Several recent studies have highlighted the microanatomical and functional segregation of two populations of γδ T cells in the mouse skin. Using intravital multiphoton imaging of fluorescently tagged mice (CXCR6-GFP knock-in), two groups have identified a novel population of γδ T cells residing in the dermis [47, 132]. In the epidermis, DETC are sessile and constantly extend and retract their dendritic projections, while dermal γδ T cells show a different morphology and migrate in the dermis at a speed of 2–4 μm/min (Fig. 4.1). A subset of these cells also makes contact with MHC-II⁺ cells, presumably antigen presenting cells, in the dermis. These behaviours suggest an active role for these cells in immune-surveillance of the skin. While DETC are uniformly Vγ5⁺, dermal γδ T cells are relatively heterogeneous, expressing Vγ5, Vγ4 or other TCRs [47, 132]. In addition, the two populations also show differential expression of γδ TCR levels, CD25, CD43, CCR6 and NK1.1. Both populations are generated perinatally and homeostatically maintained by self-renewal in situ in an IL-7 dependent manner. In addition, DETC but not dermal γδ T cells are IL-15 dependent [29, 132]. While DETC are thymically pre-programmed by skint1 ligation to produce IFN-γ [114, 138], dermal γδ T cells constitutively express RORγt and IL-23R, and rapidly produce IL-17 following exposure to IL-1β plus IL-23 [16, 47]. In an intradermal model of BCG infection, dermal γδ T cells are the earliest population showing rapid IL-17 production, which mediates recruitment of neutrophils to the site of antigen deposit. In Tcrd ^−/− mice, the recruitment of neutrophils to the infected ear skin is diminished, and the downstream adaptive response of antigen-specific conventional CD4⁺ T cells reduced [132]. Nonetheless, these two anatomically segregated of γδ T cells may have functional overlap in certain scenarios, as a recently described subset of DETC is capable of rapidly producing IL-17 in response to wounding and UV irradiation [87, 88].

Mature TCR γδ⁺ thymocytes are reduced in numbers in Tcrβ-deficient and pTα-deficient, but not Tcrα-deficient mice, indicating a role for the CD4⁺CD8⁺ DP αβ T precursors on the development of γδ T cells in the thymus. DP cells are capable of conditioning γδ T precursors via lymphotoxin (LT)-mediated RelA and RelB pathways, regulating the expression of transcription factors RORγt and RORα4 to direct their differentiation into Tγδ17 cells [109, 129]. In Tcrd ^−/− mice hyperproliferation of the αβ T cell compartment is often seen, suggesting that γδ T cells may compete with αβ T cells for homeostatic cytokines, such as IL-7. During infection, early cytokine production from γδ T cells may directly or indirectly polarise delayed adaptive responses downstream and impact on the efficacy of pathogen clearance. For instance, infection with Listeria monocytogenes or Nippostrongylus brasiliensis elicits rapid production of IFN-γ and IL-4, respectively, from γδ T cells of the peritoneal cavity [35].

In models of autoimmune-mediated inflammation, γδ T cells may exacerbate disease. Tγδ17 that are activated by IL-23 during MOG/CFA-induced experimental autoimmune encephalomyelitis (EAE) inhibit the de novo induction of regulatory T cells (Treg) from conventional T cells and confer resistance of antigen-specific effector T cells towards Treg-mediated suppression [108]. Early cytokine production from resident mucosal γδ T cells aggravates colitis induced by adoptive transfer of effector CD4⁺ T cells in lymphopenic hosts, which can be inhibited by adoptive transfer of Tregs [159]. Treg and Tγδ17 are also likely to be engaged in dynamic interactions during the steady state, as the reduction in number and function of CD4⁺CD25⁺ Treg in Pdk1-deficient mice allows the expansion of pathogenic Tγδ17 cells that drive chronic intestinal inflammation [103].

Human Vγ9⁺Vδ2⁺ cells are the majority population of γδ T cells in the peripheral blood. A subset of these cells has a CCR7⁺CD62L⁺ central memory (TCM) phenotype, expresses the B cell follicle-homing chemokine receptor CXCR5 and the costimulatory molecules CD40L and ICOS, and is capable of producing IL-2, IL-4 and IL-10. In vitro, these cells are capable of helper T cell function to drive antibody production and isotype class switching [13]. In patients with genetic lymphopenia, γδ cells remain functional and are capable of providing B cell help in place of CD4⁺ T cells, leading to the formation of small germinal centres. These patients often show hyperimmunoglobulinemia of IgE, indicative that γδ T cells are capable of driving Ig class switching in vivo [34, 95].

