Lymphocyte-mediated cytotoxicity

Fusion of green fluorescent protein to the C-terminus of granulysin alters its intracellular localization in comparison to the native molecule

The engineering of green fluorescent protein (GFP) fusion constructs in order to visibly tag a protein of interest has become a commonly used cell biology technique. Although caveats to this approach are obvious, literature reports in which the chimeric molecule behaves differently than the native molecule are scant. This brief report describes one such case. Granulysin, a small lytic and antimicrobial protein produced by cytotoxic lymphocytes, traffics to the regulated secretory system and is subsequently released from cells upon proper stimulus. In an attempt to elucidate mechanisms by which it accumulates in and is released from cytolytic granules, GFP was fused to the C-terminus of granulysin and expressed in an NK cell line. A control construct expressing the native protein was similarly expressed. The data demonstrate that, while the fusion protein is expressed and secreted, its subcellular localization is altered in comparison to native granulysin. Thus, the addition of GFP to the C-terminus of granulysin obscures the signal(s) that cytotoxic lymphocytes use to sort it to the regulated secretory pathway despite its normal biosynthesis and secretion. This example is offered as a cautionary account for other researchers who contemplate using this technology.

Lymphocytic inflammation in childhood bronchiolitis obliterans

Childhood bronchiolitis obliterans (CBO) is an infrequent, severe disorder characterized by persistent obstructive respiratory symptoms after an acute episode of bronchiolitis. The viral etiology is most common, and adenovirus is the most frequently identified causative agent. Pathologically, the disease is characterized as constrictive type BO, with variable degrees of chronic inflammation and fibrosis in the bronchioles. The nature of the cellular infiltrate is largely unknown, and its characterization may provide better understanding of the disease and offer clues for therapy. Therefore, the aim of the present study was to characterize the inflammatory infiltrate in the bronchioles of 23 open lung biopsies of children with CBO and to compare this to the infiltrate in histologically normal airways. Our results show that CD3+ T cells were the most frequent cell type observed in CBO, with a predominance of the CD8+ T-cell subtype. When compared to the control group, there was a larger number of CD8+, CD4+, CD20+, granzyme B+, and perforin+ lymphocytes in the CBO group. Further studies are needed to address the role of different cell types in the development of CBO.

Fas-Mediated Inhibition of CD4+ T Cell Priming Results in Dominance of Type 1 CD8+ T Cells in the Immune Response to the Contact Sensitizer Trinitrophenyl

One of the unusual properties of chemically reactive haptens is their capacity to simultaneously generate immunogenic determinants for hapten-specific CD8(+) and CD4(+) T cells. Surprisingly, however, a clear dominance of CD8(+) effector T cells is observed in murine contact hypersensitivity to various haptens and upon T cell priming with hapten-modified APCs in vitro. In this study we show that trinitrophenyl-specific CD8(+) T cells actively prevent CD4(+) T cell priming in vitro. This process requires cell-cell contact and is dependent on the expression of Fas on the CD4(+) T cells. Our results reveal an important Fas-dependent mechanism for the regulation of hapten-specific CD4(+) T cell responses by CD8(+) T cells, which causes the dominance of CD8(+) effector T cells and the active suppression of a CD4(+) T cell response. Moreover, our demonstration of reduced contact hypersensitivity to trinitrophenyl in the absence of Fas, but not of perforin and/or granzymes A and B, underlines the important role of Fas as a pathogenetic factor for contact hypersensitivity.

Routine flow cytometric immuno-staining of T-cell perforin is preserved using diethylene glycol for erythrocyte-lysis but lost by the use of ammonium chloride

The system of perforin-containing lytic granules of cytotoxic lymphocytes plays an important role in the immune defense machinery. Investigating the capacity and efficacy of this system in and ex vivo is helpful to understand immune responses and their modulation by therapeutic interventions. With regard to its pathophysiological function, we recently demonstrated a substantial increase of perforin-positive CD8+ T cells in the peripheral blood of patients with acute exacerbated psoriasis and severe generalized drug reactions, and, in marked contrast, a highly significant perforin-depletion and a perforin-hyperreleasability in atopic dermatitis (AD). To streamline the perforin staining procedure, isolation of peripheral blood mononuclear cells (PBMC) by Ficoll density centrifugation was to be replaced by lysis of erythrocytes. Ammonium chloride lysis, however, reduced the perforin content of CD8+ T cells substantially (up to 75-100%) as compared with Ficoll isolation of PMC. Incubation of cells in concanamycin A, a selective inhibitor of H+-ATPases, resulted in a similar loss of perforin staining pointing to the critical influence of lysosomal pH. Using diethylene glycol-mediated erythrocyte lysis, perforin was well preserved to be readily detectable by immuno flow cytometry. Representative examples of the application of this optimized perforin staining procedure as well as accumulated data are given for various dermatological disorders (psoriasis, atopic dermatitis, cutaneous drug reactions, graft-versus-host disease (GVHD) with strong involvement of the cytotoxic T-cell population. Our findings may help to explain recent conflicting reports about a widely varying range of the portion of perforin-positive cells in healthy individuals as a reflection of such artificial methodological influences.

