Effects of double mutation at two distant IgE-binding sites in the three-dimensional structure of the major house dust mite allergen Der f 2 on IgE-binding and histamine-releasing activity

Der f 35: An MD-2-like house dust mite allergen that cross-reacts with Der f 2 and Pso o 2

BACKGROUND

Dermatophagoides farinae is a source of airborne house dust mite (HDM) allergens. We elucidated IgE-reactive allergens from D. farinae by two-dimensional immunoblotting-based allergenome analysis, and identified one new allergen, named Der f 35, that possesses IgE-binding capacity comparable to that of Der f 2. The aim of this study was to clarify the allergenic capacity of new HDM allergen Der f 35.

METHODS

We cloned der f 35 from D. farinae mRNA and produced recombinant Der f 35 in Escherichia coli. The IgE-binding capacity of Der f 35 and its cross-reactivity with group 2 allergens from D. farinae and Psoroptes ovis were determined by enzyme-linked immunosorbent assay (ELISA) and ELISA inhibition assays, respectively.

RESULTS

The deduced amino acid sequence for der f 35, which possesses the MD-2-related lipid-recognition domain, showed higher identity with group 2 allergens from P. ovis (61.5%) and Blomia tropicalis (50.7%) than with Der f 2 (40.8%). Der f 35 showed IgE-binding frequencies of 77.5% (31/40) for the native form upon allergenome analysis and 51.4% (18/35) for recombinant structure by ELISA. Der f 35 showed cross-reactivity with Der f 2 and Pso o 2 in reaction with HDM-allergic patients' IgE by ELISA inhibition assay.

CONCLUSION

Der f 35 is a candidate major allergen from D. farinae, which is more similar to group 2 allergens from sheep scab mite and storage mites. Der f 35 could be responsible for the cross-reactivity among group 2 mite allergens.

Recombinant house dust mite allergens

House dust mites (HDM) are a globally important source of allergen responsible for the sensitization of more than 50% of allergic patients. Specific immunotherapy with HDM extracts is effective but allergen extracts cannot be fully standardized and severe side-effects can occur during the protracted course of treatment. The introduction of molecular biological techniques into allergy research allowed the indentification of more than 20 groups of HDM allergens. Recombinant HDM allergens can be produced in defined concentrations and consistent quality and allow the development of vaccines for HDM allergy with reduced allergenic activity and retained immunogenicity. The immunotherapy trials in pollen allergic patients with recombinant pollen allergens/hypoallergenic allergen derivatives have shown that this treatment is effective and indicated that recombinant HDM vaccines might improve immunotherapy of HDM allergic patients. Here we report the steps for the development of vaccines for HDM allergy. After selection of the most prevalent HDM species, the panel of allergens to be included into a therapeutic vaccine for HDM allergy needs to be determined. HDM allergens with high IgE-binding frequency and clinical relevance will be modified into hypoallergenic variants and evaluated for their allergenic activity and immunogenicity. Derivatives with reduced allergenic activity but with retained immunogenicity would be good candidates for a HDM vaccine for safe and efficient immunotherapy.

Asp159 is a critical core amino acid of an IgE-binding and cross-reactive epitope of a dust mite allergen Der f 7

Der f 7 and Der p 7 are important house dust mite allergens with known structure and suggested biological function recently. However, their IgE-binding determinants remain unknown. The purpose of this study is to identify the IgE-reactive epitopes of Der f 7 and the determinants of IgE-mediated cross-reactivity between Der f 7 and Der p 7. IgE-reactive determinants were identified by immunodot blot inhibition using synthetic overlapping peptides, allergen mutants, and a Der f 7 structural model. Our results showed that synthetic peptides with sequence (156)SILDP(160) on Der f 7 bind IgE in two of the 30 asthmatic serum samples tested. Recombinant Der f 7 I157A, L158A, or D159A mutants have reduced IgE-binding activity. Inhibition experiments confirmed Asp159 as a critical core residue for IgE-binding. Among Der p 7, Der f 7 and Der f 7 mutants with single substitution between residues 156 and 160, only the D159A mutant cannot inhibit significantly IgE-binding against Der p 7. Therefore, Asp159 contributes to IgE-mediated cross-reactivity between Der f 7 and Der p 7. The structural model constructed for Der f 7 suggests that the IgE-binding epitope forms a loop-like structure on the surface of the molecule. In conclusion, Asp 159 is a critical core residue of an IgE-binding and IgE-mediated cross-reactive epitope (156)SILDP(160) of Der f 7. Results obtained from this study provide more information on molecular and structural features related to allergenicity, underlying basis of IgE cross-reactivity between allergens, and in designing safer immunotherapy.

Dilution method to refold bacterially expressed recombinant Der f 2 and Der p 2 to exhibit the secondary structure and histamine-releasing activity of natural allergens

BACKGROUND

Structurally refolded recombinant forms of major house dust mite group 2 allergens, Der f 2 and Der p 2, expressed in Escherichia coli, were prepared by solubilizing the insoluble products with urea and subsequently dialyzing against buffer. In this study, we determined conditions for refolding the urea-denatured recombinant Der f 2 and Der p 2 by one-step dilution as an alternative to dialysis, which requires several steps of handling and much time and cost.

METHODS

The insoluble bacterial product containing recombinant Der f 2 was solubilized with a buffer containing 8 M urea, and the solution was diluted to various urea concentrations. The refolding efficiency in each dilution was estimated from the height of the peak corresponding to the folded recombinant Der f 2 and that containing the aggregated form on anion exchange chromatography. The structure and allergenicity of the purified recombinant Der f 2 and Der p 2 refolded using the dilution method were analyzed based on circular dichroism and a basophil histamine-releasing assay, respectively.

RESULTS

Although the refolding efficiency decreased as the urea concentration in the dilution increased, experimental conditions whereby the protein and urea concentrations in the dilution were less than 0.5 mg/ml and 0.8 M, respectively, achieved maximum refolding efficiency. The recombinant allergens prepared by the dilution method exhibited the secondary structure and histamine-releasing activity of natural allergens purified from mite culture.

CONCLUSIONS

The dilution method established in this study is more convenient in terms of handling, time, and cost than the dialysis method and will be useful for large-scale production and for the preparation of numbers of mutants to analyze IgE epitopes.

Maturation of the activities of recombinant mite allergens Der p 1 and Der f 1, and its implication in the blockade of proteolytic activity

Recombinant pro-Der p 1 expressed in yeast Pichia pastoris was convertible into the prosequence-removed mature Der p 1 with full activities of cysteine protease and IgE-binding with or without N-glycosylation of the mature sequence as well as pro-Der f 1. The active recombinant variants will be the basis for various future studies. The major N-terminus of pro-Der p 1 with low proteolytic activity was the putative signal-cleavage site, while that of pro-Der f 1 contained not only the equivalent site but also 21 residues downstream, and pro-Der f 1 retained significant activity. Contribution of the N-terminal region of the Der p 1 prosequence including an N-glycosylation motif on effective inhibition of proteolytic activity of pro-Der p 1 was suggested.