CD3δ Establishes a Functional Link between the T Cell Receptor and CD8

Abstract

T cells expressing T cell receptor (TCR) complexes that lack CD3␦, either due to deletion of the CD3␦ gene, or by replacement of the connecting peptide of the TCR␣ chain, exhibit severely impaired positive selection and TCR-mediated activation of CD8 single-positive T cells. Because the same defects have been observed in mice expressing no CD8␤ or tailless CD8␤, we examined whether CD3␦ serves to couple TCR⅐CD3 with CD8. To this end we used T cell hybridomas and transgenic mice expressing the T1 TCR, which recognizes a photoreactive derivative of the PbCS 252-260 peptide in the context of H-2K d . We report that, in thymocytes and hybridomas expressing the T1 TCR⅐CD3 complex, CD8␣␤ associates with the TCR. This association was not observed on T1 hybridomas expressing only CD8␣␣ or a CD3␦؊ variant of the T1 TCR. CD3␦ was selectively coimmunoprecipitated with anti-CD8 antibodies, indicating an avid association of CD8 with CD3␦. Because CD8␣␤ is a raft constituent, due to this association a fraction of TCR⅐CD3 is raft-associated. Cross-linking of these TCR-CD8 adducts results in extensive TCR aggregate formation and intracellular calcium mobilization. Thus, CD3␦ couples TCR⅐CD3 with raft-associated CD8, which is required for effective activation and positive selection of CD8 ؉ T cells.The differentiation of CD4 CD8 double-positive (DP) 1 thymocytes into CD8 single-positive (SP) T cells requires appropriate signals from the TCR and the coreceptor CD8 (1, 2). DP thymocytes and CD8 SP peripheral T cells express TCR␣␤ that are associated with three signal-transducing units, namely CD3␦⑀ and CD3␥⑀ heterodimers and a disulfide-linked chain homodimer (3-5). The CD3␥␦ and ⑀ subunits contain in their cytoplasmic tail a single immunoreceptor tyrosine-based activation motif, whereas the tail of the chain harbors three immunoreceptor tyrosine-based activation motifs. For surface expression of TCR␣␤, their association with CD3⑀, ␥, and but not with CD3␦ is required (6 -9). Accordingly, knockout of CD3⑀, ␥, and chain arrests T cell development at early stages (6 -11). By contrast, in CD3␦ knockout mice T cell development proceeds to the DP stage, but positive selection of CD8 (and CD4) SP T cells is severely compromised (9, 12). During TCR␣␤ assembly the TCR␤ chain first associates with CD3⑀␥ and the TCR␣ chain with CD3⑀␦, and the resulting trimers then associate and the TCR␣␤ disulfide bond is formed (13). Although CD3␦ is physically associated with the pre-TCR complex, it is not required for pre-TCR signaling, which is essential for the transition of double-negative (DN) to DP thymocytes (10,14,15).A conserved motif in the TCR␣ chain-connecting peptide domain, which connects the transmembrane and the Ig domains, referred to as ␣CPM, plays a crucial role in positive selection of CD8 and CD4 SP T cells (16 -18). The ␣CPM consists of seven highly conserved amino acids (FETDXNLN) and is present in TCR␣␤ but not in TCR␥␦ (16). In mice expressing TCR in which the ␣CPM is replaced by the corresponding sequence of the TCR␦ chain, po...

CD8α/α homodimers fail to function as co‐receptor for a CD8‐dependent TCR

“…In contrast, transfer of the CD8-dependent TCR into CD8b-deficient lymphocytes revealed that transduced T cells did not respond to antigen stimulation even at high antigen concentration, indicating that CD8a/a homodimers were unable to provide any co-receptor function. This highlighted that the CD8b chain played a central role in co-receptor function, consistent with previous studies with T cell hybridomas showing that CD8a/b heterodimers were more efficient co-receptors than CD8a/a homodimers [11,16]. It is possible that the physiological role of CD8a/a does not include co-receptor function for MHC class I-restricted TCR, but may be required for the formation of memory T cells, although this issue remains controversial, and for the function of intra-epithelial lymphocytes in the gut [17][18][19].…”

“…Recently, it has been shown that the CD3d chain can establish a functional link between the TCR and CD8 [11]. Hence, we explored if there was any difference in the amount of CD3d molecules detectable within the TCR/CD3 complex formed by the two TCR studied here.…”

Biochemical Differences in the αβ T Cell Receptor·CD3 Surface Complex between CD8+ and CD4+ Human Mature T Lymphocytes

“…Also, it is believed that a fraction of TCR⅐CD3 is raft-associated because of its association with the cytoplasmic portion of the CD8 (or CD4) coreceptor (21). We therefore reasoned that the conformational TCR⅐CD3 differences observed between CD4 ϩ and CD8 ϩ T cells could be due, at least in part, to the differential arrangement of lipid raft-associated surface TCR⅐CD3 clusters or arrays.…”

Ca2+ signals and T lymphocytes “New mechanisms and functions in Ca2+ signalling”

“…More recently, we and others have shown that T cell activation by monomeric pMHC could only be observed in adherent T cells, revealing that T cell adhesion induced a marked lowering of their threshold for TCR signalling (Doucey et al, 2003;Randriamampita et al, 2003). The phenomenon of adhesion-induced T cell priming could be due at least in part to the fact that upon adhesion, T cell Ca 2+ stores get rapidly replenished because they are less leaky than when T cells are in suspension .…”