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Updated: Jun 11, 2025
Capsular Serotyping of Streptococcus pneumoniae by Latex Agglutination
Published on: September 25, 2014
Serotype 15C Streptococcus pneumoniae resistant to classical complement deposition and agglutination by polyclonal
E J Kochis1, Y B Shaik-Dasthagirisaheb2, N Munarriz1
1Boston University Masters in Medical Sciences, Boston, MA, USA.
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.
Insights
Antibodies against Streptococcus pneumoniae 15B show limited cross-reactivity with 15C strains. This is evidenced by reduced complement deposition and agglutination, impacting their effectiveness against 15C infections.
Area of Science:
- Immunology
- Microbiology
- Vaccinology
Background:
- Streptococcus pneumoniae (SP) serotypes 15B and 15C are significant causes of invasive pneumococcal disease (IPD).
- A key difference between 15B and 15C is the O-acetylation of the 15B polysaccharide.
- The functional cross-reactivity of antibodies against SP15B with SP15C remains poorly understood.
Purpose of the Study:
- To compare the functional activity of polyclonal rabbit anti-capsular 15B sera against SP15B and SP15C isolates.
- To investigate complement activation and bacterial agglutination.
Main Methods:
- Flow cytometry was used to measure C3c and C4d deposition on SP15B and SP15C.
- Polyclonal rabbit anti-capsular 15B sera were incubated with SP serotypes and antibody-depleted complement.
- Bacterial agglutination was assessed by measuring forward and side scatter.
Main Results:
- Anti-15B sera induced high C4d and C3c deposition and agglutination on SP15B.
- Limited C4d and C3c deposition and agglutination were observed on SP15C.
- These differences indicate lower classical complement pathway activation by SP15C.
Conclusions:
- Anti-capsular 15B sera exhibit limited functional cross-reactivity with SP15C strains.
- This limited cross-reactivity is consistent with reduced opsonophagocytic killing of SP15C.
- Findings have implications for pneumococcal vaccine development and efficacy.