The interactions between γδ T cells and antigen presenting cells (APC) in the thymus and the periphery are crucial for their generation, maintenance and function. In the thymus, Vγ5⁺ precursors and medullary thymic epithelial cells (mTEC) are mutually dependent. LTi (lymphoid tissue inducer) cells as well as Vγ5⁺ γδ precursor cells provide RANK ligand signal driving the maturation and expression of Aire in mTEC [116]. While Aire expression per se has no role in the generation of Vγ5⁺ DETC, mature mTEC express skint1, which is crucial for the programming of effector functions in these cells.

Intravital imaging revealed that γδ T cells at peripheral sites constantly make contacts with resident antigen presenting cells (APCs) [24]. Examples include contact between DETC and Langerhans cells (LC) in the epidermis, and between dermal γδ T cells and MHC-II⁺ cells in the dermis. While the depletion of LCs does not affect maintenance of DETCs [134], DETCs are producers of XCL1/lymphotactin, a chemokine known to attract DC [8]. While the activation of naïve αβ T cells strictly requires interaction with mature professional APC ie dendritic cells, immature DC have been shown to be able to activate γδ T cells. Vγ9⁺ T cells engage in reciprocal interactions with DCs, and are capable of potentiating DC maturation via production of cytokines, as well as relieving inhibitory signals that block DC maturation [62, 104, 149].

γδ T cells may themselves play a role as antigen presenting cells (APCs) thereby bridging the early innate immunity to the delayed adaptive T cell-mediated response [12, 97]. Human Vγ9⁺Vδ2⁺ cells have been shown to present antigen to CD4⁺ T cells, and cross-present antigen to CD8⁺ T cells. Activated Vγ9⁺ cells display several salient features that enable them to function as professional APC, including uptake of soluble antigens, phagocytosis, upregulation of CCR7 enabling homing to secondary lymphoid organs, upregulation of MHC-II and the costimulatory molecules CD80 and CD86.

γδ T cells engage with myeloid populations in a bidirectional manner. Efficient presentation of phosphoantigen to Vγ9⁺ cells in vitro requires the presence of monocytes [94]. Previous studies showed defective maturation of monocytes derived from mice with a deficit in γδ cells [130]. In vivo, different subsets of γδ T cells have protective roles in controlling macrophage-mediated immunopathological tissue damage in Listeria infection. Vγ1⁺ T cells mediate apoptosis of activated macrophages in a Fas:FasL dependent manner [28]. The interaction between Vγ4⁺ cells and activated macrophages promotes cytokine and chemokine production by macrophages on one hand, while inducing IL-10 production from Vγ4⁺ cells themselves on the other [137]. Yet both the Vγ1⁺ and Vγ4⁺ subsets are protective against Listeria-induced liver injury in independent adoptive transfers.

Administration of chitin elicits type 2 innate inflammation in the lungs involving ILC2-mediated recruitment of eosinophils and alternatively activated macrophages. When innate lymphoid cells are depleted, there is an enhancement of Tγδ17 activation and prolonged neutrophil influx, suggesting inter-regulation between ILC2 and Tγδ17 in determining the identity of infiltrating myeloid cells in response to this allergen [143].

γδ T cells by themselves are generally insufficient for sterilising immunity. However, Tcrb ^−/− x Tcrd ^−/− mice are more susceptible to viral infection indicating that these cells do have a protective role [19, 150]. Different subsets of γδ T cells may be mobilised in temporally and geographically distinct manner for early containment of damage, or later in the response for tissue repair and wound healing [18, 19]. In various models of cutaneous infection and vaccination, γδ T cells are recruited early and produce cytokines, often before the development of an αβ T cell response [100, 125]. The protozoan Leishmania is transmitted to its mammalian host through sandfly bite, and γδ T cell numbers are elevated in the blood of patients presenting cutaneous, mucosal or visceral leishmaniasis [122]. In mice, subcutaneous Leishmania major infection leads to a systemic expansion of Vδ4⁺ γδ T cells [121]. Antibody-mediated depletion of γδ T cells culminates in larger cutaneous lesions with higher number of parasites [120]. In Tcrα ^−/− mice, γδ T cells are instrumental in limiting pathogen spread and pathogen-associated damage, as shown in mouse models of footpad and corneal infection with Herpes simplex virus-1 (HSV-1). Mice with double deficiency of αβ and γδ T cells have larger epithelial lesions, higher viral load and dissemination, and succumb to lethal viral encephalitis [125]. In cutaneous Staphylococcus aureus infection, mice with γδ T deficiency have significantly larger skin lesions, higher bacterial load, and impaired recruitment of neutrophils to the infected site [23]. The ability of DETC to rapidly produce IL-17 in an IL-1, IL-23 and TLR2 dependent manner is critical for protection. In systemic S. aureus infection, a population of CD44⁺CD27⁻ Vγ4⁺ γδ T cells that persists at the infection site and in draining lymph nodes after clearance of primary infection is found to rapidly expand and produce high amount of IL-17 upon secondary infection, displaying features of memory [98].

