Streptococcus pneumoniae serotype 3 (ST3) is one of the main pneumococcal strains that can cause severe invasive diseases, but its current vaccines are relatively inefficient. To develop more effective ST3 vaccines, tetanus toxoid (TT) conjugates of the synthetic penta-, hexa-, hepta-, and octasaccharide analogs of ST3 capsular polysaccharide (CPS) were systematically studied. These conjugates, especially those of penta- and hexasaccharides, were demonstrated to induce extremely robust T cell-dependent immune responses in mouse. Various studies also revealed that the induced antibodies could recognize ST3 CPS and mediate in vitro opsonophagocytic killing of ST3 cells. It was proved ultimately that immunization with the hexasaccharide-TT conjugate could completely protect mice from ST3-caused infection and lung damage and significantly elongate mouse survival. It was proposed that this conjugate functions through the help of CD4+ T cells and via promoting Th cell differentiation into carbohydrate antigen-specific Th2 cells to establish humoral immunity. In conclusion, ST3 CPS hexasaccharide-TT was identified as a particularly promising ST3 vaccine candidate worthy of further investigation and development.
Keywords: serotype 3; CD4+ T cell; antibacterial activity; immunoprotection; semisynthetic glycoconjugate; vaccine.