A Common Food Glycan, Pectin, Shares an Antigen with Streptococcus pneumoniae Capsule

Abstract

We are exposed daily to many glycans from bacteria and food plants. Bacterial glycans are generally antigenic and elicit antibody responses. It is unclear if food glycans' sharing of antigens with bacterial glycans influences our immune responses to bacteria. We studied 14 different plant foods for cross-reactivity with monoclonal antibodies (MAbs) against 24 pneumococcal serotypes which commonly cause infections and are included in pneumococcal vaccines. Serotype 15B-specific MAb cross-reacts with fruit peels, and serotype 10A MAb cross-reacts with many natural and processed plant foods. The serotype 10A cross-reactive epitope is terminal 1,6-linked β-galactose [βGal(1-6)], present in the rhamno-galacturonan I (RG-I) domain of pectin. Despite wide consumption of pectin, the immune response to 10A is comparable to the responses to other serotypes. An antipectin antibody can opsonize serotype 10A pneumococci, and the shared βGal(1-6) may be useful as a simple vaccine against 10A. Impact of food glycans should be considered in host-pathogen interactions and future vaccine designs.IMPORTANCE The impact of food consumption on vaccine responses is unknown. Streptococcus pneumoniae (the pneumococcus) is an important human pathogen, and its polysaccharide capsule is used as a vaccine. We show that capsule type 10A in a pneumococcal vaccine shares an antigenic epitope, βGal(1-6), with pectin, which is in many plant foods and is widely consumed. Immune response to 10A is comparable to that seen with other capsule types, and pectin ingestion may have little impact on vaccine responses. However, antibody to pectin can kill serotype 10A pneumococci and this shared epitope may be considered in pneumococcal vaccine designs.

Keywords: Streptococcus pneumoniae; bacteria; capsule; food; polysaccharide; vaccine.

FIG 1

(A) Amount of monoclonal antibody bound to microbeads coated with serotype 10A polysaccharide in the presence of various concentrations of 10A polysaccharide (filled circle), raw carrot extract (filled square), acid-treated carrot extract (open circle), and base-treated carrot extract (open triangle). As shown in the panel, both acid and alkali hydrolysates are comparable to the raw extract in their levels of inhibition at dilutions greater than 100-fold (P > 0.05). Initial concentrations were 10 μg/ml for 10A polysaccharide and neat for the carrot extract. Error bars indicate standard deviations (SDs) and are not visible where SDs are smaller than the symbol itself. (B) Amount of monoclonal antibody bound to 10A-coated microbeads in the presence of serial dilutions of 10A polysaccharide (filled circle), strawberry jam (open triangle), pureed carrot for infants (filled square), and apple juice (open circle). Initial concentrations were 10 μg/ml for 10A polysaccharide and a 1:4 dilution for the jam and pureed carrot. Error bars indicate SDs and are not visible where SDs are smaller than the symbol itself. (C) Amount of monoclonal antibody bound to 10A-coated microbeads in the presence of 10A polysaccharide (filled circle), apple pectin (filled square), and pectin from citrus fruits (open triangle) as inhibitors. The initial concentration of the inhibitors was 500 μg/ml. The monoclonal antibody was Hyp10AG1. Error bars indicate SDs and are not visible where SDs are smaller than the symbol itself. MFI, mean fluorescence intensity.

FIG 2

(A and B) Amount of Hyp10AG1 bound to 10A-coated ELISA wells in the presence of serotype 10A capsular polysaccharide (filled triangle), RG-I (open circle), RG-II (filled square), or homogalacturonan (star, indicated as uronides). Pectin was from sycamore (syc.) (A) or from tobacco (tob.) (B). (C) Amount of CCRC-M7 bound to 10A-coated ELISA wells in the presence of 10A polysaccharide (filled triangle), serotype 2 polysaccharide (open circle), and RG-I pectins from sycamore (filled square) and tobacco (star). Error bars indicate SDs and are not visible where SDs are smaller than the symbol itself.

FIG 3

(A and B) Binding of a MAb against pneumococcal capsule type 10A (Hyp10AG1) (A) and against pectin (CCRC-M7) (B) as determined by flow cytometry to pneumococci with intact wcrG (SSISP10A and KAG1030) or defective wcrG (KAG1032, 10AΔwcrG). Gray peak shows binding without the MAbs. (C) Amount of Hyp10AG1 and CCRC-M7 bound to strain KAG1030, KAG1032, or SSISP10A. Error bars show SD, and binding of both MAbs to 10AΔwcrG (KAG1032) was significantly reduced compared to the level seen with 10A (KAG1030 or SSISP10A) (P < 0.001). (D) Structure of 10A capsular polysaccharide (2). (E) Diagram of a fragment of rhamno-galacturonan-I (RG-I) pectin molecule. (Adapted from reference .) Rhamno-galacturonan backbone of pectin is shown to the right. The βGal(1-6) shared between 10A polysaccharide and RG-I is bolded and red. Gal = d-galactopyranosyl; GalA = d-galacturonosyl; Rha = l-rhamnopyranosyl; Ara = l-arabinofuranosyl; Galf = d-galactofuranosyl; GalNAc = 2-acetamido-2-deoxy-d-galactosyl; Rib-ol = d-ribitol.

FIG 4

(A) Amount of rabbit antibody bound to the microspheres coated with a monoclonal antibody against 10A polysaccharide (Hyp10AM6) in the presence of various polysaccharide samples. Error bars indicate SDs and are not visible where SDs are smaller than the symbol itself. Rabbit antibodies bound significantly more in the presence of only 0.12 ng/ml of 10A polysaccharide (solid circle) than in the presence of even 100 ng/ml of sycamore RG-I (solid square) or tobacco RG-I (open triangle) (P < 0.005). (B) Surviving pneumococci (y axis) following opsonophagocytic killing assay at various dilutions of 007sp (x axis) in the presence of inhibitors. Inhibitors were as follows: none (buffer only, solid circle), 10 μg/ml of 10A polysaccharide (solid square), 100 μg/ml of type 2 capsular polysaccharide (open triangle), and 500 μg/ml of RG-I (open circle). 007sp was prediluted 20-fold. As indicated in the panel, control polysaccharides differed significantly from 10A polysaccharide in preventing the killing (P < 0.001). At 100-fold dilution, almost no bacteria survived in the presence of control polysaccharide but almost all bacteria survived with 10A polysaccharide (P < 0.001). (C) The opsonophagocytic killing titers of preimmune (open symbols) and postimmune (filled symbols) sera from nine individual donors. The target serotypes are shown at the bottom, and the horizontal gray bars in the figure indicate geometric mean values. Killing titers significantly increased for all serotypes following vaccination (P = 0.00002 for serotype 10A, 0.0056 for 6B, 0.00045 for 14, 0.0057 for 23F, and 0.00026 for 10AΔwcrG). Pre, preimmune; Post, postimmune. (D) Surviving pneumococci (y axis) following opsonophagocytic killing assay at various dilutions of antibodies (x axis). The antibodies were 007sp (filled circle), Hyp10AG1 (filled square), Hyp10AM6 (open triangle), and CCRC-M7 (open circle). Hyp10AG1, Hyp10AM6, and CCRC-M7 culture supernatants were prediluted 2-fold before the assay. 007sp, a reference human serum for pneumococcal assays (36), was prediluted 20-fold. Error bars indicate SDs and are not visible where SDs are smaller than the symbol itself.