Skip to main content

Advertisement

Log in

Functions of skin-resident γδ T cells

  • Multi-author review
  • Published:
Cellular and Molecular Life Sciences Aims and scope Submit manuscript

Abstract

The murine epidermis contains resident T cells that express a canonical γδ TCR and arise from fetal thymic precursors. These cells are termed dendritic epidermal T cells (DETC) and use a TCR that is restricted to the skin in adult animals. DETC produce low levels of cytokines and growth factors that contribute to epidermal homeostasis. Upon activation, DETC can secrete large amounts of inflammatory molecules which participate in the communication between DETC, neighboring keratinocytes and langerhans cells. Chemokines produced by DETC may recruit inflammatory cells to the epidermis. In addition, cell–cell mediated immune responses also appear important for epidermal–T cell communication. Information is provided which supports a crucial role for DETC in inflammation, wound healing, and tumor surveillance.

This is a preview of subscription content, log in via an institution to check access.

Access this article

Price excludes VAT (USA)
Tax calculation will be finalised during checkout.

Instant access to the full article PDF.

Fig. 1

Similar content being viewed by others

Abbreviations

DETC:

Dendritic epidermal T cells

JAML:

Junctional adhesion molecule-like protein

CAR:

Coxsackie and adenovirus receptor

CS:

Delayed-type contact hypersensitivity

GVHD:

Graft versus host disease

TLR:

Toll-like receptor

Rae-1:

Retinoic acid early transcript 1

Mult1:

UL-16 binding protein-like transcript-1

ITAM:

Immunoreceptor tyrosine-based activation motif

References

  1. Asarnow DM, Goodman T, LeFrancois L, Allison JP (1989) Distinct antigen receptor repertoires of two classes of murine epithelium-associated T cells. Nature 341:60–62

    Article  PubMed  CAS  Google Scholar 

  2. Havran WL, Grell S, Duwe G, Kimura J, Wilson A, Kruisbeek AM, O’Brien RL, Born W, Tigelaar RE, Allison JP (1989) Limited diversity of T-cell receptor γ-chain expression of murine Thy-1+ dendritic epidermal cells revealed by Vγ3-specific monoclonal antibody. Proc Natl Acad Sci USA 86:4185–4189

    Article  PubMed  CAS  Google Scholar 

  3. Asarnow DM, Kuziel WA, Bonyhadi M, Tigelaar RE, Tucker PW, Allison JP (1988) Limited diversity of γδ antigen receptor genes of Thy-1+ dendritic epidermal cells. Cell 55:837–847

    Article  PubMed  CAS  Google Scholar 

  4. Havran WL, Allison JP (1988) Developmentally ordered appearance of thymocytes expressing different T-cell antigen receptors. Nature 335:443–445

    Article  PubMed  CAS  Google Scholar 

  5. Itohara S, Farr AG, Lafaille JJ, Bonneville M, Takagaki Y, Haas W, Tonegawa S (1990) Homing of a γδ thymocyte subset with homogeneous T-cell receptors to mucosal epithelia. Nature 343:754–757

    Article  PubMed  CAS  Google Scholar 

  6. Xiong N, Baker JE, Kang C, Raulet DH (2004) The genomic arrangement of T cell receptor variable genes is a determinant of the developmental rearrangement pattern. Proc Natl Acad Sci USA 101:260–265

    Article  PubMed  CAS  Google Scholar 

  7. Uche UN, Huber CR, Raulet DH, Xiong N (2009) Recombination signal sequence-associated restriction on TCRδ gene rearrangement affects the development of tissue-specific γδ T cells. J Immunol 183:4931–4939

    Article  PubMed  CAS  Google Scholar 

  8. Xiong N, Raulet DH (2007) Development and selection of γδ T cells. Immunol Rev 215:15–31

    Article  PubMed  CAS  Google Scholar 

  9. Xiong N, Zhang L, Kang C, Raulet DH (2008) Gene placement and competition control T cell receptor γ variable region gene rearrangement. J Exp Med 205:929–938

    Article  PubMed  CAS  Google Scholar 

  10. Leclercq G, Plum J, Nandi D, De Smedt M, Allison JP (1993) Intrathymic differentiation of Vγ3 T cells. J Exp Med 178:309–315

