This Phase 1 study will evaluate the safety, tolerability, and immune responses of investigational CH505 HIV vaccine regimens in adults in overall good health without HIV. Participants will receive CH505 protein nanoparticle vaccines (DV901-NP) formulated with the investigational adjuvant ACU-026-001-1, followed by either CH505 protein nanoparticle (DV902-NP) or CH505 mRNA vaccine boosters. The study will assess the ability of these regimens to induce HIV-specific immune responses, including B-cell responses associated with the development of broadly neutralizing antibodies. An immunology cohort will also evaluate how the location of booster vaccination affects immune responses.