A conjugate vaccine is a type of vaccine that combines a weak or non-toxic part of a disease-causing microorganism (such as a bacteria or virus) with a carrier protein. The purpose of creating a conjugate vaccine is to enhance the immune response, particularly in cases where the immune system might not recognize the disease-causing component on its own. Conjugate vaccines have been successful in preventing a range of infectious diseases, particularly in vulnerable populations such as infants, young children, and the elderly.
Invasive Infections by Encapsulated Bacteria
Bacteria possessing a capsule or microcapsule structure can lead to severe invasive infections in children, such as bacterial pneumonia or bacterial meningitis. Examples include Haemophilus influenzae, Meningococcus (Men), Streptococcus pneumoniae, Salmonella typhi, Klebsiella pneumoniae, and Group B Streptococcus (GBS), among others. The capsule polysaccharides (CPS) or microcapsule polysaccharides situated on the bacteria’s outer surface play a crucial role in causing these invasive infections. They are also the primary components used in vaccines to combat these bacterial infections.
Glycoconjugate Vaccine – Polysaccharide Conjugate Vaccines
Glycoconjugate vaccines, alternatively known as polysaccharide conjugate vaccines, are created by covalently attaching polysaccharide units with non-sugar molecules (such as proteins, peptides, or lipids), with an emphasis on protein-polysaccharide conjugate vaccines. Because many of these polysaccharides are considered T-cell independent antigens (Ti), they are unable to stimulate the production of immunogenic B cells, particularly in infants and children under two years of age. Therefore, the strategy behind glycoconjugate vaccines involves connecting variable polysaccharide units with proteins, peptides, or lipids through covalent bonds. The non-sugar molecules act as vaccine adjuvants, triggering the activation of helper T cells and facilitating the maturation and memory formation of B cells, consequently offering long-term immune protection.
Composition of Glycoconjugate Vaccines
- Polysaccharide Units
The external polysaccharides of pathogenic bacteria (limited to those with capsule or microcapsule structures) can result in invasive infections, with extracellular polysaccharides playing a pivotal role. Polysaccharide units in glycoconjugate vaccines constitute the primary antigenic components, and the specific types of polysaccharides dictate bacterial serotypes. These extracellular polysaccharides function as T-cell independent antigens and can stimulate B cells to produce immunoglobulins (primarily IgM with some IgG2) upon interaction with the B cell receptor (BCR). However, they do not induce immune memory. Infants possess immature C3d receptors in the spleen, and the interaction between CD21 on B cells and C3d is suppressed, impacting the immune response to polysaccharides. Consequently, polysaccharide antigens alone are insufficient to elicit a robust immune response, particularly in high-risk populations such as infants frequently exposed to these pathogens. To address this, the development of glycoconjugate vaccines involves using non-sugar molecules, such as proteins, covalently linked to polysaccharide antigens, thereby transforming polysaccharide antigens into thymus-dependent antigens (Td).
It’s important to note that not all bacterial extracellular polysaccharides are Ti antigens. For instance, the capsule polysaccharides of MenA function as Td antigens, inducing strong immunogenicity upon initial immunization in infants and forming partial immune memory. Some bacterial polysaccharides are amphoteric polysaccharides, capable of dissociating into cations and anions. These polysaccharides are Td antigens, such as Bacteroides fragilis capsule polysaccharide A1 (PSA1) and type 1 S. pneumoniae capsule polysaccharide.
- Non-Sugar Units
Currently, there are five carrier proteins used in commercially available bacterial glycoconjugate vaccines:
- Cross-reacting material 197 (CRM197), a derivative of diphtheria toxin.
- Tetanus toxoid (TT).
- Diphtheria toxoid (DT).
- Meningococcal outer membrane protein complex (OMPC).
- Haemophilus influenzae protein D (HiD).
