γδ T cells are a subset of T cells which express a TCR γ chain in combination with a TCR δ chain. In humans, peripheral γδ T cells most frequently utilize the Vγ9 and Vδ2 chains, and represent 1–5% of T cells in healthy adults , but can reach up to 50% of T cells in a matter of days following an infection . γδ T cells are enriched in the human intestine, mostly as intraepithelial lymphocytes (IELs). In contrast to αβ T cells, which are MHC-restricted, γδ T cells can recognize antigens in both MHC-dependent and MHC-independent ways.
γδ T cells are activated by both microbial and host-derived compounds. They recognize the microbial compound (E)-4-hydroxyl-3-methyl-but-2-enyl pyrophosphate (HMB-PP)TL , an essential metabolite in isoprenoid biosynthesis, generated by the majority of gram-negative bacteria and some gram-positive bacteria. They also recognize host-derived phosphoantigens such as isopentenyl pyrophosphate. Recent study has focused on the mechanism behind phosphoantigen sensing and presentation, and several studies have identified butyrophilins as responsible for presenting HMB-PP [5–9]. Once activated, γδ T cells have a range of functions, such as killing infected or stressed target cells, priming CD4+ and CD8+ T cells, providing B-cell help, inducing dendritic cell maturation, and promoting survival of neutrophils and monocytes .
Recent studies have demonstrated the role that γδ T cells play in limiting transepithelial pathogen invasion. Edelblum et al. observed that higher numbers of Salmonella typhimurium are seen in the gut of TCR δ defective mice, and that migration of γδ IELs was critical to their function. This effect may be related to their influence on the intestinal mucus layer, as Kober et al. found that γδ-deficient mice had alterations in goblet cells and crypt length in the small intestine. A later study by the same group showed that γδ-deficient mice displayed an altered O-glycan profile in the small intestine compared to wild-type littermates . Further evidence for the importance of γδ T cells in combating intestinal infections comes from research on necrotizing enterocolitis (NEC) [14▪]. Comparison of NEC ileal resections with non-NEC controls showed that γδ T IELs are reduced in NECs, and associated with decreased RAR-related orphan receptor C – a Th17 transcription factor. The authors postulated that interleukin (IL)-17 produced by γδ T cells plays a role in promoting intestinal barrier production early in life, and provided support for this by demonstrating an increase in severity of experimental gut injury in TCRδ-deficient mice.
There is increasing evidence that γδ T cells may share characteristics of, and possibly influence, the adaptive αβ T-cell response. Sheridan et al. showed that the mucosal γδ T-cell response following oral Listeria monocytogenes was retained long term and underwent extensive expansion upon oral challenge, displaying memory-like characteristics. Furthermore, γδ T cells may also act by their influence on αβ T cells in the gut, as McCarthy et al. showed that Vγ9δ2 T cells display gut homing potential upon microbial activation and populate the human intestinal mucosa. These γδ T cells mediated their effect via tumor necrosis factor (TNF)-α and interferon (IFN)-γ upon antigen exposure, and enhanced inflammation by stimulating production of IFN-γ and T-bet expression in colonic αβ T cells.
Taken together, γδ T cells likely play an important role in maintaining gut homeostasis and immunity to pathogens, though most studies of intestinal γδ T cells are limited to mouse models, and their results must be interpreted with the limited homology between mouse and human γδ T cells in mind.
Several recent studies have examined the adaptive capacity of human MAIT cells, which, despite their invariant Vα chain, features variability in both Jα and Vβ chains usage [25▪]. Gold et al.[26▪▪] found that among MAIT cells, different pathogen-specific responses were characterized by distinct TCR usage, both between and within individuals. MAIT cell clones with distinct TCRs were also found to respond differently to a riboflavin metabolite. Their heterogeneity may allow MAIT cells to fine-tune their response to bacterial metabolite variants in the gut [27▪▪]. Soudais et al.[28▪] observed that in mice, most if not all of the MAIT cell ligands in Escherichia coli are related to the riboflavin biosynthetic pathway and display very limited heterogeneity.
