2 Low affinity IgE antibodies suffice to drive the allergic responses: avidity provides the key
Antibody affinity, also known as binding affinity and defined by its equilibrium constant, is the strength of the interaction between the antigen-binding site on an antibody and a specific epitope on an antigen (monovalent binding) and can be defined by its equilibrium constant (Figure 2A). Antibody avidity represents the overall strength of the antibody-antigen interactions and is influenced by several factors in particular multivalent binding (Figure 2A). In essence, the greater an immunoglobulin’s valency (number of antigen binding sites), the greater its potential avidity as it can bind multiple epitopes on a single antigen – provided the antigen is multivalent15.
Most classical physico-chemical analysis is performed in solution, in part to avoid the effects of multivalent binding. Nevertheless, standard antibody binding assays, such as ELISA, surface plasmon resonance, and Biolayer Interferometry are performed with antibodies in solution; their ligands are, however, typically bound to two-dimensional surfaces. This opens the possibility for antibodies to bind their ligands with 2 arms, a problem that is typically avoided by complicated coating techniques, including coating at low density to avoid avidity effects (caused by multivalent binding), as affinity (referring to monovalent binding) is much better defined by classical binding models than avidity. However, a notion often overseen is that most pathogens i) are recognized on 2 dimensional surfaces and ii) are highly polyvalent and therefore prone to bind antibodies in a multivalent fashion. Indeed, natural IgM antibodies bind viral particles with great efficiency due to decavalent binding which makes many antibodies stick like glue even if the binding affinity of an individual variable region is low and barely measurable16. Hence, avidity may be more important in a real-life setting. In the case of IgE bound to FcεRI on the surface of effector cells, nature adds one level of complexity: lateral diffusion. While in ELISA plates, ligands are fixed on plastic, IgE molecules bound to FcεRI can rapidly diffuse along the cell surface membrane, a process that strongly facilitates multivalent, high-avidity binding of IgE antibodies to allergens (Figure 2B). In this context, it is interesting to note that the restriction of the movement of receptor-bound IgE within the 2 dimensions of the cell membrane results in high local concentrations of the IgE molecules as well as allergens bound to IgE. Indeed, 10’000 molecules bound to the cell surface of regular sized cells may exhibit a local concentration of >10-6 M within the membrane17. In addition, membrane bound molecules diffuse on the cell surface with a high velocity, allowing them to circle the cell once every second18. Hence all these properties foster multivalent binding of single allergens to membrane bound IgE recognizing different epitopes on the allergen. Indeed, we could show that IgE antibodies exhibiting an affinity as low as 10-6M for the allergen Fel d 1, were nevertheless able to bind multivalently to FcεRI bound IgE on mast cells and cause degranulation of the cells19. This was not possible if the membrane was brought below the Krafft point, the temperature at which cholesterol in membranes freezes, not allowing for lateral diffusion of molecules (Figure 2C). Thus, lateral diffusion of IgE allows for multivalent binding of allergen, causing high avidity interactions, and consequently, low affinity IgE antibodies can trigger cellular activation. This effect may be particularly pronounced for dimeric allergens. Indeed, a case in point is the cat allergen Fel d 1, which is naturally dimeric and causes particularly strong allergies. These data may also explain the often unexpectedly high cross-reactivity between structurally unrelated allergens such as birch allergens and latex.