# Carrier-Protein-Free Pneumococcal Glycoconjugate Vaccines Enabled by SPAAC: Serotype 15C CPS–PADRE Conjugates and the Impact of an RR Cleavage Motif

## TL;DR

Researchers tested a new vaccine design that avoids using carrier proteins by linking a pneumococcal polysaccharide to a helper epitope called PADRE, finding that it can work but with some limitations.

## Contribution

A novel carrier-protein-free vaccine design using PADRE as a helper epitope is proposed and tested for pneumococcal conjugate vaccines.

## Key findings

- CPS–PADRE conjugates elicited lower antibody levels compared to traditional CPS–protein conjugates in mice.
- Incorporating a cleavable RR motif reduced humoral responses compared to non-cleavable designs.
- The study shows carrier-protein-free vaccines are feasible but require careful optimization.

## Abstract

Background/Objectives: Polysaccharide-protein conjugate vaccines have proven highly effective, yet they remain limited by manufacturing complexity, cost, and variable performance across serotypes, while carrier proteins can add unwanted immunological and production burdens. To address these constraints, we explored a carrier-protein-free conjugate vaccine concept in which a broadly MHC class II-binding helper epitope (PADRE) replaces the conventional protein carrier to provide T-cell help for a pneumococcal capsular polysaccharide antigen. Methods: Using serotype 15C CPS as a model, we generated CPS–PADRE conjugates and compared designs with or without a putative cleavable motif (RR) at the junction, alongside a conventional protein conjugate as a benchmark. Results: In mice, the CPS–protein conjugate induced the strongest CPS-specific IgG response, whereas CPS–PADRE conjugates elicited clear but overall lower antibody levels. Notably, incorporation of the cleavable motif did not improve immunogenicity and instead reduced humoral responses relative to the non-cleavable design. Conclusion: These findings support the feasibility of carrier-protein-free polysaccharide-peptide conjugate vaccines, while highlighting that cleavable junctions are not universally advantageous and must be empirically optimized for polysaccharide-helper epitope architectures.