HIV infection can result in longstanding damage to the intestinal epithelial barrier and translocation of microbial products from the gut lumen. Several recent studies have revealed that in HIV infection, peripheral blood MAIT cells are decreased [29,30▪–32▪,33,34], possibly due to activation of MAIT cells by translocated microbial products . Such a reduction in MAIT cells is noted in elite controllers [32▪], and does not recover even after successful ART, though long-term ART leads to restoration of MAIT cells in the colon, but not the peripheral blood [30▪]. The possibility remains that decreases of MAIT cells in peripheral blood may be a consequence of migration of MAIT cells to affected tissue, instead of or in combination with depletion of MAIT cells through activation.
Data on the role of MAIT cells in immune responses against intestinal infections are limited. Our group [36▪] has shown that in Vibrio cholerae infection, circulating MAIT cells are activated, and that in children, but not in adults, the frequency of MAIT cells is decreased for at least 90 days after infection. We also found an association of MAIT cells with increases in lipopolysaccharide-specific class-switched antibody responses. This finding is in agreement with a previous finding that MAIT cells are associated with increased antibody-secreting cell response to Shigella lipopolysaccharide in humans given an experimental Shigella vaccine .
Despite their sizable presence in the intestinal mucosa, our knowledge of the mechanisms underlying MAIT cell proliferation and effect is limited by the lack of suitable animal models. MAIT cells have been proposed to be a potential target of mucosal vaccination , and further study on such interventions is needed.
Although much work on iNKT cells has focused on their role in antitumor and autoimmunity, several studies have identified their importance in modulation of immune responses against viral infections . Most recently, in a neonatal mouse model, Zhu et al.[44▪] demonstrated that enterovirus 71 (EV71) infection led to activation of iNKT cells, through a TLR3-mediated mechanism. They found that iNKT cells are involved in protection against EV71 infection, and that CD1d is essential for this protection. Similarly, in a murine model of oral S. typhimurium infection, Selvanantham et al. found that infected mice had higher frequency of iNKT cells in the lamina propria, and that iNKTs produce IFN-γ, in a process mediated by the cytosolic peptidoglycan receptors Nod1 and Nod2. Much remains to be determined in how this rare cell type may be involved in defense against intestinal pathogens in humans.
Group 3 ILCs (ILC3) are involved in the development of intestinal lymphoid organs, and reside primarily in the small intestine lamina propria. These cells express RORγt, and respond to IL-23 and IL-1β via production of IL-22 and IL-17 . The production of IL-22 by ILC3s has been shown to mediate protection against bacterial pathogens , and ILC3s can directly stimulate CD4+ T cells [51▪] and also interact with B cells to aid in T-cell-independent antibody production [52▪].
There is increasing evidence that ILC3s are critically involved in intestinal homeostasis. Mortha et al.[53▪] showed that ILC3s are the primary source of granulocyte macrophage colony-stimulating factor (GM-CSF) in the gut, and that deficient production of GM-CSF led to reduced T regulatory (Treg) numbers and impaired oral tolerance. ILC-driven GM-CSF production was dependent on the ability of macrophages to sense microbial signals and produce IL-1β. Similarly, Hepworth et al.[54▪▪] showed that loss of RORγt + ILCs was associated with dysregulated adaptive immune responses against commensal bacteria and low-grade systemic inflammation, and found that ILCs act as APCs and limit commensal bacteria-specific CD4+ T-cell responses by inducing cell death via MHC class II-dependent mechanisms [54▪▪]. On the contrary, Korn et al.[55▪] observed that CD4+ T cells were found to regulate the number and function of IL-22-producing ILCs and production of antimicrobial peptides. Additionally, recent studies by Goto et al.[56▪▪] and Pickard et al.[57▪] showed that microbial signals leading to production of IL-22 by ILC3s induce intestinal epithelial cell fucosylation. Through experiments using fucosylation-deficient mice, they also showed that fucosylation contributes to protection against S. typhimurium infection and host tolerance of Citrobacter rodentium. Taken together, ILC3 mediates protection against intestinal pathogens through interactions with both microbes and host epithelium.
