Cross-reactivity of Haemophilus influenzae type a and b polysaccharides: molecular modeling and conjugate immunogenicity studies

Semisynthetic Glycoconjugates as Potential Vaccine Candidates Against Haemophilus influenzae Type a

Glycoconjugate vaccines are based on chemical conjugation of pathogen-associated carbohydrates with immunogenic carrier proteins and are considered a very cost-effective way to prevent infections. Most of the licensed glycoconjugate vaccines are composed of saccharide antigens extracted from bacterial sources. However, synthetic oligosaccharide antigens have become a promising alternative to natural polysaccharides with the advantage of being well-defined structures providing homogeneous conjugates. Haemophilus influenzae (Hi) is responsible for a number of severe diseases. In recent years, an increasing rate of invasive infections caused by Hi serotype a (Hia) raised some concern, because no vaccine targeting Hia is currently available. The capsular polysaccharide (CPS) of Hia is constituted by phosphodiester-linked 4-β-d-glucose-(1→4)-d-ribitol-5-(PO4→) repeating units and is the antigen for protein-conjugated polysaccharide vaccines. To investigate the antigenic potential of the CPS from Hia, we synthesized related saccharide fragments containing up to five repeating units. Following the synthetic optimization of the needed disaccharide building blocks, they were assembled using the phosphoramidite approach for the installation of the phosphodiester linkages. The resulting CPS-based Hia oligomers were conjugated to CRM197 carrier protein and evaluated in vivo for their immunogenic potential, showing that all glycoconjugates were capable of raising antibodies recognizing Hia synthetic fragments.

A Multidisciplinary Structural Approach to the Identification of the Haemophilus influenzae Type b Capsular Polysaccharide Protective Epitope

Glycoconjugate vaccines so far licensed are generally composed of a native or size-reduced capsular polysaccharide conjugated to carrier proteins. Detailed information on the structural requirements necessary for CPS recognition is becoming the key to accelerating the development of next-generation improved glycoconjugate vaccines. Structural glycobiology studies using oligosaccharides (OS) complexed with functional monoclonal antibodies represent a powerful tool for gaining information on CPS immunological determinants at the atomic level. Herein, the minimal structural epitope of Haemophilus influenzae type b (Hib) CPS recognized by a functional human monoclonal antibody (hmAb) is reported. Short and well-defined Hib oligosaccharides originating from the depolymerization of the native CPS have been used to elucidate saccharide-mAb interactions by using a multidisciplinary approach combining surface plasmon resonance (SPR), saturation transfer difference-nanomagnetic resonance (STD-NMR), and X-ray crystallography. Our study demonstrates that the minimal structural epitope of Hib is comprised within two repeating units (RUs) where ribose and ribitol are directly engaged in the hmAb interaction, and the binding pocket fully accommodates two RUs without any additional involvement of a third one. Understanding saccharide antigen structural characteristics can provide the basis for the design of innovative glycoconjugate vaccines based on alternative technologies, such as synthetic or enzymatic approaches.

Conformational comparisons of Pasteurella multocida types B and E and structurally related capsular polysaccharides

Pasteurella multocida, an encapsulated gram-negative bacterium, is a significant veterinary pathogen. The P. multocida is classified into 5 serogroups (A, B, D, E, and F) based on the bacterial capsular polysaccharide (CPS), which is important for virulence. Serogroups B and E are the primary causative agents of bovine hemorrhagic septicemia that is associated with significant yearly losses of livestock worldwide, primarily in low- and middle-income countries. The P. multocida disease is currently managed by whole-cell vaccination, albeit with limited efficacy. CPS is an attractive antigen target for an improved vaccine: CPS-based vaccines have proven highly effective against human bacterial diseases and could provide longer-term protection against P. multocida. The recently elucidated CPS repeat units of serogroups B and E both comprise a N-acetyl-β-D-mannosaminuronic acid/N-acetyl-β-D-glucosamine disaccharide backbone with β-D-fructofuranose (Fruf) side chain, but differ in their glycosidic linkages, and a glycine (Gly) side chain in serogroup B. Interestingly, the Haemophilus influenzae types e and d CPS have the same backbone residues. Here, comparative modeling of P. multocida serogroups B and E and H. influenzae types e and d CPS identifies a significant impact of small structural differences on both the chain conformation and the exposed potential antibody-binding epitopes (Ep). Further, Fruf and/or Gly side chains shield the immunogenic amino-sugar CPS backbone-a possible common strategy for immune evasion in both P. multocida and H. influenzae. As the lack of common epitopes suggests limited potential for cross-reactivity, a bivalent CPS-based vaccine may be necessary to provide adequate protection against P. multocida types B and E.

Modeling of pneumococcal serogroup 10 capsular polysaccharide molecular conformations provides insight into epitopes and observed cross-reactivity

Streptococcus pneumoniae is an encapsulated gram-negative bacterium and a significant human pathogen. The capsular polysaccharide (CPS) is essential for virulence and a target antigen for vaccines. Although widespread introduction of pneumococcal conjugate vaccines (PCVs) has significantly reduced disease, the prevalence of non-vaccine serotypes has increased. On the basis of the CPS, S. pneumoniae serogroup 10 comprises four main serotypes 10A, 10B, 10C, and 10F; as well as the recently identified 10D. As it is the most prevalent, serotype 10A CPS has been included as a vaccine antigen in the next generation PCVs. Here we use molecular modeling to provide conformational rationales for the complex cross-reactivity reported between serotypes 10A, 10B, 10C, and 10F anti-sera. Although the highly mobile phosphodiester linkages produce very flexible CPS, shorter segments are conformationally defined, with exposed β-D-galactofuranose (β DGalf) side chains that are potential antibody binding sites. We identify four distinct conformational epitopes for the immunodominant β DGalf that assist in rationalizing the complex asymmetric cross-reactivity relationships. In particular, we find that strongly cross-reactive serotypes share common epitopes. Further, we show that human intelectin-1 has the potential to bind the exposed exocyclic 1,2-diol of the terminal β DGalf in each serotype; the relative accessibility of three- or six-linked β DGalf may play a role in the strength of the innate immune response and hence serotype disease prevalence. In conclusion, our modeling study and relevant serological studies support the inclusion of serotype 10A in a vaccine to best protect against serogroup 10 disease.

Comparative Molecular Modelling of Capsular Polysaccharide Conformations in Streptococcus suis Serotypes 1, 2, 1/2 and 14 Identifies Common Epitopes for Antibody Binding

Streptococcus suis is an encapsulated, commensal, potentially pathogenic bacterium that infects swine globally and causes sporadic life-threatening zoonotic septicemia and meningitis infections in humans. The capsular polysaccharide is a primary virulence factor for S. suis. As S. suis serotype 2 is the most prevalent serotype globally, the serotype 2 CPS is the primary target of current efforts to develop an effective glycoconjugate veterinary vaccine against S. suis. Possible cross-protection with related serotypes would broaden the coverage of a vaccine. The CPS in serotypes 2 and 1/2 differ at a single residue (Gal versus GalNAc), and both are similar to serotypes 1 and 14: all contain a terminal sialic acid on a side chain. However, despite this similarity, there is complex pattern of cross-protection for these serotypes, with varying estimations of the importance of sialic acid in a protective epitope. Further, a pentasaccharide without the terminal sialic acid has been identified as minimal epitope for serotype 2. Here we use molecular simulation to model the molecule conformations of the CPS in serotypes 2, 1/2, 1 and 14, as well as three vaccine candidate oligosaccharides. The common epitopes we identify assist in rationalizing the apparently contradictory immunological data and provide a basis for rational design of S. suis vaccines in the future.