T cells comprise the cellular arm of the adaptive immune system and can be categorised into either αβ or γδ T cells. Extensive cellular and biochemical characterisation of αβ T cells has unearthed the principles governing their activation and subsequent recruitment into the immune response. Comparatively there persists a poor understanding on the functions of γδ T cells that has been hampered by an incomplete ligand repertoire. This is despite γδ T cells playing a vital role in the immune response against viral, bacterial and Plasmodium infections. Therefore, we investigated how γδ T cells recognised a known antigen to shed light on this enigmatic T cell population.
The monomorphic antigen-presenting molecule CD1d presents lipid antigens to both αβ and γδ T cells. Type I Natural Killer T cells (NKT) display exquisite specificity for CD1d presenting α-Galactosylceramide (a-GalCer), while the extent of lipid specificity exhibited by CD1d-restricted γδ T cells remains unclear. Here we demonstrate that human γδ T cell receptors (TCRs) can recognise CD1d in either a lipid-dependent or auto-reactive manner with weak to moderate affinity. Using small-angle X-Ray scattering we identify ‘end-to-end’ γδ TCR-CD1d binding modality is conserved across distinct CD1d-restricted TCRs. In functional assays, CD1d presenting endogenous lipids was sufficient to stimulate γδ T cell lines. Moreover, CD1d alone induced γδ TCR-CD3 clustering and phosphorylation in a dose dependent manner while type I NKT TCR-CD3 clustering required α-GalCer. We then determined the crystal structure of a Vδ1 γδ+ TCR-CD1d complex, that showed the γδ TCR sat atop of the CD1d antigen-binding cleft but made no contacts to the presented lipid antigen. This was in stark contrast to type I NKT CD1d recognition that contacted the lipid antigen. Ultimately, through a multi-faceted biochemical, cellular and structural approach we provide a molecular basis for lipid-independent CD1d recognition by γδ TCRs and demonstrate ligand availability is a regulator of their activation.