Unfortunately, nearly the entire body of knowledge on intestinal ILCs is based on studies in animal models, and studies examining the activity of ILCs in the human intestine are needed.
Papers of particular interest, published within the annual period of review, have been highlighted as:
1. Qin J, Li R, Raes J, et al. A human gut microbial gene catalogue established by metagenomic sequencing. Nature 2010; 464:59–65.
2. Dar AA, Patil RS, Chiplunkar SV. Insights into the relationship between toll like receptors and gamma delta T cell responses. Front Immunol 2014; 5:366.
3. Morita CT, Jin C, Sarikonda G, Wang H. Nonpeptide antigens, presentation mechanisms, and immunological memory of human Vgamma2Vdelta2 T cells: discriminating friend from foe through the recognition of prenyl pyrophosphate antigens. Immunol Rev 2007; 215:59–76.
4. Gu S, Nawrocka W, Adams EJ. Sensing of pyrophosphate metabolites by Vgamma9Vdelta2 T cells. Front Immunol 2014; 5:688.
5. Vavassori S, Kumar A, Wan GS, et al. Butyrophilin 3A1 binds phosphorylated antigens and stimulates human gammadelta T cells. Nat Immunol 2013; 14:908–916.
6. Harly C, Guillaume Y, Nedellec S, et al. Key implication of CD277/butyrophilin-3 (BTN3A) in cellular stress sensing by a major human gammadelta T-cell subset. Blood 2012; 120:2269–2279.
7. Sandstrom A, Peigne CM, Leger A, et al. The intracellular B30.2 domain of butyrophilin 3A1 binds phosphoantigens to mediate activation of human Vgamma9Vdelta2 T cells. Immunity 2014; 40:490–500.
8. Hsiao CH, Lin X, Barney RJ, et al. Synthesis of a phosphoantigen prodrug that potently activates Vgamma9Vdelta2 T-lymphocytes. Chem Biol 2014; 21:945–954.
9. Rhodes DA, Chen HC, Price AJ, et al. Activation of human gammadelta T cells by cytosolic interactions of BTN3A1 with soluble phosphoantigens and the cytoskeletal adaptor periplakin. J Immunol 2015; 194:2390–2398.
10. Vantourout P, Hayday A. Six-of-the-best: unique contributions of gammadelta T cells to immunology. Nat Rev Immunol 2013; 13:88–100.
11. Edelblum KL, Singh G, Odenwald MA, et al. gammadelta Intraepithelial lymphocyte migration limits transepithelial pathogen invasion and systemic disease in mice. Gastroenterology 2015; 148:1417–1426.
12. Kober OI, Ahl D, Pin C, et al. gammadelta T-cell-deficient mice show alterations in mucin expression, glycosylation, and goblet cells but maintain an intact mucus layer. Am J Physiol Gastrointest Liver Physiol 2014; 306:G582–G593.
13. Fuell C, Kober OI, Hautefort I, Juge N. Mice deficient in intestinal gammadelta intraepithelial lymphocytes display an altered intestinal O-glycan profile compared with wild-type littermates. Glycobiology 2015; 25:42–54.
14▪. Weitkamp JH, Rosen MJ, Zhao Z, et al. Small intestinal intraepithelial TCRgammadelta+ T lymphocytes are present in the premature intestine but selectively reduced in surgical necrotizing enterocolitis. PLoS One 2014; 9:e99042.
Shows the important role of γδ T cells in preventing the development of NEC in newborn infants.
15. Sheridan BS, Romagnoli PA, Pham QM, et al. Gammadelta T cells exhibit multifunctional and protective memory in intestinal tissues. Immunity 2013; 39:184–195.
