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Mucosal associated invariant T cell

Mucosal associated invariant T cells (MAIT cells) make up a subset of T cells in the immune system that display innate, effector-like qualities.[1][2] In humans, MAIT cells are found in the blood, liver, lungs, and mucosa, defending against microbial activity and infection.[1] The MHC class I-like protein, MR1, is responsible for presenting bacterially-produced vitamin B metabolites to MAIT cells.[3][4][5] After the presentation of foreign antigen by MR1, MAIT cells secretes pro-inflammatory cytokines and are capable of lysing bacterially-infected cells.[1][5] MAIT cells can also be activated through MR1-independent signaling.[5] In addition to possessing innate-like functions, this T cell subset supports the adaptive immune response and has a memory-like phenotype.[1] Furthermore, MAIT cells are thought to play a role in autoimmune diseases, such as multiple sclerosis, arthritis and inflammatory bowel disease,[6][7] although definitive evidence is yet to be published.


Molecular characteristicsEdit

MAIT cells constitute a subset of αβ T lymphocytes characterized by a semi-invariant T cell receptor alpha (TCRα) chain. The TCRα originates from the rearrangement of TCRα variable (V) and joining (J) gene segments TRAV1-2/TRAJ12/20/33 during VDJ recombination in the nucleus. However, TRAJ33 is expressed more often than TRAJ12 and TRAJ20.[3][8] With little diversity in the TCRα chain, the TCR is more conserved in MAIT cells than in other T cell subsets. In addition, the TCRα chain can combine with a restricted number of possible TCRβ chains to form a functional MAIT cell TCR, further limiting TCR diversity.[9]

MAIT cells were initially specified as T cells that do not express the TCR co-receptors CD4 or CD8 on the cell surface.[10] However, CD8+ MAIT cells have been recently observed.[1] In humans, MAIT cells express high levels of CD161, interleukin-18 (IL-18) receptor, and chemokine receptors CCR5, CXCR6, and CCR6 on the cell surface.[1] Additionally, as an indication of their memory-like phenotype in the periphery, mature MAIT cells express a CD44+, CD45RO+, CCR7, CD62Llo phenotype.[6][11][12]

Development & presence in the bodyEdit

Like all T cell subsets, MAIT cells develop in the thymus. Here, T cells rearrange their TCRs and are subjected to TCR affinity tests as a part of positive selection and negative selection.[8] However, rather than undergoing selection on MHC class I or II molecules, MAIT cells interact with the MHC class I-like molecule, MR1, on thymocytes. MR1 also serves as the antigen-presenting molecule outside of the thymus that binds to TCR and activates MAIT cells.[8] MAIT cells display effector-like qualities before leaving the thymus, which is why they are often described as innate-like T cells in the peripheral tissue.[1] This thymic development process is found in both mice and human MAIT cell populations.[12]

In healthy humans, MAIT cells are found in the lungs, liver, joints, blood, and mucosal tissues, such as the intestinal mucosa. In total, MAIT cells make up roughly 5% of the peripheral T cell population.[6] MAIT cells are most common in the liver, where they usually comprise 20-40% of the T lymphocyte population.[6] The total murine MAIT cell population is roughly ten times smaller than the human MAIT cell population.[12]

While MAIT cells display effector characteristics immediately out of the thymus, they may also undergo clonal expansion in the periphery and establish antigen memory.[1][6] In this way, MAIT cells display both innate and adaptive characteristics.

