Tuberculosis (TB) is the main cause of mortality among all infectious diseases. The presentation of lipids by CD1b molecules and the interactions of the CD1b-lipid complexes with the immune receptors are important for the understanding of the immune response to Mycobacterium tuberculosis (Mtb), and to develop TB control methods. A specific domain antibody (dAbk11) recognizing the complex of CD1b with Mtb sulphoglycolipid (Ac2SGL) had been previously developed. In order to study the interactions of dAbk11 with Ac2SGL:CD1b, the conformation of Ac2SGL within CD1b was first modelled. The orientation of dAbκ11 with Ac2SGL:CD1b was then predicted by a docking experiment and the complex was sampled using molecular dynamics simulation. Data showed that dAbκ11 Tyr32 OH plays a decisive role in interacting with Ac2SGL alkyl tail HO17. The binding free energy calculation showed that Ac2SGL establish strong hydrophobic interactions with dAbκ11. The model also predicted a higher affinity for the natural sulfoglycolipid (Ac2SGL) than the synthetic analogue (SGL12), which was supported by the ELISA data. These results shed light on the likely mechanism of interactions between Ac2SGL:CD1b and dAbκ11, thus making possible to envision the strategies for dAbκ11 optimization for possible future applications.
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 Ac2SGL, 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-Ac2SGL 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 Ac2SGL (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-Ac2SGL 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 Ac2SGL analogs having different acyl chains, (b) molecular modeling of the CD1b-Ac2SGL/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.