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Infection and Immunity, January 2001, p. 336-344, Vol. 69, No. 1
Departments of Pediatrics and Pathology,
University of Rochester School of Medicine and Dentistry,
Rochester, New York 14642
Received 26 June 2000/Returned for modification 5 September
2000/Accepted 5 October 2000
Many pneumococcal capsular polysaccharides (PSs) are similar in
structure, and a pneumococcal antibody often binds to all of the PSs
with a similar structure. Yet, these cross-reactive antibodies may bind
to the structurally related pneumococcal capsular PSs with an avidity
too low to be effective. If memory B cells producing such weakly
cross-reactive antibodies are elicited with pneumococcal conjugate
vaccines, the memory cells for low-avidity antibodies could compromise
the subsequent immune responses to the cross-reactive PS (original
antigenic sin). To investigate these issues, we produced 14 hybridomas
secreting monoclonal antibodies (MAbs) to the capsular PS of
Streptococcus pneumoniae serotype 6B by immunizing BALB/c
mice with antigens containing 6B PS and studied their epitope, avidity,
in vitro opsonizing capacity, in vivo protective capacity, and
"antigen binding titer" by enzyme-linked immunosorbent assay
(ELISA) of 6A and 6B capsular PSs. Six MAbs bound to the
non-cross-reactive 6B-specific epitope, and seven MAbs bound to the
cross-reactive epitope present in both 6A and 6B PSs One MAb (Hyp6BM6)
revealed a novel epitope. This epitope was found on 6A PS in solution,
but not on 6A PS adsorbed onto the plastic surface of the ELISA plates.
The avidity of the MAb for 6A or 6B PS ranged from 7.8 × 106 M
0019-9567/01/$04.00+0 DOI: 10.1128/IAI.69.1.336-344.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Avidity, Potency, and Cross-Reactivity of
Monoclonal Antibodies to Pneumococcal Capsular Polysaccharide
Serotype 6B
and
1 to 4.1 × 1011
M
1. No MAbs were weakly cross-reactive, since none of the
cross-reactive MAbs showed any tendency toward having less avidity to
6A PS (the cross-reactive PS) than to 6B PS. Avidity influenced the
results of several antibody assays. When all of the hybridomas were
examined, avidity strongly correlated with the titer of a unit amount
of MAb to bind antigen-coated ELISA plates (r = 0.91) or to
opsonize pneumococci in vitro (r =
0.85). Because
both assay results are avidity dependent, the ELISA and the
opsonization assay results were strongly correlated (r = 0.91), regardless of avidity. Avidity also correlated with the
potency of a MAb to passively protect mice against pneumococcal
infections. When only the immunoglobulin G hybridomas were examined,
little increase in opsonizing capacity and in vivo protective potency
was observed above 109 M
1. Taken together, an
ELISA measuring antigen binding titer may be an adequate measure of the
protective immunity induced with pneumococcal vaccines, and the absence
of a partially cross-reactive MAb suggests that antigenic sin may not
be significant in responses to vaccines against the S. pneumoniae 6B serotype.
*
Corresponding author. Mailing address: University of
Rochester School of Medicine and Dentistry, Departments of Pediatrics and Pathology, 601 Elmwood Ave., Box 608, Rochester, NY 14642. Phone:
(716) 273-4157. Fax: (716) 273-1101. E-mail:
moon_nahm{at}urmc.rochester.edu.
Present address: Department of Anatomy, Seoul National University,
College of Medicine, Chongno-gu, Yeungeun-dong, Seoul 110-799, Korea.
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