CLDN4 and influenza: Since recombinant proteins typically induce no or only weak immune responses in the nasal mucosa38, we increased the immunogenicity by fusing the RBD to a short polypeptide derived from the C-terminus of the Clostridium perfringens Clostridium perfringens toxin (cCPE), which targets highly expressed claudin-4 on microfold cells (M-cells) and has previously been described to enhance the immunogenicity of intranasal pneumococcal and influenza vaccines10,39.