MAIT cell activationEdit

MAIT cells can be activated in ways that involve, and do not involve, MR1-mediated antigen presentation. However, MR1-independent and MR1-dependent activation elicit separate MAIT cell functions as part of an immune response.[5] During MR1-independent activation against Mycobacteria, MAIT cells bind extracellular IL-12, which is often secreted by stressed macrophages.[13] In response to IL-12, MAIT cells produce and secrete interferon-gamma (IFN-γ), a cytokine that activate macrophages, assists in the maturation of dendritic cells, and promotes the expression of MHC class II on antigen presenting cells.[14] MAIT cells also secrete IL-17, an important pro-inflammatory cytokine, after binding IL-23.[15]

MAIT cells are also activated in a MR1-dependent manner, in which a MAIT cell's semi-invariant TCR binds to the MR1 protein presenting antigen. While most T cell subsets have TCRs that recognize peptide or lipid-based antigens in association with MHC or CD1, MAIT cells are unique in that they recognize small molecules created through the process of vitamin B2 (riboflavin) and B9 (folic acid) biosynthesis.[16][3][17] The vitamin B2 related molecules that activate MAIT cells are chemically unstable, and undergo spontaneous degradation in water, although they have now been successfully chemically synthesised and isolated.[16][18] Riboflavin and folic acid are both crucial components of the metabolic pathways in bacteria.[3] When MR1 associates with these small molecules and becomes expressed on the surface of antigen-presenting cells, MAIT cell TCRs then bind to MR1, leading to MAIT cell activation, clonal expansion, memory, and an array of antimicrobial responses.[1] While protective against some pathogens, MAIT cell activation can produce inflammatory cytokines that augment immunopathology and gastritis in chronic infection by Helicobacter pylori.[19]

MAIT cell antigensEdit

MAIT cells are activated by compounds derived from bacterial vitamin B2 (riboflavin) biosynthesis.[17] In 2014, the exact identity of the antigens were found to be the compounds 5-OP-RU (5-(2-oxopropylideneamino)-6-D-ribitylaminouracil) and 5-OE-RU (5-(2-oxoethylideneamino)-6-D-ribitylaminouracil).[16] Both compounds are highly potent in activating MAIT cells, but are chemically unstable.[18] Both have been chemically synthesised, stabilised and characterised in the solvent DMSO, allowing for the unstable compounds to be used as reagents for the study of MAIT cells.[18]

A chemically stable antigen that is functionally similar to 5-OP-RU has also been created.[18]

A 2017 study also found that some common drugs and drug-like molecules can modulate MAIT cell function in mammals.[20]


Like MHC class I, MR1 is found in all a large variety of cells and associates with β2-microglobulin.[21] However, it remains to be understood whether certain cell types, such as myeloid or epithelial cells, more commonly display antigen to MAIT. While MHC class I alleles are extremely diverse in human populations, MR1 is non-polymorphic and highly conserved.[8] In fact, when comparing the genetic content of humans and mice to each other, there is a 90% similarity in MR1 coding sequences.[22] Furthermore, the ligand-binding grooves of MR1 molecules differ from those of MHC class I molecules in that they are smaller in size and specifically bind metabolic products of bacteria.[3]

MR1 is found intracellularly in the endoplasmic reticulum and interacts with some of the common MHC loading complex components and chaperone proteins (e.g. TAP, ERp57, and tapasin).[23] The loading of vitamin B metabolic molecules onto MR1 occurs in a way that is different from peptide loading onto MHC class I.[3] Yet the specifics of this process must be further looked into.

In healthy cells, MR1 is sparsely exhibited on the cell surface. However, MR1 expression is upregulated on the surface after cell infection or the introduction of a bacterially-produced MR1 ligand.[6] Once expressed on the surface, MR1, with its antigen ligand covalently-attached, binds to the appropriate MAIT cell TCR.[5]

Microbial and viral responseEdit

MAIT cells display effector-like qualities, allowing them to directly respond to microbial pathogens immediately following activation. In a MR1-dependent manner, MAIT cells respond to bacteria by producing cytokines and strengthening their cytotoxic functions.[1] After TCR binding and activation, MAIT cells secrete several cytokines, including tumor necrosis factor alpha (TNF-α), IFN-γ, and IL-17.[6] These cytokines are pro-inflammatory and activate important cells in the immune response, such as macrophages and dendritic cells.[6][14] After activation, MAIT cells also produce cytolytic molecules perforin and granzyme B, which form pores in the bacterially-infected cells, leading to apoptosis and the elimination of dangerous microbes from the body.[1]

