The magnetic nanoparticles displaying the class I major histocompatibility complex (MHC I) were biologically synthesized using the magnetotactic bacterium Magnetospirillum magneticum AMB-1. Expression level and antigen peptide (HER263-71)-binding capability of the MHC I were evaluated on bacterial magnetic particles (BacMPs, also known as magnetosomes). Furthermore, the singlechain complexes of MHC I and HER263-71 were de novo designed for the magnetosome display system in order to improve the interaction between MHC I and HER263-71. Two types of the fusion arrangements were tested, and one of the complexes was estimated to fold into the correct conformation at the level of over 70%. In addition to the high folding ratio, an advantage of this system is that any refolding processes were not required even though the N-terminus of HER263-71 peptide is not free, which conventional bacterial expression systems have never demonstrated. The as-prepared single-chain MHC I/HER263-71 complex-displaying BacMPs (MHC I/HER2-BacMPs) specifically interacted with, and magnetically separated the HER263-71-induced cells, suggesting that the native T-cell receptor could recognize the engineered MHC I/HER2 complex on the BacMPs. By optimizing the magnetic sorting method, the MHC I/HER2-BacMPs developed in this study would be useful in immunotherapeutic applications.
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