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γδ T cells (gamma delta T cells) represent a small subset of T cells that possess a distinct T cell receptor (TCR) on their surface. A majority of T cells have a TCR composed of two glycoprotein chains called α- and β- TCR chains. In contrast, in γδ T cells, the TCR is made up of one γ-chain and one δ-chain. This group of T cells is usually much less common than αβ T cells, but are found at their highest abundance in the gut mucosa, within a population of lymphocytes known as intraepithelial lymphocytes (IELs).[1]

The antigenic molecules that activate γδ T cells are still largely unknown. However, γδ T cells are peculiar in that they do not seem to require antigen processing and MHC presentation of peptide epitopes although some recognize MHC class IB molecules. Furthermore, γδ T cells are believed to have a prominent role in recognition of lipid antigens.

There also exists a γδ T cell sub-population within the epidermal compartment of the skin. Named Dendritic Epidermal γδ T cells (DETC), in mice, these cells arise during fetal development and express an invariant and canonical Vγ3 Vδ1 T cell receptor (TCR)[using Garman nomenclature]. [2]

Contents

γδ T cells in innate and adaptive immunity

The conditions that lead to responses of γδ T cells are not fully understood, and current concepts of γδ T cells as 'first line of defense', 'regulatory cells', or 'bridge between innate and adaptive responses'[1] only address facets of their complex behavior. In fact, γδ T cells form an entire lymphocyte system that develops under the influence of other leukocytes, in the thymus and in the periphery. Mature γδ T cells are divided into functionally distinct subsets that obey their own (mostly unknown) rules and that have countless direct and indirect effects on healthy tissues and immune cells, on pathogens and tissues enduring infections and the host responses to them.

Like other 'unconventional' T cell subsets bearing invariant TCRs, such as CD1d-restricted Natural Killer T cells, γδ T cells exhibit several characteristics that place them at the border between the more evolutionarily primitive innate immune system that permits a rapid beneficial response to a variety of foreign agents, and the adaptive immune system, where B and T cells coordinate a slower but highly antigen-specific immune response leading to long-lasting memory against subsequent challenges by the same antigen.

  • On one hand, γδ T cells may be considered a component of adaptive immunity in that they rearrange TCR genes to produce junctional diversity and will develop a memory phenotype.
  • However, the various subsets may also be considered part of the innate immunity[3] where a restricted TCR may be used as a pattern recognition receptor.[4] For example, according to this paradigm, large numbers of (human) Vγ9/Vδ2 T cells respond within hours to common molecules produced by microbes, and highly restricted intraepithelial Vδ1 T cells will respond to stressed epithelial cells bearing sentinels of danger.
  • Recent work has shown that human Vγ9/Vδ2 T cells are also capable of phagocytosis, a function previously exclusive to innate myeloid lineage cells such as neutrophils, monocytes and dendritic cells [5]

Clearly, the complexity of γδ T cell biology spans definitions of both innate and adaptive immune responses.

Gene families in different species

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in laboratory mice (Mus musculus)

Mouse Vγ chains

This table, summarizes the nomenclature of mouse Vγ chains and indicates monoclonal antibodies often used to identify these chains. This system has been best described in strain C57BL/6 and might not apply well to other strains. There are two systems of nomenclature in use (Heilig; Garman), and many writers do not indicate which system they use. For example, the IMGT (International Immunogenetics Information System) uses the Heilig notation, but does not indicate this fact on its website. [6]This table refers to variable chain Vγ gene segments and to monoclonal antibodies that detect the corresponding Vγ protein chains. Note that Hayday's proposed "official" nomenclature is not widely used, leaving considerable confusion in the literature. One advantage and weakness of the Hayday nomenclature is that it is based on the gene order in the B6 genome, but this might not apply to other strains.