The three Es of cancer immunoediting

After a century of controversy, the notion that the immune system regulates cancer development is experiencing a new resurgence. An overwhelming amount of data from animal models--together with compelling data from human patients--indicate that a functional cancer immunosurveillance process indeed exists that acts as an extrinsic tumor suppressor. However, it has also become clear that the immune system can facilitate tumor progression, at least in part, by sculpting the immunogenic phenotype of tumors as they develop. The recognition that immunity plays a dual role in the complex interactions between tumors and the host prompted a refinement of the cancer immunosurveillance hypothesis into one termed "cancer immunoediting." In this review, we summarize the history of the cancer immunosurveillance controversy and discuss its resolution and evolution into the three Es of cancer immunoediting--elimination, equilibrium, and escape.

Altered structure of autoantigens during apoptosis

The clustering and concentration of autoantigens at the surface of apoptotic cells, in combination with the striking tolerance-inducing function of apoptotic cells, have focused attention on abnormalities in apoptotic cell execution and clearance as potential susceptibility and initiating factors in systemic autoimmunity. Structural changes that occur during cell death may influence the immunogenicity of self antigens. This article discusses the modifications that autoantigens undergo during cell death, identifies certain proimmune forms of apoptotic death in which autoantigen structure is frequently modified, and reviews the mechanisms through which such structural changes might lead to initiation of an autoimmune response.

A proteomic approach for the discovery of protease substrates

Standardized, comprehensive platforms for the discovery of protease substrates have been extremely difficult to create. Screens for protease specificity are now frequently based on the cleavage patterns of peptide substrates, which contain small recognition motifs that are required for the cleavage of the scissile bond within an active site. However, these studies do not identify in vivo substrates, nor can they lead to the definition of the macromolecular features that account for the biological specificity of proteases. To use properly folded proteins in a proteomic screen for protease substrates, we used 2D difference gel electrophoresis and tandem MS to identify substrates of an apoptosis-inducing protease, granzyme B. We confirmed the cleavage of procaspase-3, one of the key substrates of this enzyme, and identified several substrates that were previously unknown, as well as the cleavage site for one of these substrates. We were also able to observe the kinetics of substrate cleavage and cleavage product accumulation by using the 2D difference gel electrophoresis methodology. "Protease proteomics" may therefore represent an important tool for the discovery of the native substrates of a variety of proteases.

Immunobiology of acute graft-versus-host disease

Graft-versus-host disease (GVHD) has been the primary limitation to the wider application of allogeneic bone marrow transplantation (BMT). The immunobiology of acute GVHD is complex and can be conceptualized to be a three-step process. In step 1, the conditioning regimen (irradiation and/or chemotherapy) leads to the damage and activation of host tissues and induces the secretion of inflammatory cytokines TNF-alpha and IL-1. As a consequence expression of MHC antigens and adhesion molecules is increased, thus enhancing the recognition of host alloantigens by donor T cells. Donor T-cell activation in step 2 is characterized by donor T-cell interaction with host APCs and subsequent proliferation, differentiation, and secretion of cytokines. Cytokines such as IL-2 and IFN-gamma enhance T-cell expansion, induce cytotoxic T cells (CTL) and natural killer (NK) cell responses, and prime additional mononuclear phagocytes to produce TNF-alpha and IL-1. These inflammatory cytokines in turn stimulate production of inflammatory chemokines, thus recruiting effector cells into target organs. In step 3, effector functions of mononuclear phagocytes are triggered via a secondary signal provided by lipopolysaccharide (LPS) that leaks through the intestinal mucosa damaged during step 1. This mechanism may result in the amplification of local tissue injury and further promotion of an inflammatory response, which, together with the CTL and NK components, leads to target tissue destruction in the transplant host.

Differential Requirement for IFN-γ in CTL Maturation in Acute Murine Graft-versus-Host Disease

Although IFN-gamma is the archetypal Th1 cytokine, its role in CTL maturation is uncertain. We used an in vivo mouse model of CTL development, parent-into-F(1) acute graft-vs-host disease (AGVHD), to evaluate this issue. In AGVHD, transfer of naive parental T cells into F(1) hosts stimulates the development of allospecific CTL effectors that eliminate host lymphocytes, particularly B cells. Complete elimination of IFN-gamma, using IFN-gamma-deficient donors and administering anti-IFN-gamma mAb, suppressed B cell elimination, down-regulated TNF-alpha production, and enhanced Th2 cytokine production, but did not allow the B cell expansion characteristic of chronic GVHD (CGVHD). Because complete CTL inhibition results in full-blown CGVHD that is IFN-gamma independent, these observations indicate that IFN-gamma elimination only partially blocks CTL development. IFN-gamma elimination did not inhibit donor T cell engraftment or activation in the AGVHD model, but almost completely blocked Fas/Fas ligand (FasL) gene expression, protein up-regulation, and Fas/FasL-mediated CTL killing. In contrast, IFN-gamma elimination only partially inhibited perforin gene expression and perforin-mediated CTL activity. The contributions of IFN-gamma to CTL development were indirect, because IFN-gamma receptor-deficient donor cells differentiated normally into allospecific CTLs. Consistent with the view that the Fas/FasL and perforin pathways each mediate CTL killing in AGVHD, the absence of both perforin and IFN-gamma (perforin knockout donor cells and anti-IFN-gamma mAb) converted AGVHD to CGVHD. Thus, both IFN-gamma-dependent induction of Fas/FasL and IFN-gamma-independent induction of perforin contribute to CTL-mediated elimination of host B cells in AGVHD. Suppression of both pathways is required for typical CGVHD development.

