J. M. Wands

Different Potentials of γδ T Cell Subsets in Regulating Airway Responsiveness: Vγ1+ Cells, but Not Vγ4+ Cells, Promote Airway Hyperreactivity, Th2 Cytokines, and Airway Inflammation

Allergic airway inflammation and hyperreactivity are modulated by γδ T cells, but different experimental parameters can influence the effects observed. For example, in sensitized C57BL/6 and BALB/c mice, transient depletion of all TCR-δ+ cells just before airway challenge resulted in airway hyperresponsiveness (AHR), but caused hyporesponsiveness when initiated before i.p. sensitization. Vγ4+ γδ T cells strongly suppressed AHR; their depletion relieved suppression when initiated before challenge, but not before sensitization, and they suppressed AHR when transferred before challenge into sensitized TCR-Vγ4−/−/6−/− mice. In contrast, Vγ1+ γδ T cells enhanced AHR and airway inflammation. In normal mice (C57BL/6 and BALB/c), enhancement of AHR was abrogated only when these cells were depleted before sensitization, but not before challenge, and with regard to airway inflammation, this effect was limited to C57BL/6 mice. However, Vγ1+ γδ T cells enhanced AHR when transferred before challenge into sensitized B6.TCR-δ−/− mice. In this study Vγ1+ cells also increased levels of Th2 cytokines in the airways and, to a lesser extent, lung eosinophil numbers. Thus, Vγ4+ cells suppress AHR, and Vγ1+ cells enhance AHR and airway inflammation under defined experimental conditions. These findings show how γδ T cells can be both inhibitors and enhancers of AHR and airway inflammation, and they provide further support for the hypothesis that TCR expression and function cosegregate in γδ T cells.

γδ T-cell receptors : functional correlations

Summary:  The γδ T‐cell receptors (TCRs) are limited in their diversity, suggesting that their natural ligands may be few in number. Ligands for γδTCRs that have thus far been determined are predominantly of host rather than foreign origin. Correlations have been noted between the Vγ and/or Vδ genes a γδ T cell expresses and its functional role. The reason for these correlations is not yet known, but several different mechanisms are conceivable. One possibility is that interactions between particular TCR‐V domains and ligands determine function or functional development. However, a recent study showed that at least for one ligand, receptor specificity is determined by the complementarity‐determining region 3 (CDR3) component of the TCR‐δ chain, regardless of the Vγ and/or Vδ. To determine what is required in the TCR for other specificities and to test whether recognition of certain ligands is connected to cell function, more γδTCR ligands must be defined. The use of recombinant soluble versions of γδTCRs appears to be a promising approach to finding new ligands, and recent results using this method are reviewed.

Distribution and leukocyte contacts of γδ T cells in the lung

Pulmonary γδ T cells protect the lung and its functions, but little is known about their distribution in this organ and their relationship to other pulmonary cells. We now show that γδ and αβ T cells are distributed differently in the normal mouse lung. The γδ T cells have a bias for nonalveolar locations, with the exception of the airway mucosa. Subsets of γδ T cells exhibit further variation in their tissue localization. γδ and αβ T cells frequently contact other leukocytes, but they favor different cell‐types. The γδ T cells show an intrinsic preference for F4/80+ and major histocompatibility complex class II+ leukocytes. Leukocytes expressing these markers include macrophages and dendritic cells, known to function as sentinels of airways and lung tissues. The continuous interaction of γδ T cells with these sentinels likely is related to their protective role.

