The induction of neutralising antibodies by immunisation represents one of the best strategies to prevent subsequent virus infections. However, whereas such antibodies form the basis of several vaccines available today, attempts to induce protective immune responses against the human immunodeficiency virus (HIV) failed till now. This work aimed to establish and evaluate a novel vaccine approach based on apathogenic, persistently infecting foamy virus (FV) to exploit their continuous antigen delivery and immune stimulation. To allow testing of such novel vectors in vivo, the feline FV (FFV) was used as model for later application in in cats. In a first approach, the transmembrane envelope (TM) protein of FFV was investigated as potential epitope scaffold. With the help of recombinantly produced antigen it was demonstrated that the TM protein is a strong immunogen during natural FFV infection and used to serologically determine for the first time the FFV prevalence in Germany. After immunogenic regions within FFV TM were mapped, replacing these domains through the membrane proximal external region (MPER) of the HIV TM protein, an epitope recognised by several HIV-1 broadly neutralising antibodies (bnAb), was accomplished. The generated chimeric FFV/HIV-1 Env were released from transfected cells as subviral particles (SVPs), presenting the HIV MPER epitope in a FV typical high density on the particle surface, in a trimeric context and a lipid environment and were thus tested in subsequent immunisation experiments. However, since they gave rise to fusion-defective FFV, they were impractical for application in a replicating system. In a second strategy, fusion proteins of the accessory FFV Bet protein and HIV-1 epitopes were therefore evaluated. Characterisation of these antigens showed that they were efficiently recognised by the preferred HIV-1 bnAb and elicited binding antibodies with identical epitope specificity in immunisation studies. Using further improved HIV-1 inserts this approach could be successfully transferred into the FFV and resulted in the production of infectious, chimeric FFV viruses. Importantly, HIV-1 epitope expression remained stable after extended passaging on feline cells, suggesting that these vectors are suitable for studying the effects of affinity maturation on the emergence of bnAb responses. Taken together, in this work two new HIV-1 vaccine platforms based on SVPs and replicating FFV were developed, which are characterised by a high safety profile. In particular the replicating FFV/HIV-1 chimeric viruses thereby represent attractive vectors for further analysis in vivo.
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