Effector mechanisms in transplant rejection

Antigens, provided by the allograft, trigger the activation and proliferation of allospecific T cells. As a consequence of this response, effector elements are generated that mediate graft injury and are responsible for the clinical manifestations of allograft rejection. Donor-specific CD8+ cytotoxic T lymphocytes play a major role in this process. Likewise, CD4+ T cells mediate delayed-type hypersensitivity responses via the production of soluble mediators that function to further activate and guide immune cells to the site of injury. In addition, these mediators may directly alter graft function by modulating vascular tone and permeability or by promoting platelet aggregation. Allospecific CD4+ T cells also promote B-cell maturation and differentiation into antibody-secreting plasma cells via CD40-CD40 ligand interactions. Alloantibodies that are produced by these B cells exert most of their detrimental effects on the graft by activating the complement cascade. Alternatively, antibodies can bind Fc receptors on natural killer cells or macrophages and cause target cell lysis via antibody-dependent cell-mediated cytotoxicity. In this review, we discuss these major effector pathways, focusing on their role in the pathogenesis of allograft rejection.

Differential expression of granzymes A and B in human cytotoxic lymphocyte subsets and T regulatory cells

Cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells use the perforin/granzyme pathway as a major mechanism to kill pathogen-containing cells and tumor cells.(1,2) Dysregulation of this pathway results in several human diseases, such as hemophagocytic lymphohistiocytosis. Here we characterize the single-cell expression pattern of granzymes A and B in human lymphocytes using a flow cytometry-based assay. We demonstrate that most circulating CD56(+)8(-) NK cells, and approximately half of circulating CD8(+) T lymphocytes, coexpressed both granzymes A and B. In contrast, few circulating CD4(+) T lymphocytes expressed granzymes A or B. Activation of CD8(+) T lymphocytes with concanavalin A (ConA)/interleukin-2 (IL-2), and activation of CD4(+) T lymphocytes with antibodies to CD3/CD28 or CD3/CD46 (to generate T regulatory [Tr1] cells), induced substantial expression of granzyme B, but not granzyme A. Naive CD4(+)CD45RA(+) cells stimulated with antibodies to CD3/CD46 strongly expressed granzyme B, while CD3/CD28 stimulation was ineffective. Finally, we show that granzyme B-expressing CD4(+) Tr1 cells are capable of killing target cells in a perforin-dependent, but major histocompatibility complex (MHC)/T-cell receptor (TCR)-independent, manner. Our results demonstrate discordant expression of granzymes A and B in human lymphocyte subsets and T regulatory cells, which suggests that different granzymes may play unique roles in immune system responses and regulation.

Altered autoantigen structure in Sjögren's syndrome: implications for the pathogenesis of autoimmune tissue damage

The etiology and pathogenic mechanisms underlying Sjögren's syndrome (SS) remain unclear. Recent studies have emphasized that the specific autoantibodies that occur in a high proportion of patients with SS may provide important insights into the circumstances that initiate and propagate tissue damage in this disease. Although autoantigens targeted in systemic autoimmune diseases share little in common in terms of structure, subcellular distribution, or function in normal cells, these molecules are unified by becoming clustered and concentrated in the surface blebs of apoptotic cells. Furthermore, their structure is altered during some types of cell death to generate structures not previously generated during development and homeostasis. This review highlights the susceptibility of SS autoantigens to undergoing such structural changes during activation of immune effector pathways, and synthesizes a model of SS incorporating these concepts. An understanding of the mechanisms responsible for activating the specific immune response in SS, and the role of specific immune effector pathways in propagating both the autoimmune response and tissue damage, is of potential therapeutic importance. Abbreviations used in this paper are: CTL, cytotoxic T-lymphocytes; ER, endoplasmic reticulum; GluR3, subunit III of the glutamate receptor; GrB, granzyme B; M3R, type III muscarinic receptor; NK cells, natural killer cells; PARP, poly(ADP-ribose)polymerase; SS, Sjögren's syndrome; SLE, systemic lupus erythematosus; and UV, ultraviolet.

