γδ T-APCs: a novel tool for immunotherapy?

Vδ2 T cell deficiency in granulomatosis with polyangiitis (Wegener's granulomatosis)

Previous studies have characterized phenotypic and functional alterations within T-cell receptor αβ-expressing T cells in patients with granulomatosis with polyangiitis (GPA). We analyzed the frequency, subset composition and in vitro activation of blood γδ T cells in GPA patients. We observed a significant reduction of γδ T cell numbers, due to the selective depletion of the Vδ2 subset which remained consistent over time upon repeated analysis. The loss of Vδ2 T cells was not due to migration into the inflamed lesions as very few γδ T cells were detected in inflammatory infiltrates. The memory subset distribution did not differ among Vδ2 T cells from healthy donors and GPA patients. Importantly, the remaining Vδ2 T cells were capable of responding to phosphoantigen stimulation in vitro. The marked depletion of blood Vδ2 T cells in GPA suggests cellular exhaustion, possibly due to chronic exposure to and continuous overstimulation by microbial phosphoantigens.

Increased cholesterol content in gammadelta (γδ) T lymphocytes differentially regulates their activation

Gammadelta (γδ) T lymphocytes respond quickly upon antigen encounter to produce a cytokine response. In this study, we sought to understand how functions of γδ T cells are differentially regulated compared to αβ T cells. We found that cholesterol, an integral component of the plasma membrane and a regulator of TCR signaling, is increased in γδ T cells compared to αβ T cells, and modulates their function. Higher levels of activation markers, and increased lipid raft content in γδ cells suggest that γδ T cells are more activated. Cholesterol depletion effectively decreased lipid raft formation and activation of γδ T cells, indicating that increased cholesterol content contributes to the hyper-activated phenotype of γδ T cells, possibly through enhanced clustering of TCR signals in lipid rafts. TCR stimulation assays and western blotting revealed that instead of a lower TCR threshold, enhanced TCR signaling through ERK1/2 activation is likely the cause for high cholesterol-induced rapid activation and proliferation in γδ T cells. Our data indicate that cholesterol metabolism is differentially regulated in γδ T cells. The high intracellular cholesterol content leads to enhanced TCR signaling and increases activation and proliferation of γδ T cells.

Monocytes and γδ T cells control the acute-phase response to intravenous zoledronate: insights from a phase IV safety trial

Aminobisphosphonates (NBPs) are used widely against excessive bone resorption in osteoporosis and Paget's disease as well as in metastatic bone disease and multiple myeloma. Intravenous NBP administration often causes mild to severe acute-phase responses (APRs) that may require intervention with analgesics and antipyretics and lead to treatment noncompliance and nonadherence. We here undertook a phase IV safety trial in patients with osteoporosis to investigate the APR of otherwise healthy individuals to first-time intravenous treatment with the NBP zoledronate. This study provides unique insight into sterile acute inflammatory responses in vivo, in the absence of confounding factors such as infection or cancer. Our data show that both peripheral γδ T cells and monocytes become rapidly activated after treatment with zoledronate, which ultimately determines the clinical severity of the APR. Our study highlights a key role for IFN-γ in the zoledronate-induced APR and identifies pretreatment levels of monocytes and central/memory Vγ9/Vδ2 T cells as well as their responsiveness to zoledronate in vitro as predictive risk factors for the occurrence of subclinical and clinical symptoms. These findings have diagnostic and prognostic implications for patients with and without malignancy and are relevant for Vγ9/Vδ2 T-cell-based immunotherapy approaches.

Human γδ T-cell responses in infection and immunotherapy: common mechanisms, common mediators?

Upon receiving the Nobel Prize in Physiology or Medicine in 1987, Susumu Tonegawa referred to the then recent discovery of the γδ T-cell receptor and stated that "while the function of the T cells bearing this receptor is currently unknown (…) these T cells may be involved in an entirely new aspect of immunity". [Tonegawa, S., Scand. J. Immunol. 1993. 38: 303-319]. Twenty-five years of intense research later this ambivalent view still holds true. Immunologists now appreciate that γδ T cells indeed represent a highly intriguing "new aspect of immunity" that is unique and distinct from conventional lymphocytes, yet even scientists in the field still struggle to understand the molecular basis of γδ T-cell responses, especially with respect to the enigmatic mode of antigen recognition. Here, we portray the peculiar responsiveness of human Vγ9/Vδ2 T cells to microorganisms, tumor cells and aminobisphosphonates, in an attempt to integrate the corresponding - and at times confusing - findings into a "theory of everything" that may help explain how such diverse stimuli result in similar γδ T-cell responses via the recognition of soluble low molecular weight phosphoantigens.

The multifunctionality of human Vγ9Vδ2 γδ T cells: clonal plasticity or distinct subsets?