Vγ4+ γδ T Cells Regulate Airway Hyperreactivity to Methacholine in Ovalbumin-Sensitized and Challenged Mice

The Vγ4+ pulmonary subset of γδ T cells regulates innate airway responsiveness in the absence of αβ T cells. We now have examined the same subset in a model of allergic airway disease, OVA-sensitized and challenged mice that exhibit Th2 responses, pulmonary inflammation, and airway hyperreactivity (AHR). In sensitized mice, Vγ4+ cells preferentially increased in number following airway challenge. Depletion of Vγ4+ cells before the challenge substantially increased AHR in these mice, but had no effect on airway responsiveness in normal, nonchallenged mice. Depletion of Vγ1+ cells had no effect on AHR, and depletion of all TCR-δ+ cells was no more effective than depletion of Vγ4+ cells alone. Adoptively transferred pulmonary lymphocytes containing Vγ4+ cells inhibited AHR, but lost this ability when Vγ4+ cells were depleted, indicating that these cells actively suppress AHR. Eosinophilic infiltration of the lung and airways, or goblet cell hyperplasia, was not affected by depletion of Vγ4+ cells, although cytokine-producing αβ T cells in the lung increased. These findings establish Vγ4+ γδ T cells as negative regulators of AHR and show that their regulatory effect bypasses much of the allergic inflammatory response coincident with AHR.

γδ T Lymphocytes—Selectable Cells Within the Innate System?

Lymphocytes expressing γδ T cell receptors (TCR) constitute an entire system of functionally specialized subsets that have been implicated in the regulation of immune responses, including responses to pathogens and allergens, and in tissue repair. The γδ TCRs share structural features with adaptive receptors and peripheral selection of γδ T cells occurs. Nevertheless, their specificities may be primarily directed at self-determinants, and the responses of γδ T cells exhibit innate characteristics. Continuous cross talk between γδ T cells and myeloid cells is evident in histological studies and in in vitro co-culture experiments, suggesting that γδ T cells play a functional role as an integral component of the innate immune system.

The Influence of IgE-enhancing and IgE-suppressive γδ T Cells Changes With Exposure to Inhaled Ovalbumin

It has been reported that the IgE response to allergens is influenced by γδ T cells. Intrigued by a study showing that airway challenge of mice with OVA induces in the spleen the development of γδ T cells that suppress the primary IgE response to i.p.-injected OVA-alum, we investigated the γδ T cells involved. We found that the induced IgE suppressors are contained within the Vγ4+ subset of γδ T cells of the spleen, that they express Vδ5 and CD8, and that they depend on IFN-γ for their function. However, we also found that normal nonchallenged mice harbor IgE-enhancing γδ T cells, which are contained within the larger Vγ1+ subset of the spleen. In cell transfer experiments, airway challenge of the donors was required to induce the IgE suppressors among the Vγ4+ cells. Moreover, this challenge simultaneously turned off the IgE enhancers among the Vγ1+ cells. Thus, airway allergen challenge differentially affects two distinct subsets of γδ T cells with nonoverlapping functional potentials, and the outcome is IgE suppression.

Mismatched Antigen Prepares γδ T Cells for Suppression of Airway Hyperresponsiveness

γδ T cells suppress airway hyperresponsiveness (AHR) induced in allergen-challenged mice but it is not clear whether the suppression is allergen specific. The AHR-suppressive cells express TCR-Vγ4. To test whether the suppressive function must be induced, we adoptively transferred purified Vγ4+ cells into γδ T cell-deficient and OVA-sensitized and -challenged recipients (B6.TCR-Vγ4−/−/6−/−) and measured the effect on AHR. Vγ4+ γδ T cells isolated from naive donors were not AHR-suppressive, but Vγ4+ cells from OVA-stimulated donors suppressed AHR. Suppressive Vγ4+ cells could be isolated from lung and spleen. Their induction in the spleen required sensitization and challenge. In the lung, their function was induced by airway challenge alone. Induction of the suppressors was associated with their activation but it did not alter their ability to accumulate in the lung. Vγ4+ γδ T cells preferentially express Vδ4 and -5 but their AHR-suppressive function was not dependent on these Vδs. Donor sensitization and challenge not only with OVA but also with two unrelated allergens (ragweed and BSA) induced Vγ4+ cells capable of suppressing AHR in the OVA-hyperresponsive recipients, but the process of sensitization and challenge alone (adjuvant and saline only) was not sufficient to induce suppressor function, and LPS as a component of the allergen was not essential. We conclude that AHR-suppressive Vγ4+ γδ T cells require induction. They are induced by allergen stimulation, but AHR suppression by these cells does not require their restimulation with the same allergen.