Granzyme B: pro-apoptotic, antiviral and antitumor functions

Granzyme B is a caspase-like serine protease that is released by cytotoxic lymphocytes to kill virus-infected and tumor cells. Major recent advances in our understanding of granzyme B biochemistry, biology and function include an appreciation of its uptake into and trafficking within target cells, a thorough dissection of how cell death is triggered, and the identification of the serpin protease inhibitor PI-9, which regulates its function in lymphocytes and in other cells. The roles that granzyme B plays in human pathologies, such as transplant rejection, viral immunity and particularly tumor immune surveillance, remain a topic for vigorous debate and conjecture. The recent discovery of a triply mutated human granzyme B allele, whose product is predicted to possess a reduced capacity to induce cell death, opens the way for major progress in these areas in coming years.

Perforin and the granule exocytosis cytotoxicity pathway

Perforin defects have been identified in humans with familial hematophagocytic lymphohistiocytosis. The pathology of these patients has dramatically illustrated an under-appreciated role for perforin in the regulation of T-cell responses in vivo, and experimental studies are shedding light on the mechanisms involved. The detailed molecular mechanisms of perforin's mandatory role in the cytotoxic T lymphocyte (CTL)-mediated granule exocytosis death pathway and of granzyme entry into target cells remain unclear. In model systems measuring apoptosis by granzyme B and sublytic perforin, pore formation is undetectable during granzyme entry. Selfprotection of cytotoxic lymphocytes after degranulation can be explained by surface expression of the granule protease cathepsin B, as shown by suicidal degranulation in the presence of specific inhibitors.

Alopecia areata susceptibility in rodent models

With our current view of alopecia areata as an autoimmune disease, it is probable that disease development in an individual is dependent on multiple genetic and environmental factors interacting in a complex system. Rodent models afford the opportunity to investigate alopecia areata development and to define the significance of the different factors involved. Recently, rodent model characterization has been conducted using flow cytometry, microarray analysis, and functional studies. From these a pattern of events in alopecia areata development has emerged. Although the preliminary activation events for the onset of alopecia areata remain unknown, the response of the immune system is characterized by antigen presentation and costimulation of lymphocytes in the lymph nodes and skin, a deficiency of CD4+/CD25+ regulatory cells, and an action of activated lymphocytes on hair follicles via Fas/FasL signaling and cytokines. Thus, onset of disease may require appropriate (or inappropriate) expression of stimulatory antigens within the hair follicle, the breakdown of the putative hair follicle immune privilege, the presentation of antigens to the immune system, a failure of immune system regulation, and the ability of the activated immune system to disrupt anagen-stage hair follicles. Once the sequence of events is initiated, it may become a self-perpetuating cycle, with epitope spreading leading to a wider range of targets in chronic alopecia areata. Rodent model studies have provided significant insight into alopecia areata, but much more remains to be explained about the mechanisms of disease development.

The granzyme B–serglycin complex from cytotoxic granules requires dynamin for endocytosis

Cytotoxic T lymphocytes and natural killer cells destroy target cells via the directed exocytosis of lytic effector molecules such as perforin and granzymes. The mechanism by which these proteins enter targets is uncertain. There is ongoing debate over whether the most important endocytic mechanism is nonspecific or is dependent on the cation-independent mannose 6-phosphate receptor. This study tested whether granzyme B endocytosis is facilitated by dynamin, a key factor in many endocytic pathways. Uptake of and killing by the purified granzyme B molecule occurred by both dynamin-dependent and -independent mechanisms. However most importantly, serglycin-bound granzyme B in high-molecular-weight degranulate material from cytotoxic T lymphocytes predominantly followed a dynamin-dependent pathway to kill target cells. Similarly, killing by live cytotoxic T lymphocytes was attenuated by a defect in the dynamin endocytic pathway, and in particular, the pathways characteristically activated by granzyme B were affected. We therefore propose a model where degranulated serglycin-bound granzymes require dynamin for uptake.

Antiviral Cytokines Induce Hepatic Expression of the Granzyme B Inhibitors, Proteinase Inhibitor 9 and Serine Proteinase Inhibitor 6

Expression of the granzyme B inhibitors, human proteinase inhibitor 9 (PI-9), or the murine orthologue, serine proteinase inhibitor 6 (SPI-6), confers resistance to CTL or NK killing by perforin- and granzyme-dependent effector mechanisms. In light of prior studies indicating that virally infected hepatocytes are selectively resistant to this CTL effector mechanism, the present studies investigated PI-9 and SPI-6 expression in hepatocytes and hepatoma cells in response to adenoviral infection and to cytokines produced during antiviral immune responses. Neither PI-9 nor SPI-6 expression was detected by immunoblotting in uninfected murine or human hepatocytes. Similarly, human Huh-7 hepatoma cells were found to express only very low levels of PI-9 relative to levels detected in perforin- and granzyme-resistant CTL or lymphokine-activated killer cells. Following in vivo adenoviral infection or in vitro culture with IFN-alphabeta or IFN-gamma, SPI-6 expression was induced in murine hepatocytes. Similarly, after culture with IFN-alpha, induction of PI-9 mRNA and protein expression was observed in human hepatocytes and Huh-7 cells. IFN-gamma and TNF-alpha also induced 4- to 10-fold higher levels of PI-9 mRNA expression in Huh-7 cells, whereas levels of mRNA encoding a related serine proteinase inhibitor, proteinase inhibitor 8, were unaffected by culture of Huh-7 cells with IFN-alpha, IFN-gamma, or TNF-alpha. These findings indicate that cytokines that promote antiviral cytopathic responses also regulate expression of the cytoprotective molecules, PI-9 and SPI-6, in hepatocytes that are potential targets of CTL and NK effector mechanisms.

