Increasing antigen presenting cells at the injection site improves SAM vaccine efficacy

Self-amplifying mRNAs (SAM) based vaccines are effective at eliciting potent and protective immune responses. The SAM technology is developed as a platform technology with the potential to be used for a broad range of targets, able to induce both cellular and humoral response. Recently published data demonstrate that bone marrow derived Antigen Presenting Cells (APCs) are responsible for CD8 T cell priming after SAM vaccination, even though these cells are not able to express antigen directly from the SAM construct. This suggests cross-priming is a key mechanism for antigen presentation by SAM vaccines. Professional APCs are a key target for strategies aimed to enhance the immunogenicity of vaccines because of their specific ability to capture, process and present Ags to T cells. One successful approach for targeting these APCs is to increase the number of antigen loaded APCs at the injection site. Since GM-CSF is a chemoattractant for APCs, we developed a SAM(GM-CSF) vector and administered it intramuscularly in mice together with another SAM construct encoding the influenza A virus nucleoprotein [SAM(NP)]. We hypothesized that the presence of GM-CSF together with the NP antigen would lead to increased APCs recruitment and to an increased NP-specific CD8 T cell response. We indeed observed that administration of the SAM(GM-CSF) vector enhances the recruitment of APCs (DCs and macrophages or monocytes) in the draining lymph nodes and at injection site. Moreover, co-administration of SAM(GM-CSF) with SAM(NP) significantly improves the magnitude of NP-induced CD8 T-cell responses both in frequency of cytotoxic CD8+ T cells and in functional activity in an in vivo cytotoxicity assay. A second strategy addressed in the present study, in order to enhance the potency of SAM vaccines, is to target the antigen of interest directly to surface receptors expressed on cross-presenting DCs. Fusion constructs are made of single chain fragment variable (scFv) or ligands that bind surface receptors expressed on DCs fused to OVA antigen. These fusion constructs have been designed, expressed and tested in vitro for their binding activity to DCs.