The CD1 size problem: lipid antigens, ligands, and scaffolds

Abstract

Whereas research on CD1d has emphasized a few glycosyl ceramides, the broader family of four human CD1 antigen-presenting molecules binds hundreds of distinct self-lipids. Individual lipid types bind within CD1 grooves in different ways, such that they partially fill the groove, match the groove volume, or protrude substantially from the groove. These differing modes of binding can now be connected to differing immunological functions, as individual lipids can act as stimulatory antigens, inhibitory ligands, or space-filling scaffolds. Because each type of CD1 protein folds to produce antigen-binding grooves with differing sizes and shapes, CD1a, CD1b, CD1c, CD1d, and CD1e have distinct mechanisms of capturing self-lipids and exchanging them for foreign lipids. The size discrepancy between endogeneous lipids and groove volume is most pronounced for CD1b. Recent studies show that the large CD1b cavity can simultaneously bind two self-lipids, the antigen, and its scaffold lipid, which can be exchanged for one large bacterial lipid. In this review, we will highlight recent studies showing how cells regulate lipid antigen loading and the roles CD1 groove structures have in control of the presentation of chemically diverse lipids to T cells.

Donor-unrestricted T cells in the human CD1 system

“…2). Although the aliphatic hydrocarbon chains are extremely flexible, clefts and individual pockets within CD1 proteins place an upper limit on the size of lipid antigens as reviewed previously (Ly and Moody 2014). Lipids are the products of enzymes rather than genes so are not subject to rapid change by mutation and instead change slowly over evolutionary time.…”

αβ T cell antigen receptor recognition of CD1a presenting self lipid ligands

“…3a,e), whereas there was no unaccounted electron density within the more solvent-exposed F′ pocket (data not shown), which suggested either that no ligands were present or that a heterogeneous population of ligands occupied this F′ pocket. In CD1b, the A′ pocket is a circular structure that connects to other pockets and thus forms a 'maze' for alkyl chains 6 . In contrast, the A′ pocket of CD1a is abruptly terminated, which creates a tube with only one entrance.…”

“…CD1a was expressed by recombinant methods in both baculovirus-mediated insect cells and transfection in mammalian expression systems. For expression in insect cells, sequence encoding the extracellular region of CD1a with a carboxy-terminal BirA-His 6 tag fusion (amino acid sequence, GSGLNDIFEAQKIEWHEHHHHHH) and β 2 -microglobulin was cloned into the pFastBac Dual transfer plasmid (Invitrogen), followed by expression in Spodoptera frugiperda (Sf9) insect cells as described 33 . Mammalian expression was achieved by cloning of constructs encoding CD1a and β 2 -microglobulin into a pHLsec vector 34 as carboxyterminal fusions with the leucine zippers Jun (CD1a) and Fos (β 2 -microglobulin) and a 'post-zipper' BirA-His 6 tag for CD1a.…”

“…In general, lipid antigens are amphipathic molecules with polar head groups and hydrophobic acyl tails. CD1 molecules present these lipid antigens by sequestering the acyl tails within a hydrophobic cleft, composed of the A′ and F′ pockets, while the polar head group is exposed at the CD1 surface for recognition by the TCR 6 . Unlike the genetically diverse MHC proteins, nearly all humans express the same CD1 antigen-presenting molecules, which are subcategorized as group 1 (CD1a, CD1b and CD1c) and group 2 (CD1d).…”

“…Moreover, CD1a can present 'headless' hydrophobic lipids, such as squalene and triacylglyceride, which, unlike antigens presented by other CD1 isoforms, lack any known basis for polar interactions with the TCR 10 . Indeed, while appreciation of the anatomy of the CD1 family is growing 6 , understanding of lipid-mediated T cell immunity at the molecular level is limited mainly to the recognition of CD1d-lipid by type I natural killer T cells (NKT cells) 11 , whereby the invariant NKT cell TCR α-chain directly contacts the polar head group of the glycolipid antigen protruding from the CD1d groove 11 . At present, however, there is no clear understanding of the structural basis for the recognition of CD1 group 1 molecules by the TCR.…”