In addition, γδ T cells may modulate and shape the downstream adaptive response in a multitude of different ways in different models of infection. Depletion of γδ T cells in calves reduces M. bovis-specific IgG2 and IFN-γ, and increases IL-4 production [91]. The absence of γδ T cells selectively reduces IgA but not IgG or IgM production elicited by oral immunisation with tetanus toxoid plus cholera toxin [36]. In Chagas disease patients, the presence of IL-10 producing CD4⁻CD8⁻ γδ T cells in the blood correlates positively with improved clinical parameters of cardiac function [148].

In the gut and the skin, intraepithelial γδ T cells do not only have a role in infection and inflammation, but are also indispensable in tissue homeostasis and repair. In the mouse epidermis, DETCs are in constant contact with adjacent keratinocytes as well as Langerhans cells. At steady state, DETC is the primary population of constitutive Insulin-Like Growth Factor-1 (IGF-1) producers supporting the survival, albeit not the proliferation, of keratinocytes [65]. Deficiency of γδ T cells is associated with increased epithelial apoptosis [126].

Wound healing involves coordinated phases of inflammation, proliferation, reepithelialisation, deposition of extracellular matrix and tissue remodelling. During this process, keratinocytes rapidly migrate to and colonise the wound border, produce antimicrobial factors to prevent microbial invasion, and proliferate to re-epithelialise the wound. It is known that γδ T cell deficient mice have a wound healing defect, which can be restored with wildtype DETC [66, 155]. A recent study identified a RORγt⁺ IL-17A producing subset of DETC to have a role in wound healing [87]. IL-17 blockade delays wound closure in wildtype mice, and conversely administration of exogenous IL-17 or transfer of IL-17-sufficient γδ T cells restore wound healing in IL-17A^−/− mice. IL-17 induces downstream production of a multitude of anti-microbial factors with barrier protective functions [79, 87].

In human skin, both αβ and γδ T cells are capable of producing IGF-1. Elevation of IGF-1 is seen in acute but not chronic wounds [136]. Intraepithelial γδ T cells of the skin and the intestine, but not intraepithelial αβ T cells nor lymphoid γδ T cells, are capable of Keratinocyte Growth Factor-1 (KGF-1) production [9]. Wound healing is delayed in the presence of a dominant negative KGF receptor mutant transgene [153]. Although there is no cutaneous wound healing defect in KGF-1 deficient mice due to functional compensation by KGF-2 in the skin, exogenous KGF-1 rescues the wound healing defect of Tcrd ^−/− skin organ culture in vitro [66]. In normal wound healing, DETC-derived KGF-1 induces hyaluronan production from keratinocytes, which has a role in recruiting macrophages to the site [67]. Consequentially macrophage infiltration is defective in Tcrd ^−/− mice.

In mice but not humans, wound repair is accompanied by hair follicle regeneration, a process known as wound-induced hair neogenesis (WIHN). This process is dependent on the production of FGF-9 by dermal γδ T cells [39].

The role of the IL23-IL17 axis in psoriasis pathogenesis is well recognised. IL-17A, IL-17 F, IL-22 and IL-21 are found to be elevated in the skin and blood of psoriatic patients, and IL-23 is selectively elevated in psoriatic lesions compared to non-lesional skin. Intradermal injection of IL-21 or IL-23 causes epithelial hyperplasia or acanthosis, one of the hallmarks of human psoriasis. IL-17A also upregulates the expression of keratin 17, a classical psoriatic disease marker, on keratinocytes [15]. IL-23R polymorphism is implicated in the pathogenesis of psoriasis [17]. While earlier studies have focused on Th17 cells, it is now known that innate cells including γδ T cells [16] and NKT cells [26] are major producers of IL-17 in psoriatic skin lesions.