The dominant subset of γδ T cells in human peripheral blood expresses Vγ9 paired with Vδ2 as variable TCR elements. Vγ9Vδ2 T cells recognize pyrophosphates derived from the microbial non-mevalonate isoprenoid biosynthesis pathway at pico- to nanomolar concentrations. Structurally related pyrophosphates are generated in eukaryotic cells through the mevalonate pathway involved in protein prenylation and cholesterol synthesis. However, micromolar concentrations of endogenous pyrophosphates are required to be recognized by Vγ9Vδ2 T cells. Such concentrations are not produced by normal cells but can accumulate upon cellular stress and transformation. Therefore, many tumour cells are susceptible to γδ T cell-mediated lysis owing to the overproduction of endogenous pyrophosphates. This explains why Vγ9Vδ2 T cells contribute to both anti-infective and anti-tumour immunity. Ex vivo analysed Vγ9Vδ2 T cells can be subdivided on the basis of additional surface markers, including chemokine receptors and markers for naïve and memory T cells. At the functional level, Vγ9Vδ2 T cells produce a broad range of cytokines, display potent cytotoxic activity, regulate αβ T cell responses, and - quite surprisingly - can act as professional antigen-presenting cells. Thus, an exceptional range of effector functions has been assigned to a population of T cells, which all recognize invariant exogenous or endogenous pyrophosphates that are not seen by any other immune cell. Here, we discuss whether this plethora of effector functions reflects the plasticity of individual Vγ9Vδ2 T cells or can be assigned to distinct subsets.

Complex role of γδ T-cell-derived cytokines and growth factors in cancer

γδ T cells are innate lymphocytes that recognize and kill a range of tumor cells and are currently being explored as a target for tumor immunotherapy. However, γδ T cells play a complex role in cancer and can promote, as well as inhibit, tumor growth. In addition to tumor cell killing, γδ T cells express a number of cytokines and other soluble factors in response to tumors. Soluble factors expressed by γδ T cells in these settings include interferon-γ, tumor necrosis factor-α, interleukin (IL)-4, IL-10, transforming growth factor-β, IL-17, and a number of growth factors. These factors have differing and sometimes opposing effects on antitumor immunity and tumor angiogenesis, and likely contribute to the complex role of these cells in cancer. Here, we review studies in both mice and humans that examine differential cytokine secretion by γδ T cells in response to tumors and tumor immunotherapy, and discuss the influence of these γδ T-cell-derived factors on tumor growth.

γδ-T cells: an unpolished sword in human anti-infection immunity

γδ-T cells represent a small population of immune cells, but play an indispensable role in host defenses against exogenous pathogens, immune surveillance of endogenous pathogenesis and even homeostasis of the immune system. Activation and expansion of γδ-T cells are generally observed in diverse human infectious diseases and correlate with their progression and prognosis. γδ-T cells have both 'innate' and 'adaptive' characteristics in the immune response, and their anti-infection activities are mediated by multiple pathways that are under elaborate regulation by other immune components. In this review, we summarize the current state of the literature and the recent advancements in γδ-T cell-mediated immune responses against common human infectious pathogens. Although further investigation is needed to improve our understanding of the characteristics of different γδ-T cell subpopulations under specific conditions, γδ-T cell-based therapy has great potential for the treatment of infectious diseases.

BCG vaccination of neonatal calves: potential roles for innate immune cells in the induction of protective immunity

Bovine tuberculosis is a disease of increasing incidence in the UK causing major economic losses and with significant impact on bovine and, potentially human health: the causative agent Mycobacterium bovis is a zoonotic pathogen. Neonatal vaccination with the attenuated M. bovis Bacille Calmette Guerin (BCG) vaccine confers a significant degree of protection in cattle, and is a widely used control strategy for human TB. The adaptive immune system is relatively immature in neonates and increased numbers of innate effector cells present in young animals and human infants may compensate for this, enabling effective immune responses to vaccination. Natural killer cells and subsets of γδ TCR+ T lymphocytes secrete high levels of interferon gamma and can interact with antigen presenting cells to promote both innate and adaptive immune responses. These cell populations may be pivotal in determining immune bias following neonatal vaccination with BCG.

IL-21 enhances the potential of human γδ T cells to provide B-cell help

Vγ9/Vδ2 T cells are a minor subset of T cells in human blood and differ from all other lymphocytes by their specific responsiveness to (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMB-PP), a metabolite produced by a large range of microbial pathogens. Vγ9/Vδ2 T cells can be skewed towards distinct effector functions, in analogy to, and beyond, the emerging plasticity of CD4(+) T cells. As such, depending on the microenvironment, Vγ9/Vδ2 T cells can assume features reminiscent of Th1, Th2, Th17 and Treg cells as well as professional APCs. We here demonstrate that Vγ9/Vδ2 T cells express markers associated with follicular B helper T (T(FH) ) cells when stimulated with HMB-PP in the presence of IL-21. HMB-PP induces upregulation of IL-21R on Vγ9/Vδ2 T cells. In return, IL-21 plays a co-stimulatory role in the expression of the B-cell-attracting chemokine CXCL13, the CXCL13 receptor CXCR5 and the inducible co-stimulator by activated Vγ9/Vδ2 T cells, and enhances their potential to support antibody production by B cells. The interaction between HMB-PP-responsive Vγ9/Vδ2 T cells, IL-21-producing T(FH) cells and B cells in secondary lymphoid tissues is likely to impact on the generation of high affinity, class-switched antibodies in microbial infections.