Heilig and Tonegawa's
system[7]
Garman's system
[8]
"Hayday's system[9]" antibodies comments
Vγ5 Vγ3 GV1S1 536; 17D1 specific for Vγ5(Heilig)+Vδ1 clonotype Skin, Jγ1Cγ1
Vγ6 Vγ4 GV2S1 reproductive mucosa;Jγ1Cγ1
Vγ4 Vγ2 GV3S1 UC310A6 lung;Jγ1Cγ1
Vγ7 Vγ5 GV4S1 F2.67 Pereira most common form in intestinal IEL
orthologous to human Vγ1
Jγ1Cγ1
Vγ1 Vγ1.1 GV5S1 2.11 Pereira 1995 peripheral lymphoid tissues;Jγ4Cγ4
Vγ2 Vγ1.2 GV5S2 Jγ1Cγ1
Vγ3 Vγ1.3 GV5S3 Jγ3-pseudoCγ3
Mouse Vgamma locus for C57BL/6 genome; drawn to scale. Chromosome 13: 1.927 to 1.440 Megabp Heilig notation

Types in humans

Human Vδ2+ T cells

Vγ9/Vδ2 T cells are unique to humans and primates and represent a minor and unconventional constituent of the leukocyte population in peripheral blood (0.5-5%); yet they are assumed to play an early and essential role in sensing 'danger' by invading pathogens as they expand dramatically in many acute infections and may exceed all other lymphocytes within a few days, e.g. in tuberculosis, salmonellosis, ehrlichiosis, brucellosis, tularemia, listeriosis, toxoplasmosis, and malaria. Of note, all Vγ9/Vδ2 T cells recognize the same small microbial compound (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMB-PP), a natural intermediate of the non-mevalonate pathway of isopentenyl pyrophosphate (IPP) biosynthesis.[10] HMB-PP is an essential metabolite in most pathogenic bacteria including Mycobacterium tuberculosis and malaria parasites, but is absent from the human host. Bacterial species that lack the non-mevalonate pathway and synthesize IPP via the classical mevalonate pathway instead, such as Streptococcus, Staphylococcus, and Borrelia, are unable to produce HMB-PP and do not specifically activate Vγ9/Vδ2 T cells.

IPP itself is structurally closely related to HMB-PP and ubiquitously present in all living cells (i.e. also in human cells), yet its potency in vitro is reduced 10,000 fold; whether IPP represents a physiological 'danger' signal of stressed or transformed cells is still unclear. Of pharmacological interest and with bioactivities comparable to that of IPP are synthetic aminobisphosphonates such as zoledronate (Zometa) or pamidronate (Aredia), that are widely used to treat osteoporosis and bone metastases, and incidentally act as Vγ9/Vδ2 T cell receptor agonists. However, increasing evidence suggests that these aminobisphosphonate 'antigens' are not recognised directly by Vγ9/Vδ2 T cells and in fact act indirectly, via their effects on the mevalonate biosynthetic pathway, leading to an accumulation of IPP [11]. Finally, certain alkylated amines have been described to activate Vγ9/Vδ2 T cells in vitro, however only at millimolar concentrations, i.e. with potencies 106-108fold lower than those of HMB-PP, thereby questioning their physiological relevance.

It is still not clear whether these non-peptidic antigens bind directly to the Vγ9/Vδ2 TCR or if a presenting element exists. There is evidence for a requirement for a species-specific cell-cell contact. However, none of the known antigen-presenting molecules like MHC class I and II or CD1 are required for γδ T cell activation suggesting the existence of a novel presenting element. Strong support for a direct recognition of non-peptide antigens by the Vγ9/Vδ2 TCR comes from studies which demonstrated that a transfected Vγ9/Vδ2 TCR can confer responsiveness onto a hitherto unresponsive cell; furthermore, antibodies to the γδ TCR block recognition. Thus, the presence of a functional Vγ9/Vδ2 TCR appears mandatory for a response to non-peptidic antigens although the basis for the huge differences in bioactivity between closely related molecules like HMB-PP and IPP cannot be explained by conventional epitope presentation/recognition models.