    Article  PubMed  CAS  Google Scholar 

  11. Xiong N, Kang C, Raulet DH (2004) Positive selection of dendritic epidermal γδ T cell precursors in the fetal thymus determines expression of skin-homing receptors. Immunity 21:121–131

    Article  PubMed  CAS  Google Scholar 

  12. Boyden LM, Lewis JM, Barbee SD, Bas A, Girardi M, Hayday AC, Tigelaar RE, Lifton RP (2008) Skint1, the prototype of a newly identified immunoglobulin superfamily gene cluster, positively selects epidermal γδ T cells. Nat Genet 40:656–662

    Article  PubMed  CAS  Google Scholar 

  13. Sharp LL, Jameson JM, Witherden DA, Komori HK, Havran WL (2005) Dendritic epidermal T-cell activation. Crit Rev Immunol 25:1–18

    Article  PubMed  CAS  Google Scholar 

  14. Ye SK, Maki K, Lee HC, Ito A, Kawai K, Suzuki H, Mak TW, Chien Y, Honjo T, Ikuta K (2001) Differential roles of cytokine receptors in the development of epidermal γδ T cells. J Immunol 167:1929–1934

    PubMed  CAS  Google Scholar 

  15. Ye SK, Agata Y, Lee HC, Kurooka H, Kitamura T, Shimizu A, Honjo T, Ikuta K (2001) The IL-7 receptor controls the accessibility of the TCRγ locus by Stat5 and histone acetylation. Immunity 15:813–823

    Article  PubMed  CAS  Google Scholar 

  16. Kang J, DiBenedetto B, Narayan K, Zhao H, Der SD, Chambers CA (2004) STAT5 is required for thymopoiesis in a development stage-specific manner. J Immunol 173:2307–2314

    PubMed  CAS  Google Scholar 

  17. Jiang X, Campbell JJ, Kupper TS (2010) Embryonic trafficking of γδ T cells to skin is dependent on E/P selectin ligands and CCR4. Proc Natl Acad Sci USA 107:7443–7448

    Article  PubMed  CAS  Google Scholar 

  18. Jin Y, Xia M, Sun A, Saylor CM, Xiong N (2010) CCR10 is important for the development of skin-specific γδ T cells by regulating their migration and location. J Immunol 185:5723–5731

    Article  PubMed  CAS  Google Scholar 

  19. Lee P, Lee DJ, Chan C, Chen SW, Ch’en I, Jamora C (2009) Dynamic expression of epidermal caspase 8 simulates a wound healing response. Nature 458:519–523

    Article  PubMed  CAS  Google Scholar 

  20. Jameson JM, Sharp LL, Witherden DA, Havran WL (2004) Regulation of skin cell homeostasis by γδ T cells. Front Biosci 9:2640–2651

    Article  PubMed  CAS  Google Scholar 

  21. Sharp LL, Jameson JM, Cauvi G, Havran WL (2005) Dendritic epidermal T cells regulate skin homeostasis through local production of insulin-like growth factor 1. Nat Immunol 6:73–79

    Article  PubMed  CAS  Google Scholar 

  22. Edmondson SR, Thumiger SP, Werther GA, Wraight CJ (2003) Epidermal homeostasis: the role of the growth hormone and insulin-like growth factor systems. Endocr Rev 24:737–764

    Article  PubMed  CAS  Google Scholar 

  23. Su HY, Cheng WT, Chen SC, Lin CT, Lien YY, Liu HJ, Gilmour RS (2004) Mouse keratinocytes express c98, a novel gene homologous to bcl-2, that is stimulated by insulin-like growth factor 1 and prevents dexamethasone-induced apoptosis. Biochim Biophys Acta 1676:127–137

    PubMed  CAS  Google Scholar 

  24. Matsue H, Cruz PD Jr, Bergstresser PR, Takashima A (1993) Profiles of cytokine mRNA expressed by dendritic epidermal T cells in mice. J Invest Dermatol 101:537–542

    Article  PubMed  CAS  Google Scholar 

  25. Boismenu R, Feng L, Xia YY, Chang JC, Havran WL (1996) Chemokine expression by intraepithelial γδ T cells. Implications for the recruitment of inflammatory cells to damaged epithelia. J Immunol 157:985–992

