pH-Dependent Interdomain Tethers of CD1b Regulate Its Antigen Capture

Abstract

As CD1 proteins recycle between the cell surface and endosomes, they show altered receptiveness to lipid antigen loading. We hypothesized that changes in proton concentration encountered within distinct endosomal compartments influence the charge state of residues near the entrance to the CD1 groove and thereby control antigen loading. Molecular dynamic models identified flexible areas of the CD1b heavy chain in the superior and lateral walls of the A' pocket. In these same areas, residues that carry charge in a pH-dependent manner (D60, E62) were found to tether the rigid alpha1 helix to flexible areas of the alpha2 helix and the 50-60 loop. After disruption of these tethers with acid pH or mutation, we observed increased association and dissociation of lipids with CD1b and preferential presentation of antigens with bulky lipid tails. We propose that ionic tethers act as molecular switches that respond to pH fluxes during endosomal recycling and regulate the conformation of the CD1 heavy chain to control the size and rate of antigens captured.

Structural Differences in Lipomannans from Pathogenic and Nonpathogenic Mycobacteria That Impact CD1b-restricted T Cell Responses

“…Furthermore, antigenic ligands for CD1b have been shown to bind in such a way that their hydrophobic acyl domains are stabilized by nonpolar van der Waals interactions with nonpolar amino acids that line the interior of the CD1 groove (37). In the case of the SmegLM population, the high content of succinates may reduce the pH enough to protonate amino acids in the CD1 binding groove or it may generate the formation of new hydrogen bonds thereby stabilizing SmegLM molecules for presentation to T cells (38). Thus, apart from the potential importance of the fatty acids present in the MPI anchor of the LMs, succinates may directly participate in stabilizing the molecule within the CD1 binding groove allowing for the antigenic epitopes to be highly exposed to the T cell receptor.…”

Discovery of deoxyceramides and diacylglycerols as CD1b scaffold lipids among diverse groove-blocking lipids of the human CD1 system

“…In particular, CD1b and CD1d are relatively resistant to exogenous antigen loading at neutral pH conditions normally found in the secretory pathway (3)(4)(5)(6). They more rapidly exchange antigens at pH 4-6, which is characteristic of the late endosomal or lysosomal environment (7,8). The differing pH requirements for antigen loading, combined with enrichment of endogenous lipids in the secretory pathway and exogenous lipids endosomes, are coalescing into a two-step model of lipid antigen presentation.…”

The cellular and biochemical rules of lipid antigen presentation

“…The low pH of these organelles may also facilitate opening of the CD1 pockets and association and dissociation of lipid antigens. For example, pH fluxes during endosomal recycling controls the conformation of the CD1b chain and the size and rate of loaded antigens [49].…”

“…Some bacterial lipids with long alkyl chains, such as mycolic acid that can be 80 carbons long, bind to CD1b in the absence of partial degradation [56]. Binding requires the acid pH of Ly, which is important in facilitating opening of the CD1b antigen-binding groove [49].…”

“…The low pH of these organelles may also facilitate opening of the CD1 pockets and association and dissociation of lipid antigens. For example, pH fluxes during endosomal recycling controls the conformation of the CD1b chain and the size and rate of loaded antigens [49].An important issue is where the endogenous lipids that stimulate iNKT cells are loaded on CD1d. Exogenous microbial antigens are loaded within LE/Ly, whereas endogenous self-lipids stimulating human iNKT cells are loaded in other compartments [50], probably within the ER.…”