Structural basis for the stability of the swine major histocompatibility complex class I with heterologous β2-microglobulin.

Establishing mouse models expressing heterologous major histocompatibility complex class I (MHC-I) molecules is an effective strategy for immunological research. However, murine β2-microglobulin (mβ2m) may interfere with the proper folding and peptide loading of the heterologous heavy chain. We found that mβ2m can associate with SLA-1*1502 and the Porcine reproductive and respiratory syndrome virus (PRRSV)-derived epitope peptide (PPs) to form a complex, albeit with lower efficiency compared with swine β2m (sβ2m). We determined the crystal structure of the chimeric complex SLA-1*1502/mβ2m/PP7. Comparison with the previously reported SLA-1*1502/sβ2m/PP7 structure revealed that, although the overall conformations are similar, the interaction pattern between the heavy and light chains differs, exhibiting features characteristic of both murine and porcine origins. Interestingly, the peptide binding mode in the chimeric complex also exhibited distinct local features: the buried surface area between PP7 and the binding groove increased, and a new salt bridge was formed between PP7 and residue R114 of SLA-1*1502. Structural comparison of diverse cross-species chimeric complexes reveals that conformational shifts of bound epitope peptides are governed by the stability of the core hydrogen-bond network at the heavy-light chain interface, and substitution with human β2m triggers milder local structural distortions than murine β2m. In summary, our findings demonstrate that the differences between mβ2m and sβ2m influence both complex formation and peptide binding mode. Therefore, constructing mouse models co-expressing SLA-I and sβ2m would be a more optimized strategy for related immunological studies.