Structural and immunological characterization of derived recombinant CRM protein used as carrier in conjugate vaccines.

It is established that the immunogenicity of polysaccharides is enhanced by coupling them to carrier proteins. Cross reacting material (CRM), a nontoxic variant of diphtheria toxin (DT) is widely used carrier protein for polysaccharide conjugate vaccines. Conventionally, CRM is isolated by fermentation of C7 (β) cultures, which often suffers from low yield. Recently, several recombinant approaches have been reported with robust processes and higher yields, which will improve the affordability of CRM-based vaccines. Vaccine manufacturers require detailed analytical information to ensure that the CRM meets quality standards and regulatory requirements. In the present manuscript we have described detailed structural characteristics of based recombinant CRM (rCRM) carrier protein. The crystal structure of the based rCRM was found to be identical with the reported crystal structure of the C7 CRM produced in C7 strain (Protein Data Bank (PDB) ID: 4EA0) The crystal structure of rCRM was determined at 2.3 Å resolution and structure was submitted to the PDB with accession number ID 5I82. This is the first report of a crystal structure of derived recombinant CRM carrier protein. Furthermore, the rCRM was conjugated to Vi polysaccharide to generate Typhoid conjugate vaccine (Vi-rCRM) and its immunogenicity was evaluated in Balb/C Mice. The Vi-rCRM conjugate vaccine was found to generate strong primary α-Vi antibody response and also showed a booster response after subsequent vaccination in mice. Overall data suggest that based recombinant CRM exhibits structural and immunological similarity with the C7 CRM and can be used as a carrier protein in conjugate vaccine development.