Immune biology of Ag-specific γδ T cells in infections

Accumulating evidence suggests that human γδ T cells act as non-classical T cells and contribute to both innate and adaptive immune responses in infections. Vγ2 Vδ2 T (also termed Vγ9 Vδ2 T) cells exist only in primates, and in humans represent a dominant circulating γδ T-cell subset. Primate Vγ2 Vδ2 T cells are the only γδ T cell subset capable of recognizing microbial phosphoantigen. Since nonhuman primate Vγ2 Vδ2 T cells resemble their human counterparts, in-depth studies have been undertaken in macaques to understand the biology and function of human Vγ2 Vδ2 T cells. This article reviews the recent progress for immune biology of Vγ2 Vδ2 T cells in infections.     

Modulation of γδ T cell responses by TLR ligands

Toll-like receptors (TLR) are pattern-recognition receptors that recognize a broad variety of structurally conserved molecules derived from microbes. The recognition of TLR ligands functions as a primary sensor of the innate immune system, leading to subsequent indirect activation of the adaptive immunity as well as none-immune cells. However, TLR are also expressed by several T cell subsets, and the respective ligands can directly modulate their effector functions. The present review summarizes the recent findings of γδ T cell modulation by TLR ligands. TLR1/2/6, 3, and 5 ligands can act directly in combination with T cell receptor (TCR) stimulation to enhance cytokine/chemokine production of freshly isolated human γδ T cells. In contrast to human γδ T cells, murine and bovine γδ T cells can directly respond to TLR2 ligands with increased proliferation and cytokine production in a TCR-independent manner. Indirect stimulatory effects on IFN-γ production of human and murine γδ T cells via TLR-ligand activated dendritic cells have been described for TLR2, 3, 4, 7, and 9 ligands. In addition, TLR3 and 7 ligands indirectly increase tumor cell lysis by human γδ T cells, whereas ligation of TLR8 abolishes the suppressive activity of human tumor-infiltrating Vδ1 γδ T cells on αβ T cells and dendritic cells. Taken together, these data suggest that TLR-mediated signals received by γδ T cells enhance the initiation of adaptive immune responses during bacterial and viral infection directly or indirectly. Moreover, TLR ligands enhance cytotoxic tumor responses of γδ T cells and regulate the suppressive capacity of γδ T cells.     

Dynamics of the interaction of γδ T cells with their neighbors in vivo

γδ T cells are a diverse component of the immune system in humans and mice with presumably important but still largely unknown functions. Understanding the dynamic interaction of γδ T cells with their neighbors should help to understand their physiological role. This review addresses recent advances and strategies to visualize the dynamic interactions of γδ T cells with their neighbors in vivo. Current knowledge regarding the dynamic contacts of tissue resident γδ T cells and epithelial cells, but also of the communication between circulating γδ T cells and DCs, monocytes and FoxP3⁺ regulatory T cells is revisited with emphasis on the role of γδ T cell motility.     

γδ T-APCs: a novel tool for immunotherapy?

The series of seminal articles in this book clearly illustrate the multi-functional nature of γδ T cells. Some of the functions correlate with the tissue tropism of distinct γδ T cell subsets whereas others appear to result from oligoclonal selection. Here, we discuss the antigen-presenting cell (APC) function of the major subset of circulating γδ T cells, Vγ9/Vδ2 T cells, present in human blood. During tissue culture, Vγ9/Vδ2 T cells uniformly respond to a class of non-peptide antigens, so-called prenyl pyrophosphates, derived from stressed host cells or from microbes. It is this feature that distinguishes human (and primate) Vγ9/Vδ2 T cells from αβ and γδ T cells of all other species and that forms the basis for detailed studies of human Vγ9/Vδ2 T cells. One of the consequences of Vγ9/Vδ2 T cell activation is the rapid acquisition of APC characteristics (γδ T-APCs) reminiscent of mature dendritic cells (DCs). In the following discussion, we will discriminate between the potential use of γδ T-APCs as a cellular vaccine in immunotherapy and their role in anti-microbial immunity. Exploiting the APC function in γδ T-APCs represents a true novelty in current immunotherapy research and may lead to effective, anti-tumor immunity in cancer patients.     

