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. 2024 Dec 2;42(26):126419.
doi: 10.1016/j.vaccine.2024.126419. Epub 2024 Oct 4.

Serotype 15C Streptococcus pneumoniae resistant to classical complement deposition and agglutination by polyclonal rabbit anti-capsular 15B sera

E J Kochis  1 , Y B Shaik-Dasthagirisaheb  2 , N Munarriz  1 , I Kretzmann  2 , M Nahm  3 , S I Pelton  4 , Rotem Lapidot  5
Affiliations

Serotype 15C Streptococcus pneumoniae resistant to classical complement deposition and agglutination by polyclonal rabbit anti-capsular 15B sera

E J Kochis et al. Vaccine. .

Abstract

Background: S. pneumoniae (SP) serotypes 15B and 15C are frequent causative agents of invasive pneumococcal disease (IPD), otitis media, and nasopharyngeal colonization in the post PCV13 era. The principal difference between serotypes 15B and 15C is the presence of an O-acetyl group on the pentasaccharide repeating unit of 15B polysaccharide. It remains unclear if antibodies against SP15B polysaccharide demonstrate functional cross reaction with SP15C strains. We compared functional activity of polyclonal rabbit anti-capsular 15B sera against SP15B and SP15C isolates.

Methods: Using flow cytometry we measured complement factors C3c and C4d deposition on SP15B and 15C in the presence of polyclonal rabbit anti capsular 15B sera. We measured the binding of C3c, common to all complement pathways, and C4d, specific to classical pathway, on SP serotypes 15B and 15C when co-incubated with polyclonal rabbit anti capsular 15B sera and antibody depleted complement. Both the proportion of bacteria with complement deposition and the fluorescence intensity were measured. We also measured agglutination as the increase in forward and side scatter.

Results: Polyclonal rabbit anti-capsular 15B sera activated classical pathway resulting in deposition of C4d and C3c at high intensity on all SP15B cells but only achieved limited deposition and intensity of C4 with SP15C. Similarly, polyclonal rabbit anti-capsular 15B sera induced agglutination of SP15B strains in a dose dependent manner and limited agglutination of SP15C.

Conclusions: Anti-capsular 15B sera induce limited C4d and C3c deposition, and minimal agglutination with SP15C strains, reflecting lower classical pathway activation in contrast to high C4d and C3c deposition and agglutination of SP15B. These observations support limited functional cross reactivity of anti-15B to SP15C strains and are consistent with the reduction in opsonophagocytic killing of SP15C reported following immunization with vaccines containing 15B polysaccharide.

Keywords: 15B; 15C; Agglutination; Complement; Flow cytometry; IPD; PCV20; S. pneumoniae.

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Conflict of interest statement

Declaration of competing interest Dr. Pelton has research support to Boston Medical Center from Pfizer, Inc. and Merck Vaccines related to pneumococcal vaccine. He received honorarium and consulting fees from Pfizer, Inc., Merck Vaccines, GSK, and Sanofi for participation in advisory boards related to Pneumococcal vaccine, Participation in DSMB, and consulting on research design in collaboration with Aval ere Health. Dr. Lapidot has research support to Boston Medical Center and Rambam Healthcare Campus from Pfizer, Inc. and Merck Vaccines. She received honorarium from Pfizer, Inc. and Merck Vaccines for Participation in advisory boards and symposium related to Pneumococcal disease/vaccines. Dr. Moon Nahm received honorarium and consulting fees from MSD.

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