Displaying Fel d1 on virus-like particles prevents reactogenicity despite greatly enhanced immunogenicity: a novel therapy for cat allergy

Allergen-specific desensitization is the only disease-modifying therapy currently available for the treatment of allergies. These therapies require application of allergen over several years and some may induce life-threatening anaphylactic reactions. An ideal vaccine for desensitization should be highly immunogenic and should alleviate allergic symptoms upon few injections while being nonreactogenic. We describe such a vaccine for the treatment of cat allergy, consisting of the major cat allergen Fel d1 coupled to bacteriophage Qβ-derived virus-like particles (Qβ–Fel d1). Qβ–Fel d1 was highly immunogenic, and a single vaccination was sufficient to induce protection against type I allergic reactions. Allergen-specific immunoglobulin G antibodies were shown to be the critical effector molecules and alleviated symptoms by two distinct mechanisms. Although allergen-induced systemic basophil degranulation was inhibited in an FcγRIIb-dependent manner, inhibition of local mast cell degranulation in tissues occurred independently of FcγRIIb. In addition, treatment with Qβ–Fel d1 abolished IgE memory responses upon antigen recall. Despite high immunogenicity, the vaccine was essentially nonreactogenic and vaccination induced neither local nor systemic anaphylactic reactions in sensitized mice. Moreover, Qβ–Fel d1 did not induce degranulation of basophils derived from human volunteers with cat allergies. These data suggest that vaccination with Qβ–Fel d1 may be a safe and effective treatment for cat allergy.Allergic reactions are associated with several hypersensitivity diseases including asthma, rhinoconjunctivitis, contact dermatitis, urticaria, anaphylaxis, and insect, drug, and food allergy. These diseases can affect all age groups and have reached epidemic proportions worldwide with increasing incidence over the last decades (Holgate, 1999). The most common forms of allergies, such as pollen, house dust, or animal dander allergies, are dependent on type 2 T cell responses (Georas et al., 2005), leading to the generation of IL-4 and IgE. IgE antibodies have a short half-life in serum but are stable if bound to Fcε receptors on circulating basophils and, in particular, tissue mast cells (Vieira and Rajewsky, 1988). Cross-linking of the IgE–FcεI receptor complex on these cells by allergen leads to degranulation within seconds, liberating a variety of preformed inflammatory mediators. The clinical effects attended by such allergic reactions vary according to the site of basophil and mast cell activation. Although inhalation or ingestion of allergens activates mucosal mast cells, i.v. or s.c. antigen entry activates circulating basophils and connective tissue mast cells.Most current therapies for the treatment of allergies block mast cell effector molecules (e.g., histamines) or nonspecifically suppress immune responses (e.g., steroids). Although effective, these treatments fail to affect the immunological conditions causing the allergies. In addition, different vaccination or desensitization strategies have been investigated, including the usage of allergen-derived peptides (Francis and Larché, 2005), recombinant hypoallergenic derivates (Niederberger et al., 2004; Saarne et al., 2005), oligonucleotides with CpG motives (Hessel et al., 2005), or allergen conjugated to carbohydrate-based particles (Andersson et al., 2004). However, only a few disease-modifying therapies for allergy have been approved so far. These immunotherapies consist of either multiple s.c. injections of increasing doses of allergen or multiple sublingual or oral administration of the allergen, resulting in long-term desensitization (Till et al., 2004). Although these allergen-specific immunotherapies have shown successes (Durham and Till, 1998), the procedures are time consuming and require 1–3 yr of regular treatments (Hedlin et al., 1986, 1991). Moreover, this therapy bears a high risk for anaphylactic reactions, especially after administration of higher allergen doses (Cox and Coulter, 1997; Bousquet et al., 1998).The mechanism by which this specific immunotherapy affects allergies is poorly understood. The reduction in allergic symptoms by this treatment has been hypothesized to be at least partly mediated by a shift from Th2 toward a Th1 response or the induction of regulatory T (T reg) cells (Akdis et al., 2005). Alternatively, and not mutually exclusive, the balance between allergen-specific IgE and IgG antibodies may regulate mast cell and basophil activity. In fact, Hulett et al. (1993) identified an FcR, with a binding site for IgG, which regulates high affinity IgE receptor–mediated mast cell activation (Daëron et al., 1995b). It is well established that the FcγRIIb (FcγIIb receptor), expressed on mouse and human basophils and mast cells (Bischoff, 2007), can down-regulate FcεRI signaling by cross-linking of the FcεRI–IgE and FcγRIIb–IgG-Ag complex (Daëron et al., 1995a; Katz, 2002; Bruhns et al., 2005; Kraft and Kinet, 2007; Nimmerjahn and Ravetch, 2008). Indeed, a fusion molecule between the cat allergen Fel d1 and the constant part of IgG1 has recently been shown to inhibit allergic symptoms by cross-linking of FcγRIIb with FcεRI in a mouse model of asthma (Zhu et al., 2005; Terada et al., 2006). In addition to signaling through FcγRIIb, allergen-specific IgG antibodies may sequester allergens and hence prevent their binding to IgE–FcεRI complexes. Moreover, it has been shown that the ratio of allergen-specific IgE and IgG antibodies may affect presentation of allergen-derived epitopes to T cells (Wachholz and Durham, 2004). Thus, allergen-specific antibodies may have multiple ways to modulate allergic responses.We have previously shown that antigens displayed in a repetitive fashion on virus-like particles (VLPs) derived from the coat protein of the bacteriophage Qβ are highly immunogenic in mice (Jegerlehner et al., 2002a,b; Lechner et al., 2002; Spohn et al., 2005) and humans (Maurer et al., 2005; Kündig et al., 2006; Ambühl et al., 2007; Tissot et al., 2008). Because bacterial host RNA is incorporated into the VLPs during self-assembly inside bacteria, Qβ-VLPs provide Toll-like receptor ligands, which induce strong IgG2a/c-dominated antibody responses (Forsbach et al., 2007, 2008; Jegerlehner et al., 2007). In the present study, the major cat allergen Fel d1 was coupled in an oriented fashion to Qβ-VLPs, resulting in a highly repetitive form of the allergen. This vaccine was strongly immunogenic in mice, yielding high and long-lasting antigen-specific serum IgG titers after only a single immunization. Vaccination with Qβ–Fel d1 resulted in strongly reduced immediate type I allergic responses in a mouse model of mast cell degranulation, vascular leakage, and anaphylaxis and completely abolished IgE B cell memory responses upon antigen recall. Strikingly, coupling of Fel d1 to Qβ-VLPs essentially abrogated its ability to induce allergic responses in mice or the degranulation of human basophils derived from allergic individuals. Thus, displaying Fel d1 on VLPs enhanced its immunogenicity and therapeutic efficacy while strongly reducing its reactogenicity.