Heterologous immunization modulates B-cell epitope competition between helper peptides and the MPER segment in MPER/liposome vaccines.

Subdominant B-cell immune responses to conserved epitopes are major obstacles in eliciting broadly neutralizing antibodies (bnAbs) against HIV-1 through natural infection or vaccination. Although the sequence conserved membrane proximal external region (MPER) of HIV-1 gp41 is partially occluded on the virion surface, epitope-focused immunogens could mitigate access limitations. Here, we found that a MPER/liposome vaccine delivered with a single CD4 T cell helper epitope results in a post-priming response hierarchy, eliciting low-affinity MPER-specific B cells. Heterologous boosting, however, promotes MPER-specific B cell clonal expansion and enhances plasma antibody functionality. This improvement is associated with increased B-cell affinity for MPER and reduced competition from B cells targeting the helper epitope. While helper peptide co-delivery increases the affinity of serum antibodies, the outcome of subsequent MPER antibody responses is shaped by the priming antigen. Our results offer insights into heterologous immunization strategies to potentiate subdominant B cell responses against frequently mutating viruses.