Table 2.

Obstacles to the elicitation of HIV-1 broadly neutralizing antibodies

Obstacles and Questions	Approach
Epitope dominance/subdominance Do dominant variable region epitopes compete out subdominant conserved epitopes Do specific epitopes provide weak B cell receptor (BCR) stimuli Are some neutralization epitopes only recognized by rare germline precursor BCRs What are structural and genetic characteristics of BNAbs to subdominant epitopes; e.g., antibodies to the CD4 binding site	Dampen dominant epitopes; e.g. add glycans, remove variable loops Minimal epitope constructs; e.g., structure based designs of gp120 core, transplanted epitopes on to protein scaffolds Native Env trimer to focus immune response on accessible viral spike epitopes
Tolerance: deletion/anergy Are self-reactive BCRs limiting responses to some epitopes; e.g. MPER, glycan epitopes Does requirement for poly (lipid)-reactivity for MPER BNAbs lead to clonal deletion How does a non-lipid reactive BNAb effectively target the MPER Are long CDRH3 in BNAbs encoded at germline or result from affinity maturation and insertions How do we overcome selection against long CDRH3 antibodies	Antigen design for strong BCR signals to overcome anergy; High affinity epitope specific antigens; e.g., scaffolds to present optimal MPER epitope Antigen presentation; e.g, multivalency and adjuvants Understand prevalence of long CDRH3 BCRs and how to engage these BCRs
Selected BCRs and Affinity Maturation If some germline antibodies (e.g. VRC01 class) do not bind gp120, how is the BCR stimulated What are defining structural and genetic characteristics of VH1-2 restricted antibodies How much somatic mutation is required to achieve virus neutralization How to promote affinity maturation via immunization Does virus evolution specifically drive antibody maturation toward broad neutralization What is optimal CD4 T-help to promote affinity maturation	Rational immunogen design to engage appropriate BCRs (e.g. VH1-2 gene for VRC01 class antibodies) Genetic analysis of BNAb antibody lineages to understand germline and antibody evolution, and interaction with virus evolution Structural studies of mature and germline antibody binding B-cell lineage-based vaccine design (Fig. 2). Design immunogens based on unmutated and intermediate ancestors of known BNAb lineages. Guide B-cell maturation