The Case for Pneumococcal Surface Protein A (PspA): A Comprehensive Review of a Leading Candidate in Pneumococcal...

Streptococcus pneumoniae remains a leading cause of morbidity and mortality worldwide, with current polysaccharide-based vaccines offering limited serotype coverage, high production costs, and reduced efficacy in vulnerable populations. These limitations have prompted the search for conserved pneumococcal proteins as universal vaccine candidates. Among them, pneumococcal surface protein A (PspA) stands out as a major virulence factor, present in virtually all clinically relevant strains, and capable of interfering with complement activation, opsonophagocytosis, and host defense mechanisms. Over three decades of research have demonstrated PspA's strong immunogenicity, protective efficacy in multiple animal models, and safety in early-phase clinical trials. Here, we critically review advances in PspA-based vaccine development, including recombinant protein fragments, fusion constructs, nanoparticle formulations, and live-vector platforms. We highlight the structural and immunological determinants underlying its protective potential, while discussing major challenges such as antigenic variability and cross-reactivity across pneumococcal strains expressing distinct PspA clades. By integrating recent experimental and translational findings, this review outlines the opportunities and obstacles for the implementation of serotype-independent PspA-based vaccines.