MAIT cells can target a wide variety of microbes, including Staphylococcus aureus, Escherichia coli Mycobacterium tuberculosis, Candida albicans, and Salmonella enterica, to name a few.[4][24] However, some types of bacteria, including strains of Listeria and Enterobacter, may escape MAIT cell targeting. These strains avoid MAIT cell-mediated elimination because they have unusual riboflavin metabolic pathways that do not produce viable ligands for MR1 molecules.[3]

While MAIT cells have not been found to target viruses in a TCR-dependent manner, they can respond against viruses upon stimulation with IL-18 and other cytokines, such as IL-12 and IFN-α/β.[25] After receiving these cytokine signals, MAIT cells secrete anti-viral cytotoxic molecules and cytokines that aid the immune response.[25]

Role in autoimmunityEdit

While MAIT cells play a crucial role in the immune system by targeting bacterially-infected cells and other pathogens, they may also attack healthy cells and play a role in certain autoimmune diseases.[6]

Multiple sclerosisEdit

For individuals with the autoimmune disease multiple sclerosis (MS), the immune system attacks the myelin sheaths covering nerves, causing impaired nerve signaling.[26] While T helper 1 (Th1) and T helper 17 (Th17) cells have been reported as contributors to MS by increasing inflammation at myelin sites, human MAIT cells have also been observed at these sites.[6][7] In addition, during periods of myelin degeneration, MAIT cell levels in the peripheral blood have been found to decrease, suggesting their tendency to migrate to sites of MS-related inflammation. At these sites, MAIT cells further contribute to the autoimmune response by secreting pro-inflammatory cytokines.[7] However, in contrast to these findings, MAIT cells have also been found to display a protective role in MS by limiting Th1 cell secretion of IFN-γ at sites of inflammation.[27] To explain these findings, the role of MAIT cells in MS must be further explored.

Inflammatory bowel diseaseEdit

In autoimmune-related inflammatory bowel disease, the immune system initiates a response against healthy parts of the gastrointestinal tract, such as the mucosal microbiome.[28] During relapse periods of certain types of inflammatory bowel disease, such as Crohn’s disease, MAIT cells have been found to migrate to sites of inflammation, triggering the harmful responses of other immune cells through the expression of NKG2D and increasing inflammation by secreting IL-17.[6]

Rheumatic diseaseEdit

In systematic autoimmune rheumatic diseases, such as rheumatoid arthritis and systemic lupus erythematosus (SLE), MAIT cells are activated through TCR-independent signaling.[6][15] Stimulated by IL-12, IL-18, and IL-23, MAIT cells can produce and secrete pro-inflammatory cytokines, drawing immune cells into areas of the autoimmune attack.[6][15] In this way, MAIT cells facilitate and intensify the harmful effects of systematic autoimmune rheumatic diseases.