16. McCarthy NE, Bashir Z, Vossenkamper A, et al. Proinflammatory Vdelta2+ T cells populate the human intestinal mucosa and enhance IFN-gamma production by colonic alphabeta T cells. J Immunol 2013; 191:2752–2763.
17. Le Bourhis L, Guerri L, Dusseaux M, et al. Mucosal-associated invariant T cells
: unconventional development and function. Trends Immunol 2011; 32:212–218.
18. Ussher JE, Klenerman P, Willberg CB. Mucosal-associated invariant T-cells: new players in antibacterial immunity. Front Immunol 2014; 5:450.
19. Reantragoon R, Corbett AJ, Sakala IG, et al. Antigen-loaded MR1 tetramers define T cell receptor heterogeneity in mucosal-associated invariant T cells
. J Exp Med 2013; 210:2305–2320.
20▪. Corbett AJ, Eckle SB, Birkinshaw RW, et al. T-cell activation by transitory neo-antigens derived from distinct microbial pathways. Nature 2014; 509:361–365.
Shows that MAIT cell activation requires key genes encoding enzymes that form 5-amino-6-D-ribitylaminouracil (5-A-RU), an early intermediate in bacterial riboflavin synthesis.
21. McWilliam HE, Birkinshaw RW, Villadangos JA, et al. MR1 presentation of vitamin B-based metabolite ligands. Curr Opin Immunol 2015; 34C:28–34.
22▪. Ussher JE, Bilton M, Attwod E, et al. CD161++ CD8+ T cells, including the MAIT cell subset, are specifically activated by IL-12+IL-18 in a TCR-independent manner. Eur J Immunol 2014; 44:195–203.
Shows that MAIT cell activation is not limited to recognition of bacterial metabolites by the TCR, potentially broadening the roles of MAIT cells to possibly include viral infections and other inflammatory stimuli.
23. Le Bourhis L, Dusseaux M, Bohineust A, et al. MAIT cells detect and efficiently lyse bacterially-infected epithelial cells. PLoS Pathog 2013; 9:e1003681.
24▪. Kurioka A, Ussher JE, Cosgrove C, et al. MAIT cells are licensed through granzyme exchange to kill bacterially sensitized targets. Mucosal Immunol 2015; 8:429–440.
Shows the ability of MAIT cells to directly kill bacterially exposed cells in an MR1 and degranulation-dependent manner.
25▪. Lepore M, Kalinichenko A, Colone A, et al. Parallel T-cell cloning and deep sequencing of human MAIT cells reveal stable oligoclonal TCRbeta repertoire. Nat Commun 2014; 5:3866.
Defines the Vβ usage for MAIT cells, finding that a small number of clonotypes accounts for the majority of MAIT cells in blood and liver, and that additional Vα7.2 rearrangements utilizing Jα12 and Jα20 exist and are functional for MAIT cells.
26▪▪. Gold MC, McLaren JE, Reistetter JA, et al. MR1-restricted MAIT cells display ligand discrimination and pathogen selectivity through distinct T cell receptor usage. J Exp Med 2014; 211:1601–1610.
Shows that MAIT cells may display adaptive properties by being able to respond to different pathogens based upon TCR sequence.
27▪▪. Eckle SB, Birkinshaw RW, Kostenko L, et al. A molecular basis underpinning the T cell receptor heterogeneity of mucosal-associated invariant T cells
. J Exp Med 2014; 211:1585–1600.
Shows how MAIT TCR heterogeneity can fine-tune MR1 recognition in an antigen-dependent manner, potentially modulating MAIT cell recognition.
28▪. Soudais C, Samassa F, Sarkis M, et al. In vitro and in vivo analysis of the gram-negative bacteria-derived riboflavin precursor derivatives activating mouse MAIT cells. J Immunol 2015; 194:4641–4649.
Shows that in Vα19 transgenic mice, most, if not all, MAIT cell ligands are related to the riboflavin biosynthetic pathway and display very limited heterogeneity.