    PubMed  CAS  Google Scholar 

  26. Jameson JM, Cauvi G, Sharp LL, Witherden DA, Havran WL (2005) γδ T cell-induced hyaluronan production by epithelial cells regulates inflammation. J Exp Med 201:1269–1279

    Article  PubMed  CAS  Google Scholar 

  27. Taylor KR, Mills RE, Costanzo AE, Jameson JM (2010) γδ T cells are reduced and rendered unresponsive by hyperglycemia and chronic TNFα in mouse models of obesity and metabolic disease. PLoS One 5(7):e11422

    Article  PubMed  Google Scholar 

  28. Wang T, Scully E, Yin Z, Kim JH, Wang S, Yan J, Mamula M, Anderson JF, Craft J, Fikrig E (2003) IFN-γ-producing γδ T cells help control murine West Nile virus infection. J Immunol 171:2524–2531

    PubMed  CAS  Google Scholar 

  29. Shibata K, Yamada H, Hara H, Kishihara K, Yoshikai Y (2007) Resident Vδ1+ γδ T cells control early infiltration of neutrophils after Escherichia coli infection via IL-17 production. J Immunol 178:4466–4472

    PubMed  CAS  Google Scholar 

  30. Gao Y, Yang W, Pan M, Scully E, Girardi M, Augenlicht LH, Craft J, Yin Z (2003) γδ T cells provide an early source of interferon γ in tumor immunity. J Exp Med 198:433–442

    Article  PubMed  CAS  Google Scholar 

  31. Cho JS, Pietras EM, Garcia NC, Ramos RI, Farzam DM, Monroe HR, Magorien JE, Blauvelt A, Kolls JK, Cheung AL, Cheng G, Modlin RL, Miller LS (2010) IL-17 is essential for host defense against cutaneous Staphylococcus aureus infection in mice. J Clin Invest 120(5):1762–1773

    Article  PubMed  Google Scholar 

  32. Molne L, Corthay A, Holmdahl R, Tarkowski A (2003) Role of γδ T cell receptor-expressing lymphocytes in cutaneous infection caused by Staphylococcus aureus. Clin Exp Immunol 132:209–215

    Article  PubMed  CAS  Google Scholar 

  33. Leclercq G, Plum J (1995) Stimulation of TCR Vγ3 cells by gram-negative bacteria. J Immunol 154:5313–5319

    PubMed  CAS  Google Scholar 

  34. Martin B, Hirota K, Cua DJ, Stockinger B, Veldhoen M (2009) Interleukin-17-producing γδ T cells selectively expand in response to pathogen products and environmental signals. Immunity 31:321–330

    Article  PubMed  CAS  Google Scholar 

  35. Shimura H, Nitahara A, Ito A, Tomiyama K, Ito M, Kawai K (2005) Up-regulation of cell surface Toll-like receptor 4-MD2 expression on dendritic epidermal T cells after the emigration from epidermis during cutaneous inflammation. J Dermatol Sci 37:101–110

    Article  PubMed  CAS  Google Scholar 

  36. Ptak W, Askenase PW (1992) γδ T cells assist αβ T cells in adoptive transfer of contact sensitivity. J Immunol 149:3503–3508

    PubMed  CAS  Google Scholar 

  37. Szczepanik M, Lewis J, Geba GP, Ptak W, Askenase PW (1998) Positive regulatory γδ T cells in contact sensitivity: augmented responses by in vivo treatment with anti-γδ monoclonal antibody, or anti-Vγ5 or Vδ4. Immunol Invest 27:1–15

    Article  PubMed  CAS  Google Scholar 

  38. Szczepanik M, Nowak B, Askenase PW, Ptak W (1998) Cross-talk between γδ T lymphocytes and immune cells in humoral response. Immunology 95:612–617

    Article  PubMed  CAS  Google Scholar 

  39. Ushio H, Tsuji RF, Szczepanik M, Kawamoto K, Matsuda H, Askenase PW (1998) IL-12 reverses established antigen-specific tolerance of contact sensitivity by affecting costimulatory molecules B7–1 (CD80) and B7–2 (CD86). J Immunol 160:2080–2088

    PubMed  CAS  Google Scholar 

  40. Dieli F, Asherson GL, Sireci G, Dominici R, Gervasi F, Vendetti S, Colizzi V, Salerno A (1997) γδ cells involved in contact sensitivity preferentially rearrange the Vγ3 region and require interleukin-7. Eur J Immunol 27:206–214

