​One of the major challenges facing the development of molecular therapies is the accessibility of drugs to their intracellular targets. For many years, the teams of Ludger Johannes (Institut Curie) and Éric Tartour (HEGP) have been studying the B subunit of Shiga bacterial toxin (STxB) because of its unique ability to interact with a receptor expressed on the surface of dendritic cells, the sentinels of our immune system, the glycolipid Gb3. After binding to the receptor, STxB enters the cell's cytosol by endocytosis. The original idea was to use the toxin's weakly immunogenic and non-toxic B subunit as a tool for intracellular delivery of tumor or pathogen antigens into the immune presentation pathways of dendritic cells, and to induce stronger immune responses than with an antigen (Ag) not vectorized by STxB. Functionalization of STxB with an antigen has already proved effective in inducing an enhanced immune response to a tumor Ag when it is fused to it, compared with its non-vectorized form. This STxB-Ag treatment also inhibited tumour growth in murine models.

In the present study, SIMoS researchers explored the possibility of further enhancing STxB membrane translocation by optimizing the chemical synthesis of STxB (69 residues), into which they introduced unnatural amino acids at different positions of the peptide. These amino acids were then functionalized with hydrophobic entities to locally destabilize endosomal membranes. Intracellular trafficking of this functionalized STxB was measured by confocal microscopy, and its entry into the cytosol was monitored using a new detection method based on a highly robust, sensitive and quantitative translocation assay recently developed in the laboratory. In the most efficient configuration of vectorized STxB, determined using different types of hydrophobic motifs, the researchers measured translocation increased by a factor of 2.5. They also showed that intranasal administration of different types of viral antigens vectorized by synthetic STxB (SARS-CoV-2 or HPV antigenic peptides) led to a stronger immune response in a mouse model.

These results confirm the value of exploiting STxB in a mucosal anti-tumor and anti-infectious vaccination strategy, and are important since, for example, mRNA vaccines do not induce mucosal immunity, which would enable better protection of healthy populations.

Contact : Denis Servent (denis.servent@cea.fr )