Analysis of the molecular basis of the immune response to Streptococcus pneumoniae capsular polysaccharide

Streptococcus pneumoniae is a major cause of infection world wide, particularly in the very young and the elderly. Serotype specific anti-capsular antibodies are known to protect against infection. Studies of human antibodies specific for bacterial derived capsular polysaccharide antigens reveal these antibodies to be oligoclonal and of limited diversity. Relatively little is known of the molecular basis of the human immune response to pneumococcal polysaccharide. With new generation pneumococcal conjugate vaccines now becoming available a better understanding of the human immune response to Streptococcus pneumoniae is of increasing importance. This thesis describes an analysis of the immune response to two types of pneumococcal vaccine (plain polysaccharide and conjugate) in healthy adult volunteers. Using heterohybridoma technology to produce antigen specific monoclonal antibodies, the diversity of the immune response to pneumococcal polysaccharides was analysed and the molecular characteristics of the antibodies were correlated with in vitro functional activity. A high proportion of the hybridomas produced were isotype switched and highly mutated, inconsistent with their being derived from a primary immune response. Identical genes were used by a number of individuals to generate antibodies to a variety of serotypes and a common replacement mutation was identified in the CDR2 of two clones derived from different individuals and of different sero-specificity. Both ranked highly in the functional assays. Two of the hybridomas generated were isotype switch variants of the same clone and demonstrated marked differences in antibody avidity and opsonophagocytic activity. These data suggest that the antibody repertoire induced by pneumococcal vaccination in adults may be restricted in V gene use with common V genes and somatic mutations demonstrated to a variety of serotypes and shared between individuals. Mutation analysis demonstrates that the antibody repertoire of adult vaccinees may be dictated not by vaccine formulation but by immune history of each individual in whom priming for memory has already occurred.