Dealing with the unnatural – the MHC way
By Riley Fricke
Adaptive immunity allows an organism to target invading pathogens with high specificity. Antigens are particles from a pathogen that are recognized by the host as foreign. For the adaptive immune system to achieve high specificity, the antigen must be correctly recognized at multiple steps of the process. Frequently, antigens are small peptides that are cleaved from the pathogen’s proteins. Because these small peptide antigens are not native to the host, they trigger an immune response. But can this process by modulated if the structure of the antigen is modified?
To answer this question, Cheloha et al.  investigated a 17-mer model antigen from ovalbumin (OVA323-339). Using β-amino acids, it was possible to systematically insert one extra methylene group into the peptide backbone at multiple different loci. This set of modified peptides allowed the researchers investigate the impact of modifications of the peptide backbone on the adaptive immune system in mice.
It had been shown previously that some peptides containing β-amino acids can be more proteolytically stable than those lacking β-amino acids and yet can still participate in molecular recognition events crucial to the adaptive immune system. , Proteolytic stability and adaptive immunity are key considerations in the development of any therapeutic protein, so understanding the role of β-amino acid insertions in these two complex processes is important. However, systematic inquiries of this scale have not been accomplished until now.
In total, 22 peptides containing single α- to β-amino acid substitution were made. These 22 peptide analogs were used to stimulate immune responses from mouse T-cells in vitro and in vivo. The results were both expected and unexpected. In general, β-amino acid substitutions were well tolerated outside the region of the peptide that is recognized by the immune system. Surprisingly, there were two positions in the recognition region at which α- to β-amino acid substitutions still allowed for binding. In some cases, the analog would be recognized at one step in the immune response, but not generate a T-cell response. Additionally, one β-amino acid-containing peptide analog showed greater activity than the native antigen, stimulating higher levels of cytokine release. Fascinatingly, this higher potency analog contained the β-amino acid outside of the binding region.
While this was the most systematic study of α- to β-amino acid substitutions in antigens to date, it is still quite simple. As technology improves for β-amino acid incorporation into protein, larger and more complex studies of this nature will be possible.
- Cheloha, R. W.; Woodham, A. W.; Bousbaine, D.; Wang, T.; Liu, S.; Sidney, J.; Sette, A.; Gellman, S. H.; Ploegh, H. L. “Recognition of Class II MHC Peptide Ligands That Contain β-Amino Acids” The Journal of Immunology 2019. doi:10.4049/jimmunol.1900536
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- McDonald, C. A., N. L. Payne, G. Sun, D. J. Clayton, M. P. Del Borgo, M. I. Aguilar, P. Perlmutter, and C. C. A. Bernard. 2014. Single b3-amino acid substitutions to MOG peptides suppress the development of experimental autoimmune encephalomyelitis. J. Neuroimmunol. 277: 67–76. doi:10.1016/j.jneuroim.2014.09.022