Detection of Cell Surface Ligands for the γδ TCR Using Soluble TCRs

The natural ligands recognized by γδ TCRs are still largely unknown, in part because immunization does not normally result in Ag-specific γδ T cell responses. Taking advantage of an established ligand for a particular γδ TCR, we demonstrated that a multimerized recombinant form of this γδ TCR can be used like a mAb to specifically detect its own ligand. Using the same approach for more common γδ TCRs whose ligands remain unknown, we detected on certain cell lines molecules that appear to be ligands for three additional γδ TCRs. One of these represents the mouse Vγ6/Vδ1 invariant γδ TCR, which predominates in the female reproductive tract, the tongue, and the lung, and other tissues during inflammation. The second represents the closely related Vγ5/Vδ1 invariant γδ TCR expressed by most epidermal T cells. The third is a Vγ1/Vδ6.3 TCR, representative of a variable type frequently found on lymphoid γδ T cells. We found evidence that ligands for multiple γδ TCRs may be simultaneously expressed on a single cell line, and that at least some of the putative ligands are protease sensitive. This study suggests that soluble versions of γδ TCRs can be as tools to identify and characterize the natural ligands of γδ T cells.

Evidence That CD8+ Dendritic Cells Enable the Development of γδ T Cells That Modulate Airway Hyperresponsiveness

Airway hyperresponsiveness (AHR), a hallmark of asthma and several other diseases, can be modulated by γδ T cells. In mice sensitized and challenged with OVA, AHR depends on allergen-specific αβ T cells; but Vγ1+ γδ T cells spontaneously enhance AHR, whereas Vγ4+ γδ T cells, after being induced by airway challenge, suppress AHR. The activity of these γδ T cell modulators is allergen nonspecific, and how they develop is unclear. We now show that CD8 is essential for the development of both the AHR suppressor and enhancer γδ T cells, although neither type needs to express CD8 itself. Both cell types encounter CD8-expressing non-T cells in the spleen, and their functional development in an otherwise CD8-negative environment can be restored with transferred spleen cell preparations containing CD8+ dendritic cells (DCs), but not CD8+ T cells or CD8− DCs. Our findings suggest that CD8+ DCs in the lymphoid tissues enable an early step in the development of γδ T cells through direct cell contact. DC-expressed CD8 might take part in this interaction.

Allergic Airway Hyperresponsiveness-Enhancing γδ T Cells Develop in Normal Untreated Mice and Fail to Produce IL-4/13, Unlike Th2 and NKT Cells

Allergic airway hyperresponsiveness (AHR) in OVA-sensitized and challenged mice, mediated by allergen-specific Th2 cells and Th2-like invariant NKT (iNKT) cells, develops under the influence of enhancing and inhibitory γδ T cells. The AHR-enhancing cells belong to the Vγ1+ γδ T cell subset, cells that are capable of increasing IL-5 and IL-13 levels in the airways in a manner like Th2 cells. They also synergize with iNKT cells in mediating AHR. However, unlike Th2 cells, the AHR enhancers arise in untreated mice, and we show here that they exhibit their functional bias already as thymocytes, at an HSAhigh maturational stage. In further contrast to Th2 cells and also unlike iNKT cells, they could not be stimulated to produce IL-4 and IL-13, consistent with their synergistic dependence on iNKT cells in mediating AHR. Mice deficient in IFN-γ, TNFRp75, or IL-4 did not produce these AHR-enhancing γδ T cells, but in the absence of IFN-γ, spontaneous development of these cells was restored by adoptive transfer of IFN-γ-competent dendritic cells from untreated donors. The i.p. injection of OVA/aluminum hydroxide restored development of the AHR enhancers in all of the mutant strains, indicating that the enhancers still can be induced when they fail to develop spontaneously, and that they themselves need not express TNFRp75, IFN-γ, or IL-4 to exert their function. We conclude that both the development and the cytokine potential of the AHR-enhancing γδ T cells differs critically from that of Th2 cells and NKT cells, despite similar influences of these cell populations on AHR.