Known Allergen Structures Predict Schistosoma mansoni IgE-Binding Antigens in Human Infection

Abstract

The IgE response has been associated with both allergic reactions and immunity to metazoan parasites. Recently, we hypothesized that all environmental allergens bear structural homology to IgE-binding antigens from metazoan parasites and that this homology defines the relatively small number of protein families containing allergenic targets. In this study, known allergen structures (Pfam domains) from major environmental allergen families were used to predict allergen-like (SmProfilin, SmVAL-6, SmLipocalin, SmHSP20, Sm triosephosphate isomerase, SmThioredoxin, Sm superoxide dismutase, SmCyclophilin, and Sm phosphoglycerate kinase) and non-allergen-like [Sm dynein light chain (SmDLC), SmAldolase SmAK, SmUbiquitin, and Sm14-3-3] proteins in Schistosoma mansoni. Recombinant antigens were produced in Escherichia coli and IgG1, IgG4, and IgE responses against them measured in a cohort of people (n = 222) infected with S. mansoni. All allergen-like antigens were targeted by IgE responses in infected subjects, whilst IgE responses to the non-allergen-like antigens, SmAK, SmUbiquitin, and Sm14-3-3 were essentially absent being of both low prevalence and magnitude. Two new IgE-binding Pfam domain families, not previously described in allergen family databases, were also found, with prevalent IgE responses against SmDLC (PF01221) and SmAldolase (PF00274). Finally, it was demonstrated that immunoregulatory serological processes typically associated with allergens also occurred in responses to allergen-like proteins in S. mansoni infections, including the production of IgG4 in people responding with IgE and the down-regulation of IgE in response to increased antigen exposure from S. mansoni eggs. This study establishes that structures of known allergens can be used to predict IgE responses against homologous parasite allergen-like molecules (parallergens) and that serological responses with IgE/IgG4 to parallergens mirror those seen against allergens, supporting our hypothesis that allergenicity is rooted in expression of certain protein domain families in metazoan parasites.

Keywords: IgE; IgG4; allergenicity; helminth proteins; metazoan parasite; parasite allergens; schistosomiasis mansoni.

Figure 1

SDS-PAGE of recombinant antigens following expression and purification. Proteins were run on a 4–12% gradient Bis-Tris gel and stained with Coomassie Blue.

Figure 2

Prevalence of IgE (A), IgG4 (B), and IgG1 (C) responses (with 95% binomial CIs), both pre- and post-treatment with PZQ to recombinant antigens within the population (n = 222). Prevalences/proportions of responders were calculated following the application of assay sensitivity cut-offs (mean + 3σ) determined using sera from 26 normal European controls. Proteins were grouped into predicted IgE-binding antigens (I), antigens without known allergenic domains (II), and confirmed IgE-binding antigens (III).

Figure 3

Proportions of individuals responding with IgE alone, IgG4 alone, or IgG4 and IgE together post-treatment with PZQ. Prevalences/proportions of responders were calculated following the application of assay sensitivity cut-offs (mean + 3σ) determined using sera from 26 normal European controls. Two-sample test of proportions was used to compare the prevalence of individuals responding with IgE alone vs. those responding with IgE and IgG4 together (*p < 0.05, **p < 0.01, ***p < 0.001). Proteins were grouped into predicted IgE-binding antigens (I), antigens without known allergenic domains (II), and confirmed IgE-binding antigens (III).

Figure 4

Comparison of the structures of SmLipocalin, SmThioredoxin, SmSOD with their respective IgE-binding counterparts identified from Allergome. Parasite allergen-like molecules were used to identify similar allergens by BLAST search of the Allergome database. Identities of the matched allergens with the corresponding E-values are shown below each matched pair. Structures were superimposed using LSQMAN and the quality of the alignment assessed by root mean squared deviation (RMSD). (A) Alignment of SmLipocalin (1VYF, cyan) and Der f 13 (2A0A, green). (B) Alignment of SmSOD (1TO4, cyan) and Tomato SOD (3HOG, green). (C) Alignment of SmThioredoxin (2XBI, cyan) and White Shrimp Thioredoxin (3ZZX, green).

Figure 5

Differential transcription of antigens in S. mansoni adult worms (A) and eggs (B) as determined by previously published microarray studies (35). Levels of antigen transcription were expressed as relative fluorescent units (RFU).

Figure 6

Relationships between antigen-specific natural log normalized geometric mean (GM) IgE levels and transcription of antigen in egg (A)*, natural log normalized GM IgG4 levels (B), GM age (C), or GM infection intensity (D) in individuals that respond to either IgG4 or IgE both pre- (open circles, short dashes) and post-treatment (filled circles, long dashes) with PZQ to a given antigen (β, SE, and p value calculated by linear regression). *SmHSP20 was excluded from this plot as it exerted undue influence on the linear regression model (Cook’s distance > 1).