Human non-Vδ2+ T cells

The extensive structural diversity of Vδ1 and Vδ3 TCRs and the existence of Vδ1+ clones reactive against MHC, MHC-like, or non-MHC molecules suggest recognition of a highly diverse and heterogeneous set of antigens by non-Vδ2 cells, although cognate interactions between non-Vδ2 TCRs and any of these antigens have not been shown yet. MHC class-I-chain-related gene A (MICA) has also been proposed as an important tumor antigen recognized by Vδ1+ T cells. However, the very low affinity of MICA–Vδ1 TCR interactions estimated by surface plasmon resonance analyses raises doubts about the functional relevance of MICA or MHC class-I-chain-related gene B (MICB) recognition by Vδ1+ TCRs.

Non-Vδ2 γδ T cells are expanded in various infectious contexts involving intracellular bacteria (Mycobacteria and Listeria) as well as extracellular bacteria, such as Borrelia burgdorferi and viruses (HIV, cytomegalovirus). In most instances, the stimuli that trigger Vd1 expansion are not derived from pathogens but instead correspond to endogenous gene products presumably upregulated on infection. The antigens recognized by non-Vδ2 T cells expanded in the above infectious contexts have not been characterized, but the fact that Vδ1+ T-cell responses are not blocked by monoclonal antibody directed against known classical or non-classical MHC molecules suggests recognition of a new class of conserved stress-induced antigens.

See also

References

  1. ^ a b Holtmeier W, Kabelitz D. γδ T cells link innate and adaptive immune responses. Chem Immunol Allergy. 2005;86:151-183. PMID 15976493
  2. ^ Jameson J, Havran WL. Skin gammadelta T-cell functions in homeostasis and wound healing. Immunol Rev. 2007 Feb;215:114-22. PMID: 17291283
  3. ^ Born WK, Reardon CL, O'Brien RL. The function of γδ T cells in innate immunity. Curr Opin Immunol. 2006;18:31-38. PMID 16337364
  4. ^ Morita CT, Mariuzza RA, Brenner MB. Antigen recognition by human γδ T cells: pattern recognition by the adaptive immune system. Springer Semin Immunopathol. 2000;22:191-217. PMID 11116953
  5. ^ Wu Y, Wu W, Wong WM, Ward E, Thrasher AJ, Goldblatt D, Osman M, Digard P, Canaday DH, Gustafsson K. Human gammadelta T cells: a lymphoid lineage cell capable of professional phagocytosis. J Immunol. 2009 Nov 1;183(9):5622-9. PMID: 19843947 PubMed
  6. ^ [1] IMGT website
  7. ^ Heilig and TonegawaDiversity of murine γ and expression in fetal and adult T lymphocytes Nature. 1986;322:836-840. PMID 2943999
  8. ^ Garman et al.. Diversity, rearrangement, and expression of murine T cell γ genes Cell 1986;45:733-742.
  9. ^ Hayday γδ cells: A right time and a right place for a conserved third way of protection Ann. Rev. Immunol. 2000;18:975-1026. PMID 10837080
  10. ^ Eberl M, Hintz M, Reichenberg A, Kollas AK, Wiesner J, Jomaa H. Microbial isoprenoid biosynthesis and human γδ T cell activation. FEBS Lett. 2003;544:4-10. PMID 12782281
  11. ^ Hewitt RE, Lissina A, Green AE, Slay ES, Price DA, Sewell AK. The bisphosphonate acute phase response: rapid and copious production of proinflammatory cytokines by peripheral blood γδ T cells in response to aminobisphosphonates is inhibited by statins Clin Exp Immunol. 2005;139:101-11. PMID 15606619
  • Hayday AC. γδ cells: a right time and a right place for a conserved third way of protection. Annu Rev Immunol. 2000;18:975-1026. PMID 10837080
  • Girardi M. Immunosurveillance and immunoregulation by γδ T cells. J Invest Dermatol. 2006 Jan;126:25-31. PMID 16417214
  • Thedrez A, Sabourin C, Gertner J, Devilder MC, Allain-Maillet S, Fournie JJ, Scotet E, Bonneville M. Self/non-self discrimination by human γδ T cells: simple solutions for a complex issue? Immunol Rev. 2007;215:123-135. PMID 17291284

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