29. Wong EB, Akilimali NA, Govender P, et al. Low levels of peripheral CD161++CD8+ mucosal associated invariant T (MAIT) cells are found in HIV and HIV/TB co-infection. PLoS One 2013; 8:e83474.
30▪. Greathead L, Metcalf R, Gazzard B, et al. CD8+/CD161++ mucosal-associated invariant T-cell levels in the colon are restored on long-term antiretroviral therapy and correlate with CD8+ T-cell immune activation. AIDS 2014; 28:1690–1692.
Shows that upon antiretroviral treatment, MAIT cell levels in the colon, but not the blood, are restored.
31▪. Fernandez CS, Amarasena T, Kelleher AD, et al. MAIT cells are depleted early but retain functional cytokine expression in HIV infection. Immunol Cell Biol 2015; 93:177–188.
Shows that Vα7.2+ CD161– cells, though increased in HIV infection, do not stain positive with an MR1 tetramer, and thus likely do not represent MAIT cells which have down-regulated CD161 upon activation.
32▪. Eberhard JM, Hartjen P, Kummer S, et al. CD161+ MAIT cells are severely reduced in peripheral blood and lymph nodes of HIV-infected individuals independently of disease progression. PLoS One 2014; 9:e111323.
Shows that the loss of MAIT cells as a result of HIV infections seems to be an early event that is independent of later disease stages, and is partially due to the vulnerability of MAIT cells to stimulation by microbial products and cytokines during HIV infection.
33. Cosgrove C, Ussher JE, Rauch A, et al. Early and nonreversible decrease of CD161++ /MAIT cells in HIV infection. Blood 2013; 121:951–961.
34. Leeansyah E, Ganesh A, Quigley MF, et al. Activation, exhaustion, and persistent decline of the antimicrobial MR1-restricted MAIT-cell population in chronic HIV-1 infection. Blood 2013; 121:1124–1135.
35. Vyboh K, Jenabian MA, Mehraj V, Routy JP. HIV and the gut microbiota, partners in crime: breaking the vicious cycle to unearth new therapeutic targets. J Immunol Res 2015; 2015:614127.
36▪. Leung DT, Bhuiyan TR, Nishat NS, et al. Circulating mucosal associated invariant T cells are activated in Vibrio cholerae O1 infection and associated with lipopolysaccharide antibody responses. PLoS Negl Trop Dis 2014; 8:e3076.
Shows activation of MAIT cells in response to cholera infection, and that changes in MAIT cell frequency correlated with changes in antibody levels.
37. Abautret-Daly AE, Davitt CJ, Lavelle EC. Harnessing the antibacterial and immunological properties of mucosal-associated invariant T cells
in the development of novel oral vaccines against enteric infections. Biochem Pharmacol 2014; 92:173–183.
38. Montalvillo E, Garrote JA, Bernardo D, Arranz E. Innate lymphoid cells
and natural killer T cells in the gastrointestinal tract immune system. Rev Esp Enferm Dig 2014; 106:334–345.
39. Brennan PJ, Brigl M, Brenner MB. Invariant natural killer T cells
: an innate activation scheme linked to diverse effector functions. Nat Rev Immunol 2013; 13:101–117.
40. Middendorp S, Nieuwenhuis EE. NKT cells in mucosal immunity. Mucosal Immunol 2009; 2:393–402.
41. Nau D, Altmayer N, Mattner J. Mechanisms of innate lymphoid cell and natural killer T cell activation during mucosal inflammation. J Immunol Res 2014; 2014:546596.
42. Dellabona P, Abrignani S, Casorati G. iNKT-cell help to B cells: a cooperative job between innate and adaptive immune responses. Eur J Immunol 2014; 44:2230–2237.
43. Juno JA, Keynan Y, Fowke KR. Invariant NKT cells: regulation and function during viral infection. PLoS Pathog 2012; 8:e1002838.
44▪. Zhu K, Yang J, Luo K, et al. TLR3 signaling in macrophages is indispensable for the protective immunity of invariant natural killer T cells
against enterovirus 71 infection. PLoS Pathog 2015; 11:e1004613.