Lesions of T-cell-mediated kidney allograft rejection in mice do not require perforin or granzymes A and B

Organ allograft rejection is strongly associated with the presence of alloreactive cytotoxic T cells but the role of cytotoxicity in the pathologic lesions is unclear. Previous studies showed that the principal lesions of kidney rejection - interstitial infiltration, tubulitis, and endothelial arteritis - are T-cell-dependent and antibody-independent. We studied the role of cytotoxic granule components perforin and granzymes A and B in the evolution of the T-cell-mediated lesions of mouse kidney transplant rejection. By real-time RT-PCR, allografts rejecting in wild-type hosts at days 5, 7, 21, and 42 showed massively elevated and persistent expression of perforin and granzymes A and B, but evolution of tubulitis and arteritis did not correlate with increasing granzyme or perforin expression. Allografts transplanted into hosts with disrupted genes for perforin or granzymes A and B showed no change in tubulitis, arteritis, or MHC induction. Thus the development of the histologic lesions diagnostic of T-cell-mediated kidney transplant rejection are associated with but not mediated by perforin or granzyme A or B. Together with previous graft survival studies, these results indicate that the granule-associated cytotoxic mechanisms of T cells are not the effectors of T-cell-mediated allograft rejection.

Calorie-related rapid onset of alveolar loss, regeneration, and changes in mouse lung gene expression

Calorie restriction, followed by ad libitum refeeding, results, respectively, in loss and regeneration of pulmonary alveoli. We now show 35% of alveoli are lost within 72 h of onset of calorie restriction (⅔ decreased daily chow intake), and an additional 12% of alveoli are lost over a subsequent 12 days of calorie restriction. Tissue necrosis was not seen. Within 72 h of refeeding, after 15 days of calorie restriction, the number of alveoli returns to precalorie restriction values. Microarray lung gene profiling, in conjunction with Western and RNase protection assay, demonstrate an increase of granzyme and caspase gene expression 2–3 h after onset of calorie restriction. By 12 h, granzyme and caspase expression is no longer increased, but tumor necrosis factor death receptor expression is elevated. At 336 h, Fas death receptor expression is increased. Because granzymes are found only in cytotoxic lymphocytes (CTLs) and natural killer (NK) cells, we suggest calorie restriction activates these cells, initiating a series of molecular events that results in alveolar destruction. The evidence of involvement of CTLs and NK cells and the absence of necrosis are similar to alveolar destruction in chronic obstructive pulmonary disease.

Characterization of murine cathepsin W and its role in cell-mediated cytotoxicity

Cathepsin W is a member of the papain-like family of cysteine proteases. In this report, we have isolated the cDNA for murine CtsW (mCtsW) from a splenocyte library. The deduced 371-amino-acid sequence shares 68% identity with human CtsW and includes the conserved catalytic triad cysteine, histidine, and asparagine found in all members of this family. In addition to the fulllength form of mCtsW, we have isolated an alternatively spliced form of the mRNA that lacks a complete catalytic triad. An S1 nuclease protection assay and a Western blot analysis showed that mCtsW is mainly restricted to the CD8(+) T cell and natural killer cell compartments. In addition, we confirmed that, like its human homologue, mCtsW is localized mainly to the endoplasmic reticulum and its expression is up-regulated upon activation. We also characterized the mCtsW locus using bacterial artificial chromosome clones. The gene consists of 10 coding exons and 9 introns spanning 3.2 kb. To elucidate the physiologic role of this protease, we generated mice deficient in mCtsW. Our data establish that mCtsW is not required for cytotoxic lymphocyte-induced target cell death in vitro. In addition, mCtsW deficiency does not alter the susceptibility of cytotoxic lymphocytes to suicide or fratricide after degranulation. Thus, mCtsW does not have a unique role in target cell apoptosis or cytotoxic cell survival in vitro.

An animal model of hemophagocytic lymphohistiocytosis (HLH): CD8+ T cells and interferon gamma are essential for the disorder

Hemophagocytic lymphohistiocytosis (HLH) is a rare disorder with familial and acquired forms. The familial form is associated with mutations in the perforin gene and both forms are associated with severe defects in lymphocyte cytotoxic function. We examined perforin-deficient mice as a model of HLH in order to gain insight into this poorly understood disorder. While these mice do not spontaneously develop HLH-like symptoms, we found that they manifest all of the features of HLH after infection with lymphocytic choriomeningitic virus (LCMV). Following LCMV infection, perforin-deficient mice develop fever, splenomegaly, pancytopenia, hypertriglyceridemia, hypofibrinogenemia, and elevation of multiple serum cytokine levels, and hemophagocytosis is evident in many tissues. Investigation into how this phenotype develops has revealed that CD8+ T cells, but not natural killer (NK) cells, are necessary for the development of this disorder. Cytokine neutralization studies have revealed that interferon gamma (IFNgamma) is uniquely essential as well. Finally, the excessive amount of IFNgamma seen in affected mice appears to be driven by increased antigen presentation to CD8+ T cells. These studies provide insight into the pathophysiology of HLH, and provide new targets for specific therapeutic intervention in this fatal disorder.