In murine models of spontaneous psoriasitic dermatitis, disease progression correlates with a loss of Vγ5⁺ DETC and infiltration of IL-17⁺ dermal γδ T cells [4, 38]. There is compelling evidence indicating a pathogenic role of IL-17 producing dermal γδ T cells in various models of spontaneous and induced psoriatic dermatitis in mice [15, 16, 84, 86, 101, 140]. These models include topical application of imiquimod cream, a TLR7 agonist, or intradermal injection of IL-23 . IL-17A, IL-17 F and IL-22 deficient mice are protected from psoriatic dermatitis indicating the pathogenic role of these cytokines [101, 140]. Moreover, infiltrating dermal γδ T cells and RORγt⁺ ILC, rather than Th17 cells, have been shown to be the primary IL-17 producers, and are necessary and sufficient for psoriatic pathogenesis. More recently, Gray et al. found SOX13-dependent Vγ4⁺ Tγδ17 cells to be the specific pathogenic population in their model of psoriatic dermatitis [45]. These cells proliferate in skin draining lymph nodes and home to the inflamed skin. In a CD45.1⁺ C57BL/6 substrain, which carries a Sox13 mutation, the neonatal development of these Vγ4⁺ T cells is impaired, and these mice are protected from psoriasis development.

In human psoriasis patients, dermal IL-17-producing γδ T cells are found to be increased in psoriatic lesions [16]. Specifically, a population of CLA⁺CCR6⁺ Vγ9⁺Vδ2⁺ cells is reduced in the peripheral blood and increased in psoriatic skin lesions suggesting disease-associated redistribution of these cells. These cells produce IL-17A, and are capable of activating keratinocytes in vitro in a TNF-α and IFN-γ dependent manner [78].

There have been much recent efforts geared towards targeting the IL-23/IL-17 axis in the clinic. While targeting the p40 chain of IL-23/IL-12 is potentially risky due to its pleiotropic effect, the anti-IL-17 antibodies AIN457 (secukinumab) and LY2439821 (ixekizumab), and anti-IL-17R antibody AMG 827 (brodalumab) have shown promising efficacy in phase II clinical trials involving cases of chronic psoriatic plaques [15, 60, 81, 102].

Vγ9⁺Vδ2⁺ cells constitute the major γδ T population in the peripheral blood of healthy humans. These cells are uniquely able to recognise self- and microbial- derived phosphoantigens in a TCR-dependent, MHC-unrestricted manner. This TCR binds isopentenyl pyrophosphate, an isoprenoid intermediate of the human mevalaonate pathway, and also the microbial isoprenoid (HMBPP), an intermediate of the 2-C-methyl-D-erythritol-4 phosphate (MEP) pathway for isoprenoid biosynthesis [96]. This pathway is shared by a broad spectrum of prokaryotic and eukaryotic pathogens. The ability of these cells to bind an evolutionarily conserved group of invariant molecules via the TCR is consistent with their purported role as early rapid effectors. It has been shown that phosphoantigen recognition requires the presence of APC but is independent of MHC, MR1 or CD1. The nature of presentation of phosphoantigen remains to be resolved. The mitochondrial protein F1 ATPase (F1 adenosine triphosphatase) is a candidate phospho-antigen presenting molecule, but its involvement remains to be proven. Recent studies provided compelling evidence for an antigen-presenting role of BTN3A1, a butyrophilin-like extended B7 family member, in the binding of phospho-antigen to the Vγ9Vδ2 TCR [123, 145].

Vγ9⁺Vδ2⁺ T cells may have promising clinical potential especially in anti-tumour therapy [44, 96, 147, 158]. These cells are able to recognise a broad range of tumour cells, and have been demonstrated to be cytolytic towards a variety of tumour cells in vitro. Interestingly, transformed cells have been shown to upregulate the mevalonate pathway and accumulate IPP intracellularly, pointing to possibilities of ex vivo expansion of these γδ T cells and targeting a conserved pathway of tumour cell metabolism. The identification of the minimal binding moiety for γδ TCR, which is a five-carbon alkenyl chain with a pyrophosphate moiety, facilitates the mining and synthesis of phosphoantigen-based agonists for ex vivo manipulation and expansion of these cells.

In mouse models γδ T cells have been shown to mediate anti-tumour immunity via rapid early production of cytokines [37, 85]. In a murine sarcoma model, chemotherapy induces an early rapid infiltration of Tγδ17 cells into the tumour bed that is required for the subsequent infiltration of CD8⁺ CTL and anti-tumour chemotherapeutic efficacy [85].