The carrier proteins in glycoconjugate vaccines are covalently bound to polysaccharide antigens. Within B cells, this covalent bond remains intact. The peptide structures within glycopeptide complexes primarily activate CD4+ T cells, promoting the differentiation and maturation of polysaccharide-specific B cells and the formation of memory. Carrier proteins also induce memory immune responses against the protein components. For instance, CRM197 can inhibit tumor cell proliferation and migration, exhibiting anticancer effects. Additionally, when different glycoconjugate vaccines using the same carrier protein are administered, the likelihood of immune interference increases, resulting in decreased levels of polysaccharide-specific antibodies.
The potential carrier proteins are being explored in animal experiments and clinical trials, including pneumococcal surface protein A (PspA), pneumococcal histidine triad D (PhtD), exotoxin A of P. aeruginosa (EPA), non-toxic peptides from Clostridium difficile toxin A (CDTA) and toxin B (CDTB), detoxified pneumolysin (dPly), and Staphylococcus aureus alpha-toxin (Hla).
Immune Mechanisms of Glycoconjugate Vaccines
The classical immune mechanism of glycoconjugate vaccines involves cross-linking with BCR, leading to intracellular enzymatic cleavage of the glycoconjugate into glycopeptide complexes. The peptide segments within these complexes are presented to CD4+ T cells by B cell surface MHC class II molecules. Cooperative interactions between T cells and B cells stimulate the migration of polysaccharide-specific B cells to germinal centers for differentiation and maturation. In addition to promoting the conversion of polysaccharide-specific IgM to IgG (primarily IgG1 and IgG3), the formation of polysaccharide-specific memory B cells occurs.
- Polysaccharide-specific antibodies in peripheral circulation exert neutralizing effects, effectively blocking invasive infections caused by pathogenic bacteria.
- Polysaccharide-specific memory B cells ensure rapid immune responses during pathogenic invasions, thereby controlling infections.
- Immune Response Mediated by Tcarbs
The immune mechanism of glycoconjugate vaccines also involves the immune response mediated by carbohydrate-specific helper CD4+ T cells (Tcarbs), which recognize oligosaccharide structures within glycopeptide complexes. Research on glycoconjugate vaccines for type III group B Streptococcus (GBS III) capsule polysaccharide has revealed the presence of Tcarbs subpopulations when glycopeptide complexes are presented within B cells. Tcarbs promote the differentiation and maturation of polysaccharide-specific B cells, as well as the formation of immune memory. Similar Tcarbs subpopulation immune response mechanisms have been observed in glycoconjugate vaccines for type C Streptococcus, Haemophilus influenzae type b (Hib), and type I group B Streptococcus (GBSI) capsule polysaccharides.
- Glycopeptide Antigenic Epitopes
It’s important to note that a Tcarbs subpopulation recognizing the polysaccharide structure of glycoconjugate vaccines for meningococcus C (MenC) does not exist. This could be attributed to the structure of MenC capsule polysaccharides, which consist of linear polysialic acid copolymers linked by α,2-9 glycosidic bonds. Enzymatic cleavage within B cells leads to the formation of smaller sialic acid monosaccharide residues, which are inadequate to form antigenic epitopes for presentation by MHC class II molecules. Consequently, during MenC glycoconjugate vaccine immunization, the activated T cells primarily recognize carrier protein peptides. This suggests that enhancing the immune response to MenC polysaccharide vaccines necessitates the use of the same carrier protein to ensure a rapid immune response by memory B cells. The presence of Tcarbs subpopulations indicates that an ideal glycoconjugate vaccine should enrich glycopeptide antigenic epitopes.
References:
- Rappuoli, R., Glycoconjugate vaccines: Principles and mechanisms, Sci. Transl. Med., 2018, 10, eaat4615.
- Pollard, A. J., Perrett, K. P., Beverley, P. C., Maintaining protection against invasive bacteria with protein–polysaccharide conjugate vaccines, Nat. Rev. Immunol., 2009, 9(3), 213-220.
- Berti, F., et al., Carbohydrate based meningococcal vaccines: past and present overview, Glycoconj J., 2021, 38(4): 401-409.
- R.M.v.d.P., et al., Carriers and Antigens: New Developments in Glycoconjugate Vaccines, Vaccines, 2023, 11(2), 219.