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Complement deposition by antibodies to Pseudomonas aeruginosa mucoid exopolysaccharide (MEP) and by non-MEP specific
G B Pier1, M Grout, D Desjardins
1Channing Laboratory, Department of Medicine, Brigham and Women's Hospital, Boston, MA.
Abstract:
The failure of cystic fibrosis patients to limit chronic infection due to mucoid Pseudomonas aeruginosa might be due to ineffective opsonins produced against this bacterium. Nonopsonizing antibody to the bacterial capsule, mucoid exopolysaccharide (MEP), appears at elevated titers during chronic colonization of cystic fibrosis patients, as do opsonins not specific for MEP. Nonopsonic antibodies to MEP occur naturally in most adults and can be induced in animals by immunization. A limited number of humans produce MEP-specific opsonic antibodies after immunization. The purpose of this study was to compare the activation and deposition of C components onto the bacterial surface in the presence of these different antibodies. Opsonic killing uses the classical C pathway. MEP-specific opsonic and nonopsonic antibodies bound to whole bacteria and activated C to a comparable degree, but opsonic antibody deposited 3 to 40 times more C3 onto bacteria, mostly as C3bi, compared to nonopsonic antibody. In addition, two to three times as much nonopsonic mAb as opsonic mAb (both IgG2b) bound to the bacteria at comparable input concentrations, indicating the difference in C deposition was not due to differences in antibody binding. Non-MEP-specific opsonins also bound C3 to the bacteria, but only a mean of 27 +/- 14% was ester linked, compared with 81 +/- 11% of C3 deposited by MEP-specific opsonins. Immunoprecipitation experiments indicated that two-thirds of the C3 bound in the presence of MEP-specific opsonins was linked to MEP, whereas non-MEP-specific opsonins obtained from infected patients deposited the C3 onto LPS and other unidentified Ag. These data show that MEP-specific opsonins function by depositing C3 onto the outer bacterial surface that differentiates them from non-MEP-specific opsonins produced in response to chronic infection.
Insights
Cystic fibrosis patients may fail to clear Pseudomonas aeruginosa due to ineffective opsonins. MEP-specific opsonins deposit more C3 onto bacteria, differentiating their function from non-MEP-specific opsonins in chronic infections.
Area of Science:
- Immunology
- Microbiology
- Complement System
Background:
- Cystic fibrosis patients often suffer chronic Pseudomonas aeruginosa infections.
- Mucoid exopolysaccharide (MEP) is a key component of P. aeruginosa in chronic infections.
- Antibody responses in cystic fibrosis patients are complex, involving both opsonic and nonopsonic antibodies.
Purpose of the Study:
- To compare complement component activation and deposition on P. aeruginosa in the presence of MEP-specific opsonic and nonopsonic antibodies.
- To elucidate the mechanism by which MEP-specific opsonins mediate complement deposition.
- To differentiate the function of MEP-specific opsonins from non-MEP-specific opsonins found in chronic infection.
Main Methods:
- Antibody binding assays to whole bacteria.
- Complement component C3 deposition assays.
- Immunoprecipitation to identify C3 linkage to bacterial components.
Main Results:
- Both opsonic and nonopsonic antibodies activated complement, but opsonic antibodies deposited significantly more C3 (3-40 fold).
- MEP-specific opsonic antibodies resulted in 81% ester-linked C3 deposition, primarily on MEP.
- Non-MEP-specific opsonins deposited less C3 (27% ester-linked), with deposition on LPS and other antigens.
Conclusions:
- MEP-specific opsonins enhance complement deposition on P. aeruginosa via C3 linkage to MEP.
- This mechanism of C3 deposition distinguishes MEP-specific opsonins from non-MEP-specific opsonins in chronic cystic fibrosis infections.
- Understanding these differences may inform therapeutic strategies against chronic P. aeruginosa infections.