(21E)-3-hydroxy-35-[(icosan-2-yl)oxy]-35-oxo-2-pentacosylpentatriacont-21-enoic acid

Abstract

CD1 proteins present microbial lipids to T cells. Germline-encoded mycolyl lipid-reactive (GEM) T cells with conserved αβ T cell receptors (TCRs) recognize CD1b presenting mycobacterial mycolates. As the molecular basis underpinning TCR recognition of CD1b remains unknown, here we determine the structure of a GEM TCR bound to CD1b presenting glucose-6-O-monomycolate (GMM). The GEM TCR docks centrally above CD1b, whereby the conserved TCR α-chain extensively contacts CD1b and GMM. Through mutagenesis and study of T cells from tuberculosis patients, we identify a consensus CD1b footprint of TCRs present among GEM T cells. Using both the TCR α- and β-chains as tweezers to surround and grip the glucose moiety of GMM, GEM TCRs create a highly specific mechanism for recognizing this mycobacterial glycolipid.

A two-chain tweezers mechanism grips GMM

Whereas most GEM T cells recognize GMM, the extent of fine specificity for antigen structure remains unknown. One ‘atypical' GEM TCR expressed by clone 18 (GEM18 TCR) recognized free mycolic acid (MA). This antigen lacks the glucosyl moiety, pointing toward possible promiscuity. On the other hand, CD1b-reactive clones known as LDN5-like T cells, do show specificity for the glucosyl group of GMM,. We measured GEM clone 42 and clone 18 responses to molecules with altered head groups, including MA, which lacks glucose, as well as mannose-6-O-monomycolate (ManMM) and galactose-6-O-monomycolate (GalMM), which differ from GMM only in the orientation of one hydroxyl group in the hexosyl ring at position C2 or C4, respectively. In all cases alteration of the headgroup abolished the GEM clone 42 T cell response, demonstrating precise specificity for the glucosyl moiety ().