Many bacteria have capsular polysaccharides or O-specific polysaccharides, which are both major virulence factors and important targets for protective antibodies. Vaccine consisting of purified capsular polysaccharides of selected pneumococcal serotypes was first tested in 1940s, followed by other polysaccharide vaccines against meningococcal serogroups ACWY, Streptococcus pneumoniae, and Haemophilus influenzae type b (Hib). The shortcoming of these pure polysaccharide vaccine is, they are not protective enough in high-risk groups, particularly children under two years of age and immunocompromised patients.

Under this consideration, glycoconjugate vaccines are developed, which consist of a polysaccharide covalently linked to an appropriate carrier protein. The combination gives the polysaccharide the ability to induce a long-lasting and enhanced IgG antibody response, even in children under two years of age. The first polysaccharide-protein conjugate vaccines were developed against Haemophilus influenzae type B. and many other similar vaccines were approved successively or currently under development.

Several parameters can influence the immunogenicity of glycoconjugate vaccines, including the size of the polysaccharide and its structural modifications, the carrier protein used, and the ratio of polysaccharide to protein. Additionally, conjugation chemistry, including the presence of linkers and attachment points on the carrier protein, can play an important role.

Depending on genetic characteristics of bacterial strains, they can produce polysaccharides of different chain lengths. Furthermore, the fermentation and purification conditions employed can have a profound impact on the length of the sugar chain. Depending on their properties, polysaccharides are polymers with identical repeating units, ranging from ten to several thousand units in length. Thus, polysaccharide molecules are characterized by a wide range of sizes. It is generally preferred to use shorter saccharides to increase conjugation yield and consistency and to facilitate conjugate purification, particularly of unreacted saccharides. Additionally, working with saccharides of smaller size may facilitate the characterization of the entire process, including sugar activation and conjugation, and may be beneficial in the production of conjugates with more defined structures. Therefore, polysaccharides are often chemically or mechanically fragmented and reduced in size to generate a homogeneous population of lower molecular weight before binding to carrier proteins.

Mechanical size control based on high-pressure homogenizers is one of the most widely used methods for producing glycoconjugate vaccines. High shear rates are applied by pumping the process fluid through flow paths of the required dimensions. The high-pressure homogenization process is particularly suitable for reducing the size of polysaccharides containing non-sugar substituents, such as O-acetyl, phosphoglyceride, pyruvyl commonly found in pneumococci, meningococci, staphylococci, and group B streptococci. Homogenization is an ideal method for size reduction because once the polysaccharide concentration, pressure, and number of cycles are determined, the process can be highly consistent. This method reduces the polydispersity of polysaccharide molecules in solution because only larger molecules rather than smaller molecules are reduced in size. However, each polysaccharide must be evaluated independently to ensure that the composition and structure of the repeating segments are not adversely changed.

In summary, high-pressure homogenization is a simple and scalable technology with good reproducibility and no side effects. This technology has been used for Streptococcus pneumoniae capsular polysaccharide and meningococcus capsular polysaccharide. Group B Streptococcus capsular polysaccharide and Staphylococcus aureus capsular polysaccharide treatment.

References:

? E.Kay, J.Cuccui, B.W.Wren, Recent advances in the production of recombinant glycoconjugate vaccines. Npj Vaccine, 2019.

? G.Stefanetti, C.A.Maclennan, F.Micoli, Impact and Control of Sugar Size in Glycoconjugate Vaccine. Molecules, 2022.

? C.Anish, M.Beurret, J.Poolman, Combined effects of glycan chain length and linkage type on the immunogenicity of glycoconjugate vaccines. NPJ, Vaccines, 2021.

ATS Engineering Limited (A Duoning Company) provides a full range of high-pressure homogenizer products, which can support applications of different scales from laboratory to production, aiming at reducing the molecular weight of polysaccharides and controlling the size of polysaccharides in glycoconjugate vaccines, combined with optimized process parameters, products with more uniform molecular weight can be obtained after high-pressure homogenization, thereby optimizing the immune response.

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