Effector cell-derived lymphotoxin alpha and Fas ligand, but not perforin, promote Tc1 and Tc2 effector cell-mediated tumor therapy in established pulmonary metastases

Cytolytic CD8(+) effector cells fall into two subpopulations based on cytokine secretion. Type 1 CD8(+) T cells (Tc1) secrete IFN-gamma, whereas type 2 CD8(+) T cells (Tc2) secrete interleukin (IL)-4 and IL-5. Although both effector cell subpopulations display Fas ligand (FasL) and tumor necrosis factor (TNF), tumor lysis is predominantly perforin dependent in vitro. Using an ovalbumin-transfected B16 lung metastasis model, we show that heightened numbers of adoptively transferred ovalbumin-specific Tc1 and Tc2 cells accumulated at the tumor site by day 2 after therapy and induced tumor regression that enhanced survival in mice with pulmonary metastases. Transfer of either TNF-alpha- or perforin-deficient Tc1 or Tc2 effector cells generated from specified gene-deficient mice showed no differences in therapeutic efficiency when compared with corresponding wild-type cells. In contrast, both Tc1 and Tc2 cells, derived from either FasL or TNF-alpha/lymphotoxin (LT) alpha double knockout mice, showed that therapeutic effects were dependent, in part, on effector cell-derived FasL or LTalpha. Six days after effector cell therapy, elevated levels of activated endogenous CD8/CD44(High) and CD4/CD44(High) T cells localized and persisted at sites of tumor growth, whereas donor cell numbers concomitantly decreased. Both Tc1 and Tc2 effector cell subpopulations induced endogenous antitumor responses that were dependent, in part, on recipient-derived IFN-gamma and TNF-alpha. However, neither effector cell-mediated therapy was dependent on recipient-derived perforin, IL-4, IL-5, or nitric oxide. Collectively, tumor antigen-specific Tc1 and Tc2 effector cell-mediated therapy is initially dependent, in part, on effector cell-derived FasL or LTalpha that may subsequently potentiate endogenous recipient-derived type 1 antitumor responses dependent on TNF-alpha and IFN-gamma.

Characterization of naı̈ve, memory and effector CD8+ T cells: effect of age

Aging is associated with progressive decline in T cell functions and increased frequency of infections, autoimmune phenomenon, and cancer. Memory T cells rapidly acquire effector functions to kill infected and malignant cells and/or inhibit their replication. Recently, memory T cells have been further classified into central and effector memory T cells (and early and intermediate T cells by some investigators). In aging, memory T cells are accumulated; however, these subpopulations of memory and effector T cells have not been fully characterized and changes in central memory and effector memory T cells in aged humans have not been described. In this article, we have further defined naïve, central memory, effector memory, and effector CD8+ T cells in humans and their changes in aged humans.

ECH, an Epoxycyclohexenone Derivative That Specifically Inhibits Fas Ligand-Dependent Apoptosis in CTL-Mediated Cytotoxicity

CTL eliminate cells infected with intracellular pathogens and tumor cells by two distinct mechanisms mediated by Fas ligand (FasL) and lytic granules that contain perforin and granzymes. In this study we show that an epoxycyclohexenone derivative,(2R,3R,4S)-2,3-epoxy-4-hydroxy-5-hydroxymethyl-6-(1E)-propenyl-cyclohex-5-en-1-one (ECH) specifically inhibits the FasL-dependent killing pathway in CTL-mediated cytotoxicity. Recently, we have reported that ECH blocks activation of procaspase-8 in the death-inducing signaling complex and thereby prevents apoptosis induced by anti-Fas Ab or soluble FasL. Consistent with this finding, ECH profoundly inhibited Fas-mediated DNA fragmentation and cytolysis of target cells induced by perforin-negative mouse CD4+ CTL and alloantigen-specific mouse CD8+ CTL pretreated with an inhibitor of vacuolar type H+-ATPase concanamycin A that selectively induces inactivation and proteolytic degradation of perforin in lytic granules. However, ECH barely influenced perforin/granzyme-dependent DNA fragmentation and cytolysis of target cells mediated by alloantigen-specific mouse CD8+ CTL. The components of lytic granules and the granule exocytosis pathway upon CD3 stimulation were also insensitive to ECH. In conclusion, our present results demonstrate that ECH is a specific nonpeptide inhibitor of FasL-dependent apoptosis in CTL-mediated cytotoxicity. Therefore, ECH can be used as a bioprobe to evaluate the contributions of two distinct killing pathways in various CTL-target settings.

