CD8 Expression Allows T Cell Signaling by Monomeric Peptide-MHC Complexes

The self-restricted T cell repertoire exhibits a high frequency of alloreactivity. Because these alloreactive T cells are derived from the pool of cells selected on several different self MHC alleles, it is unknown how development of the alloantigenic repertoire is influenced by homology between a self MHC allele and an alloantigen. To address this, we used the 2C transgenic TCR that is selected by Kb, is alloreactive for Ld, and cross-reacts with Lq. Lq is highly homologous to Ld and binds several of the same peptide ligands, including p2Ca, the peptide recognized by 2C. We find that Ld/p2Ca is a high avidity agonist ligand, whereas Lq/p2Ca is a low avidity agonist ligand for 2C T cells. When mice transgenic for the 2C TCR are bred to Lq-expressing mice, 2C+ T cells develop; however, they express lower levels of either the 2C TCR or CD8 and require a higher Ld/p2Ca ligand density to be activated than 2C+ T cells selected by Kb. Furthermore, the 2C T cells selected in the presence of Lq fail to detect Lq/p2Ca complexes even at high ligand density. Thus, despite possessing the identical TCR, there is a functional avidity difference between 2C+ T cells selected in the presence of Lq vs Kb. These data provide evidence that homology between the selecting ligand and an alloantigen can influence the avidity of the T cell repertoire for the alloantigen, and suggest that thymic selection can fine tune T cell avidity independent of intrinsic TCR affinity.

Section: Discussionsupporting

confidence: 75%

Homology Between an Alloantigen and a Self MHC Allele Calibrates the Avidity of the Alloreactive T Cell Repertoire Independent of TCR Affinity

Hornell

Myers

Hansen

et al. 2003

“…Doucey, D. F. Legler, N. Boucheron, J.-C. Cerottini, C. Bron, and I. F. Luescher, manuscript in preparation). It has been reported that CD8 ϩ T cells are activated by monomeric K d -peptide complexes, but in this study the cells were adhered and not in suspension (33).…”

Section: Discussionmentioning

confidence: 91%

“…T cell hybridomas were cultured in DMEM (Life Technologies, Gaithersburg, MD) supplemented with 5% (v/v) heat-inactivated FCS (HIFCS, Life Technologies), penicillin-streptomycin-neomycin (PSN, Life Technologies), and 2-ME. The T1.4.1 ␣␤ and T1.4.1 ␣Ј␤ (␣ tailless) and T1.4.1 ␣Љ␤ (CD8␣ lck binding site mutant) transfectants (33) were cultured in the presence of 2 mg/ml G418 (Life Technologies). T1.4.2 ␣␣ transfectants were cultured in the presence of 2 mM histidinol (Sigma), whereas T1.4.2 ␣␤ and T1.4.2 ␣␤Ј (␤ tailless) transfectants were cultured in DMEM containing 1.5% (v/v) HIFCS supplemented with 2 mM histidinol and 3 g/ml puromycin (Calbiochem, La Jolla, CA).…”

Section: Cell Culturementioning

confidence: 99%

Essential Role of CD8 Palmitoylation in CD8 Coreceptor Function

Arcaro

Grégoire

Boucheron

et al. 2000

Self Cite

159

164

To investigate the molecular basis that makes heterodimeric CD8αβ a more efficient coreceptor than homodimeric CD8αα, we used various CD8 transfectants of T1.4 T cell hybridomas, which are specific for H-2Kd, and a photoreactive derivative of the Plasmodium berghei circumsporozoite peptide PbCS 252–260 (SYIPSAEKI). We demonstrate that CD8 is palmitoylated at the cytoplasmic tail of CD8β and that this allows partitioning of CD8αβ, but not of CD8αα, in lipid rafts. Localization of CD8 in rafts is crucial for its coreceptor function. First, association of CD8 with the src kinase p56lck takes place nearly exclusively in rafts, mainly due to increased concentration of both components in this compartment. Deletion of the cytoplasmic domain of CD8β abrogated localization of CD8 in rafts and association with p56lck. Second, CD8-mediated cross-linking of p56lck by multimeric Kd-peptide complexes or by anti-CD8 Ab results in p56lck activation in rafts, from which the abundant phosphatase CD45 is excluded. Third, CD8-associated activated p56lck phosphorylates CD3ζ in rafts and hence induces TCR signaling and T cell activation. This study shows that palmitoylation of CD8β is required for efficient CD8 coreceptor function, mainly because it dramatically increases CD8 association with p56lck and CD8-mediated activation of p56lck in lipid rafts.

“…Engagement of the MHC coreceptor CD8 has been proposed to play an important role in activation of CD8 T cells (12,21,34). In the current experiments, we had an opportunity to study the activation responses of CD8 ϩ 2C T cells (ϳ75-80% of lymph node T cells) vs a naturally occurring population of CD8 Ϫ T cells also found in the 2C transgenic mice (ϳ20 -25% of lymph node T cells) (Fig.…”

Section: Allogeneic Mhc-peptide Complexes Activate Cd8 T Cells As Olimentioning

confidence: 99%

CD8 T Cells, Like CD4 T Cells, Are Triggered by Multivalent Engagement of TCRs by MHC-Peptide Ligands but Not by Monovalent Engagement

Stone

Stern

2006

T cell activation is initiated by recognition of antigenic peptide presented in complex with MHC molecules on the surface of APCs. The mechanism by which this recognition occurs is still unclear, and many models exist in the literature. CD4 T cells have been shown to respond to soluble oligomers of activating class II MHC-peptide complexes, but not to soluble monomers. In determining the reactivity of CD8 T cells to soluble activating class I MHC-peptide complexes, a complicating phenomenon had been observed whereby peptide from soluble complexes was loaded onto cell surface MHCs on the T cells and re-presented to other T cells, clouding the true valency requirement for activation. This study uses soluble allogeneic class I MHC-peptide monomers and oligomers to stimulate murine CD8 T cells without the possible complication of peptide re-presentation. The results show that MHC class I monomers bind to, but do not activate, CD8 T cells whether the cells are in solution or adhered to a surface. Monomeric MHC class I binding can antagonize the stimulation triggered by soluble oligomers, a phenomenon also observed for CD4 T cells. Dimeric engagement is necessary and sufficient to stimulate downstream activation processes including TCR down-regulation, Zap70 phosphorylation, and CD25 and CD69 up-regulation, even in T cells that do not express the MHC coreceptor CD8. Thus, the valency dependence of the response of CD8 T cells to soluble MHC-peptide reagents is the same as previously observed for CD4 T cells.