Antigenic cross-reactivity between Schistosoma mansoni and peanut: a role for cross-reactive carbohydrate determinants (CCDs) and implications for the hygiene hypothesis

Abstract

The antigenic reactivity of constituents of Schistosoma mansoni and peanut (Arachis hypogaea) was investigated to determine whether identical antigenic epitopes possessed by both organisms provided a possible explanation for the negative correlation between chronic schistosome infection and atopy to allergens. Aqueous extracts of peanuts were probed in Western immunoblots with rabbit IgG antibodies raised against the egg, cercarial and adult worm stages of S. mansoni. Several molecules in the peanut extract were antigenically reactive with antibodies from the various rabbit anti-schistosome sera. A pair of cross-reactive peanut molecules at ~30 000-33 000 molecular weight was purified and both proteins were identified by mass spectrometric analysis as the peanut allergen Ara h 1. Anti-S. mansoni soluble egg antigen antibodies that were eluted off the peanut molecules reacted with two S. mansoni egg antigens identified by mass spectrometry as IPSE/α-1 and κ-5. Alignments of the amino acid sequences of Ara h 1 and either IPSE/α-1 or κ-5 revealed a low level of peptide sequence identity. Incubation of nitrocellulose paper carrying electrophoresed peanut molecules, six constituents of other allergic plants and S. mansoni egg antigens in a mild solution of sodium metaperiodate before probing with antibodies, inhibited most of the cross-reactivities. The results are consistent with the antigenic cross-reactive epitopes of S. mansoni egg antigens, peanut and other allergic plants being cross-reactive carbohydrate determinants (CCDs). These findings are novel and an explanation based on 'blocking antibodies' could provide an insight for the inverse relationship observed between schistosome infection and allergies.

Keywords: Schistosoma mansoni; IgG antibodies; antigenic cross-reactivity; cross-reactive carbohydrate determinants; hygiene hypothesis; peanut Ara h 1.

Figure 1

Western immunoblots of electrophoresed peanut and mouse tissue extracts probed with rabbit antisera. (a) Peanut, (b) mouse blood, (c) mouse kidney, (d) mouse spleen. (M) Molecular weight markers, (1) anti‐Schistosoma mansoni soluble egg antigens (anti‐SmSEA) serum, (2) anti‐SmSEA serum, (3) anti‐S. mansoni cercariae homogenate (anti‐SmCH), (4) anti‐SmCH, (5) anti‐S. mansoni adult worm (anti‐SmWh), (6) anti‐normal mouse serum (anti‐NMS), (7) anti‐complete Freund's adjuvant (anti‐CFA). Amounts of protein loaded per lane: 0·1 mg peanut extract; 0·003 mg NMS; 0·021 mg kidney extract; 0·214 mg spleen extract. [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 2

Steps showing the identification and purification of two cross‐reactive peanut antigens. (a) Western immunoblot showing the identification of peanut antigens that are cross‐reactive with IgG antibodies in an anti‐Schistosoma mansoni soluble egg antigen (anti‐SmSEA) serum, (b) a portion of gel “a” stained in Coomassie blue for identification and excision of a pair of cross‐reactive peanut gel bands at ~30 000–33 000 MW intended for purification, (c) Coomasie‐stained SDS–PAGE showing further resolution of the pair of peanut bands into a lower band (lane 1) and a top band (lane 2) for mass spectrometry analysis. [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 3

Western immunoblots probing extracts of peanut and mouse tissues with anti‐Schistosoma mansoni soluble egg antigen (anti‐SmSEA) IgG antibodies eluted by low pH buffer from a ~30 000–33 000 MW pair of peanut antigens. (M) Molecular weight marker. (a) Peanut, (b) normal mouse serum (NMS), (c) kidney, (d) spleen. Each strip was probed with: (1) eluted anti‐SmSEA IgG antibodies, (2) incubated only in 1 ml acid‐elution buffer, (3) rabbit anti‐NMS. [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 4

Western immunoblot probing Schistosoma mansoni soluble egg antigens (SmSEA) and two purified egg antigens with eluted anti‐SmSEA IgG antibody to identify cross‐reactive egg antigens. (M) Molecular weight markers. (a) SmSEA probed with; (1) eluted anti‐SmSEA IgG antibodies, (2) rabbit anti‐IPSE/α‐1, (3) rabbit anti‐κ‐5, (4) incubated only in elution buffer, (5) rabbit anti‐SmSEA. (b) SDS–PAGE showing the purified egg antigens after staining in SimplyBlue SafeStain; (1) IPSE/α‐1, (2) κ‐5. (c) Purified IPSE/α‐1 probed with; (1) eluted anti‐SmSEA IgG antibodies, (2) rabbit anti‐IPSE/α‐1, (3) rabbit anti‐SmSEA, (4) incubated only in elution buffer. (d) purified κ‐5 probed with; (1) eluted anti‐SmSEA IgG antibodies, (2) rabbit anti‐κ‐5, (3) rabbit anti‐SmSEA, (4) incubated only in elution buffer. [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 5

Constituents of different plants probed with the acid‐eluted anti‐Schistosoma mansoni soluble egg antigen (anti‐SmSEA) IgG antibodies and the effect of treatment with periodate. (a) Incubated in 50 mm sodium acetate buffer before probing with eluted anti‐SmSEA IgG antibodies, (b) Incubated in 20 mm sodium metaperiodate dissolved in 50 mm sodium acetate buffer before probing with the same primary antibody as ‘a’. (M) Molecular weight marker, (1) unfractionated peanut, (2) purified pair of peanut antigens at ~30 000–33 000 MW, (3) crude latex (0·2 mg), (4) banana (0·16 mg), (5) tomato (0·27 mg), (6) melon (0·24 mg), (7) avocado (0·22 mg), (8) kiwi (0·19 mg), (9) SmSEA (0·010 mg). [Colour figure can be viewed at wileyonlinelibrary.com]