Structure of a TCR with high affinity for self-antigen reveals basis for escape from negative selection

Abstract

The failure to eliminate self-reactive T cells during negative selection is a prerequisite for autoimmunity. To escape deletion, autoreactive T-cell receptors (TCRs) may form unstable complexes with self-peptide-MHC by adopting suboptimal binding topologies compared with anti-microbial TCRs. Alternatively, escape can occur by weak binding between self-peptides and MHC. We determined the structure of a human autoimmune TCR (MS2-3C8) bound to a self-peptide from myelin basic protein (MBP) and the multiple sclerosis-associated MHC molecule HLA-DR4. MBP is loosely accommodated in the HLA-DR4-binding groove, accounting for its low affinity. Conversely, MS2-3C8 binds MBP-DR4 as tightly as the most avid anti-microbial TCRs. MS2-3C8 engages self-antigen via a docking mode that resembles the optimal topology of anti-foreign TCRs, but is distinct from that of other autoreactive TCRs. Combined with a unique CDR3β conformation, this docking mode compensates for the weak binding of MBP to HLA-DR4 by maximizing interactions between MS2-3C8 and MBP. Thus, the MS2-3C8-MBP-DR4 complex reveals the basis for an alternative strategy whereby autoreactive T cells escape negative selection, yet retain the ability to initiate autoimmunity.

Identification of the Docking Site for CD3 on the T Cell Receptor β Chain by Solution NMR

“…Interestingly, this is the region where we engineered a C␣Cys 159 -C␤Cys 171 interchain disulfide (36) to increase yields of paired TCR ␣␤ heterodimers for the NMR sample. The wild-type protein used for x-ray analysis did not contain this disulfide (30). Generally, however, the secondary structure elements derived from chemical shifts are very similar to those observed in the x-ray structure, indicating comparable fold topologies for MS2-3C8 in solution and crystal forms.…”

“…Although virtually all regions of the MS2-3C8 ␤ chain that were ordered in the NMR structure (high S 2 values) were also ordered in the x-ray structure (low B factors) (30), intriguing exceptions are the ␣A and ␣B helical regions that constitute the CD3 docking site. These ␣-helices are ordered in solution but display high B factors in the crystal.…”

“…2A). These results were compared with the x-ray structure of MS2-3C8 in complex with MBP-HLA-DR4 (Protein Data Bank accession code 3O6F) (30). The only region that appears to be slightly different is strand D in the C␤ domain, which is a short 3-residue ␤-strand in the x-ray structure but has a coil conformation in solution.…”

“…The pathogenic potential of MS2-3C8 was demonstrated using mice transgenic for this TCR and HLA-DR4 (31). An x-ray crystal structure of TCR MS2-3C8 bound to its pMHC ligand, MBP-HLA-DR4, has been determined (30), as well as that of the ternary complex formed by MS2-3C8, MBP-HLA-DR4, and CD4 (32). The interactions of MS2-3C8 with several of the key players in T cell signaling are therefore well characterized structurally, making this TCR a good candidate for studying critical associations with CD3⑀␥ and CD3⑀␦.…”

“…2 H, 13 C, 15 N]-For detailed NMR studies, we used a human autoimmune TCR, MS2-3C8, which recognizes a self-peptide from myelin basic protein (MBP) and the multiple sclerosis-associated MHC class II molecule HLA-DR4 (30). The pathogenic potential of MS2-3C8 was demonstrated using mice transgenic for this TCR and HLA-DR4 (31).…”