    Article  PubMed  CAS  Google Scholar 

  41. Dieli F, Ptak W, Sireci G, Romano GC, Potestio M, Salerno A, Asherson GL (1998) Cross-talk between Vβ8+ and γδ+ T lymphocytes in contact sensitivity. Immunology 93:469–477

    Article  PubMed  CAS  Google Scholar 

  42. Girardi M, Lewis J, Glusac E, Filler RB, Geng L, Hayday AC, Tigelaar RE (2002) Resident skin-specific γδ T cells provide local, nonredundant regulation of cutaneous inflammation. J Exp Med 195:855–867

    Article  PubMed  CAS  Google Scholar 

  43. Lewis JM, Girardi M, Roberts SJ, Barbee SD, Hayday AC, Tigelaar RE (2006) Selection of the cutaneous intraepithelial γδ+ T cell repertoire by a thymic stromal determinant. Nat Immunol 7:843–850

    Article  PubMed  CAS  Google Scholar 

  44. Marcinkiewicz J, Bereta M, Malinowski J, Ptak W (1984) The induction of oxazolone-specific T suppressor afferent cells in mice by hapten-modified isologous IgG. Eur J Immunol 14:759–762

    Article  PubMed  CAS  Google Scholar 

  45. Rosenstein RW, Murray JH, Cone RE, Ptak W, Iverson GM, Gershon RK (1981) Isolation and partial characterization of an antigen-specific T-cell factor associated with the suppression of delayed type hypersensitivity. Proc Natl Acad Sci USA 78:5821–5825

    Article  PubMed  CAS  Google Scholar 

  46. Szczepanik M, Anderson LR, Ushio H, Ptak W, Owen MJ, Hayday AC, Askenase PW (1996) γδ T cells from tolerized αβ T cell receptor (TCR)-deficient mice inhibit contact sensitivity-effector T cells in vivo, and their interferon-γ production in vitro. J Exp Med 184:2129–2139

    Article  PubMed  CAS  Google Scholar 

  47. McMenamin C, Pimm C, McKersey M, Holt PG (1994) Regulation of IgE responses to inhaled antigen in mice by antigen-specific γδ T cells. Science 265:1869–1871

    Article  PubMed  CAS  Google Scholar 

  48. Shiohara T, Moriya N, Gotoh C, Hayakawa J, Nagashima M, Saizawa K, Ishikawa H (1990) Loss of epidermal integrity by T cell-mediated attack induces long-term local resistance to subsequent attack. I. Induction of resistance correlates with increases in Thy-1+ epidermal cell numbers. J Exp Med 171:1027–1041

    Article  PubMed  CAS  Google Scholar 

  49. Shiohara T, Moriya N, Hayakawa J, Itohara S, Ishikawa H (1996) Resistance to cutaneous graft-vs.-host disease is not induced in T cell receptor δ gene-mutant mice. J Exp Med 183:1483–1489

    Article  PubMed  CAS  Google Scholar 

  50. Kaminski MJ, Cruz PD Jr, Bergstresser PR, Takashima A (1993) Killing of skin-derived tumor cells by mouse dendritic epidermal T-cells. Cancer Res 53:4014–4019

    PubMed  CAS  Google Scholar 

  51. Girardi M, Oppenheim DE, Steele CR, Lewis JM, Glusac E, Filler R, Hobby P, Sutton B, Tigelaar RE, Hayday AC (2001) Regulation of cutaneous malignancy by γδ T cells. Science 294:605–609

    Article  PubMed  CAS  Google Scholar 

  52. Oppenheim DE, Roberts SJ, Clarke SL, Filler R, Lewis JM, Tigelaar RE, Girardi M, Hayday AC (2005) Sustained localized expression of ligand for the activating NKG2D receptor impairs natural cytotoxicity in vivo and reduces tumor immunosurveillance. Nat Immunol 6:928–937

    Article  PubMed  CAS  Google Scholar 

  53. Champsaur M, Lanier LL (2010) Effect of NKG2D ligand expression on host immune responses. Immunol Rev 235:267–285

    PubMed  CAS  Google Scholar 

  54. Whang MI, Guerra N, Raulet DH (2009) Costimulation of dendritic epidermal γδ T cells by a new NKG2D ligand expressed specifically in the skin. J Immunol 182:4557–4564

