7 TLR7 triggering broadens antibody repertoire and affinity
Despite the above-described ability of low affinity allergen-specific IgG antibodies to block type I hypersensitivity responses, it remains common thinking that high affinity antibodies would be more effective to also neutralize the allergen. In addition, induction of high affinity antibodies continues to be a general and common goal in vaccinology. For that, induction of germinal centres (GC), also known as secondary follicles, is a key goal, as GCs are the anatomical location of hypermutation and antibody affinity maturation. Induction of GCs is driven by antigen deposition on follicular dendritic cells (FDCs) and effective stimulation of B cells. Repetitive display of antigens in general and allergens specifically has been found to induce strong and rapid B cell responses due to effective B cell receptor cross-linking as well as engagement of the innate immune system 30. Indeed, natural IgM recognizes repetitive epitopes on viruses and virus-like particles (VLPs), causing activation of the classical pathway of complement (C1q), leading to the deposition of these particles on FDCs and to GC-reactions 16. In addition, VLPs may package RNA from E. coli during VLP production, when assembling inside bacteria. This RNA has not only been observed to effectively drive class switching to IgG and IgA, but also to increase the affinity of the induced antibodies as well as to facilitate maintenance of a broad immunoglobulin repertoire against both VLPs as well as displayed allergens 31. Furthermore, in a preclinical model of peanut-allergy, presence of RNA in VLPs displaying Ara h 2 proved essential for induction of protective IgG responses against peanut allergy. Hence, repetitive display of allergens on VLPs packaged with RNA appears an attractive way to increase induction and maintenance of high affinity antibodies (Figure 3). In studies on VLPs with Fel d 1 and peanut allergens, a substantially reduced interaction with IgE bound to mast cells has been shown and a failure to significantly activate the FcεRI mediated signalling cascade. Hence, under conditions where similar or higher amounts of allergen were bound to IgE on mast cells, free allergen induced strong cellular activation while allergens on VLPs failed to do so 13,14. Together, it was therefore an attractive choice to bring Ara h 2 displayed on VLPs (VLP-peanut) into clinical development 24.