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 domain (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 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 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. Significance Statement: A key challenge in vaccination against mutable viruses like HIV-1 is the immune system's focus on highly variable regions. The conserved MPER segment of gp41 elicits weak antibody responses due to poor B cell accessibility. This study evaluated a liposome-based MPER vaccine strategy. Initial priming generated low-affinity MPER-specific memory B cells, influenced by strong B cell affinity for a dominant T cell helper peptide. However, heterologous boosting overcomes subdominant MPER responses by increasing B cell affinity for booster immunogens while also reducing competition from other helper peptides. Co-delivery of helper peptides enhanced antibody affinity, though the priming antigen was critical in shaping responses. These findings suggest heterologous immunization is a promising strategy to enhance subdominant B cell responses.