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Volume 14
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10.3390/vaccines14020127
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Article

Development and Validation of a Functional Antibody Assay for Evaluating Protein-Based Pneumococcal Vaccines

by
Jiangjiao Li
1,†,
Kang Li
1,†,
Youyou Wang
1,
Yang Huang
1,
Xiuwen Sui
2,
Xiao Xu
1,
Huijing Du
1,
Bochao Wei
2,
Ying Yang
2,
Jinming Zhang
2,
Liang Kong
2,
Tao Zhu
2,* and
Bin Wang
1,*
1
State Key Laboratory of Drug Regulatory Science, NHC Key Laboratory of Research on Quality and Standardization of Biotech Products, National Institutes for Food and Drug Control, Beijing 102629, China
2
CanSino Biologics Research Center, Tianjin 300457, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Vaccines 2026, 14(2), 127; https://doi.org/10.3390/vaccines14020127
Submission received: 17 December 2025 / Revised: 20 January 2026 / Accepted: 21 January 2026 / Published: 27 January 2026
(This article belongs to the Special Issue Immune Correlates of Protection in Vaccines, 2nd Edition)
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Abstract

Background: Streptococcus pneumoniae (Spn) is a leading bacterial pathogen responsible for severe invasive diseases, including meningitis, sepsis, and pneumonia. Current pneumococcal vaccines, which are all based on capsular polysaccharide antigens, provide limited protection and are further compromised by post-vaccination serotype replacement. Pneumococcal surface protein A (PspA), a highly conserved virulence factor expressed across diverse serotypes, has emerged as a promising candidate antigen for novel protein-based vaccines. However, progress in this field has been hindered by the absence of standardized in vitro functional antibody assays. Methods: This study established a robust functional antibody detection method for PspA-based protein vaccines by modifying the conventional multiplex opsonophagocytic killing assay (MOPA), originally designed for polysaccharide-based vaccines. Using polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA) typing, a target strain panel was selected and developed to include representative strains from PspA Family 1-Clade 2 and Family 2-Clades 3 and 4. The MOPA protocol was optimized by extending the phagocytic reaction time to enhance sensitivity. Specificity was confirmed through recombinant PspA competitive inhibition assays. Results: The assay demonstrated high linearity (R2 ≥ 0.98) between opsonophagocytic index (OI) and serum dilution, along with acceptable repeatability (CV ≤ 30%) and intermediate precision (CV ≤ 50%). Both preclinical and clinical serum samples exhibited potent bactericidal activity against diverse PspA families, independent of capsule type. Conclusions: This study provided a standardized framework to support the development and regulatory assessment of protein-based pneumococcal vaccines.
Keywords: Streptococcus pneumoniae; Pneumococcal surface protein A; pneumococcal vaccines; functional antibody; validation Streptococcus pneumoniae; Pneumococcal surface protein A; pneumococcal vaccines; functional antibody; validation

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MDPI and ACS Style

Li, J.; Li, K.; Wang, Y.; Huang, Y.; Sui, X.; Xu, X.; Du, H.; Wei, B.; Yang, Y.; Zhang, J.; et al. Development and Validation of a Functional Antibody Assay for Evaluating Protein-Based Pneumococcal Vaccines. Vaccines 2026, 14, 127. https://doi.org/10.3390/vaccines14020127

AMA Style

Li J, Li K, Wang Y, Huang Y, Sui X, Xu X, Du H, Wei B, Yang Y, Zhang J, et al. Development and Validation of a Functional Antibody Assay for Evaluating Protein-Based Pneumococcal Vaccines. Vaccines. 2026; 14(2):127. https://doi.org/10.3390/vaccines14020127

Chicago/Turabian Style

Li, Jiangjiao, Kang Li, Youyou Wang, Yang Huang, Xiuwen Sui, Xiao Xu, Huijing Du, Bochao Wei, Ying Yang, Jinming Zhang, and et al. 2026. "Development and Validation of a Functional Antibody Assay for Evaluating Protein-Based Pneumococcal Vaccines" Vaccines 14, no. 2: 127. https://doi.org/10.3390/vaccines14020127

APA Style

Li, J., Li, K., Wang, Y., Huang, Y., Sui, X., Xu, X., Du, H., Wei, B., Yang, Y., Zhang, J., Kong, L., Zhu, T., & Wang, B. (2026). Development and Validation of a Functional Antibody Assay for Evaluating Protein-Based Pneumococcal Vaccines. Vaccines, 14(2), 127. https://doi.org/10.3390/vaccines14020127

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