Multivalent presentation of saccharide fragments related to streptococcus pneumoniae 19F serotype

The polysaccharide encapsulated bacteria Streptococcus pneumonia (SP) is one of the leading causes of bacterial infections in young children. The capsular polysaccharides (CPS) are virulence factors, and their chemical structure defines the different bacterial serotypes. One of the most virulent SP serotype is the 19F, which is characterized by the trisaccharide repeating unit (4)--D-ManpNAc-(14)--D-Glcp-(12)--L-Rhap-(1-OPO3-), with a phosphodiester bridge connecting a residue of rhamnose at the reducing end of one unit to the non-reducing end of the following unit. It has been demonstrated that there is an evident effect of the pneumococcal polysaccharide chain length on the immunogenicity. We have observed for SP 19F that even the single trisaccharide repeating unit shows a good ability to inhibit the binding between the 19F polysaccharide and the anti-19F human polyclonal antibody, with efficacy and affinity lower than the natural polysaccharide. The lower activity of the monomer can be ascribed to the absence of the multivalent presentation of the epitope, that on the contrary is present in the natural polysaccharide chain. In this context, a multivalent scaffold which act as the core for the multivalent resentation of biological ligands is a strategy to positively affect the immunological potential of the saccharide antigen. Herein we will describe our ongoing work toward the preparation of a multivalent system based on a calixarene scaffold for the multivalent presentation of the trisaccharide repeating unit of SP 19F. A set of calix[4]arenes decorated with 3-amino-propyl N-acetyl-Dmannosamine, i.e. the immunodominant residue of the trisaccharide SP 19F repeating unit, have been prepared. The systems show different conformational properties, that reflect in different orientation of the saccharide antigens toward the protein receptor, and have been biologically evaluated to select the more promising calixarene platform for the conjugation of the SP 19F trisaccharide repeating unit.