γδ T cell surveillance via CD1 molecules

Abstract

γδ T cells are a prominent epithelial-resident lymphocyte population, possessing multi-functional capacities in the repair of host tissue, pathogen clearance, and tumor surveillance. Although three decades have now passed since their discovery, the nature of γδ T cell receptor (TCR)-mediated ligand recognition remains poorly defined. Recent studies have provided structural insight into this recognition, demonstrating that γδ T cells survey both CD1 and the presented lipid, and in some cases are exquisitely lipid specific. We review these findings here, examining the molecular basis for and the functional relevance of this interaction. We discuss potential implications on the notion that non-classical major histocompatibility complex (MHC) molecules may function as important restricting elements of γδ TCR specificity, and on our understanding of γδ T cell activation and function.

Keywords: CD1d; T cell receptor; crystallography; lipids; restriction; γδT cells.

Figure 1. Ligands recognized by γδ TCRs

A selection of reported γδ TCR ligands are shown, grouped by species-specificity and by structural class. MHC-like ligands include the non-classical molecules T10/T22 [86, 87], Endothelial protein C receptor (EPCR) [88], MHC class I-related molecule A (MICA) [47], and CD1 family members CD1c and CD1d [8, 11, 36, 51, 74], as well as the classical class II MHC molecule I-E^k [89]. MHC-unrelated ligands include HSV glycoprotein I [90], phosphoantigens [4, 5, 91], F1-ATPase [92], histidyl tRNA synthetase [93], and phycoerythrin [94]. A comprehensive list can be found in recent reviews [1, 2].

Figure 2. Overall structures of TCR-ligand complexes. Top panels

(A) Side-view of the DP10.7 TCR-CD1d-sulfatide complex (Protein Data Bank accession code 4MNG). The crystallized TCR construct was a single-chain version; thus shown are the variable domains only. The TCR docks at a titled angle such that only the δ chain (violet) makes contact with CD1d-sulfatide. (B) Side-view of the 9C2 TCR-CD1d-αGalCer complex (Protein Data Bank accession code 4LHU). The full-length TCR structure was determined, but the variable domains only are shown here for comparison purposes with (A). The 9C2 TCR is docked such that both the γ (blue) and δ (purple) chains contact the CD1d-αGalCer surface. In both A and B, CD1d is shown in light grey and the lipid ligands are shown in yellow. (C) Structure of a classical αβ TCR-peptide-MHC (2C TCR-H-2 K^b-dEV8 peptide) complex (Protein Data Bank accession code 2CKB). Shown are the variable domains only. The TCR is docked centrally and diagonally above the MHC footprint. Both the TCR α chain (blue) and β chain (green) contribute to MHC (light blue) and peptide contact (orange). (D) Structure of an iNKT TCR-CD1d-αGalCer complex (Protein Data Bank accession code 2PO6). Shown are the variable domains only. The NKT15 clone show here prominently uses the α chain (blue) to engage the the αGalCer ligand (yellow) and the CD1d surface (grey). The β (green) chain is docked at the extreme end of the CD1d and makes less extensive CD1d contacts. Bottom panels, shown are cartoon representations of TCR docking upon CD1d/MHC surfaces, corresponding to TCR shown in upper panel. View is looking down upon the MHC/CD1d surface; each TCR chains is depicted as a circle. Cartoons of lipid ligands or peptides also depicted. Acyl chains shown as black lines, lipid head groups shown as yellow/orange circles, peptide as orange line. TCR chain colors are the same as upper panels. Dotted line indicates TCR chain does not make contact with CD1d-lipid.

Figure 3. Two TCRs, two footprints

(A) Different footprints of DP10.7 and 9C2 TCR upon CD1d. CD1d surface shown in light grey, lipid (sulfatide) shown in yellow. DP10.7 TCR δ chain (purple) and γ chain (light pink) CDR loops, and 9C2 TCR δ chain (green) and γ chain (light green) CDR loops from complex structures are shown upon CD1d surface. Loops that are not involved in the interaction are shown as transparent (CDR1,2,3γ of DP10.7 TCR, CDR2γ of 9C2 TCR). (B) CD1d surface residues contacted by the DP10.7 (left) and 9C2 (right) TCRs. TCR loops that are involved in recognition are shown above the CD1d surface (white), and participating CD1d residues are colored according to which TCR chain makes contact (CD1d residues contacted by the TCR δ chain shown in pink, contacted by γ chain shown in blue, contacted by both chains shown in green). The pattern of CD1d contact residues clearly differs between the two TCRs: the DP10.7 TCR is docked more centrally above the lipid portal, whereas the 9C2 TCR is docked closer to the extreme A’ end of CD1d.

Figure 4. CDR3 loop length may govern TCR docking mode

(A). Shown are the CDR3δ loops of three CD1d-specific Vδ1 TCRs, the DP10.7 (purple), AB18.1 (green) and δ1A/B-2 (cyan) aligned to the CD1d-αGalCer bound 9C2 TCR (gold). The surface of CD1d is shown in transparent light grey. Docking of the AB18.1 and DP10.7 TCRs based on V domain alignment with the 9C2 TCR results in steric clashes of their CDR3δ loops with CD1d, as can be seen by the submerging of these loops under the CD1d surface. Additional TCR loops have been removed for increased clarity.

Figure 5. Cartoon representation of the Signal 1/Signal 2 model

In this model, CD1d (shown in cyan) recognition can provide either a potent signal derived from alter-self lipids or lipids from microbial origin sufficient to stimulate a Vδ1 T cell (yellow). This we call “Signal 1”. In some cases, this CD1d-lipid specific signal is of low to moderate intensity, deriving from engagement of a CD1d-self lipid complex to produce a “self-reactive” signal that requires additional enhancement from a “Signal 2”, here shown as the activating receptor NKG2D (orange) engaging the stress-induced MICA (red) on the target cell. From the structures derived thus far, we hypothesize that the Vδ1 domain (pink) will dominate in this interaction, in some cases contributing all contacts in the interface with CD1d-lipid. In cases where the CD3δ loop is of a different length or amino acid composition, or if the lipid has a large or structural complex head group, an alternate TCR footprint will be used in engagement, resulting in contacts derived from the Vγ domain (green).