Background: Human T-cell leukemia virus type 1 (HTLV-1) is endemic to numerous regions worldwide, with some sustaining high infection rates (around 40% in adults) as identified in the remote First Nations communities of Central Australia. The lack of an approved vaccine preventing transmission or pathogenesis highlights HTLV-1 as a neglected public health issue. We have demonstrated that potent neutralising antibodies targeting the HTLV-1 Envelope (Env) protein are induced during natural infection with HTLV-1 subtype C. Here, we aim to recapitulate this with a vaccination regimen with mRNA-lipid nanoparticles (LNP) expressing HTLV-1C-Env protein stabilised in a trimeric prefusion conformation.
Methods: A panel of mutant env expression constructs was generated and used to transfect ExpiCHO cells. Soluble Env expression and stabilisation was assessed by SDS-PAGE, BN-PAGE and ELISA. Key mutants were selected, and mRNA was in vitro transcribed (IVT) using N1-methyl pseudouridine and CleanCap. Protein expression was confirmed by transfection of HEK293 cells. LNPs were formulated using GenVoy ILM and used to vaccinate groups of C57Bl/6 mice (n=5). Membrane-tethered vs soluble forms of env with stabilising mutations were compared at two different doses. Serum was collected 2 weeks post-boost and Env binding of serum IgG assessed by ELISA and virus-neutralisation assayed using reporter-pseudovirus.
Results: Characterisation of a comprehensive panel of HTLV-1 Env mutants has identified amino-acid substitutions that enhance expression of stable native-like trimer. We have demonstrated successful protein expression by transfection of IVT mRNA into HEK293 cells and delivered these mRNA-LNP vaccines to mice to test the protective immune response induced. The full assessment of vaccine antigenicity in mice is ongoing.
Conclusions: We have shown that HTLV-1 Env expressed during natural infection is capable of generating a robust serological response. We aspire to recapitulate this response with several doses of mRNA-LNP expressing prefusion-stabilised Env mutants identified in this study.