Heterologous expression of an immunogenic pneumococcal type 3 capsular polysaccharide in Lactococcus lactis

In order to develop a new system for the analysis of capsular biosynthetic pathways we have explored the possibility of expressing type 3 capsular pol ysaccharide (CPS) from the pathogen Streptococcus pneumoniae in Lactococcus lactis, an unencapsulated lactic acid bacterium being developed as a vacci ne delivery vehicle for mucosal immunization. Only three of the four type 3 CPS biosynthesis genes were found to be necessary for the abundant formati on (120 mg liter(-1)) of an extracellular type 3 CPS in L. lactis, implying a role for the type 3-specific synthase in the extracellular transport of the CPS or implying the existence of an alternative export system in L. lac tis, The authenticity of the expressed heterologous polysaccharide was esta blished by chemical and immunological analyses. Proton and carbon nuclear m agnetic resonance spectroscopy of CPSs purified from L. lactis and S. pneum oniae showed that the two CPS structures were identical. When mice were imm unized intraperitoneally with 3.5 x 10(6) CFU of live recombinant lactococc i expressing a total of approximately 0.5 mu g of type 3 CPS, the immune re sponses elicited appeared identical to those observed in mice inoculated wi th 0.5 mu g of type 3 CPS purified from S. pneumoniae. These findings show that L. lactis is a useful host in which to study the role and function of genes involved in the production of bacterial capsules. Additionally, L. la ctis shows potential as a host for the safe production of capsule antigens and as a vaccine delivery vehicle for polysaccharide antigens.