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CD8αα and CD8αβ: truly different?
News & Comment
TRENDS in Immunology Vol.23 No.4 April 2002
177
Journal Club
αα and CD8α αβ: truly different? CD8α CD8 is a coreceptor of T cells for the recognition of peptide–MHC (pMHC) on antigen-presenting cells. It exists in two forms: an αα homodimer and an αβ heterodimer. The different functions of these two forms have long been a mystery, although it is believed that the heterodimer has a stronger affinity than the homodimer for pMHC. The structural basis of the interaction between pMHC and the αα homodimer in humans and mice was determined by X-ray crystallography a few years ago, but no structural explanation for the functional differences between homo- and heterodimers could be deduced. Two recent papers provide some clues on this matter. In their investigation of the functional consequences of carbohydrate changes associated with T-cell differentiation in the thymus, Moody et al. [1] show that it is the β-chain of CD8 that modulates the binding of the Ig-like domain of the αβ heterodimer head to pMHC through the (O-linked) glycans on its stalk region, in such a way as
to reorient the heterodimer towards pMHC. The O-glycosylation is highly programmed during T-cell development and controlled by ST3Gal-l sialyltransferase in the CD8 stalk region, which is highly glycosylated in both the homo- and heterodimeric forms of CD8. In contrast to the unique function of the β-chain in modulating the pMHC-binding avidity of the αβ heterodimer, Leishman et al. [2] show that the αα homodimer (not αβ heterodimer) in intestinal intraepithelial T lymphocytes (iIELs) mediates the activation of iIELs upon binding to TL antigen, an MHC-related molecule expressed on the surface of intestinal epithelial cells. This makes CD8αα a modulator of T-cell activation, in addition to its known function as a coreceptor. As well as cell-based experiments, the authors used BIAcoreTM analysis to confirm this interaction. TL antigen binds to αα homodimer preferentially with a much higher affinity than it binds to αβ heterodimer. The affinity between TL and CD8αα is in the range of the
T-cell receptor–pMHC interaction, much greater than the affinity of the interaction between CD8αα and pMHC, whereas the binding of TL to CDαβ is almost undetectable. These two papers provide compelling evidence that the CD8αα homodimer and CD8αβ heterodimer are truly different in their functions, at least in mice. It is noteworthy that a TL homologue has not been found in humans, and so the question remains as to whether these findings apply to human CD8. 1 Moody, A.M. et al. (2001) Developmentally regulated glycosylation of the CD8αβ coreceptor stalk modulates ligand binding. Cell 107, 501–512 2 Leishman, A.J. et al. (2001) T-cell responses modulated through interaction between CD8αα and the nonclassical MHC class I molecule, TL. Science 294, 1936–1939
George F. Gao [email protected]
Immunological synapse generates microenvironment for costimulation The two-signal hypothesis of T-cell activation predicts that signal one is transduced by the T-cell receptor (TCR) and enhanced by adhesion molecules, whereas signal two is generated by costimulatory molecules on the surface of the antigenpresenting cell (APC). The interaction of the costimulatory receptor CD28 with its ligands, CD80 and CD86, is topologically similar to that of many other adhesion molecules, such as CD2 and CD48/CD58, suggesting that CD28 plays a dual role as an adhesion and signalling molecule. The immunological synapse is characterized by a peripheral ring of adhesive LFA-1–ICAM-1 interactions surrounding a central cluster of TCR–peptide–MHC (pMHC) interactions, referred to as a central supramolecular activation cluster (cSMAC). The interaction of CD28 with CD80 is unusual as CD28 is colocalized with the engaged TCR. Engagement of CD28 promotes the cytoskeletal-dependent recruitment of cellsurface protein and lipid rafts rich in kinases and adaptor proteins that contribute to the formation of the immunological synapse. http://immunology.trends.com
