Effect of N-terminal poly histidine-tag on immunogenicity of Streptococcus pneumoniae surface protein SP0845

SP0845, a pneumococcal surface protein and a potential candidate vaccine for Streptococcus pneumoniae infection, was used to evaluate the role of histidine affinity tag on its biophysical properties and immunogenicity. The protein was expressed in E. coli with and without histidine affinity tag and purified to homogeneity. Size exclusion chromatographic studies revealed that tag free SP0845 was mainly monomeric in solution whereas, histidine tagged SP0845 stayed predominantly in an oligomeric form. Histidine-tagged SP0845 have higher β sheet content than the tag free protein. Removal of histidine tag increased the α-helical content of SP0845 from 35% to 46%. Histidine tagged SP0845 elicited higher serum antibody titer in comparison to the tag free SP0845 in mice. Effect of alum in improving the immunogenicity of tagged SP0845 was low in comparison to that observed with tag free protein. Immunogenicity of tag free SP0845 was enhanced by delivering it using polylactide polymeric particles. The presence of histidine tag thus influences the secondary structure and immunogenicity of protein and need careful consideration before use.