Structural studies of MR1 uncovered an important clue. The MR1 protein was unstable when produced in vitro by cell lines grown in typical tissue culture conditions. It was discovered that the protein was stabilized when it was refolded in media containing B vitamins or folic acid (vitamin B9). Chemical analysis revealed that a small molecule—identified as a folate derivative, 6-formyl pterin (6-FP)—was bound to the stabilized MR1. X-ray crystallographic studies showed that 6-FP was bound in the central groove of the MR1 molecule; this helped explain how folate derivatives might stabilize MR1. However, MAIT cells were not activated by cells expressing the 6-FP:MR1 complex, suggesting other molecules might be the physiological ligands to activate MAIT cells. Analysis of MR1 proteins that were refolded in the presence of supernatants from cultures of Salmonella typhimurium eventually led to the identification of several riboflavin metabolites that are formed by biosynthetic pathways in most bacteria and yeast. These metabolites not only bind to MR1, but also activate MAIT cells. Thus, MAIT cells are activated in response to infection by these organisms by detecting products specific to their folate metabolism. As such, MAIT cells appear to hold an intermediate place in the spectrum of innate and adaptive immunity, similar to iNKT cells, in that they use an antigen receptor assembled by somatic gene rearrangement, but recognize a molecular structure that falls within the definition of a PAMP.
6-20 γ:δ T cells can recognize a variety of diverse ligands.
γ:δ T cells and α:β T cells have been known to be distinct developmental lineages almost since the T-cell receptor genes were identified. But unlike α:βT cells, the function of γ:δ T cells has remained somewhat obscure, due primarily to difficulty in identifying the ligands they recognize. Yet the abundance of γ:δ T cells across vertebrate species, their rapid expansion to form more than 50% of the blood lymphocytes during infections, and their abundant cytokine production all argue for an important role in immunity. Over time, many different ligands recognized by γ:δ T-cell clones have been identified (Fig. 6.29), and their diversity suggests that, like iNKT and MAIT cells, they hold an intermediate, or transitional, position in the spectrum of innate versus adaptive immunity.
In Section 4-20, we discussed how one γ:δ T-cell receptor binds to the non-classical MHC class I molecule T22. Instead of binding centrally over the MHC binding groove, similar to an α:β T-cell receptor, the γ:δ T-cell receptor interacts obliquely from one side of the T22 molecule. However, fewer than 1% of γ:δ T cells recognize this ligand. Other antigens recognized by murine γ:δT cells are the protein phycoerythrin (PE) from algae, the inner mitochondrial membrane lipid cardiolipin, glycoprotein I of herpes simplex virus, and a peptide derived from the hormone insulin. Among antigens that can activate human γ:δ T cells are the nonclassical MHC class I proteins MICA and ULBP4 and the endothelial protein C receptor (EPCR), which is expressed by endothelial cells. Like MICA and ULBPs, EPCR appears to be induced upon stress, such as during infection of cells by cytomegalovirus, suggesting that reactive γ:δ T cells could serve in an innate capacity similar to that of NK cells activated by stress-induced nonclassical MHC class Ib molecules. Several other antigens can activate human γ:δ T cells (see Fig. 6.29), although there is still limited structural information about their interaction with the T-cell receptor, and even reservations about whether such an interaction is always