA formation leads to clinical non-response.

Differential effect of drug interference in immunogenicity assays

The presence of anti-drug antibodies (ADA) in adalimumab-treated patients is associated with reduced serum adalimumab levels and a lower clinical response. Currently, there is no standard for measurement of anti-drug antibodies and many factors influence the results. Consequently, the incidence of ADA as reported in different studies varies considerably. Here we investigated the differential effect of drug interference in two common types of assays used to measure anti-adalimumab: an antigen binding test (ABT) and a more often-used bridging elisa. We measured ADA to adalimumab in a cohort of 216 rheumatoid arthritis patients treated with adalimumab for 28 weeks. Only 15 samples (7%) were positive in the bridging elisa, compared to 29 (13%) in the ABT, despite the fact that the bridging elisa was the most sensitive assay. Furthermore, in an ABT specific for IgG4, 48 samples (22%) were found positive. The bridging elisa was found to detect only the bivalent form of (drug-specific) IgG4, resulting in an underestimation of ADA levels. However, the predominant reason for the different outcomes of these assays was a differential susceptibility to drug interference. In particular, the bridging elisa only detected ADA in the absence of detectable amounts of circulating adalimumab and is therefore not suited for measurement of ADA in complex with the drug. In summary, we showed that a bridging elisa is susceptible to drug interference and typically measures ADA only in absence of detectable drug levels.

Human IgG4 Binds to IgG4 and Conformationally Altered IgG1 via Fc-Fc Interactions

The Fc fragment of IgG4 can interact with the Fc fragment of another IgG molecule. This interaction is a confounding factor when measuring IgG4 rheumatoid factor levels. Recently, we demonstrated that half-molecules of IgG4 can exchange to form a bispecific Ab. We expected these two phenomena to be related and investigated the physicochemical aspects of IgG4 Fc-Fc interactions. We found that IgG4 is >99% monomeric by size-exclusion chromatography; therefore, IgG4 Fc-Fc interactions in the fluid phase (if any) would be short-lived. However, 125I-labeled IgG4 does bind to IgG1 and IgG4 coupled to a solid phase. By contrast, IgG1 does not bind to coupled IgG4. Furthermore, conditions that induce partial unfolding/dissociation of the CH3 domains enhance IgG4 Fc binding, suggesting that Fc binding is primarily CH3 mediated. IgG4 slowly associates with both IgG4 and IgG1 coupled to a biosensor chip. Remarkably, subsequent dissociation was much faster for IgG4 than for IgG1. Moreover, after binding of an IgG4 mAb to Sepharose-coupled Ag, we observed additional binding of IgG4 with irrelevant specificity, whereas similar binding was not observed with Ag-bound IgG1. We propose that the IgG4-IgG4 Fc interaction resembles an intermediate of the Fab-arm (half-molecule) exchange reaction that is stabilized because one of the IgG4 molecules is coupled to a solid phase. By contrast, IgG4 Fc recognizes IgG1 only after a conformational change that renders CH3(IgG1) accessible to an interaction with the CH3(IgG4). Such Fc interactions may enhance Ag binding of IgG4 in vivo.

Enrichment of Sialylated IgG by Lectin Fractionation Does Not Enhance the Efficacy of Immunoglobulin G in a Murine Model of Immune Thrombocytopenia

Intravenous immunoglobulin G (IVIg) is widely used against a range of clinical symptoms. For its use in immune modulating therapies such as treatment of immune thrombocytopenic purpura high doses of IVIg are required. It has been suggested that only a fraction of IVIg causes this anti immune modulating effect. Recent studies indicated that this fraction is the Fc-sialylated IgG fraction. The aim of our study was to determine the efficacy of IVIg enriched for sialylated IgG (IVIg-SA (+)) in a murine model of passive immune thrombocytopenia (PIT). We enriched IVIg for sialylated IgG by Sambucus nigra agglutinin (SNA) lectin fractionation and determined the degree of sialylation. Analysis of IVIg-SA (+) using a lectin-based ELISA revealed that we enriched predominantly for Fab-sialylated IgG, whereas we did not find an increase in Fc-sialylated IgG. Mass spectrometric analysis confirmed that Fc sialylation did not change after SNA lectin fractionation. The efficacy of sialylated IgG was measured by administering IVIg or IVIg-SA (+) 24 hours prior to an injection of a rat anti-mouse platelet mAb. We found an 85% decrease in platelet count after injection of an anti-platelet mAb, which was reduced to a 70% decrease by injecting IVIg (p<0.01). In contrast, IVIg-SA (+) had no effect on the platelet count. Serum levels of IVIg and IVIg-SA (+) were similar, ruling out enhanced IgG clearance as a possible explanation. Our results indicate that SNA lectin fractionation is not a suitable method to enrich IVIg for Fc-sialylated IgG. The use of IVIg enriched for Fab-sialylated IgG abolishes the efficacy of IVIg in the murine PIT model.

