(183ar) Design of the Next Generation Liposomal Encapsulation of Polysaccharide (LEPS) Vaccine for Pneumococcal Disease.

Streptococcus pneumoniae (S. pneumoniae) is a Gram-positive bacterium often found in the human nasopharynx. It can cause pneumococcal disease, especially in vulnerable populations such as young children (≤ 5 years) and the elderly (≥ 65 years), whose immune systems are often underdeveloped or weakened. It can lead to a range of illnesses, from common infections such as otitis media and rhinosinusitis to life-threatening diseases like pneumonia, meningitis, and bacteremia, particularly in infants and children. In the United States, about 150,000 people annually are hospitalized because of infection with pneumococcal disease, killing about 5-7%, or 1 in 20 of those infected³.

Two pneumococcal vaccines are currently available on the market: PPSV23 and PCV20. PPSV23 contains capsular polysaccharides from 23 defined S. pneumoniae serotypes and induces a T-cell–independent immune response. However, this response is typically short-lived and does not generate strong immunological memory. In contrast, PCV20 is a conjugate vaccine in which the polysaccharides are covalently linked to the carrier protein CRM197. This design enables a T-cell–dependent immune response, leading to enhanced and longer-lasting immune memory.

We propose a strategy to design a next-generation vaccine for pneumococcal disease to address limitations of current options. We will increase the coverage range of serotype polysaccharides. The current vaccine targets a specific and limited number of S. pneumoniae serotypes. As a result, its use is prone to infections from non-vaccine serotypes⁹. Also, PPSV23 and PCV20 provide limited protection to certain host groups. Their protection efficiency to the elderly is only 45 and 33%, respectively¹⁰. Moreover, these vaccine options only target nasopharyngeal colonization or the invasive phase of infection but S. pneumoniae can change surface polysaccharides or proteins during systemic infection¹¹.

In this work, we propose the encapsulation of 24 polysaccharides using a liposomal platform termed liposomal encapsulated polysaccharides (LEPS). In addition, key protein antigens from S. pneumoniae are non-covalently bound to the liposome surface to expand disease coverage. Stability testing indicates that LEPS remained stable at both 2 °C and 23 °C for up to 35 days, with no significant difference in particle size and zeta potential. Additionally, optimizing formulation composition to ensure vaccine safety is another key objective. Cytotoxicity and immune response were assessed using MTT assays, hemolysis assays, and nitric oxide (NO) assays in RAW264.7 macrophage and human fibroblast cells. The optimized formulation exhibited over 90% cell viability and NO production, suggesting strong immune activation.

Overall, LEPS provides a novel approach to pneumococcal vaccines. LEPS offers a safer, broader, and more efficient pneumococcal vaccine by encapsulating polysaccharides without the need for covalent protein conjugation. The platform has shown promising results in aged mouse subjects while simultaneously offering a relatively simple formulation process, thus, representing a promising platform for next-generation vaccines against diverse pathogens.