Inhibition of human Vγ9Vδ2 T-cell antitumoral activity through HLA-G: implications for immunotherapy of cancer

Vγ9Vδ2 T cells play a crucial role in the antitumoral immune response through cytokine production and cytotoxicity. Although the expression of the immunomodulatory molecule HLA-G has been found in diverse tumors, its impact on Vγ9Vδ2 T-cell functions remains unknown. Here we showed that soluble HLA-G inhibits Vγ9Vδ2 T-cell proliferation without inducing apoptosis. Moreover, soluble HLA-G inhibited the Vγ9Vδ2 T-cell production of IFN-γ induced by phosphoantigen stimulation. The reduction in Vγ9Vδ2 T-cell IFN-γ production was also induced by membrane-bound or soluble HLA-G expressed by tumor cell lines. Finally, primary tumor cells inhibited Vγ9Vδ2 T-cell proliferation and IFN-γ production through HLA-G. In this context, HLA-G impaired Vγ9Vδ2 T-cell cytotoxicity by interacting with ILT2 inhibitory receptor. These data demonstrate that HLA-G inhibits the anti-tumoral functions of Vγ9Vδ2 T cells and imply that treatments targeting HLA-G could optimize Vγ9Vδ2 T-cell-mediated immunotherapy of cancer.     

Regulation and function of IL-17A- and IL-22-producing γδ T cells

The regulation of IL-17A and IL-22 production differs between human and murine γδ T cells. We find that human γδ T cells expressing Vγ2Vδ2 T cell receptors are peripherally polarized to produce IL-17A or IL-22, much like CD4 αβ Th17 T cells. This requires IL-6, IL-1β, and TGF-β, whereas expansion and maintenance requires IL-23, IL-1β, and TGF-β. In contrast, IL-17A and IL-22 production by murine γδ T cells is innately programmed during thymic ontogeny but requires IL-23 and IL-1β for maintenance. Murine γδ cells producing IL-17A and IL-22 play important roles in microbial, autoimmune, and inflammatory responses. However, the roles played by human IL-17A- and IL-22-producing γδ T cells are less clear but are also likely to be important. These observations highlight differences between humans and murine γδ T cells and underscore the importance of IL-17A- and IL-22-producing γδ T cells.     

Peptide antigens for gamma/delta T cells

γδ T cells express adaptive antigen receptors encoded by rearranging genes. Their diversity is highest in the small region of TCR V–J junctions, especially in the δ chain, which should enable the γδ TCRs to distinguish differences in small epitopes. Indeed, recognition of small molecules, and of an epitope on a larger protein has been reported. Responses to small non-peptides known as phospho-antigens are multi-clonal yet limited to a single γδ T cell subset in humans and non-human primates. Responses to small peptides are multi-clonal or oligo-clonal, include more than one subset of γδ T cells, and occur in rodents and primates. However, less effort has been devoted to investigate the peptide responses. To settle the questions of whether peptides can be ligands for the γδ TCRs, and whether responses to small peptides might occur normally, peptide binding will have to be demonstrated, and natural peptide ligands identified.     

Searching for “signal 2”: costimulation requirements of γδ T cells

T cell activation requires the integration of signals that arise from various types of receptors. Although TCR triggering is a necessary condition, it is often not sufficient to induce full T-cell activation, as reflected in cell proliferation and cytokine secretion. This has been firmly demonstrated for conventional αβ T cells, for which a large panel of costimulatory receptors has been identified. By contrast, the area remains more obscure for unconventional, innate-like γδ T cells, as the literature has been scarce and at times contradictory. Here we review the current state of the art on the costimulatory requirements of γδ T cell activation. We highlight the roles of members of the immunoglobulin (like CD28 or JAML) or tumour necrosis factor receptor (like CD27) superfamilies of coreceptors, but also of more atypical costimulatory molecules, such as NKG2D or CD46. Finally, we identify various areas where our knowledge is still markedly insufficient, hoping to provoke future research on γδ T cell costimulation.     

