Capless self-amplifying mRNA vaccine induces dose-sparing protective immunity against HPAI clade 2.3.4.4 H5 virus in mice

Self-amplifying mRNA (samRNA) vaccines can induce potent immune responses at lower doses than conventional non-replicating mRNA vaccines; however, large RNA size and manufacturing considerations associated with 5' capping remain important challenges. Here, we developed a capless samRNA (CLsamRNA) vaccine platform derived from a Coxsackievirus B5 replicon that uses IRES-mediated cap-independent translation. Systematic optimization of key genetic elements enhanced antigen expression from the CLsamRNA backbone. Using reporter RNAs, LNP-formulated CLsamRNA showed rapid early expression and RNA amplification, with expression kinetics distinct from VEEV-based saRNA and nucleoside-modified mRNA comparators. When encoding the hemagglutinin antigen of highly pathogenic avian influenza clade 2.3.4.4 H5 viruses, CLsamRNA induced potent immune responses after LNP formulation. In mouse models, CLsamRNA induced potent neutralizing antibody responses, cross-reactive activity against clade 2.3.4.4b H5N1 virus, Th1-skewed humoral immunity, and strong antigen-specific cellular immune responses. CLsamRNA also elicited platform-specific early inflammatory and lymph-node immune gene signatures associated with antigen presentation, costimulation, and cellular immune priming. Notably, a minimal 0.01 μg dose of CLsamRNA conferred complete protection against lethal H5N8 influenza virus challenge in BALB/c mice. These findings support CLsamRNA as a distinct cap-independent RNA vaccine platform with dose-sparing protective efficacy against HPAI H5 viruses.