See alsoEdit


  1. ^ a b c d e f g h i j k Napier, Ruth J.; Adams, Erin J.; Gold, Marielle C.; Lewinsohn, David M. (2015-07-06). "The Role of Mucosal Associated Invariant T Cells in Antimicrobial Immunity". Frontiers in Immunology. 6: 344. doi:10.3389/fimmu.2015.00344. ISSN 1664-3224. PMC 4492155. PMID 26217338.
  2. ^ Gold, Marielle C.; Lewinsohn, David M. (2017-02-12). "Mucosal associated invariant T cells and the immune response to infection". Microbes and Infection / Institut Pasteur. 13 (8–9): 742–748. doi:10.1016/j.micinf.2011.03.007. ISSN 1286-4579. PMC 3130845. PMID 21458588.
  3. ^ a b c d e f g Eckle, Sidonia B. G.; Corbett, Alexandra J.; Keller, Andrew N.; Chen, Zhenjun; Godfrey, Dale I.; Liu, Ligong; Mak, Jeffrey Y. W.; Fairlie, David P.; Rossjohn, Jamie (2015-12-18). "Recognition of Vitamin B Precursors and Byproducts by Mucosal Associated Invariant T Cells". The Journal of Biological Chemistry. 290 (51): 30204–30211. doi:10.1074/jbc.R115.685990. ISSN 0021-9258. PMC 4683245. PMID 26468291.
  4. ^ a b Ussher, James E.; Klenerman, Paul; Willberg, Chris B. (2014-10-08). "Mucosal-Associated Invariant T-Cells: New Players in Anti-Bacterial Immunity". Frontiers in Immunology. 5: 450. doi:10.3389/fimmu.2014.00450. ISSN 1664-3224. PMC 4189401. PMID 25339949.
  5. ^ a b c d e Howson, Lauren J.; Salio, Mariolina; Cerundolo, Vincenzo (2015-06-16). "MR1-Restricted Mucosal-Associated Invariant T Cells and Their Activation during Infectious Diseases". Frontiers in Immunology. 6: 303. doi:10.3389/fimmu.2015.00303. ISSN 1664-3224. PMC 4468870. PMID 26136743.
  6. ^ a b c d e f g h i j k l m Hinks, Timothy S. C. (2016). "Mucosal‐associated invariant T cells in autoimmunity, immune‐mediated diseases and airways, disease". Immunology. 148 (1): 1–12. doi:10.1111/imm.12582. ISSN 0019-2805. PMC 4819138. PMID 26778581.
  7. ^ a b c Bianchini, Elena; De Biasi, Sara; Simone, Anna Maria; Ferraro, Diana; Sola, Patrizia; Cossarizza, Andrea; Pinti, Marcello (2017-03-01). "Invariant natural killer T cells and mucosal-associated invariant T cells in multiple sclerosis". Immunology Letters. 183: 1–7. doi:10.1016/j.imlet.2017.01.009. PMID 28119072.
  8. ^ a b c d Treiner, Emmanuel; Duban, Livine; Bahram, Seiamak; Radosavljevic, Mirjana; Wanner, Valerie; Tilloy, Florence; Affaticati, Pierre; Gilfillan, Susan; Lantz, Olivier (2003-03-13). "Selection of evolutionarily conserved mucosal-associated invariant T cells by MR1". Nature. 422 (6928): 164–169. doi:10.1038/nature01433. ISSN 0028-0836. PMID 12634786.
  9. ^ Lepore, Marco; Kalinicenko, Artem; Colone, Alessia; Paleja, Bhairav; Singhal, Amit; Tschumi, Andreas; Lee, Bernett; Poidinger, Michael; Zolezzi, Francesca (2014-05-15). "Parallel T-cell cloning and deep sequencing of human MAIT cells reveal stable oligoclonal TCRβ repertoire". Nature Communications. 5: 3866. doi:10.1038/ncomms4866. ISSN 2041-1723. PMID 24832684.
  10. ^ Porcelli, S.; Yockey, C. E.; Brenner, M. B.; Balk, S. P. (1993-07-01). "Analysis of T cell antigen receptor (TCR) expression by human peripheral blood CD4-8- alpha/beta T cells demonstrates preferential use of several V beta genes and an invariant TCR alpha chain". Journal of Experimental Medicine. 178 (1): 1–16. doi:10.1084/jem.178.1.1. ISSN 0022-1007. PMC 2191070. PMID 8391057.