The distance spanned by the interaction of CD28 on T cells with its ligands CD80 or CD86 on APCs (~15 nm) is similar to that spanned by TCR–pMHC interactions, so CD28 could generate the appropriate spacing for the TCR to interact efficiently with pMHC. However, a recent study indicates that CD28–CD80 interactions do not support adhesion and have little capacity for enhancing TCR–pMHC interactions, prompting a new view of the two-signal hypothesis. Bromley et al. [1] studied the role of CD28–CD80 interactions in the formation of immunological synapses using planar lipid bilayers expressing fluorescently labelled ligands. In lipid bilayers containing pMHC and ICAM-1, CD80 was biologically active as it supported T-cell proliferation. When CD28 was highly expressed (e.g. 22 000 molecules per cell), as on the leukemic T-cell line Jurkat, it mediated adhesion with high affinity but low maximum binding. Only one-third of the available CD28 interacted with CD80 within the bilayer. By contrast, when CD58 is present in the bilayer and presented to CD2-expressing Jurkat cells, over two-thirds
of the available CD2 interacts with CD58. Elegant photobleaching experiments revealed a low lateral mobility of CD28, which was dependent on the presence of the CD28 cytoplasmic domain. Physiological levels of CD28 expression are much lower than on leukemic cell lines. Hence, under conditions of low CD28 expression in naive T cells (e.g. 1500 molecules per cell), the low lateral mobility of CD28 can prevent cell adhesion to the bilayer substrate. However, CD28–CD80 interactions were detected in naive T-cell contacts with substrates formed by CD2 and CD48, and were focused within the central cluster of the synapse, albeit with delayed kinetics compared with the engagement of ICAM-1 receptors in the peripheral ring. Importantly, CD28 engagement did not enhance synapse formation or the interaction of the TCR with pMHC. From their studies, the authors propose that there are three key steps leading to full-blown T-cell activation: (1) adhesionfacilitated TCR triggering; (2) synapse formation; and (3) synapse-facilitated costimulatory signalling. Thus, formation of
1471-4906/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved.
TRENDS in Immunology Vol.23 No.4 April 2002
177
Journal Club
αα and CD8α αβ: truly different? CD8α CD8 is a coreceptor of T cells for the recognition of peptide–MHC (pMHC) on antigen-presenting cells. It exists in two forms: an αα homodimer and an αβ heterodimer. The different functions of these two forms have long been a mystery, although it is believed that the heterodimer has a stronger affinity than the homodimer for pMHC. The structural basis of the interaction between pMHC and the αα homodimer in humans and mice was determined by X-ray crystallography a few years ago, but no structural explanation for the functional differences between homo- and heterodimers could be deduced. Two recent papers provide some clues on this matter. In their investigation of the functional consequences of carbohydrate changes associated with T-cell differentiation in the thymus, Moody et al. [1] show that it is the β-chain of CD8 that modulates the binding of the Ig-like domain of the αβ heterodimer head to pMHC through the (O-linked) glycans on its stalk region, in such a way as
to reorient the heterodimer towards pMHC. The O-glycosylation is highly programmed during T-cell development and controlled by ST3Gal-l sialyltransferase in the CD8 stalk region, which is highly glycosylated in both the homo- and heterodimeric forms of CD8. In contrast to the unique function of the β-chain in modulating the pMHC-binding avidity of the αβ heterodimer, Leishman et al. [2] show that the αα homodimer (not αβ heterodimer) in intestinal intraepithelial T lymphocytes (iIELs) mediates the activation of iIELs upon binding to TL antigen, an MHC-related molecule expressed on the surface of intestinal epithelial cells. This makes CD8αα a modulator of T-cell activation, in addition to its known function as a coreceptor. As well as cell-based experiments, the authors used BIAcoreTM analysis to confirm this interaction. TL antigen binds to αα homodimer preferentially with a much higher affinity than it binds to αβ heterodimer. The affinity between TL and CD8αα is in the range of the
