HLA-DM co-expression enhances MHC class II function in the magnetosome display system.

Efficient identification of antigenic peptides that bind to major histocompatibility complex class II (MHC II) molecules is crucial for vaccine and therapeutic development. However, mammalian expression systems are constrained by high costs and low scalability, particularly for stable peptide exchange. To address these challenges, we developed a bacterial magnetosome surface display system using Magnetospirillum magneticum, a magnetotactic bacterium that produces lipid bilayer-coated magnetosomes. This system enables the co-expression of MHC II and human leukocyte antigen DM (HLA-DM) on magnetosome surfaces via cohesin-dockerin interactions. This spatial co-localization significantly enhances MHC II stabilization and immobilization on magnetosomes. Moreover, co-expression with HLA-DM improves functional MHC II peptide loading, as demonstrated by increased binding of the influenza-derived HA₃₀₆-₃₁₈ epitope. Structural analysis suggests that HLA-DM stabilizes MHC II by facilitating peptide loading and reducing degradation. Overall, this system provides a robust alternative to mammalian expression systems and represents a promising tool for high-throughput antigen discovery and rational vaccine design.IMPORTANCEHigh-throughput screening of MHC II-peptide interactions remains a major bottleneck in antigen discovery. Conventional mammalian systems are costly and time-consuming, which limits scalability. This study introduces a bacterial system that enables stable co-localization of MHC II and human leukocyte antigen DM (HLA-DM) on genetically engineered magnetosomes. The system supports functional pMHC II complex formation and epitope screening by promoting peptide exchange and conformational stability. It integrates expression, immobilization, and peptide loading in a single step, and it is compatible with diverse immunological targets, offering broad applications in vaccine development and immunotherapy.