  11. ^ Sakala, Isaac G.; Kjer-Nielsen, Lars; Eickhoff, Christopher S.; Wang, Xiaoli; Blazevic, Azra; Liu, Ligong; Fairlie, David P.; Rossjohn, Jamie; McCluskey, James (2015-07-15). "Functional heterogeneity and anti-mycobacterial effects of mouse mucosal associated invariant T (MAIT) cells specific for riboflavin metabolites". Journal of Immunology. 195 (2): 587–601. doi:10.4049/jimmunol.1402545. ISSN 0022-1767. PMC 4490942. PMID 26063000.
  12. ^ a b c Rahimpour, Azad; Koay, Hui Fern; Enders, Anselm; Clanchy, Rhiannon; Eckle, Sidonia B.G.; Meehan, Bronwyn; Chen, Zhenjun; Whittle, Belinda; Liu, Ligong (2015-06-29). "Identification of phenotypically and functionally heterogeneous mouse mucosal-associated invariant T cells using MR1 tetramers". The Journal of Experimental Medicine. 212 (7): 1095–1108. doi:10.1084/jem.20142110. ISSN 0022-1007. PMC 4493408. PMID 26101265.
  13. ^ Chua, Wei-Jen; Truscott, Steven M.; Eickhoff, Christopher S.; Blazevic, Azra; Hoft, Daniel F.; Hansen, Ted H. (2017-02-25). "Polyclonal Mucosa-Associated Invariant T Cells Have Unique Innate Functions in Bacterial Infection". Infection and Immunity. 80 (9): 3256–3267. doi:10.1128/IAI.00279-12. ISSN 0019-9567. PMC 3418730. PMID 22778103.
  14. ^ a b Boehm, U.; Klamp, T.; Groot, M.; Howard, J. C. (1997-01-01). "Cellular responses to interferon-gamma". Annual Review of Immunology. 15: 749–795. doi:10.1146/annurev.immunol.15.1.749. ISSN 0732-0582. PMID 9143706.
  15. ^ a b c Chiba, Asako; Tajima, Ryohsuke; Tomi, Chiharu; Miyazaki, Yusei; Yamamura, Takashi; Miyake, Sachiko (2012-01-01). "Mucosal-associated invariant T cells promote inflammation and exacerbate disease in murine models of arthritis". Arthritis & Rheumatism. 64 (1): 153–161. doi:10.1002/art.33314. ISSN 1529-0131. PMID 21904999.
  16. ^ a b c Corbett, Alexandra J.; Eckle, Sidonia B. G.; Birkinshaw, Richard W.; Liu, Ligong; Patel, Onisha; Mahony, Jennifer; Chen, Zhenjun; Reantragoon, Rangsima; Meehan, Bronwyn (2014). "T-cell activation by transitory neo-antigens derived from distinct microbial pathways". Nature. 509 (7500): 361–365. doi:10.1038/nature13160. PMID 24695216.
  17. ^ a b Kjer-Nielsen, Lars; Patel, Onisha; Corbett, Alexandra J.; Nours, Jérôme Le; Meehan, Bronwyn; Liu, Ligong; Bhati, Mugdha; Chen, Zhenjun; Kostenko, Lyudmila (2012). "MR1 presents microbial vitamin B metabolites to MAIT cells". Nature. 491 (7426): 717–723. doi:10.1038/nature11605. PMID 23051753.
  18. ^ a b c d Mak, Jeffrey Y. W.; Xu, Weijun; Reid, Robert C.; Corbett, Alexandra J.; Meehan, Bronwyn S.; Wang, Huimeng; Chen, Zhenjun; Rossjohn, Jamie; McCluskey, James (2017-03-08). "Stabilizing short-lived Schiff base derivatives of 5-aminouracils that activate mucosal-associated invariant T cells". Nature Communications. 8: 14599. doi:10.1038/ncomms14599. ISSN 2041-1723. PMC 5344979. PMID 28272391.