    Article  PubMed  CAS  Google Scholar 

  55. Nitahara A, Shimura H, Ito A, Tomiyama K, Ito M, Kawai K (2006) NKG2D ligation without T cell receptor engagement triggers both cytotoxicity and cytokine production in dendritic epidermal T cells. J Invest Dermatol 126:1052–1058

    Article  PubMed  CAS  Google Scholar 

  56. Diefenbach A, Tomasello E, Lucas M, Jamieson AM, Hsia JK, Vivier E, Raulet DH (2002) Selective associations with signaling proteins determine stimulatory versus costimulatory activity of NKG2D. Nat Immunol 3:1142–1149

    Article  PubMed  CAS  Google Scholar 

  57. Gilfillan S, Ho EL, Cella M, Yokoyama WM, Colonna M (2002) NKG2D recruits two distinct adapters to trigger NK cell activation and costimulation. Nat Immunol 3:1150–1155

    Article  PubMed  CAS  Google Scholar 

  58. Rosen DB, Araki M, Hamerman JA, Chen T, Yamamura T, Lanier LL (2004) A structural basis for the association of DAP12 with mouse, but not human, NKG2D. J Immunol 173:2470–2478

    PubMed  CAS  Google Scholar 

  59. Zompi S, Hamerman JA, Ogasawara K, Schweighoffer E, Tybulewicz VL, Di Santo JP, Lanier LL, Colucci F (2003) NKG2D triggers cytotoxicity in mouse NK cells lacking DAP12 or Syk family kinases. Nat Immunol 4:565–572

    Article  PubMed  CAS  Google Scholar 

  60. Shojaei H, Oberg HH, Juricke M, Marischen L, Kunz M, Mundhenke C, Gieseler F, Kabelitz D, Wesch D (2009) Toll-like receptors 3 and 7 agonists enhance tumor cell lysis by human γδ T cells. Cancer Res 69:8710–8717

    Article  PubMed  CAS  Google Scholar 

  61. Krahenbuhl O, Gattesco S, Tschopp J (1992) Murine Thy-1+ dendritic epidermal T cell lines express granule-associated perforin and a family of granzyme molecules. Immunobiology 184:392–401

    PubMed  CAS  Google Scholar 

  62. Atkins MB (2006) Cytokine-based therapy and biochemotherapy for advanced melanoma. Clin Cancer Res 12:2353s–2358s

    Article  PubMed  CAS  Google Scholar 

  63. Bonmort M, Ullrich E, Mignot G, Jacobs B, Chaput N, Zitvogel L (2007) Interferon-γ is produced by another player of innate immune responses: the interferon-producing killer dendritic cell (IKDC). Biochimie 89:872–877

    Article  PubMed  CAS  Google Scholar 

  64. Murugaiyan G, Saha B (2009) Protumor vs. antitumor functions of IL-17. J Immunol 183:4169–4175

    Article  PubMed  CAS  Google Scholar 

  65. Baum CL, Arpey CJ (2005) Normal cutaneous wound healing: clinical correlation with cellular and molecular events. Dermatol Surg 31, 674–86 (discussion 686)

    Google Scholar 

  66. Gailit J, Clark RA (1994) Wound repair in the context of extracellular matrix. Curr Opin Cell Biol 6:717–725

    Article  PubMed  CAS  Google Scholar 

  67. Strecker-McGraw MK, Jones TR, Baer DG (2007) Soft tissue wounds and principles of healing. Emerg Med Clin North Am 25:1–22

    Article  PubMed  Google Scholar 

  68. Diegelmann RF, Evans MC (2004) Wound healing: an overview of acute, fibrotic and delayed healing. Front Biosci 9:283–289

    Article  PubMed  CAS  Google Scholar 

  69. Izadi K, Ganchi P (2005) Chronic wounds. Clin Plast Surg 32:209–222

    Article  PubMed  Google Scholar 

  70. Phillips TJ (1994) Chronic cutaneous ulcers: etiology and epidemiology. J Invest Dermatol 102:38S–41S

    Article  PubMed  CAS  Google Scholar 

  71. Wolfe RA, Roi LD, Flora JD, Feller I, Cornell RG (1983) Mortality differences and speed of wound closure among specialized burn care facilities. JAMA 250:763–766

