Isolation and characterization of human monoclonal antibodies to pneumococcal capsular polysaccharide 3

The current pneumococcal capsular polysaccharide (PPS) conjugate vaccine (PCV13) is less effective againstStreptococcus pneumoniaeserotype 3 (ST3), which remains a major cause of pneumococcal disease and mortality. Therefore, dissecting structure-function relationships of human PPS3 antibodies may reveal characteristics of protective antibodies. Using flow cytometry, we isolated PPS3-binding memory B cells from pneumococcal vaccine recipients and generated seven human PPS3-specific monoclonal antibodies (humAbs). Five humAbs displayed ST3 opsonophagocytic activity, four induced ST3 agglutinationin vitro, and four mediated both activities. For two humAbs, C10 and C27, that used the same variable heavy (VH) and light (VL) chain domains (VH3-9*01/VL2-14*03), C10 had fewer VLsomatic mutations, higher PPS3 affinity, more ST3 opsonophagocytic and agglutinating activity, whilst both humAbs altered ST3 gene expressionin vitro. After VLswaps, C10VH/C27VLexhibited reduced ST3 binding and agglutination, but C27VH/C10VLbinding was unchanged. In C57Bl/6 mice, C10 and C27 reduced nasopharyngeal colonization with ST3 A66 and a clinical strain, B2, and prolonged survival following lethal A66 intraperitoneal infection, but only C10 protected against lethal intranasal infection with the clinical strain. Our findings, associate efficacy of PPS3-specific humAbs with ST3 agglutination and opsonophagocytic activity and reveal an unexpected role for the VLin functional activityin vitroandin vivo. These findings also provide insights that may inform antibody-based therapy and identification of surrogates of vaccine efficacy against ST3.IMPORTANCEDespite the global success of pneumococcal conjugate vaccination, serotype 3 (ST3) pneumococcus remains a leading cause of morbidity and mortality. In comparison to other vaccine-included serotypes, the ST3 pneumococcal capsular polysaccharide (PPS3) induces a weaker opsonophagocytic response, which is considered a correlate of vaccine efficacy. Previous studies of mouse PPS3 monoclonal antibodies identified ST3 agglutination as a correlate of reduced ST3 nasopharyngeal colonization in mice, however neither the agglutinating ability of human vaccine-elicited PPS3 antibodies nor their ability to prevent experimental murine nasopharyngeal colonization has been studied. We generated and analysed the functional andin vivoefficacy of human vaccine-elicited PPS3 monoclonal antibodies and found that ST3 agglutination associated with antibody affinity, protectionin vivo, and limited somatic mutations in the light chain variable region. These findings provide new insights that may inform the development of antibody-based therapies and next generation vaccines for ST3.