The molecular and structural basis for antigen recognition by NKT cells

© 2012 Dr. Daniel Grant PellicciNKT cells are a subset of T lymphocytes that influence a large array of immune responses ranging from cancer to autoimmune disease. The involvement of NKT cells in these immune responses is primarily due to their ability to rapidly produce immunoregulatory cytokines in response to glycolipid antigens presented by the antigen presentation molecule, CD1d. Understanding the precise molecular interactions that govern NKT cell recognition of glycolipid antigens will provide important insight into how these cells can be manipulated in a therapeutic setting. NKT cells express a semi-invariant T cell receptor (TCR), Vα24-Jα18 in humans, Vα14-Jα18 in mice; Vβ11 in humans, Vβ8, Vβ7 or Vβ2 in mice. The restricted T cell repertoire of NKT cells is thought to reflect their unusual antigen specificity, as most other T cell populations recognise peptide antigens presented by major histocompatibility complexes (MHC). Although NKT express a restricted T cell repertoire due to limited TCR chain usage, it is important to note that the complementarity determining regions (CDR) of the TCR β-chain and more specifically, CDR3β, can be enormously diverse. This thesis provides important insight into how NKT TCR diversity is critical for the recognition of functionally distinct glycolipid antigens. Structural analysis of the first human NKT TCR in complex with hCD1d loaded with the foreign glycolipid antigen, α-galactosylceramide (α-GalCer) revealed a strikingly different docking mode compared to that observed for TCR/MHC-peptide complexes. In contrast to a diagonal docking mode that is common for peptide recognition, the NKT TCR docked parallel to, and at the far end of CD1d binding cleft allowing the highly conserved invariant α-chain to dominate interactions with CD1d/α-GalCer. Given that the three β-chains expressed by mouse NKT cells exhibit non-conserved differences in the proposed CD1d docking regions, it was important to establish how these TCRs contributed to glycolipid recognition. The structures of the three main mouse NKT TCRs in complex with mouse CD1d loaded with α-GalCer were solved, revealing, that while the docking mode is broadly similar for each TCR, clear differences in the contribution of each TCR β-chain were observed. Furthermore, a comparison of mouse NKT TCRs using retroviral NKT cell lines, fresh thymic NKT cells and soluble NKT TCRs revealed that even minor alterations in glycolipids could lead to drastic alterations in cellular responses and affinity. Collectively, these studies demonstrate that different TCR β-chains provide an altered landscape that can influence interactions with structurally distinct glycolipid antigens. Whilst considerable effort has been made in understanding how NKT cells recognise foreign α-linked antigens, it is still unclear how NKT cells are involved in a range of immune responses such as cancer, autoimmunity, infection and inflammation. In these cases, it is likely that NKT cells respond to self-antigens such as β-linked glycolipids. How can a semi-invariant NKT TCR recognise β-linked antigens, which sit differently in the binding groove of CD1d compared to α-linked antigens? So far, all structural studies prior to this work focused of recognition of α-linked foreign antigens and these all contained a single sugar residue. Structural analysis of NKT TCRs in complex with CD1d loaded with mono and multi-glycoslylated β-linked glycolipids revealed that the NKT TCR pushes over the sugar head group, regardless of size, so that it resembles a foreign α-linked glycolipid. Collectively, this study provides important insight into the mechanisms that govern recognition of self-antigens by NKT cells and reveals that induced fit molecular mimicry underpins recognition of self β-linked glycolipid antigens