Functional autoantibodiy

Functional autoantibodies (GPCR-AAB) are antibodies directed against G protein-coupled receptors (GPCR). Functional autoantibodies (GPCR-AAB) bind specifically to G protein-coupled receptors (GPCR) receptors located on the surface of cells (L. Sterin-Borda L et al. 1976; Borda E et al. 1984).

In contrast to classical autoantibodies, which primarily trigger immune responses leading to destruction of the afflicted tissue, the attacks of functional autoantibodies on cells, tissues, or organs are not always associated with inflammation or apoptosis. Rather, after receptor binding, GPCR-AAB trigger uncontrolled "receptor-mediated signaling cascades" that lead to pathological conditions. Diseases associated with such a functionally active type of autoantibody (functional autoantibodies) are termed "functional autoantibody diseases" (see classification below).

G protein-coupled receptors (GPCRs) are detected in almost all living organisms. In humans, of the approximately 21,000 genes, about 1000 genes could be defined as "GPCR genes". G protein-coupled receptors mediate a large number of biological reactions, for example in the regulation of blood pressure and heart rate, the processing of light, odour and taste stimuli, in cell growth and differentiation as well as in inflammatory processes. The large number of receptors alone indicates the great physiological importance of GPC receptors.

All GPC receptors are integral membrane proteins. Their amino acid chain forms seven transmembrane regions consisting of the extracellular N-terminal and intracellular C-terminal domains as well as three extracellular and three intracellular loops (see Fig.). The classical physiological ligands bind to a hydrophobic pocket in the extracellular domain portion of a G protein-coupled receptor. GPCRs function as highly dynamic systems that exist in a variety of functionally distinct conformations, where ligands can regulate receptor activity by conformational choice of different states (Hilger D et al. 2018; Breitwieser GE 2004).

Functional autoantibodies bind to different portions of the extracellular domains of the receptor protein. In this process, GPCR-AABs compete with the specific physiological receptor ligands. After binding to the GPCR, the autoantibodies can reduce the activity of the physiological ligands. Depending on the receptor type, they induce stimulatory or also inhibitory effects and thus lead to a modulation of the receptor-mediated signalling cascade with effects on physiological functions.

GPCRs respond to a broad spectrum of chemical entities ranging from photons, protons and calcium ions to small organic molecules (including odorants and neurotransmitters), peptides and glycoproteins (including functional autoantibodies). The classical role of a GPCR is to recognize the presence of an extracellular agonist, transmit the information across the plasma membrane, and activate a cytoplasmic heterotrimeric G protein, resulting in modulation of downstream effector proteins. Binding of an extracellular ligand to the extracellular loops of the receptor protein triggers a cycle of G protein activation and inactivation localized to the intracellular receptor site, which modulates the activity of enzymes and ion channels downstream, regulating the formation and concentration of cytosolic second messengers (Wang W et al. 2018).

After physiological and pharmacological ligand binding to a receptor, excessive receptor modulation is controlled by preventive mechanisms such as receptor down-regulation and desensitization of signal transduction. This control mechanism is crucial for physiological receptor activity.

Such control mechanisms are absent when GPCR-AAB bind to target receptors. It is suggested that GPCA-AAB are capable of cross-linking and permanently activating receptors due to their bivalent IgG structure. This has been demonstrated for β1-AAB, for autoantibodies directed against the β2-adrenergic receptor (β2-AAB) and the muscarinic 2 receptor (M2-AAB) (Hoebeke J (1996). Consequently, GPCR-AAB lead to disturbed metabolic balance and pathological conditions. They are a key event in GPCR-AAB-associated autoimmunity.

It is likely that GPCR-AAB-induced receptor cross-linking is one of the key events responsible for the absence of regulatory mechanisms such as receptor desensitization and internalization, resulting in excessive and prolonged receptor stimulation that may lead to disturbed metabolic balance and pathological conditions. This is in marked contrast to the state of receptors after binding of physiological or pharmacological ligands, where receptor internalization and desensitization counteract overboarding and prolonged receptor activation and signal transduction, thus protecting individuals from disturbed metabolic balance and pathological states. The absence of tachyphylaxis has been observed in several GPCR-AABs and thus most likely plays a key role in the pathogenesis of GPCR-AAB-associated diseases (Wallukat G et al. (1999).

Evidence suggests that GPCR autoantibodies appear to have pathogenic effects only in damaged tissue. In systemic sclerosis, receptor stimulation by AT1- and ETA-AAB affect interleukins, oxygen species, and ultimately growth factor balance, chemotaxis, cell migration, proliferation, angiogenesis, thrombosis, and fibrosis, among others (Cabral-Marques O et al. 2016).

For Chagas cardiomyopathy, there is evidence of how self-tolerance is breached. Antigens presented by the T. cruzi parasite, such as ribosomal P and B13 proteins, and animal and human cardiac antigens are cross-reactive; this involves the β1- and β2-adrenergic receptors as well as the muscarinic 2 receptor (Cunha-Neto E et al. 2011). The corresponding agonistic autoantibodies are frequently found in patients with Chagas heart failure. They bind on the first or second extracellular loop of β1-adrenergic receptors, among others (Wallukat G et al. 2010, L. Sterin-Borda L et al. 1976; Borda E et al. 1984).

Antibody evidence can also be detected in many other diseases such as diabetes mellitus and Alzheimer's disease. In patients with type II diabetes mellitus, α1-AAB have been found, although their prevalence in diabetics is not precisely known (Hempel P et al. 2009).

Inhibitory GPCR-AAB such as the autoantibodies directed against the β2-adrenergic receptor found in patients with allergic asthma block receptor activation by corresponding agonists. These autoantibodies are directed against the third extracellular receptor loop (Wallukat G et al. 1991).

The role of G protein-coupled receptors is recognized in the pathogenesis and progression of idiopathic dilated cardiomyopathy and is increasingly being considered in other diseases. For other diseases with GPCR-AAB positivity, such as thromboangiitis obliterans (Klein-Weigel P et al. 2016), postural orthostatic tachycardia syndrome (POTS) (A. Fedorowski H Li et al. 2017), dementia and Alzheimer's disease, benign prostatic hyperplasia (Müller J et al. 2017), complex regional pain syndrome -CRPS (Sudeck's disease) (Hendrickson JE 2016), and COVID-onset fatigue syndrome, the significance of GPCR-AAB positivity is still unclear.

Two different lines of treatment have emerged for functional autoantibody diseases:

Elimination of GPCR-AAB (plasmapheresis, immunoadsorption of all immunoglobulins).

Systemic treatments for direct suppression of GPCR-AAB such as:

• intravenous IgG treatment (IVIG) or
• B-cell depletion.

GPCR-AABs, like the classical autoantibodies, are also found with low prevalence in healthy individuals. Thus, β1-AAB and M2-AAB are found in about 10% of the healthy population, with GPCR-AAB titers increasing with age (Liu HR et al. 1999). The prevalence of β1-AAB and M2-AAB in healthy individuals appears to be somewhat higher than, for example, classical autoantibodies such as AAB to cardiac myosin and troponin (Becker NP et al. 2017).

It is as yet unclear whether finding a specific GPCR-AAB in healthy individuals can predict the development of any of the associated diseases. However, in patients with systemic scleroderma, higher GPCR-AAB levels appear to be associated with disease severity and patient mortality (Becker NP et al 2017).

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