    Article  PubMed  CAS  Google Scholar 

  72. Rubin JS, Osada H, Finch PW, Taylor WG, Rudikoff S, Aaronson SA (1989) Purification and characterization of a newly identified growth factor specific for epithelial cells. Proc Natl Acad Sci USA 86:802–806

    Article  PubMed  CAS  Google Scholar 

  73. Werner S, Peters KG, Longaker MT, Fuller-Pace F, Banda MJ, Williams LT (1992) Large induction of keratinocyte growth factor expression in the dermis during wound healing. Proc Natl Acad Sci USA 89:6896–6900

    Article  PubMed  CAS  Google Scholar 

  74. Finch PW, Rubin JS, Miki T, Ron D, Aaronson SA (1989) Human KGF is FGF-related with properties of a paracrine effector of epithelial cell growth. Science 245:752–755

    Article  PubMed  CAS  Google Scholar 

  75. Staiano-Coico L, Krueger JG, Rubin JS, D’Limi S, Vallat VP, Valentino L, Fahey T 3rd, Hawes A, Kingston G, Madden MR et al (1993) Human keratinocyte growth factor effects in a porcine model of epidermal wound healing. J Exp Med 178:865–878

    Article  PubMed  CAS  Google Scholar 

  76. Jameson J, Ugarte K, Chen N, Yachi P, Fuchs E, Boismenu R, Havran WL (2002) A role for skin γδ T cells in wound repair. Science 296:747–749

    Article  PubMed  CAS  Google Scholar 

  77. Jameson JM, Cauvi G, Witherden DA, Havran WL (2004) A keratinocyte-responsive γδ TCR is necessary for dendritic epidermal T cell activation by damaged keratinocytes and maintenance in the epidermis. J Immunol 172:3573–3579

    PubMed  CAS  Google Scholar 

  78. Havran WL, Chien YH, Allison JP (1991) Recognition of self antigens by skin-derived T cells with invariant γδ antigen receptors. Science 252:1430–1432

    Article  PubMed  CAS  Google Scholar 

  79. Witherden DA, Verdino P, Rieder SE, Garijo O, Mills RE, Teyton L, Fischer WH, Wilson IA, Havran WL (2010) The junctional adhesion molecule JAML is a costimulatory receptor for epithelial γδ T cell activation. Science 329:1205–1210

    Article  PubMed  CAS  Google Scholar 

  80. Verdino P, Witherden DA, Havran WL, Wilson IA (2010) The molecular interaction of CAR and JAML recruits the central cell signal transducer PI3 K. Science 329(5996):1210–1214

    Article  PubMed  CAS  Google Scholar 

  81. Morita CT, Parker CM, Brenner MB, Band H (1994) TCR usage and functional capabilities of human γδ T cells at birth. J Immunol 153:3979–3988

    PubMed  CAS  Google Scholar 

  82. Trejdosiewicz LK, Smart CJ, Oakes DJ, Howdle PD, Malizia G, Campana D, Boylston AW (1989) Expression of T-cell receptors TcR1 (γδ) and TcR2 (αβ) in the human intestinal mucosa. Immunology 68:7–12

    PubMed  CAS  Google Scholar 

  83. Toulon A, Breton L, Taylor KR, Tenenhaus M, Bhavsar D, Lanigan C, Rudolph R, Jameson J, Havran WL (2009) A role for human skin-resident T cells in wound healing. J Exp Med 206:743–750

    Article  PubMed  CAS  Google Scholar 

Download references

Acknowledgments

The authors are supported by grants from the National Institutes of Health (AI007244, AI64811, AI36964, and GM80301), L’Oreal, Deutsche Dermatologische Gesellschaft and the Arbeitsgemeinschaft Dermatologische Forschung.

Author information

Authors and Affiliations

Authors

Corresponding author

Correspondence to Wendy L. Havran.

Rights and permissions

Reprints and permissions

About this article

Cite this article

MacLeod, A.S., Havran, W.L. Functions of skin-resident γδ T cells. Cell. Mol. Life Sci. 68, 2399–2408 (2011). https://doi.org/10.1007/s00018-011-0702-x

Download citation

  • Received:

  • Revised:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1007/s00018-011-0702-x

Keywords

Navigation