A Direct Role for the CD1b Endogenous Spacer in the Recognition of a Mycobacterium tuberculosis Antigen by T-Cell Receptors

Abstract

Lipids, glycolipids and lipopeptides derived from Mycobacterium tuberculosis (Mtb) are presented to T cells by monomorphic molecules known as CD1. This is the case of the Mtb-specific sulfoglycolipid Ac 2 SGL, which is presented by CD1b molecules and is recognized by T cells found in tuberculosis (TB) patients and in individuals with latent infections. Our group, using filamentous phage display technology, obtained two specific ligands against the CD1b-Ac 2 SGL complex: (i) a single chain T cell receptor (scTCR) from a human T cell clone recognizing the CD1b-AcSGL complex; and (ii) a light chain domain antibody (dAbκ11). Both ligands showed lower reactivity to a synthetic analog of Ac 2 SGL (SGL12), having a shorter acyl chain as compared to the natural antigen. Here we put forward the hypothesis that the CD1b endogenous spacer lipid (EnSpacer) plays an important role in the recognition of the CD1b-Ac 2 SGL complex by specific T cells. To support this hypothesis we combined: (a) molecular binding assays for both the scTCR and the dAbκ11 antibody domain against a small panel of synthetic Ac 2 SGL analogs having different acyl chains, (b) molecular modeling of the CD1b-Ac 2 SGL/EnSpacer complex, and (c) modeling of the interactions of this complex with the scTCR. Our results contribute to understand the mechanisms of lipid presentation by CD1b molecules and their interactions with T-cell receptors and other specific ligands, which may help to develop specific tools targeting Mtb infected cells for therapeutic and diagnostic applications.

αβ T-cell receptor recognition of self-phosphatidylinositol presented by CD1b

“…Our data inform and expand the hypothesis of Camacho et al. ( 9 ), who proposed in context of M. tuberculosis sulfoglycolipid recognition that longer chain endogenous scaffold lipids (>C36) could possibly protrude beyond Tyr151 through the F′ portal, potentially allowing for antigenic headgroup and scaffold lipid cocontact by the CDR3 of the TCR. While the long scaffold lipid observed here matched a length of 36 carbons based on electron density, the scaffold does not protrude from the F′ portal, and instead, the TCR contact inside CD1b is demonstrated directly.…”

“…While peptide antigen recognition forms the central dogma of classical T-cell–mediated adaptive immunity, the role of lipids as nonclassical antigens, presented by major histocompatibility complex class I–like molecules known as CD1, are emerging as new targets of protection and autoimmunity. CD1 mediated T-cell response to bacteria like Mycobacterium tuberculosis ( 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ), and self-lipid antigens mediate autoreactivity and autoimmune disease via lipid activation ( 15 , 16 , 17 , 18 , 19 , 20 ), lipid-independent binding ( 21 , 22 , 23 , 24 , 25 ), and lipid-blocking mechanisms ( 26 ).…”