Within the rapidly growing vaccine space, conjugate vaccines—polysaccharide antigens covalently linked to carrier proteins—have been shown to be effective against several bacterial pathogens.
Conjugate vaccines use carrier proteins to increase the immunogenicity of antigens (e.g., peptides, oligosaccharides, polysaccharides) and other haptens. These antigens are poor immunogens unless conjugated to proteins.
The role of the carrier protein is to enhance immunogenicity by providing T-cell epitopes via MHC Class II presentation to T-helper cells. Carrier proteins both increase the magnitude of the immune response as well as engender B-cell “memory.”
The number of carrier proteins used in licensed vaccines is relatively limited and includes tetanus toxoid, diphtheria toxoid, CRM197 (a nontoxic mutant of diphtheria toxin), Haemophilus influenzae protein D, and Neisseria outer membrane protein. Access to clinically-proven, safe, and efficacious carrier proteins is critical for research in the field of conjugate vaccines.
CRM197, which has a single amino acid substitution of glutamic acid for glycine, is a well-characterized carrier protein and is utilized in a number of approved conjugate vaccines for diseases such as meningitis and pneumococcal bacterial infection. It is best known for its use in Pfizer’s conjugate vaccine for pneumococcal infection, Prevnar®, as well as HibTiter® (Haemophilus influenzae b) and Novartis’ meningococcal vaccine Menveo®.