The Role Of Immune Cells In Hepatic Ischemia Reperfusion

The Role of Immune Cells in Hepatic Ischemia Reperfusion

Role of CD1- and MR1-restricted T cells in Immunity and Disease

CD1 and MR1 are major histocompatibility complex (MHC) class I-related proteins that bind and present non-peptide antigens to subsets of T cells with specialized functions. CD1 proteins typically present lipid antigens to CD1-restricted T cells, whereas MR1 presents vitamin B-based ligands and a variety of drugs and drug-like molecules to MR1-restricted T cells. The CD1 family of antigen presenting molecules has been divided into two groups: Group 1 contains CD1a, CD1b and CD1c, and Group 2 contains CD1d. Additionally, CD1e is expressed intracellularly and is involved in the loading of lipid antigens onto Group 1 CD1 proteins. Humans express both Groups 1 and 2 CD1 proteins, whereas mice only express CD1d. Group 1 CD1 proteins present lipid antigens to T cells that generally express diverse T cell receptors (TCRs) and exhibit adaptive-like functions, whereas CD1d presents lipid antigens to subsets of T cells that express either diverse or highly restricted TCRs and exhibit innate-like functions. CD1d-restricted T cells are called natural killer T (NKT) cells, which includes Type I or invariant NKT (iNKT) cells expressing semi-invariant TCRs, and Type II NKT cells expressing more diverse TCRs. CD1-restricted T cells have been implicated in a wide variety of diseases, including cancer, infections, and autoimmune, inflammatory and metabolic diseases. Additionally, NKT cells have been targeted for immunotherapy of disease with ligands such as α-galactosylceramide for iNKT cells, or sulfatide for Type II NKT cells. Like iNKT cells, MR1-restricted T cells express semi-invariant TCRs and display innate-like functions. MR1-restricted T cells, also called mucosal-associated invariant T (MAIT) cells, have been implicated in immune responses against a variety of pathogens such as Mycobacterium tuberculosis, Pseudomonas aeruginosa, Helicobacter pylori, hepatitis C virus and influenza virus. Moreover, these cells contribute to autoimmune and inflammatory diseases, including colitis, rheumatoid arthritis, psoriasis, lupus, and diabetes.

The Role of Glycans in Immune Cell Functions

Glycans represent a major constituency of post-translational modifications that occur on most, if not all, proteins. Whether on mammalian or invertebrate cell surfaces, they exist as sugar chain moieties designed from the exquisite and coordinated activity of cell-specific glycosylation. Some of the more common glycan structures are linked to cell surface polypeptides via an asparagine (N)-linked residue or a serine/threonine (O)-linked residue, along with a notable contingent found linked to ceramides in the lipid bilayer known as glycosphingolipids. These glycans can associate with complementary glycan-binding proteins (GBP) or lectins to mediate and translate this carbohydrate recognition to cell function. In immunity, there is increasing evidence that precise immune cell glycans are recognized by corresponding GBPs in a cell-intrinsic or -extrinsic manner. Unique carbohydrate recognition domains within GBPs are comprised of precisely spaced amino acid functional groups that allow for selective engagement of a particular glycan target. This structure-function relationship is present in immune signaling pathways, whereby glycans and GBPs on the surface of immune cells (and non-immune cells) help control processes such as immune cell activation, recognition of pathogens, suppression and tissue-specific migration. The diversity of glycan structures and glycosylation among individual immune cell subsets is controlled by the expression of genes involved in glycan biosynthesis including glycosyltransferases, glycosidases, glycan-precursor biosynthetic enzymes and nucleotide-sugar transporters. These genes represent more than 3% of the human genome, and cell-specific expression of these genes dictates a cell’s glycan repertoire, ultimately influencing its molecular interactions with GBPs. Altogether, these emerging lines of investigation highlight the regulatory capacity of glycans in immune health and disease, which in turn, pave the way for novel diagnostic, prognostic, and therapeutic strategies.