Immune exclusion by naturally acquired secretory IgA against pneumococcal pilus-1
Ulrike Binsker, … , Alexandria J. Hammond, Jeffrey N. Weiser
Ulrike Binsker, … , Alexandria J. Hammond, Jeffrey N. Weiser
Published February 3, 2020; First published November 5, 2019
Citation Information: J Clin Invest. 2020;130(2):927-941. https://doi.org/10.1172/JCI132005.
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Research Article Microbiology

Immune exclusion by naturally acquired secretory IgA against pneumococcal pilus-1

Abstract

Successful infection by mucosal pathogens requires overcoming the mucus barrier. To better understand this key step, we performed a survey of the interactions between human respiratory mucus and the human pathogen Streptococcus pneumoniae. Pneumococcal adherence to adult human nasal fluid was seen only by isolates expressing pilus-1. Robust binding was independent of pilus-1 adhesive properties but required Fab-dependent recognition of RrgB, the pilus shaft protein, by naturally acquired secretory IgA (sIgA). Pilus-1 binding by specific sIgA led to bacterial agglutination, but adherence required interaction of agglutinated pneumococci and entrapment in mucus particles. To test the effect of these interactions in vivo, pneumococci were preincubated with human sIgA before intranasal challenge in a mouse model of colonization. sIgA treatment resulted in rapid immune exclusion of pilus-expressing pneumococci. Our findings predict that immune exclusion would select for nonpiliated isolates in individuals who acquired RrgB-specific sIgA from prior episodes of colonization with piliated strains. Accordingly, genomic data comparing isolates carried by mothers and their children showed that mothers are less likely to be colonized with pilus-expressing strains. Our study provides a specific example of immune exclusion involving naturally acquired antibody in the human host, a major factor driving pneumococcal adaptation.

Authors

Ulrike Binsker, John A. Lees, Alexandria J. Hammond, Jeffrey N. Weiser

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Figure 6

Secretory IgA is necessary but not sufficient for pneumococcal adherence to hNF.

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Secretory IgA is necessary but not sufficient for pneumococcal adherence...
(A) Adherence of Spn TIGR4 and isogenic pilus-1–deficient mutant to hNF from 6 individual donors was assessed in a solid-phase assay. Bacteria (2 × 104 per 100 μL DMEM) were incubated with 10 μg immobilized hNF in the presence of 0.1% BSA for 2 hours at 30°C. n = 6. (B) Anti-RrgB IgA was determined using an ELISA. Recombinant purified RrgB protein was immobilized in a microtiter plate (Immulon 2HB, Thermo Fisher Scientific) and, after blocking, incubated with 200 μg/mL hNF, or 25 μg/mL sIgA and serum IgA, respectively. Binding of RrgB-specific IgA was detected using a biotin-labeled anti-human IgA and peroxidase-coupled streptavidin. The values of control wells without hNF or sIgA were subtracted from each measured value. Results are illustrated as mean values ± SD of 2 independent experiments; n = 4. (C) Inhibition adherence assay using purified sIgA in increasing concentrations or purified serum IgA in a 2-fold molar ratio (compared with 50 μg/mL sIgA). Bacteria were pretreated with either sIgA or serum IgA for 30 minutes at 37°C before incubation with immobilized pooled hNF for 2 hours at 30°C. n = 6. (D) Binding of WT Spn to immobilized sIgA or BSA. Secretory IgA and BSA (each 10 μg) were immobilized overnight followed by blocking with 0.1% BSA and incubation with 2 × 104 per 100 μL bacteria for 2 hours at 30°C. n = 6. (A–C) Experiments were performed in duplicate, and mean values of 3 independent experiments are shown with error bars corresponding to SD. **P < 0.01, ****P < 0.0001 by 1-way ANOVA followed by Šidák’s multiple comparison (C).