CD1E

CD1e is a unique intracellular member of the CD1 lipid antigen-presenting family that functions as a lysosomal lipid-editing and transfer protein in dendritic cells, shaping the repertoire of glycolipid antigens presented by CD1b, CD1c, and CD1d molecules. Synthesized in the Golgi, CD1e traffics directly to lysosomes without transiting the plasma membrane; this routing is controlled by monoubiquitination of cytoplasmic tail lysines, and upon arrival in late endosomes/lysosomes, CD1e is proteolytically cleaved into a stable soluble form (PMID:10948205, PMID:15752135, PMID:18208508). Soluble CD1e binds glycolipids with fast on/off kinetics — enabled by an unusually wide antigen-binding groove — and selectively assists lysosomal α-mannosidase in digesting complex mycobacterial phosphatidyl-myo-inositol mannosides (PIM6) in an acylation-dependent manner, while also transferring processed lipid antigens to CD1b (PMID:16311334, PMID:21788486, PMID:22782895). By accelerating both CD1–lipid complex formation and turnover, CD1e positively or negatively modulates lipid antigen presentation across multiple CD1 isoforms, thereby tuning CD1-restricted T cell responses (PMID:21844346).

1. 2000 High

   Establishing that CD1e is an exclusively intracellular CD1 molecule answered the question of why no surface CD1e expression had been detected: alternatively spliced isoforms with three α-domains associate with β2-microglobulin but accumulate in late Golgi and late endosomes via atypical dilysine tail motifs, where they are cleaved into soluble forms, never reaching the plasma membrane.

   Evidence Transfection of isoform constructs, subcellular fractionation, immunofluorescence, and biochemical analysis in dendritic cells

   PMID:10948205

   Open questions at the time

   • Protease responsible for endosomal cleavage to soluble form not identified
   • Function of alternatively spliced isoforms lacking full alpha domains unclear
   • Signal directing Golgi retention versus endosomal transit not fully defined
2. 2003 Medium

   Conservation of CD1e's intracellular trafficking and cleavage in rhesus macaque dendritic cells established that this unique non-surface pathway is evolutionarily conserved across primates, strengthening the inference that it serves a selected function.

   Evidence Immunofluorescence and biochemical characterization of simian CD1e in dendritic cells

   PMID:12671734

   Open questions at the time

   • Functional consequence of conservation not tested (no antigen presentation assay in macaque)
   • Whether non-primate mammals have a functional CD1e ortholog not addressed
3. 2005 High

   Two key advances defined the cell biology and function of CD1e: live-cell and immunoelectron microscopy showed that DC maturation triggers CD1e redistribution from Golgi to lysosomes via sorting endosomes, and in vitro reconstitution demonstrated that soluble CD1e is required for lysosomal α-mannosidase to efficiently process PIM6, thereby expanding the CD1b-presented antigen repertoire.

   Evidence Live-cell fluorescence and immunoelectron microscopy of immature/mature DCs; recombinant CD1e glycolipid-binding assays and in vitro digestion assays with CD1b-restricted T cell readout

   PMID:15752135 PMID:16311334

   Open questions at the time

   • Mechanism by which CD1e facilitates α-mannosidase activity not resolved at molecular level
   • Whether CD1e assists processing of non-PIM glycolipids not tested
   • In vivo relevance in intact organism not demonstrated
4. 2008 High

   The molecular mechanism governing CD1e's unusual Golgi-to-lysosome trafficking was resolved: monoubiquitination of cytoplasmic tail lysines triggers exit from the Golgi, as shown by lysine-to-arginine mutagenesis causing Golgi retention and surface expression, rescued by ubiquitin fusion. A natural CD1e variant (Pro194) that fails to assemble properly and cannot reach lysosomes is functionally inactive in PIM6 presentation.

   Evidence Mutagenesis, ubiquitin-fusion rescue constructs, immunofluorescence in DCs; T cell stimulation assays and localization analysis of allelic variants

   PMID:18208508 PMID:18325888

   Open questions at the time

   • E3 ubiquitin ligase responsible for CD1e monoubiquitination not identified
   • Whether the Pro194 variant has clinical immunological consequences in carriers unknown
   • Mechanism of CD1e cleavage in late endosomes still undefined
5. 2011 High

   Structural and functional analyses revealed how CD1e acts as a lipid transfer protein: its crystal structure showed an unusually wide groove portal (2 Å wider than other CD1 proteins) with no stably bound endogenous lipid, and kinetic assays demonstrated fast lipid association/dissociation enabling CD1e to transfer lipids to CD1b and displace lipids from stable CD1b complexes. Transgenic mouse studies showed CD1e accelerates both formation and turnover of CD1b/c/d–lipid complexes, positively or negatively modulating antigen presentation.

   Evidence X-ray crystallography at 2.90 Å, native mass spectrometry, in vitro lipid transfer assays, CD1e transgenic mouse APC functional assays with CD1b/c/d-restricted T cells

   PMID:21788486 PMID:21844346

   Open questions at the time

   • Structural basis for selectivity toward specific lipid substrates not defined
   • Whether CD1e interacts directly with other CD1 molecules during transfer not established
   • Negative modulation mechanism (accelerated turnover) not fully dissected
6. 2012 High

   The lipid transfer specificity of CD1e was refined: it selectively assists α-mannosidase digestion and membrane-to-CD1b transfer of diacylated but not higher-acylated PIM species, establishing acylation state as a selectivity determinant. Separately, LAPTM5 was identified as a molecular partner of CD1e in the Golgi and late endosomes.

   Evidence In vitro digestion and liposome transfer assays with defined PIM species of varying acylation; Co-IP, colocalization, and MS identification of LAPTM5 interaction

   PMID:22782895 PMID:22880058

   Open questions at the time

   • Functional role of LAPTM5–CD1e interaction remains undefined (LAPTM5 does not control CD1e ubiquitination or cleavage)
   • Structural basis for acylation-dependent selectivity not determined
   • Whether CD1e transfers non-PIM lipid antigens with similar selectivity not systematically tested

• Key unresolved questions include the identity of the protease that cleaves CD1e in lysosomes, the E3 ligase mediating its monoubiquitination, whether CD1e directly contacts other CD1 molecules during lipid transfer, and the in vivo immunological consequences of CD1e deficiency or polymorphism in infection or autoimmunity.
• Protease responsible for lysosomal cleavage of CD1e unknown
• E3 ubiquitin ligase for CD1e monoubiquitination not identified
• No in vivo loss-of-function model in an organism with a complete CD1 system