Key findings towards optimising adalimumab treatment: the concentration-effect curve.

OBJECTIVE To determine a concentration-effect curve of adalimumab in rheumatoid arthritis (RA) patients taking into account the effect of methotrexate (MTX) on concentration and effect and to identify a therapeutic range for adalimumab concentrations. METHODS In a prospective observational cohort study, 221 consecutive patients with RA were treated with 40 mg adalimumab subcutaneously every other week. The relationship between adalimumab trough level and clinical efficacy after 28 weeks of follow-up was determined in a concentration-effect curve. A receiver-operator characteristics (ROC) curve established a therapeutic cut-off concentration. The effect of MTX on adalimumab trough levels was shown by dividing patients that are and are not concomitantly using MTX in the concentration-effect curve and a concentration table. RESULTS Clinical efficacy improved with increasing adalimumab concentration and reached a maximum (mean disease activity score in 28 joints improvement of 2) with levels between 5-8 μg/mL. Levels exceeding 8 μg/mL were illustrated to have no additional beneficial effect on disease activity. The ROC curve showed an area under the curve of 0.695 (95% CI 0.626 to 0.764) for European League Against Rheumatism response and adalimumab levels: good responders versus non-responders and moderate responders. A cut-off of 5 μg/mL had a sensitivity of 91% and a specificity of 43%. Adalimumab levels are influenced by concomitant MTX use: patients on adalimumab monotherapy had a median adalimumab level of 4.1 μg/mL (IQR 1.3-7.7), whereas patients concomitantly taking MTX had a median level of 7.4 μg/mL (IQR 5.3-10.6, p<0.001). CONCLUSIONS Adalimumab trough levels in a range of 5-8 μg/mL are sufficient to reach adequate clinical response. These levels are influenced substantially by concomitant MTX use.

A prominent lack of IgG1 Fc-fucosylation of platelet-alloantibodies in pregnancy

Immunoglobulin G (IgG) formed during pregnancy against human platelet antigens (HPAs) of the fetus mediates fetal or neonatal alloimmune thrombocytopenia (FNAIT). Because antibody titer or isotype does not strictly correlate with disease severity, we investigated by mass spectrometry variations in the glycosylation at Asn297 in the IgG Fc because the composition of this glycan can be highly variable, affecting binding to phagocyte IgG-Fc receptors (FcγR). We found markedly decreased levels of core fucosylation of anti-HPA-1a-specific IgG1 from FNAIT patients (n = 48), but not in total serum IgG1. Antibodies with a low amount of fucose displayed higher binding affinity to FcγRIIIa and FcγRIIIb, but not to FcγRIIa, compared with antibodies with a high amount of Fc fucose. Consequently, these antibodies with a low amount of Fc fucose showed enhanced phagocytosis of platelets using FcγRIIIb(+) polymorphonuclear cells or FcγRIIIa(+) monocytes as effector cells, but not with FcγRIIIa(-) monocytes. In addition, the degree of anti-HPA-1a fucosylation correlated positively with the neonatal platelet counts in FNAIT, and negatively to the clinical disease severity. In contrast to the FNAIT patients, no changes in core fucosylation were observed for anti-HLA antibodies in refractory thrombocytopenia (post platelet transfusion), indicating that the level of fucosylation may be antigen dependent and/or related to the immune milieu defined by pregnancy.