gammadelta T cells as regulators of airway hyperresponsiveness

Coreceptors and Their Ligands in Epithelial γδ T Cell Biology

Epithelial tissues line the body providing a protective barrier from the external environment. Maintenance of these epithelial barrier tissues critically relies on the presence of a functional resident T cell population. In some tissues, the resident T cell population is exclusively comprised of γδ T cells, while in others γδ T cells are found together with αβ T cells and other lymphocyte populations. Epithelial-resident γδ T cells function not only in the maintenance of the epithelium, but are also central to the repair process following damage from environmental and pathogenic insults. Key to their function is the crosstalk between γδ T cells and neighboring epithelial cells. This crosstalk relies on multiple receptor–ligand interactions through both the T cell receptor and accessory molecules leading to temporal and spatial regulation of cytokine, chemokine, growth factor, and extracellular matrix protein production. As antigens that activate epithelial γδ T cells are largely unknown and many classical costimulatory molecules and coreceptors are not used by these cells, efforts have focused on identification of novel coreceptors and ligands that mediate pivotal interactions between γδ T cells and their neighbors. In this review, we discuss recent advances in the understanding of functions for these coreceptors and their ligands in epithelial maintenance and repair processes.

γδT cells regulate chronic airway inflammation and development of airway remodelling

BACKGROUND

γδT cells play a crucial immunoregulatory role in the lung, maintaining normal airway tone and preventing hyperresponsiveness to innocuous allergen. During acute inflammatory episodes, γδT cells promote resolution of acute inflammation. However, their contribution to inflammation-associated airway remodelling remains unexplored. Here we investigate the effects of γδT cell blockade on established allergic airway inflammation and development of remodelling.

METHODS

Sensitised mice were exposed to prolonged ovalbumin challenge or continuous house-dust mite exposure to induce chronic inflammation and remodelling. Functional blocking anti-TCRδ antibody was administered therapeutically, and parameters of airway inflammation and remodelling were examined.

RESULTS

Therapeutic blockade of γδT cells prevented the typical resolution of acute airway inflammation characterised by elevated eosinophil and Th2 cell numbers. Moreover, the lung displayed exacerbated airway remodelling, typified by excess peribronchiolar collagen deposition.

CONCLUSIONS

These results demonstrate a unique role for γδT cells in constraining allergen-induced airway remodelling. Manipulating the γδT cell compartment may therefore contribute to strategies to prevent and treat remodelling.

Cross-talk between intraepithelial γδ T cells and epithelial cells

Intraepithelial γδ T cells play pivotal roles in homeostasis, tissue repair, inflammation, and protection from malignancy. In some tissues, γδ T cells are the only resident T cell population, whereas in others, they coexist with αβ T cells and other lymphocyte populations. γδ T cell function in the epithelium requires constant communication between cells in the form of cell-to-cell contacts and cell-to-matrix interactions. These interactions coordinate with the timely production of specific cytokines, chemokines, growth factors, and glycosaminoglycans, which have specialized effects on neighboring epithelial cells. Antigens that activate these T cells are not well-defined, and they do not express classic costimulatory or coreceptor molecules. As such, an understanding of the mechanisms used by epithelial γδ T cells to maintain homeostasis and facilitate wound repair has necessitated the identification of novel molecular interactions between γδ T cells and their neighboring epithelial cells.

Can peripheral blood γδ T cells predict osteonecrosis of the jaw? An immunological perspective on the adverse drug effects of aminobisphosphonate therapy

Nitrogen-bisphosphonates (n-BP), often referred to as aminobisphosphonates, are the most commonly prescribed drugs for the treatment of disorders of bone fragility. However, long-term continuous treatment predisposes certain individuals to serious rare side effects, such as bisphosphonate-associated osteonecrosis of the jaw (BAONJ). n-BP use is known to unintentionally activate a subset of innate T cells called Vγ9Vδ2 T cells, but the consequence of this chronic immune stimulation has remained unexplored. The primary objectives of this study were to 1) determine the fate of Vγ9Vδ2 T cells in osteoporotic patients on n-BP therapy as a function of time and type of therapy; 2) evaluate the proportion of Vγ9Vδ2 T cells in patients who had recently experienced n-BP-associated ONJ. We found there is a notable loss of Vγ9Vδ2 T cells over time in osteoporotic patients on n-BP therapy, particularly those on intravenous (iv) therapy (Spearman r = -0.55, p < 0.0001 iv; r = -0.3, p < 0.03 oral) (n = 68); no difference was observed in total T cells, monocytes, or granulocytes. Importantly, the observed negative effect on Vγ9Vδ2 T cells coincides with the reported route of administration and timing of the rare occurrence of BAONJ. Patients (n = 6) who had experienced BAONJ were all found to be significantly deficient in Vγ9Vδ2 T cells (median = 0.07%) in comparison to age- and sex-matched treatment-naïve controls (N = 11; median = 2.40%), U = 0, p = 0.001; this was the only consistent difference in the leukocytes assessed. All BAONJ cases had an underlying condition that further contributed to impaired immunity. We propose Vγ9Vδ2 T cells show a strong potential to serve as harbingers of possible adverse immune effects of n-BP therapy, particularly in those patients already having a compromised immune system as they may be most vulnerable to the development of conditions such as BAONJ.

