Nonstimulatory peptides contribute to antigen-induced CD8–T cell receptor interaction at the immunological synapse

Abstract

It is unclear if the interaction between CD8 and the TCR-CD3 complex is constitutive or antigeninduced. FRET microscopy between fluorescent chimeras of CD3ζ and CD8β showed that this interaction was induced by antigen recognition, within the immunological synapse. Non-stimulatory endogenous or exogenous peptides, presented simultaneously with antigenic peptides, increased the CD8-TCR interaction. This indicates that the interaction between the intracellular regions of a TCR-CD3 complex recognizing its cognate peptide-major histocompatibility complex (pMHC) antigen, and CD8 (plus Lck), is enhanced by a non-cognate CD8-MHC interaction. Thus the CD8 interaction with non-stimulatory pMHC helps mediate T cell recognition of antigen, improving the coreceptor function of CD8.The αβ T cell receptor (TCR) is responsible for the affinity and specificity of antigen recognition 1,2 , whereas the coreceptors, CD8 or CD4, enhance the sensitivity of TCR recognition 3 . Disruption of coreceptor-pMHC interactions inhibit or change the quality of the T cell response 3-5 . Coreceptors act in two main ways. Firstly, they bind to non-polymorphic regions of MHC 4 . This can aid in adhesion, but the main role is generally believed to be increasing the sensitivity of T cell activation via the entropic facilitation of TCR-pMHC binding, rather than through energetic stabilization of the tri-molecular complex 6-9 ). Secondly, CD4 and CD8 are associated with the kinase Lck. Coreceptor binding to pMHC recruits Lck close to the TCR, enabling it to phosphorylate components of the TCR's signaling complex (CD3), thus enhancing signal transduction 3 .There are conflicting data on the interaction between the TCR-CD3 complex and coreceptors, and in particular whether any interaction between these molecules is constitutive or induced by antigen recognition. Various co-immunoprecipitation and flow cytometry fluorescence resonance energy transfer (FRET) experiments suggest a constitutive interaction between some TCR-CD3 and coreceptors in unstimulated T cells 7,10-13 . Others show interaction only after T cell activation 14-17 . Co-capping and co-modulation experiments also support an interaction of coreceptor with TCR 18-21 . CD8αβ and CD4 reside on glycolipid-enriched microdomains or rafts, whereas TCR association with rafts is greatly increased upon stimulation 22,23 . It is therefore questionable whether all of these assays measure a direct interaction or simply colocalization of the molecules on the same rafts.Whether the interaction between CD8 and TCR is constitutive or antigen-induced has important consequences for the role of CD8. Constitutive association implies that CD8 acts as a universal amplifier in antigen recognition. Inducible interaction suggests an extra level of fine-tuning, where CD8 could sharpen and amplify sensitivity and specificity of recognition. In order to address definitively whether CD8 and TCR interact, and to what extent the interaction is induced upon antigen recognition, we have used microscopy to measure FR...

Increased Mobility of Major Histocompatibility Complex I-Peptide Complexes Decreases the Sensitivity of Antigen Recognition

“…S3), this argues that the density of cytoskeletonassociated ICAM-1 modulates the mobility of noncognate complexes in the synapse. 2) CD8 in the synapse can bind to and retain noncognate MHC I-peptide complexes (47). These authors proposed that CD8 increases the local density of MHC I-peptide complexes in the synapse and vice versa and that this enhances the sensitivity of antigen recognition.…”

Human Major Histocompatibility Complex (MHC) Class I Molecules with Disulfide Traps Secure Disease-related Antigenic Peptides and Exclude Competitor Peptides

“…In another study, SCTs were used to demonstrate the importance of size-based molecular segregation for immune synapse formation (54). Along these same lines, a future application of SCTs should be to definitively test the importance of non-cognate co-receptor interactions in immune synapses (55). Finally SCTs have been expressed as transgenes and used to define MHC-I-mediated licensing during NK cell development (49).…”

Do the peptide-binding properties of diabetogenic class II molecules explain autoreactivity?

“…Moreover, the T cell activation thresholds for such weak ligands may be enhanced by the repertoire of self-peptides present on an APC. Recent reports with both CD4 + and CD8+ T cells have suggested that normal self peptides contribute towards the formation of an immunological synapse and T cell activation [44,45]. The extent and specificity of these self peptide-MHC complexes in facilitating activation of antigen-specific T cells needs to be examined in greater detail.…”