Project Summary/Abstract A key requirement for HIV-1 vaccine development is the induction of broadly neutralizing antibodies (bNAbs) targeting the HIV-1 envelope (Env) spike. The BG505 SOSIP.664 gp140 soluble timer is engineered to display conformational epitopes recognized by several bNAbs but not non-neutralizing antibodies. The prototype SOSIP vaccine is currently being evaluated in two clinical trials. An effective messenger RNA (mRNA) vaccine expressing BG505 SOSIP.664 will complement the clinical vaccine and open a path to heterologous immunization strategies to elicit immune responses defined by increased breadth and magnitude. Recent achievements in mRNA vaccines is a result of advancements in nanoparticle delivery technologies. We recently reported immunogenicity of a replicating RNA (repRNA) vaccine encoding the spike (S) protein of SARS-CoV- 2; this vaccine, delivered with a novel Lipid Inorganic Nanoparticle (LION) carrier, induced potent binding and neutralizing antibodies in both mice and non-human primates. In the proposed R61/R33 application, we will investigate and optimize attributes of the LION formulation to engineer the immunity of a BG505 SOSIP.664 expressing repRNA vaccine (repRNA-SOSIP.664). The goal is to identify a lead vaccine and administration route that promotes B cell differentiation and maturation needed for induction of high affinity Env antibodies that confer protection in a macaque SHIV challenge model. The proposed work will be performed with a three-way collaboration between Dr. Amit Khandhar at HDT Bio, Dr. Noah Sather at Seattle Children’s Research Institute (SCRI) and Prof. Deborah Fuller at the University of Washington (UW). Proposed activities in the R61 phase will include (specific aim 1) optimizing the repRNA/LION platform for intramuscular (IM), intradermal (ID) and intranasal (IN) delivery and (specific aim 2) screening immunogenicity of vaccine candidates in mouse and rabbit models. In the R33 phase, we will advance (specific aim 3) one or more lead vaccine candidates to evaluate protective efficacy in a macaque SHIV challenge model. Milestones in the R61 phase are (milestone 1) identification of formulations that target lymph nodes or remain at injection site, (milestone 2) develop an optimized LION formulation for intranasal delivery of repRNA, (milestone 3) one or more repRNA/LION immunization regimens that induce neutralizing antibodies greater than or equal to the BG505 SOSIP.664 protein/adjuvant vaccine. Using clearly defined go/no-go metrics for each milestone we will determine transition to the R33 phase of the project. Milestones in the R33 phase are (milestone 1) one or more vaccine regiment affords statistically significant protection compared to controls in a macaque SHIV challenge model as measured by one or more protection measures and (milestone 2) one or more immune responses in vaccinated animals correlate with one or more protection measure.

Project narrative The prototypic BG505 SOSIP.664 protein is a potent HIV-1 immunogen currently in two phase I clinical trials. RNA vaccines induce immune pathways distinct from protein-based vaccines, and if successfully developed, our proposed replicating RNA (repRNA) vaccine can complement the clinical vaccine and potentially boost waning immune responses. We will develop a repRNA vaccine expressing BG505 SOSIP.664 and evaluate protective immunity in a non-human primate challenge model.