Vγ9Vδ2 T cell-based immunotherapy in hematological malignancies: from bench to bedside

Many hematological malignancies consist of tumor cells that are spontaneously recognized and killed by Vγ9Vδ2 T cells. These tumor cells generate high amounts of intracellular phosphorylated metabolites mimicking the natural ligands and display a wide range of stress-induced self-ligands that are recognized by Vγ9Vδ2 T cells via TCR-dependent and TCR-independent mechanisms. The intrinsic features of Vγ9Vδ2 T cells and that of tumor cells of hematological origin constitute an ideal combination from which to develop Vγ9Vδ2 T cell-based immune interventions. In this review, we will discuss the rationale, preclinical and clinical data in favor of this therapeutic strategy and the future perspectives of its development.     

Functions of skin-resident γδ T cells

The murine epidermis contains resident T cells that express a canonical γδ TCR and arise from fetal thymic precursors. These cells are termed dendritic epidermal T cells (DETC) and use a TCR that is restricted to the skin in adult animals. DETC produce low levels of cytokines and growth factors that contribute to epidermal homeostasis. Upon activation, DETC can secrete large amounts of inflammatory molecules which participate in the communication between DETC, neighboring keratinocytes and langerhans cells. Chemokines produced by DETC may recruit inflammatory cells to the epidermis. In addition, cell–cell mediated immune responses also appear important for epidermal–T cell communication. Information is provided which supports a crucial role for DETC in inflammation, wound healing, and tumor surveillance.     

High frequency of human cytomegalovirus (HCMV)-specific CD8+-T cells detected in a healthy CMV-seropositive donor

Human cytomegalovirus (HCMV) persists after infection but is controlled by cellular immune responses, particularly by CD8⁺ T cells. If infected individuals are immunosuppressed, HCMV can be reactivated. Upon testing the blood of healthy donors with human lymphocyte antigen tetramers, we found one individual with about 50 % of his CD8⁺ T cells being specific for the immunodominant pp65 epitope NLVPMVATV. Over a period of 2 years the high level of HCMV-specific T cells was maintained, and no HCMV DNA could be detected. At one timepoint, however, HCMV-specific DNA was detected, while 65 % of CD8⁺ T cells were specific for HCMV. When virus was detectable, a lower percentage of HCMV-specific CD8⁺ T cells showed interferon γ (IFN-γ) production after peptide stimulation in vitro. These data suggest that HCMV reactivation may also occur in immunocompetent persons, accompanied by the presence of HCMV-specific CD8⁺ T cells which are not producing IFNγ, and therefore potentially anergic or in vivo exhausted. Received 6 March 2002; received after revision 15 April 2002; accepted 17 April 2002     

The cellular immune response to heat shock proteins

T lymphocytes, which are central to almost every immune response, frequently recognize microbial hsp60. Such cells could provide an early defense mechanism against pathogenic microbes. However, T cells also recognize epitopes of hsp60 shared by microbe and host. Not only conventional / T cells respond to hsp60; / T cells do so, as well. In fact, certain / T cells seem to have a particular preference for this molecule. Recognition of stressed host cells expressing hsp60 could facilitate the scavenger function of the T cell system. On the other hand, such recognition could be involved in autoimmune disease.     