T-cell receptor–pMHC interaction, much greater than the affinity of the interaction between CD8αα and pMHC, whereas the binding of TL to CDαβ is almost undetectable. These two papers provide compelling evidence that the CD8αα homodimer and CD8αβ heterodimer are truly different in their functions, at least in mice. It is noteworthy that a TL homologue has not been found in humans, and so the question remains as to whether these findings apply to human CD8. 1 Moody, A.M. et al. (2001) Developmentally regulated glycosylation of the CD8αβ coreceptor stalk modulates ligand binding. Cell 107, 501–512 2 Leishman, A.J. et al. (2001) T-cell responses modulated through interaction between CD8αα and the nonclassical MHC class I molecule, TL. Science 294, 1936–1939
George F. Gao [email protected]
Immunological synapse generates microenvironment for costimulation The two-signal hypothesis of T-cell activation predicts that signal one is transduced by the T-cell receptor (TCR) and enhanced by adhesion molecules, whereas signal two is generated by costimulatory molecules on the surface of the antigenpresenting cell (APC). The interaction of the costimulatory receptor CD28 with its ligands, CD80 and CD86, is topologically similar to that of many other adhesion molecules, such as CD2 and CD48/CD58, suggesting that CD28 plays a dual role as an adhesion and signalling molecule. The immunological synapse is characterized by a peripheral ring of adhesive LFA-1–ICAM-1 interactions surrounding a central cluster of TCR–peptide–MHC (pMHC) interactions, referred to as a central supramolecular activation cluster (cSMAC). The interaction of CD28 with CD80 is unusual as CD28 is colocalized with the engaged TCR. Engagement of CD28 promotes the cytoskeletal-dependent recruitment of cellsurface protein and lipid rafts rich in kinases and adaptor proteins that contribute to the formation of the immunological synapse. http://immunology.trends.com
The distance spanned by the interaction of CD28 on T cells with its ligands CD80 or CD86 on APCs (~15 nm) is similar to that spanned by TCR–pMHC interactions, so CD28 could generate the appropriate spacing for the TCR to interact efficiently with pMHC. However, a recent study indicates that CD28–CD80 interactions do not support adhesion and have little capacity for enhancing TCR–pMHC interactions, prompting a new view of the two-signal hypothesis. Bromley et al. [1] studied the role of CD28–CD80 interactions in the formation of immunological synapses using planar lipid bilayers expressing fluorescently labelled ligands. In lipid bilayers containing pMHC and ICAM-1, CD80 was biologically active as it supported T-cell proliferation. When CD28 was highly expressed (e.g. 22 000 molecules per cell), as on the leukemic T-cell line Jurkat, it mediated adhesion with high affinity but low maximum binding. Only one-third of the available CD28 interacted with CD80 within the bilayer. By contrast, when CD58 is present in the bilayer and presented to CD2-expressing Jurkat cells, over two-thirds
of the available CD2 interacts with CD58. Elegant photobleaching experiments revealed a low lateral mobility of CD28, which was dependent on the presence of the CD28 cytoplasmic domain. Physiological levels of CD28 expression are much lower than on leukemic cell lines. Hence, under conditions of low CD28 expression in naive T cells (e.g. 1500 molecules per cell), the low lateral mobility of CD28 can prevent cell adhesion to the bilayer substrate. However, CD28–CD80 interactions were detected in naive T-cell contacts with substrates formed by CD2 and CD48, and were focused within the central cluster of the synapse, albeit with delayed kinetics compared with the engagement of ICAM-1 receptors in the peripheral ring. Importantly, CD28 engagement did not enhance synapse formation or the interaction of the TCR with pMHC. From their studies, the authors propose that there are three key steps leading to full-blown T-cell activation: (1) adhesionfacilitated TCR triggering; (2) synapse formation; and (3) synapse-facilitated costimulatory signalling. Thus, formation of
1471-4906/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved.