ABSTRACT OVERALL The main goal of our IPCAVD Program grant is to evaluate in humans self-amplifying mRNA (saRNA) vaccines that express two HIV-1 Env-derived protein immunogens that activate and initiate the maturation of VRC01-class B cell receptors (BCRs). The first immunogen, 426c.Mod.Core, was specifically designed to bind with high affinity to the unmutated (germline, gl) forms of those BCRs as they are expressed on the surface of naïve B cells. The second immunogen, HxB2.WT.Core, although unable to bind germline VRC01-class BCRs, binds the VRC01-class BCRs that became activated by 426c.Mod.Core and have accumulated some somatic mutations. As a result, the boost immunization with HxB2.WT.Core furthers the maturation of the VRC01-class antibodies elicited by the 426c.Mod.Core. These observations were made with the adjuvanted recombinant (rec) forms of these two immunogens. As mRNA- based vaccines are less costly and more easily GMP-manufactured that rec proteins, we believe that they will accelerate the preclinical and clinical evaluation of HIV-1 Env-derived immunogens. Here, we propose to first compare preclinically the VRC01 B cell and antibody responses elicited by these two Env immunogens when delivered by saRNA vaccines to those elicited by the corresponding adjuvanted rec proteins. And then, if the results are promising, the saRNA vaccines expressing the two immunogens will be GMP-manufactured for clinical evaluation. As the 426c.Mod.Core adjuvanted rec protein will be evaluated clinically (phase I) in the spring of 2022 (HVTN301) and the HxB2.WT.Core rec protein is currently being GMP manufactured for a follow-up phase I clinical evaluation in 2023, we will be in a unique position to compare the VRC01 B cell and antibody responses elicited by humans immunized with these two HIV-1 Env-derived immunogens when delivered as adjuvanted rec proteins and as expressed by saRNA vaccines. To accomplish our goals in this IPCAVD grant we will take advantage of our expertise in immunogen-design and testing, expertise in the analysis of B cell and antibody responses elicited by vaccination and during infection, our ability to rapidly sequence BCR genes using high through put technologies, the availability of appropriate animal models, our expertise in saRNA vaccine technology, our unique expertise in conducting clinical testing of HIV-1 vaccines, the existing collaboration among the participating groups and the documented expertise of the participants to successfully manage complex Programs.