Biochemical and Immunological Insights into Capsular Polysaccharide of Streptococcus pneumoniae Serotype 38 (American type 71)- An emerging non-vaccine serotype

Invasive pneumococcal disease presents a threat to humankind predominantly affecting children and the elderly. Despite the availability of high-valency pneumococcal polysaccharide vaccine of PPSV23 (PNEUMOVAX® 23) and conjugate vaccines such as VAXNEUVANCE and PREVNAR 20®, non-vaccine serotypes continue to contribute to higher mortality rates. The characterization of non-vaccine serotypes is becoming increasingly crucial considering an increase in their prevalence. In this study, biochemical characteristics, immunological properties, and critical quality attributes of the capsular polysaccharide isolated from prevalent non-vaccine serotype 38 (American type 71) have been examined. Advanced analytical techniques, including multi-angle light scattering (MALS), ion chromatography, dynamic light scattering in addition to conventional biochemical methods and SLOTBLOT analysis were employed. We observed that serotype 38 capsular polysaccharide has a molar mass of 768 kDa with a distribution of 1.451 (± 4.460%) and a z-average radius of gyration ( R _g ) is 90 nm. The polysaccharide composition included approximately 72% galactose, 9.78% N-acetylglucosamine, and 8.05% galacturonic acid, while the unknown peak accounted for approximately 7.83% of the total peak area of the chromatogram. The O-acetyl content of polysaccharide was determined to be nearly 6% and it lacked methyl pentoses (rhamnose). Zeta potential measurements revealed its zwitterionic state which suggested its potential to trigger T cell-dependent B cell-mediated immunological response. Serotype 38 polysaccharide showed immunological cross-reactivity with serotype 5 and serotype 1 polyclonal sera, likely due to a shared common epitope region having an unknown sugar component (Sug _p ) in their polysaccharide repeating units and zwitterionic properties. The findings highlight novel features of serotype 38 polysaccharide, including its amino acid content and zwitterionic nature, which may contribute to the development of new therapeutics and improved vaccines.