The Germline Targeting Vaccine Concept: Overview and Updates from HIV Pre-Clinical and Clinical Trials

An effective HIV-1 vaccine should elicit diverse immune responses, including broadly neutralizing antibodies (bNAbs). Such antibodies recognize regions of the viral envelope glyco-protein (Env) that are conserved among the diverse HIV-1 clades and strains. They are isolated from people living with HIV-1 to protect animals from experimental viral exposure and reduce HIV-1 acquisition in clinical settings. However, despite efforts spanning several decades, bNAbs have not been elicited through immunization. The HIV Env efficiently binds bNAbs, but not their unmutated (germline, gl) precursors. In contrast, Env readily engages the germline precursors of antibodies with no, or very narrow, cross-neutralizing activities (non-neutralizing antibodies, nnAbs). That, in part, explains why Env-based immunogens consistently elicit nnAbs, but not bNAbs. In the past decade, Env-derived proteins have been specifically designed to engage the germline precursors of diverse bNAbs. These 'germline-targeting' Env immunogens activate the corresponding naive B cells in vivo, but are unable to guide their proper maturation towards their broadly neutralizing forms. For this, immunizations with currently not well-defined heterologous Envs are required. Here, we discuss the development of germline-targeting Env immunogens, their in vivo evaluation, and the strategies currently under evaluation that aim to rapidly guide the mat-uration of germline-precursor BCRs into their broadly neutralizing forms.