Der p 7 appears closely related to several proteins known to bind hydrophobic compounds, including the Take-out proteins and BPI and LBP. The TO proteins are involved in signaling in a wide variety of biological functions of insects.43
Since the class Insecta
and the class Arachnida
(mites) are both in the phylum Arthropoda
this may provide some clues as to the natural function of Der p 7 in mites, but there is not a good enough match in the literature with a known natural ligand. Der p 7 does not appear to bind to LPS like BPI or LBP, but exhibits affinity for PB, which is a nine residue cyclic peptide with a lipid tail from the Gram-positive bacteria Bacillus polymyxa
. The weak binding of Polymyxin B to Der p 7 does not imply that PB is the natural substrate. Future studies to identify the natural ligand(s) will require screening of a broader spectrum of potential ligands, as well as a determination of the corresponding dissociation constants. However, the mapping of the interaction site to a similar position in homologous proteins validates the conclusion that Der p 7 has affinity for a lipid substrate, in this case a bacterially-derived lipopeptide product. The importance of this is that lipid binding is a common feature of some allergens44
and natural lipid adjuvants are suspected to be important in sensitizing patients to allergens.17
LBP has been shown to interact with di- and tri-acylated lipopeptides, as well as LPS.45, 46
Bacterial and mycobacterial lipopeptides can induce the release of cytokines through interactions with TLR2 in cooperation with TLR1 or TLR6. Several TLRs may be involved in stimulating the allergic response through promotion of Th2 immunity.44
Indeed stimulation of TLR2 in the presence of allergen enhances Th2 responses,47, 48
as does TLR5 stimulation.49
We speculate that if Der p 7 can bind bacterial lipopeptides other than PB as its natural ligand, it may promote Th2 immunity (to itself) through co-stimulation of TLR2 pathways.
In addition to understanding allergen function, structural characterization of allergens can provide detailed knowledge of potential B-cell epitopes and aid in mapping antibody interactions. We previously used hydrogen exchange NMR to map monoclonal antibody binding sites on Der p 2 and NMR techniques have recently been used to define a mAb epitope on Blomia tropicalis
allergen Blo t 5.50, 51
Several X-ray crystal structures of allergen/antibody complexes have also been reported including Bet v 1, β-lactoglobulin and Bla g 2.52-54
Modification of structure has been an effective way to design recombinant hypoallergenic variants of allergens, which have potential clinical application for immunotherapy in mite allergy.55, 56
Our IgE binding data confirms that Der p 7 is an important allergen and shows that the prevalence of IgE ab to rDer p 7 is comparable to that of the natural allergen.12
The purified rDer p 7 was of sufficiently high quality for crystallographic studies. These observations suggest that rDer p 7 could be used in a cocktail of recombinant mite allergens for use in subcutaneous immunotherapy and for rational design of hypoallergenic variants. A cocktail of five recombinant timothy pollen allergens was successfully used in a placebo controlled trial and resulted in a 39% reduction in symptom scores and medication use.57
Clinical improvement was associated with a strong increase in allergen-specific IgG4
antibody production, reduced levels of IgE ab and induction of IL-10 producing regulatory T-cells. Current data suggests that recombinant Der p 1, Der p 2 and Der p 7 are important immunologic targets and should be included in similar vaccine trials for mite allergen immunotherapy. The structure presented here will be useful in the rational design of hypoallergenic mutant proteins, and to understand future studies of the cross reactivity between the Group 7 mite allergens.
Knowledge of allergen function has contributed to an improved understanding of allergenicity. This was demonstrated for the protease activity of Group 1 mite allergens on the adaptive immune system7, 8
and the interaction with the innate immune system by Group 2 mit337 e allergens.17
We suggest that there may be similarities between the biological functions of the group 7 and group 2 mite allergens that involve binding of lipid substrates and that both allergens may co-opt innate immune responses into promoting allergenicity. Further studies are needed to investigate this hypothesis. The structure of Der p 7 presented here will contribute to an improved understanding of the complex interactions between the innate and adaptive immune systems in the development of allergy.