A γδ T-cell glimpse of glycolipids

T cells as well as B cells are the key players of the adaptive immune system. Whereas B cells produce antibodies that can bind to any antigen in solution, T cells generally require antigen presentation by dedicated antigen-presenting molecules termed major histocompatibility (MHC) or MHC-like proteins. As such, T cells are both specific for the antigen as well as the antigen-presenting molecule. T cells express a heterodimeric antigen receptor (TCR) on the surface, connected to an intracellular signaling apparatus. The TCR that can be formed by either an αβ chain pair or a γδ chain pair leads to the classification of αβ and γδ T cells, depending on the chain combination expressed. Compared with αβ T cells, which have been extensively studied, γδ T-cell function, as well as their antigen specificity and requirement for antigen presentation, is far less understood. Uldrich et al.¹ have now structurally and functionally characterized a subset of human γδ T cells that, in addition to αβ T cells, can recognize the antigen-presenting molecule CD1d, and furthermore that activation of these γδ T cells can be modulated by select glycolipid antigens, including αGalCer. Recognition of CD1d-presented αGalCer to date has been the hallmark of semi-invariant Vα24Vβ11 natural killer T (iNKT) cells, a population of evolutionarily conserved T lymphocytes that vary greatly in number in human peripheral blood.² The CD1d-reactive γδ T cells predominantly express Vδ1 and CD161, are either CD4⁻/CD8⁻ or CD8⁺, and are generally less frequent than iNKT cells in human peripheral blood of healthy donors. However, Vδ1 is expressed less in peripheral blood T cells and more in tissue homing T cells, and therefore, the lower frequencies observed in blood are not necessarily a reflection of the overall cell population size in humans.³ Furthermore, they are found consistently in several donors and produce similar cytokines, albeit at lower levels, compared with iNKT cells. Interestingly, and in contrast to iNKT cells, the CD1d-restricted Vδ1⁺ human γδ T cells are not cross-reactive with murine CD1d and it is currently not clear whether mice have a similar γδ T-cell population.

Although γδ T cells have been previously shown to recognize CD1c and CD1d isotypes, with or without the presentation of a defined glycolipid antigen,^3,4 none of those interactions have been biochemically and structurally characterized. The only known structure of a γδ TCR recognizing a non-classical MHC-like molecule is that of the T10 TCR binding to the MHC-like T22, where a long CDR3δ loop inserts into the empty cleft of T22.⁵ A recent case also reports a γδ TCR in which the antigen, a phosphorylated prenyl metabolite, is presented by butyrophilin BTN3A1; however, BTN3A1 is not a bona fide antigen-presenting molecule.⁶

Uldrich et al., using CD1d tetramers loaded with or without specific glycolipids, revealed differences in γδ T-cell staining intensities among different donors. The staining intensity was often but not always increased by the addition of glycolipids, the most potent one being αGalCer, suggesting that glycolipids contribute but are not exclusively required for the activation of these cells and this might be reflected in their TCR diversity. T-cell autoreactivity has previously been described for murine iNKT cells, but not to the extent seen for the human γδ T cells. Biochemical studies revealed a binding constant (K_D) of ~35 μM between a representative γδ TCR (Vδ1-Dδ3-Jδ1/Vγ5-Jγ1) and CD1d-self antigen complexes. This binding affinity increased roughly twofold to 16 μM in the presence of αGalCer, whereas other glycolipids, such as αGlcCer or βGlcCer, have a reduced CD1d–glycolipid tetramer staining intensity and will likely have an intermediate K_D. This led to the speculation as to how important the antigen is in activating this population of γδ T cells. To address this question, the authors elegantly determined the crystal structure of the γδ TCR bound to human CD1d-presenting αGalCer. In the structure, the γδ TCR docks diagonally above the A′ pocket of CD1d, similar to a type II NKT TCR that recognizes sulfatide but more offset to the periphery of CD1d, while binding is radically different from the parallel docking mode of iNKT or type I NKT cells that sit above the F′ pocket of CD1d (Figure 1).^7–9 In agreement with both NKT-cell TCRs, the γδ TCR also recognizes the antigen with only one TCR chain, in this case the γ chain, whereas the Vδ1 chain, which is predominantly found in CD1d-restricted γδ T cells, dominates the overall contacts with the non-polymorphic CD1d molecule. As the non-germline encoded CDR3γ forms the only contact with the antigen, it is likely that CDR3γ diversity can affect the lipid spectrum that is recognized by these CD1d-restricted γδ T cells. This is in contrast to iNKT cells, where CDR3α is formed by an evolutionarily conserved rearrangement between Vα24 and Jα18, owing to positive selection by endogenous ligands.

Figure 1.

Antigen recognition spectrum of T cells. (a) αβ T cells have been shown to recognize a range of diverse antigens, such as peptides, lipids and small metabolites, but only when bound to an antigen-presenting molecule. This population includes classical peptide-reactive T cells, lipid-reactive NKT cells and metabolite-reactive Mucosal associated invariant T (MAIT) cells. γδ T cells, in contrast, can recognize antigen-presenting molecules in the absence or presence of antigen. (b) Schematic representation of the modes of binding of αβ and γδ TCRs to CD1d molecules. CD1d is shown in gray with the two main pockets, A′ and F′, composing the binding groove. The two chains forming the TCR are shown as ellipsoids together with a line connecting the center of masses of each V domain. Note how the Vδ1 γδ TCR (top) binds on the opposite side of CD1d compared with the type I NKT TCR (middle), but similar to the binding of a mouse type II NKT TCR to mouse CD1d (bottom).

Thus, in summary, the identified population of Vδ1⁺ γδ T cells is a novel, distinct population of T cells able to respond to glycolipids presented by CD1d, in addition to type I and II NKT cells, which recognize αGalCer and sulfatide (among others), respectively, while γδ T cells have been shown to be able to recognize both antigens. Moreover, their expression of CD161 suggests a common theme among CD1d-restricted glycolipid-reactive T cells, which are rapidly activated to produce cytokines within hours after antigen challenge contributing to their innate-like phenotype. Although the functional contribution of those cells is not clear, activation of γδ T cells can be concomitant with iNKT-cell activation and their overlapping cytokine spectrum may hint to a functional synergy.