  19. ^ D’Souza, Criselle; Pediongco, Troi; Wang, Huimeng; Scheerlinck, Jean-Pierre Y.; Kostenko, Lyudmila; Esterbauer, Robyn; Stent, Andrew W.; Eckle, Sidonia B. G.; Meehan, Bronwyn S. (2018-03-01). "Mucosal-Associated Invariant T Cells Augment Immunopathology and Gastritis in Chronic Helicobacter pylori Infection". The Journal of Immunology. 200 (5): 1901–1916. doi:10.4049/jimmunol.1701512. ISSN 0022-1767. PMID 29378910.
  20. ^ Keller, Andrew N; Eckle, Sidonia B G; Xu, Weijun; Liu, Ligong; Hughes, Victoria A; Mak, Jeffrey Y W; Meehan, Bronwyn S; Pediongco, Troi; Birkinshaw, Richard W (2017). "Drugs and drug-like molecules can modulate the function of mucosal-associated invariant T cells" (PDF). Nature Immunology. 18 (4): 402–411. doi:10.1038/ni.3679. PMID 28166217.
  21. ^ Yamaguchi, Hisateru; Hashimoto, Keiichiro (2002-01-18). "Association of MR1 protein, an MHC class I-related molecule, with beta(2)-microglobulin". Biochemical and Biophysical Research Communications. 290 (2): 722–729. doi:10.1006/bbrc.2001.6277. ISSN 0006-291X. PMID 11785959.
  22. ^ Yamaguchi, H.; Hirai, M.; Kurosawa, Y.; Hashimoto, K. (1997-09-29). "A highly conserved major histocompatibility complex class I-related gene in mammals". Biochemical and Biophysical Research Communications. 238 (3): 697–702. doi:10.1006/bbrc.1997.7379. ISSN 0006-291X. PMID 9325151.
  23. ^ Miley, Michael J.; Truscott, Steven M.; Yu, Yik Yeung Lawrence; Gilfillan, Susan; Fremont, Daved H.; Hansen, Ted H.; Lybarger, Lonnie (2003-06-15). "Biochemical Features of the MHC-Related Protein 1 Consistent with an Immunological Function". The Journal of Immunology. 170 (12): 6090–6098. doi:10.4049/jimmunol.170.12.6090. ISSN 0022-1767. PMID 12794138.
  24. ^ Gold, Marielle C.; Cerri, Stefania; Smyk-Pearson, Susan; Cansler, Meghan E.; Vogt, Todd M.; Delepine, Jacob; Winata, Ervina; Swarbrick, Gwendolyn M.; Chua, Wei-Jen (2010-06-29). "Human Mucosal Associated Invariant T Cells Detect Bacterially Infected Cells". PLOS Biology. 8 (6): e1000407. doi:10.1371/journal.pbio.1000407. ISSN 1545-7885. PMC 2893946. PMID 20613858.
  25. ^ a b van Wilgenburg, Bonnie; Scherwitzl, Iris; Hutchinson, Edward C.; Leng, Tianqi; Kurioka, Ayako; Kulicke, Corinna; de Lara, Catherine; Cole, Suzanne; Vasanawathana, Sirijitt (2016-06-23). "MAIT cells are activated during human viral infections". Nature Communications. 7: 11653. doi:10.1038/ncomms11653. ISSN 2041-1723. PMC 4931007. PMID 27337592.
  26. ^ Karussis, Dimitrios (2014-02-01). "The diagnosis of multiple sclerosis and the various related demyelinating syndromes: a critical review". Journal of Autoimmunity. 48–49: 134–142. doi:10.1016/j.jaut.2014.01.022. ISSN 1095-9157. PMID 24524923.
  27. ^ Miyazaki, Y.; Miyake, S.; Chiba, A.; Lantz, O.; Yamamura, T. (2011-09-01). "Mucosal-associated invariant T cells regulate Th1 response in multiple sclerosis". International Immunology. 23 (9): 529–535. doi:10.1093/intimm/dxr047. ISSN 0953-8178. PMID 21712423.
  28. ^ Baumgart, Daniel C; Carding, Simon R (2007-05-18). "Inflammatory bowel disease: cause and immunobiology". The Lancet. 369 (9573): 1627–1640. doi:10.1016/S0140-6736(07)60750-8. PMID 17499605.