Shows that iNKT cells may play a role in the control of enteric viral infections, especially in newborns when adaptive immunity is not fully developed.
45. Selvanantham T, Escalante NK, Cruz Tleugabulova M, et al. Nod1 and Nod2 enhance TLR-mediated invariant NKT cell activation during bacterial infection. J Immunol 2013; 191:5646–5654.
46. Spits H, Artis D, Colonna M, et al. Innate lymphoid cells
: a proposal for uniform nomenclature. Nat Rev Immunol 2013; 13:145–149.
47▪. Meylan F, Hawley ET, Barron L, et al. The TNF-family cytokine TL1A promotes allergic immunopathology through group 2 innate lymphoid cells
. Mucosal Immunol 2014; 7:958–968.
Shows that ILC2 are stimulated by TL1A via its receptor DR3, and that this may be a therapeutic target for allergic lung disease.
48▪. Oliphant CJ, Hwang YY, Walker JA, et al. MHCII-mediated dialog between group 2 innate lymphoid cells
and CD4(+) T cells potentiates type 2 immunity and promotes parasitic helminth expulsion. Immunity 2014; 41:283–295.
Shows that MHC II expression in ILC2 is necessary for interaction with antigen-specific T cells in promoting the expulsion of helminths.
49. Takatori H, Kanno Y, Watford WT, et al. Lymphoid tissue inducer-like cells are an innate source of IL-17 and IL-22. J Exp Med 2009; 206:35–41.
50. Diefenbach A, Colonna M, Koyasu S. Development, differentiation, and diversity of innate lymphoid cells
. Immunity 2014; 41:354–365.
51▪. von Burg N, Chappaz S, Baerenwaldt A, et al. Activated group 3 innate lymphoid cells
promote T-cell-mediated immune responses. Proc Natl Acad Sci U S A 2014; 111:12835–12840.
Shows the ability of ILC3 to process antigen and primer CD4+ T-cell responses via MHC class II.
52▪. Magri G, Miyajima M, Bascones S, et al. Innate lymphoid cells
integrate stromal and immunological signals to enhance antibody production by splenic marginal zone B cells. Nat Immunol 2014; 15:354–364.
Shows that ILCs can facilitate innate-like antibody production at the interface between the immune and circulatory systems.
53▪. Mortha A, Chudnovskiy A, Hashimoto D, et al. Microbiota-dependent crosstalk between macrophages and ILC3 promotes intestinal homeostasis. Science 2014; 343:1249288.
Discusses the role of ILC3 in production of GM-CSF, and how lack of GM-CSF production by ILC3 leads to reduced Treg numbers and impaired oral tolerance.
54▪▪. Hepworth MR, Fung TC, Masur SH, et al. Group 3 innate lymphoid cells
mediate intestinal selection of commensal bacteria-specific CD4+ T cells. Science 2015; 348:1031–1035.
Demonstrates the mechanism by which ILC3 are able to regulate gut homeostasis by directly inducing commensal-specific CD4+ T-cell death in the gut.
55▪. Korn LL, Thomas HL, Hubbeling HG, et al. Conventional CD4+ T cells regulate IL-22-producing intestinal innate lymphoid cells
. Mucosal Immunol 2014; 7:1045–1057.
Shows that CD4+ T cells have the capacity to regulate intestinal ILCs and production of antimicrobial peptides.
56▪▪. Goto Y, Obata T, Kunisawa J, et al. Innate lymphoid cells
regulate intestinal epithelial cell glycosylation. Science 2014; 345:1254009.
Shows that ILC3 induce fucosylation of epithelial cells in the gut, and that disruption of fucosylation led to increased susceptibility to Salmonella infection.
57▪. Pickard JM, Maurice CF, Kinnebrew MA, et al. Rapid fucosylation of intestinal epithelium sustains host-commensal symbiosis in sickness. Nature 2014; 514:638–641.