An ever increasing number of different monoclonal antibodies (mAb) are used to influence the immune system and to treat diseases, which are also constantly on the increase. Antibodies, also called immunoglobulins (Ig), are Y-shaped. The short upper arms are equipped with binding sites, which bind to specific target structures, so-called antigens. This portion is very variable, since the antibodies are specific to various antigens. The “foot” of the Y is the Fc part which is less variable, and which determines the Ig subclass of an antibody. The Fc portion mediates the effector function of an antibody such as complement activation or binding to Fc receptors (FcRs). The different Fc portions of the various Ig subclasses differ in their amino acid sequence and thus also in their binding properties regarding FcR.
Up to now, it has generally been assumed that a strong Fc:FcγR interaction also conveys a strong effector function, and that, on the other hand, a less pronounced Fc:FcγR interaction conveys no effector function, or at least, to an only negligible extent. Corresponding to the affinities of the FcγR for certain IgG subclasses and the effects relating to them, the appropriate mAb of different IgG subclasses are used for different indications. In cancer therapy with antibodies, Fc:FcγR mediated cell toxicity is desired to combat cancer cells, and thus, the subclass IgG1 is used most frequently for such therapies. On the other hand, IgG2 or IgG4 are used if an Fc:FcγR interaction is undesired. This is frequently the case, for instance, if the aim is neutralisation of a target structure.
TGN1412 is a mAb which became well-known under dramatic circumstances: TGN1412 was developed for the treatment of rheumatoid arthritis and a particular form of leukemia. In 2006, the antibody was administered to six healthy individuals in the United Kingdom in a so-called “first-in-man” study. All volunteers experienced a massive release of immunological messenger substances (known as cytokine storm) with life-threatening symptoms. Such signs of a risk were not detected in the preceding animal studies – which form an important part of the tests required before the first use in humans. The severe immunological reactions were an absolute surprise.
Since then, the mechanisms which caused this unexpected reaction have been studied extensively. TGN1412 was developed as an IgG4 mAb, since, in its working mechanism, only binding to the target structure (CD28 on T cells) was desired, however, not the interaction with FcγR. In follow-up tests after the disastrous first-in-man study with TGN1412, however, it could be shown that at least in vitro, exactly such Fc:FcγR interactions contribute to the induction of a cytokine storm.
Professor Zoe Waibler, head of Section “Product Testing of Immunological Biomedicines” of Division Immunology at the Paul-Ehrlich-Institut (PEI) and her research team have now obtained unexpected research results. At first, they were able to provide evidence that the previously known patterns of Fc:FcγR interactions also apply to TGN1412. TGN1412 as a member of IgG subclass 4 hardly interacts with the low-affinity FcγRII, but is bound by the high-affinity FcγRI. Interestingly, the barely detectable interaction of TGN1412 with the low-affinity FcγRII mediates a strong effector function, i.e. the proliferation of T cells. Inversely, the comparatively strong interaction between TGN1412 and the high-affinity FcγRI only causes a very low T cell proliferation. The researchers observed this effect both with transgenic cell lines and with primarily human blood cells.
Professor Waibler commented the unexpected findings by stating that “These results will contribute to an even better understanding of immunological processes and, in addition to regulatory measures, shall serve to contribute to preventing such disasters in future.”
The Paul-Ehrlich-Institut, the Federal Institute for Vaccines and Biomedicines, in Langen near Frankfurt/Main is a senior federal authority reporting to the Federal Ministry of Health (Bundesministerium für Gesundheit, BMG). It is responsible for the research, assessment, and marketing authorisation of biomedicines for human use and immunological veterinary medicinal products. Its remit also includes the authorisation of clinical trials and pharmacovigilance, i.e. recording and evaluation of potential adverse effects. Other duties of the institute include official batch control, scientific advice and inspections. In-house experimental research in the field of biomedicines and life science form an indispensable basis for the manifold tasks performed at the institute. The Paul-Ehrlich-Institut, with its roughly 800 members of staff, also has advisory functions nationally (federal government, federal states (Länder)), and internationally (World Health Organisation, European Medicines Agency, European Commission, Council of Europe etc.).
Dudek S, Weißmüller S, Anzaghe M, Miller L, Sterr S, Hoffmann K, Hengel H, Waibler Z (2019): Human Fcγ receptors compete for anti-CD28 monoclonal antibody TGN1412 binding which determines the antibody’s effector function. Eur J Immunol. 2019 Apr 19.