Six-of-the-best: unique contributions of γδ T cells to immunology

γδ T cells are a unique and conserved population of lymphocytes that have been the subject of a recent explosion of interest owing to their essential contributions to many types of immune response and immunopathology. But what does the integration of recent and long-established studies really tell us about these cells and their place in immunology? The time is ripe to consider the evidence for their unique and crucial functions. We conclude that whereas B cells and αβ T cells are commonly thought to contribute primarily to the antigen-specific effector and memory phases of immunity, γδ T cells are distinct in that they combine conventional adaptive features (inherent in their T cell receptors and pleiotropic effector functions) with rapid, innate-like responses that can place them in the initiation phase of immune reactions. This underpins a revised perspective on lymphocyte biology and the regulation of immunogenicity.

Linking innate and adaptive immunity: human Vgamma9Vdelta2 T cells enhance CD40 expression and HMGB-1 secretion

γδ T cells play an important role in regulating the immune response to stress stimuli; however, the mean by which these innate lymphocytes fulfill this function remains poorly defined. The main subset of human peripheral blood γδ T cells responds to nonpeptidic antigens, such as isopentylpyrophosphate (IPP), a metabolite in the mevalonate pathway for both eukaryote and prokaryote cells. IPP-primed γδ T cells significantly augment the inflammatory response mediated by monocytes and αβ T cells to TSST-1, the staphylococcal superantigen that is the major causative agent of toxic shock syndrome. Here we show that the small pool of activated peripheral γδ T cells induces an early upregulation of CD40 on monocytes and the local release of High Mobility Group Box-1 (HMGB-1), the molecule designated as the late mediator of systemic inflammation. This finding provides a new basis for how γδ T cells may serve as influential modulators of both endogenous and exogenous stress stimuli.

Aerosolized anti-T-cell-receptor antibodies are effective against airway inflammation and hyperreactivity

Aerosolized monoclonal antibodies (mAbs) specific for T-cell receptors (TCR) were used to manipulate T-cell function in airways of ovalbumin (OVA)-sensitized and -challenged mice with airway hyperresponsiveness (AHR). The inhaled mAbs were found to be effective at low doses, had little or no systemic effect and specifically abrogated both effector and regulatory functions of the targeted T cells. Specific mAbs targeting alphabeta T cells suppressed and those targeting gammadelta T cells enhanced AHR. Moreover, specific mAbs directed against subsets of gammadelta T cells varied in their effect on AHR. Using this approach of targeting either alphabeta or gammadelta T cells reduced airway eosinophila, although the effect of mAbs specific for alphabeta T cells was stronger. The use of aerosolized anti-TCR mAbs may offer an effective approach for the treatment of airway inflammation and AHR.

Role of gamma delta T cells in inflammatory bowel disease

gammadelta T cells have previously been shown to play a protective role in various animal models of chronic inflammation (e.g., experimental autoimmune encephalomyelitis, collagen-induced arthritis, and non-obese diabetes). This immunoregulatory potential is exerted by synthesizing various anti-inflammatory cytokines and growth factors (e.g., transforming growth factor-beta). As the normal balance between inflammatory and regulatory cytokines is perturbed in inflammatory bowel disease (IBD) a protective effect of gammadelta T cells seems likely. This notion is supported by our finding of increased mortality of rats with 2,4,6-trinitrobenzene sulfonic acid-induced colitis following gammadelta T cell depletion. In contrast, no effect was observed after depletion of gammadelta T cells in a Crohn's disease animal model with terminal ileitis (TNF(DeltaARE) mice). Therefore, future studies must further define where in the intestinal immune system gammadelta T cells exert their protective function and how this can be used in the treatment of IBD.