Gammadelta T cell receptor-bearing dendritic epidermal T cells (DETCs) found in murine skin recognize antigen expressed by damaged or stressed keratinocytes. Activated DETCs produce keratinocyte growth factors (KGFs) and chemokines, raising the possibility that DETCs play a role in tissue repair. We performed wound healing studies and found defects in keratinocyte proliferation and tissue reepithelialization in the absence of wild-type DETCs. In vitro skin organ culture studies demonstrated that adding DETCs or recombinant KGF restored normal wound healing in gammadelta DETC-deficient skin. We propose that DETCs recognize antigen expressed by injured keratinocytes and produce factors that directly affect wound repair.

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...

How T cells translate T cell receptor (TCR) recognition of almost identical pMHC ligands into distinct biological responses has remained enigmatic. Although differences in affinity or off rate are important, they offer at best an incomplete explanation. By using Förster resonance energy transfer (FRET), we have visualized the ligand-induced interaction between OT-I TCR and CD8. We found that both recruitment of TCR to the immunological synapse and the TCR-CD8 interaction induced by weak agonists (positive-selecting ligands) was delayed but not necessarily weaker than strong agonists (negative selectors). A delayed and perhaps longer lasting CD8-TCR interaction results in delayed phospho-ERK recruitment to the synapse. The kinetics of the TCR-CD8 interaction can reconcile previously anomalous data, where biological activity did not correlate with TCR-pMHC binding kinetics for certain ligands. Our findings indicate that the T cell translates antigen recognition into T cell responses by differential recruitment of CD8 to the TCR.

T cells are extremely sensitive in their ability to find minute amounts of antigenic peptide in the midst of many endogenous peptides presented on an antigen-presenting cell. The role of endogenous peptides in the recognition of foreign peptide and hence in T cell activation has remained controversial for CD8+ T cell activation. We showed previously that in a CD8+ T cell hybridoma, nonstimulatory endogenous peptides enhance T cell sensitivity to antigen by increasing the coreceptor function of CD8. However, others were not able to detect such enhancement in naive and activated CD8+ T cells. Here, we show that endogenous peptides substantially enhance the ability of T cells to detect antigen, an effect measurable by up-regulation of activation or maturation markers and by increased effector function. This enhancement is most pronounced in thymocytes, moderate in naive T cells, and mild in effector T cells. The importance of endogenous peptides is inversely proportional to the agonist activity of the stimulatory peptide presented. Unlike for CD4+ T cells, the T cell receptor of CD8+ T cells does not distinguish between endogenous peptides for their ability to enhance antigen recognition.

Protein kinase Cη (PKCη) is highly abundant in T cells and is recruited to the immunological synapse that is formed between a T cell and a cognate antigen-presenting cell; however, its function in T cells is unknown. Here, we showed that PKCη was required for the activation of mature CD8+ T cells by stimulation through the T cell receptor. PKCη−/− T cells showed poor proliferation in response to stimulation by antigen as compared to wild-type T cells, a trait shared with T cells deficient in PKCθ, the most abundant PKC isoform in T cells, and the only PKC previously thought to have a specific role in T cell activation. In contrast, defective homeostatic proliferation, a function requiring recognition of self antigens, was only observed in PKCη- deficient T cells. PKCη was dispensable for the development of thymocytes; however, thymocytes from mice doubly deficient in PKCη and PKCθ exhibited poor positive selection, indicating some redundancy between the PKC isoforms. PKCη and PKCθ had opposing effects on relative numbers of CD4+ and CD8+ T cells, because PKCη−/− mice had a higher ratio of CD4+ to CD8+ T cells compared to that of wild-type mice, whereas PKCθ−/− mice had a lower ratio. In mice deficient in both PKC isoforms, the ratio of CD4+ to CD8+ T cells returned to normal. Together, these data suggest that whereas PKCη shares redundant roles with PKCθ in T cell biology, it also performs nonredundant functions that are important for homeostasis and activation of T cells.

The CD8 coreceptor contributes to the recognition of peptide-MHC (pMHC) ligands by stabilizing the TCR-pMHC interaction and enabling efficient signaling initiation. It is unclear though, which structural elements of the TCR ensure a productive association of the coreceptor. The α-chain connecting peptide motif (α-CPM) is a highly conserved sequence of eight amino acids in the membrane proximal region of the TCR α-chain. TCRs lacking the α-CPM respond poorly to low-affinity pMHC ligands and are unable to induce positive thymic selection. In this study we show that CD8 participation in ligand binding is compromised in T lineage cells expressing mutant α-CPM TCRs, leading to a slight reduction in apparent affinity; however, this by itself does not explain the thymic selection defect. By fluorescence resonance energy transfer microscopy, we found that TCR-CD8 association was compromised for TCRs lacking the α-CPM. Although high-affinity (negative-selecting) pMHC ligands showed reduced TCR-CD8 interaction, low-affinity (positive-selecting) ligands completely failed to induce molecular approximation of the TCR and its coreceptor. Therefore, the α-CPM of a TCR is an important element in mediating CD8 approximation and signal initiation.

The co-receptors CD4 and CD8 are important in the activation of T cells, primarily because of their ability to interact with the proteins of the MHC, enhancing recognition of the MHC–peptide complex by the T cell receptor (TCR). An antigen-presenting cell presents a small number of antigenic peptides on its MHC molecules, in the presence of a much larger number of endogenous, mostly nonstimulatory, peptides. Recent work has demonstrated that these endogenous MHC–peptide complexes have an important role in modulating the sensitivity of the TCR. But the role of the endogenous nonstimulatory MHC–peptide complexes differs in MHC class I and class II-restricted T cells. This chapter discusses the data on the role of CD4 or CD8 co-receptors in T cell activation at the immunological synapse, and the role of non stimulatory MHC–peptide complexes in aiding antigen recognition.

Bimolecular fluorescence complementation was used to engineer CD8 molecules so that CD8aa and CD8ab dimers can be independently visualized on the surface of a T cell during antigen recognition. Using this approach, we show that CD8aa is recruited to the immunological synapse almost as well as CD8ab, but because the kinase Lck associates preferentially with CD8ab in lipid rafts, CD8aa is the weaker co-receptor. During recognition of the strong CD8aa ligand H2-TL, CD8aa is preferentially recruited. Thus, recruitment of the two CD8 species correlates with their relative binding to the available ligands, rather than with the co-receptor functions of the CD8 species.