Neurological Disorder after Borna Disease Virus Infection in the Absence of Either Interferon-γ, Fas, Inducible NO Synthase, or Chemokine Receptor CXCR3

Borna disease virus (BDV) can induce severe neurological disorder in Lewis rats and MRL mice. Antiviral CD8 T cells have been shown to be the mediators of disease in these animals. To define molecules involved in the disease process, we performed infection studies in MRL mice lacking either interferon-gamma, a functional Fas/FasL system, chemokine receptor CXCR3, or inducible NO synthase. We further used transgenic MRL mice expressing interferon-gamma-inducible, T cell-attracting chemokine CXCL10 in brain astrocytes. After intracerebral infection with BDV, wild-type and mutant mice developed CD8 T cell responses and neurological disease at similar frequency and with similar kinetics, suggesting that these factors are not required for initiation and maintenance of the immunopathological process. Similarly, the course of disease could not be altered by treating infected MRL mice or Lewis rats with the drug L-N(6)-(1-iminoethyl)-lysine (L-NIL) that specifically blocks the activity of the inducible NO synthase. We therefore have excluded a number of important factors that have been demonstrated to be crucial in the pathogenesis of a broad number of pathologic conditions. Thus, BDV-induced disease may not result from the action of a single dominant T cell-dependent effector molecule. Disease rather reflects a combined influence of several as yet undefined factors from CD8 T cells.

Graft-versus-leukemia in a retrovirally induced murine CML model: mechanisms of T-cell killing

The graft-versus-leukemia (GVL) effect, mediated by donor T cells, has revolutionized the treatment of leukemia. However, effective GVL remains difficult to separate from graft-versus-host disease (GVHD), and many neoplasms are GVL resistant. Murine studies aimed at solving these problems have been limited by the use of leukemia cell lines with limited homology to human leukemias and by the absence of loss-of-function leukemia variants. To address these concerns, we developed a GVL model against murine chronic-phase chronic myelogenous leukemia (mCP-CML) induced with retrovirus expressing the bcr-abl fusion cDNA, the defining genetic abnormality of chronic-phase CML (CP-CML). By generating mCP-CML in gene-deficient mice, we have studied GVL T-cell effector mechanisms. mCP-CML expression of Fas or tumor necrosis factor (TNF) receptors is not required for CD8-mediated GVL. Strikingly, maximal CD4-mediated GVL requires cognate interactions between CD4 cells and mCP-CML cells as major histocompatibility complex-negative (MHC II(-/-)) mCP-CML is relatively GVL resistant. Nevertheless, a minority of CD4 recipients cleared MHC II(-/-) mCP-CML; thus, CD4 cells can also kill indirectly. CD4 GVL did not require target Fas expression. These results suggest that CPCML's GVL sensitivity may in part be explained by the minimal requirements for T-cell killing, and GVL-resistance may be related to MHC II expression.

Impairment of death-inducing signalling complex formation in CD95-resistant human primary lymphoma B cells

Multiple mechanisms exist by which tumour cells can escape CD95-mediated apoptosis. Previous studies by our laboratory have shown that primary B cells from non-Hodgkin's Lymphoma (B-NHL) were resistant to CD95-induced cell death. In the current study, we have analysed the mechanisms underlying CD95 resistance in primary human lymphoma B cells. We report that FADD (FAS-associated death domain protein) and caspase-8 were constitutively expressed in lymphoma B cells and that the CD95 pathway was blocked upstream to caspase-8 activation. However, caspase-8 was processed and functional after treatment with staurosporine (STS). We found that the expression levels of FLICE (FADD-like interleukin-1 beta-converting enzyme)-Inhibitory Protein (c-FLIP) and Bcl-2-related proteins were heterogeneous in B-NHL cells and were not related to CD95 resistance. Finally, we report the absence of a CD95-induced signalling complex [death-inducing signalling complex (DISC)] in lymphoma B cells, with no FADD and caspase-8 recruitment to CD95 receptor. In contrast, DISC formation was observed in CD95-resistant non-tumoural (NT) B cells. Therefore, we propose that the absence of DISC formation in primary lymphoma B cells may contribute to protect these cells from CD95-induced apoptosis.

MHC class Ia-restricted memory T cells inhibit expansion of a nonprotective MHC class Ib (H2-M3)-restricted memory response

Listeria monocytogenes infection generates major histocompatibility complex (MHC) class Ia-restricted and MHC class Ib-(H2-M3)-restricted effector and memory CD8+ T cells. However, only MHC class Ia-restricted memory cells expand after rechallenge, and it is unknown if MHC class Ib-restricted memory CD8+ T cells generated by vaccination are protective. We show here that H2-M3-restricted memory CD8+ T cells were capable of secondary expansion but, in contrast to primary H2-M3-restricted effector cells, failed to provide protective immunity. In lm-immune mice, MHC class Ia-restricted memory CD8+ T cells prevented the expansion of H2-M3-restricted memory T cell populations by limiting dendritic cell antigen presentation. Thus, protective immunity by H2-M3-restricted T cells is limited to primary infection, indicating that memory MHC class Ia-restricted T cells prevent nonessential immune responses during secondary infection.

