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Updated: Sep 27, 2026
Use of Single Chain MHC Technology to Investigate Co-agonism in Human CD8+ T Cell Activation
Published on: February 28, 2019
Requirement for Q226, but not multiple charged residues, in the class I MHC CD loop/D strand for TCR-activated CD8
Micheal Durairaj1, Ranjana Sharma, Jay C Varghese
1Department of Microbiology & Immunology, Faculty of Medicine, University of Alberta, Edmonton, Alberta T6G 2S2, Canada.
Insights
CD8 binding to MHC class I molecules has two roles: coreceptor and accessory. Researchers found that specific residues in the alpha3 domain are crucial for CD8 accessory functions after T cell receptor activation.
Area of Science:
- Immunology
- Molecular Biology
- Cell Biology
Background:
- CD8+ cytotoxic T lymphocytes (CTLs) are crucial for adaptive immunity.
- T cell activation relies on T cell receptor (TCR) and CD8 coreceptor interactions with MHC class I-peptide complexes.
- CD8 also functions as an accessory molecule, mediating adhesion and cosignaling.
Purpose of the Study:
- To investigate the distinct roles of CD8 in coreceptor versus accessory functions.
- To identify specific residues in MHC class I molecules critical for CD8 accessory interactions following TCR engagement.
Main Methods:
- Expression and isolation of mutant MHC class I molecules with altered residues in alpha2 and alpha3 domains.
- Solid-phase CTL adhesion assays.
- Degranulation assays.
Main Results:
- Multiple acidic residues in the MHC class I alpha3 domain, while important for CD8 coreceptor function, are dispensable for TCR-activated CD8 accessory interactions.
- The polar residue Q226, located at the end of the alpha3 domain CD loop, is essential for TCR-activated CD8 accessory functions.
- These findings highlight functional differences between CD8 coreceptor and accessory roles.
Conclusions:
- CD8 coreceptor and accessory functions are mechanistically distinct.
- TCR activation induces conformational changes in MHC class I molecules, altering structural requirements for CD8 accessory binding.
Abstract:
Activation of CD8+ cytotoxic T lymphocytes typically begins with recognition of class I MHC-peptide complexes by the TCR and CD8 as a coreceptor. In its coreceptor role, CD8 binds thesame class I-peptide antigen complex as the TCR, enhancing the strength of TCR-class I interaction. Subsequent to initial TCR engagement, CD8 acts as an accessory molecule by binding any properly conformed class I molecules on the target cell surface, leading to CD8-mediated adhesion and cosignaling functions. We expressed and isolated a number of mutant class I molecules in which one or moreacidic or polar residues in the class I alpha3 domain CD loop and D strand region, or alpha2 domain were altered. Using solid phase CTL adhesion and degranulation assays with isolated class I molecules, we demonstrate that multiple acidic residues in the alpha3 domain, although involved in CD8 coreceptor interaction, are not required for TCR-activated CD8 accessory interactions. Instead, we show that Q226, a polar group on the end of the CD loop, is required for TCR-activated CD8 accessory functions. These results indicate that CD8 coreceptor and accessory interactions differ substantially and suggest that TCR activation results in changes that alter the structural constraints for CD8 accessory interactions.
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