The cellular and biochemical rules of lipid antigen presentation

Abstract

The recognition of both protein and lipid antigens follows similar strategies that rely on different molecular mechanisms. APC present lipid antigens exploiting the same mechanisms implicated in lipid translocation, lipoprotein assembly and lipid degradation. An important issue is how the lipid structure contributes to antigenicity. Lipid hydrophobicity influences the modes of internalization by APC, the trafficking through different membrane compartments, the binding to CD1 molecules and the stability of antigenic complexes. Some glycolipids with large hydrophilic parts require processing of the sugar moieties exerted by lysosomal hydrolases. Finally, extraction of lipids from membranes, their solubilization and loading on CD1 molecules are facilitated by the same lysosomal lipid-binding proteins that are also instrumental in lipid catabolism. More recent investigations reveal how lipid-specific immunity is regulated during infections. In this review we describe the main cellular and biochemical rules of lipid antigen presentation and discuss their implications in anti-microbial and autoimmune responses.Key words: Antigen recognition . CD1 . Lipid antigens . TCR IntroductionMembers of the MHC or CD1 families present protein and lipid antigens to T cells, respectively. CD1 molecules are differentially expressed by a variety of cell types including DC, B cells, monocytes, Langerhans cells, stellate hepatic cells, epithelial cells, microglial cells and keratinocytes. In humans, five genes (CD1A, B, C, D and E) encode CD1 proteins. CD1a, CD1b, CD1c and CD1d molecules reach the plasma membrane and are involved in lipid presentation to T cells, whereas CD1e remains intracellular and is involved in lipid processing. Lipid-specific T cells express a variety of TCR heterodimers, with the exception of invariant NKT (iNKT) cells, a CD1d-restricted population that uses a semi-invariant TCR (invariant Va24-Ja18 and variable Vb11 chains in humans, invariant Va14-Ja18 and variable Vb8.2, Vb7 or Vb2 chains in mice).Proteins become antigenic after a series of events starting with partial degradation by the cellular machinery represented by the proteasome or by proteases located in lysosomes (Ly) and endoplasmic reticulum (ER). This immunological function, commonly known as antigen processing, leads to the generation of peptide fragments that are carried to the proximity of MHC molecules with which they form antigenic complexes. A similar scheme applies to the presentation of lipid antigens. Lipids derived from extracellular routes are internalized or alternatively, self-lipid antigens are synthesized within the APC. These lipids are then transported into the cellular compartments where CD1 molecules traffic and after loading onto CD1, form antigenic complexes.The physicochemical properties of lipids and peptides impose the utilization of different strategies by the APC to digest, transport and load each of these classes of molecules. Antigenicity of lipids is achieved with the participation of extracellular and intracellular li...

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“…structures presented by CD1b contain a maximum of two fatty acyl chains (2). Several lysosomal glycosidases have been shown to be involved in glycolipid processing (7,29,30), but lipases processing multiacylated glycolipids have never been identified.…”

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