The processing and presentation of endogenous and exogenous antigen by Schwann cells in vitro

The expression of major histocomatibility complex class II in vitro and in vivo by Schwann cells indicates a potential facultative role of Schwann cells in the presentation of antigen to neuritogenic T cells during inflammatory demyelinating neuropathies. Using a T cell proliferation assay, this study demonstrated that processing and presentation of endogenous and exogenous antigen by Schwann cells influences T cell proliferation. Statistical analysis of proliferation and its relation to processing and presentation of antigen by Schwann cells had not been previously addressed. Different combinations of factors including treatment of cultures (untreated, irradiated or fixed), concentration of exogenous antigen (0 or 40 μmg/ml), the presence of interferon-γ and the timing of exogenous antigen addition influence the proliferation P₂-specific, non-mammalian protein ovalbumin-specific T cell lines and naive T cells. Received 25 July 2002; received after revision 9 September 2002; accepted 7 October 2002     

How tumors might withstand γδ T-cell attack

Several clinical trials are currently assessing the therapeutic activity of human TCRVγ9Vδ2⁺ lymphocytes in cancer. Growing tumors usually follow a triphasic “Elimination, Equilibrium, Escape” evolution in patients. Thus, at diagnostic, most tumors have already developed some means to escape to immune protection. We review here the conventional immunoescape mechanisms which might also protect against cytolytic TCRVγ9Vδ2⁺ lymphocytes activated by phosphoantigens. Neutralization of these deleterious processes might prove highly valuable to improve the efficacy of ongoing γδ cell-based cancer immunotherapies.     

Heat shock protein 60: regulatory role on innate immune cells

Human heat shock protein 60 (Hsp60) exhibits immunoregulatory properties, primarily by inducing pro-inflammatory responses in innate immune cells. Extensive analyses identified specific receptor structures for the interaction of Hsp60 with these cells. The existence of distinct receptor structures responsible for Hsp60 binding and for Hsp60-induced release of pro-inflammatory mediators has been demonstrated, implying that the interaction of Hsp60 with innate immune cells is a multifaceted process. Distinct Hsp60 epitopes responsible for binding to innate immune cells and for the activation of these cells have been identified. Depending on the cell-type, the amino acid (aa) region 481–500 or the regions aa241–260, aa391–410 and aa461–480 are involved in Hsp60-binding to innate immune cells. An entirely different Hsp60-region, aa354–365 was found to bind lipopolysaccharide, thereby mediating the pro-inflammatory effects of Hsp60. Because of its immunoregulatory properties, Hsp60 has been proposed to act as intercellular danger signal, controlling innate and adaptive immune reactions. Received 19 September 2006; received after revision 13 October 2006; accepted 13 December 2006     

Presenilin-dependent regulated intramembrane proteolysis and γ-secretase activity

Inhibiting the production of amyloid-β by antagonising γ-secretase activity is currently being pursued as a therapeutic strategy for Alzheimer’s disease (AD). However, early pre-clinical studies have demonstrated that disruption of presenilin-dependent γ-secretase alters many presenilin-dependent processes, leading to early lethality in several AD model organisms. Subsequently, transgenic animal studies have highlighted several gross developmental side effects arising from presenilin deficiency. Partial knockdown or tissue-specific knockout of presenilins has identified the skin, vascular and immune systems as very sensitive to loss of presenilin functions. A more appreciative understanding of presenilin biology is therefore demanded if γ-secretase is to be pursued as a therapeutic target. Herein we review the current understanding of γ-secretase complexes; their regulation, abundance of interacting partners and diversity of substrates. We also discuss regulation of the γ-secretase complexes, with an emphasis on the functional role of presenilins in cell biology. Received 25 July 2008; received after revision 24 November 2008; accepted 10 December 2008     

Type I phosphoinositide 3-kinases: potential antithrombotic targets?

Arterial thrombosis is the single most common cause of death and disability in industrialized societies and is the primary pathogenic mechanism underlying acute myocardial infarction and ischemic stroke. Platelets play a central role in this process, and as a consequence, a great deal of effort has gone into identifying the mechanisms regulating the adhesive function of platelets. Platelet adhesion is controlled by intracellular signaling pathways, with growing evidence for a major role for phosphoinositide 3-kinases (PI3Ks) in this process. Platelets express all type I PI3K isoforms, including p110α, p110β, p110δ and p110γ, with recent evidence suggesting important roles for p110γ and p110β in regulating distinct phases of the platelet activation process. Deficiency of p110 γ or inhibition of p110β produces a marked defect in arterial thrombosis without a corresponding increase in bleeding time, raising the possibility that inhibition of one or more PI3K isoforms may represent an effective antithrombotic approach. Received 3 January 2006; received after revision 20 February 2006; accepted 20 February 2006     

Biased binding of class IA phosphatidyl inositol 3-kinase subunits to inducible costimulator (CD278)

To better understand T lymphocyte costimulation by inducible costimulator (ICOS; H4; CD278), we analyzed proteins binding to ICOS peptides phosphorylated at the Y₁₉₁MFM motif. Phosphorylated ICOS binds class IA phosphatidyl inositol 3-kinase (PI3-K) p85α, p50-55α and p85β regulatory subunits and p110α, p110δ and p110β catalytic subunits. Intriguingly, T cells expressed high levels of both p110α or p110δ catalytic subunits, yet ICOS peptides, cell surface ICOS or PI3-kinase class IA regulatory subunits preferentially coprecipitated p110α catalytic subunits. Silencing p110α or p110δ partially inhibited Akt/PKB activation induced by anti-CD3 plus anti-ICOS antibodies. However, silencing p110α enhanced and silencing p110δ inhibited Erk activation. Both p110α- and p110δ-specific inhibitors blocked cytokine secretion induced by TCR/CD3 activation with or without ICOS costimulus, but only p110α inhibitors blocked ICOS-induced cell elongation. Thus, p110α and p110δ are essential to optimal T cell activation, but their abundance and activity differentially tune up distinct ICOS signaling pathways.     

Macrophages derived from bone marrow modulate differentiation of myeloid dendritic cells

Dendritic cells (DC) are specialized antigen-presenting cells. Bone marrow monocytes have been widely used to generate murine myeloid DC. We found that mouse macrophages derived from bone marrow CD11b⁺ monocytes influenced the differentiation of these precursors into DC. Modulation of differentiation was demonstrated by the down-regulation of CD11c, CD40, and CD86 expression and by IL-12 production. DC differentiated in the presence of conditioned medium from bone marrow-derived macrophage culture (MCM) had impaired ability to stimulate proliferation of, and IFN- γ production by, allogeneic CD4⁺ T cells. This inhibition of DC differentiation was mainly mediated by secretory products from macrophages but not by cell-cell contact. MCM contained higher concentrations of macrophage-colony-stimulating factor (M-CSF), IL-10, and TGF- β1, whereas IL-6 remained unchanged compared with conditioned medium from fresh monocytes. M-CSF may be the major mediator in MCM inhibiting DC differentiation. This study demonstrates an important influence of bone marrow-derived macrophages on DC precursors during DC differentiation. Received 12 September 2006; received after revision 20 October 2006; accepted 13 November 2006     

Immunology and functional genomics of Behçet's disease

Behçet's disease (BD) is a multisystemic inflammatory disorder. Although the cause and pathogenesis of BD are still unclear, there is evidence for genetic, immunologic and infectious factors at the onset or in the course of BD. This review focusses on the functional genomics and immunology of BD. HLA-B51 is the major disease susceptibility gene locus in BD. An increased number of T cells in the peripheral blood and in the involved tissues have been reported. However, the T cells at the sites of inflammation appear to be a phenotypically distinct subset. There is also a significant T cell proliferative response to mycobacterial 65-kDa heat shock protein peptides. Homologous peptides derived from the human 60-kDa heat shock protein were observed in BD patients. There is evidence that natural killer T cells may also play a role in BD.Received 27 November 2002; accepted 4 March 2003