Munc13-4 is essential for cytolytic granules fusion and is mutated in a form of familial hemophagocytic lymphohistiocytosis (FHL3)

Secretion of cytolytic granules content at the immunological synapse is a highly regulated process essential for lymphocyte cytotoxicity. This process requires the rapid transfer of perforin containing lytic granules to the target cell interface, followed by their docking and fusion with the plasma membrane. Defective cytotoxicity characterizes a genetically heterogeneous condition named familial hemophagocytic lymphohistiocytosis (FHL), which can be associated with perforin deficiency. The locus of a perforin (+) FHL subtype (FHL3), observed in 10 patients, was mapped to 17q25. This region contains hMunc13-4, a member of the Munc13 family of proteins involved in vesicle priming function. HMunc13-4 mutations were shown to cause FHL3. HMunc13-4 deficiency results in defective cytolytic granule exocytosis, despite polarization of the secretory granules and docking with the plasma membrane. Expressed tagged hMunc13-4 localizes with cytotoxic granules at the immunological synapse. HMunc13-4 is therefore essential for the priming step of cytolytic granules secretion preceding vesicle membrane fusion.

Getting secretory granules ready for prime time

Several rare human diseases have shed important light on the secretory pathway required for lymphocyte cytotoxicity. In this issue of Cell, Feldmann et al. identify mutations in Munc13-4 as a cause of familial hemophagocytic lymphohistiocytosis. Munc13-4 appears to be involved in the priming of cytotoxic granules prior to fusion with the plasma membrane.

Granzyme B-induced cell death

Granzyme B (GrB) is a serine protease that is released by cytotoxic lymphocytes to kill virus-infected and tumor cells. Recent advances in the understanding of GrB have stressed the importance of reassessing the mechanisms by which GrB accomplishes its death functions. These include the uptake and trafficking of GrB within target cells, pathways used to trigger cell death, and the mechanism(s) controlling its killing activity. In addition, the role that GrB plays in human pathologies is still to be defined. The purpose of this review is to discuss recent insights into the biology of GrB and to evaluate its functional significance in health and disease.

An internal ribosome entry site directs translation of the murine gammaherpesvirus 68 MK3 open reading frame

The gammaherpesviruses characteristically drive the proliferation of latently infected lymphocytes. The murine gammaherpesvirus 68 (MHV-68) MK3 protein contributes to this process in vivo by evading CD8(+)-T-cell recognition during latency, as well as during lytic infection. We analyzed some of the molecular mechanisms that control MK3 expression. No dedicated MK3 mRNA was detected. Instead, the MK3 open reading frame (ORF) was transcribed as part of a bicistronic mRNA, downstream of a previously unidentified ORF, 13M. The 13M/MK3 promoter appeared to extend approximately 1 kb 5' of the transcription start site and included elements both dependent on and independent of the ORF50 lytic transactivator. MK3 was translated from the bicistronic transcript by virtue of an internal ribosome entry site (IRES) element. RNA structure mapping identified two stem-loops between 13M and MK3 that were sufficient for IRES activity in a bicistronic reporter plasmid and a third stem-loop just within the MK3 coding sequence, with a subtler, perhaps regulatory role. Overall, translation of the MHV-68 MK3 bore a striking resemblance to that of the Kaposi's sarcoma-associated herpesvirus vFLIP, suggesting that IRES elements are a common theme of latent gammaherpesvirus immune evasion in proliferating cells.

Exposure of human primary colon carcinoma cells to anti-Fas interactions influences the emergence of pre-existing Fas-resistant metastatic subpopulations

Fas, an important death receptor-mediated signaling pathway, has been shown to be down-regulated during human colon tumorigenesis; however, how alterations in Fas expression influence the metastatic process remains unresolved. In mouse models, loss of Fas function was found to be both necessary and sufficient for tumor progression. In this study, we investigated the link between functional Fas status and malignant phenotype using a matched pair of naturally occurring primary (Fas-sensitive) and metastatic (Fas-resistant) human colon carcinoma cell lines in both in vitro and in vivo (xenograft) settings. Metastatic sublines were produced in vitro from the primary tumor cell line by functional elimination of Fas-responsive cells. Conversely, sublines derived from the primary tumor in vivo at distal metastatic sites were Fas-resistant. In contrast, simply disrupting the Fas pathway by molecular-based strategies in the Fas-sensitive primary tumor failed to achieve the same metastatic outcome. Interestingly, both in vitro- and in vivo-produced sublines resembled the naturally occurring metastatic population, based on functional and morphologic studies and genome-scale gene expression profiling. Overall, using this human colon carcinoma model, we: 1) showed that loss of Fas function was linked to, but alone was insufficient for, acquisition of a detectable metastatic phenotype; 2) demonstrated that metastatic subpopulations pre-existed within the heterogeneous primary tumor, and that anti-Fas interactions served as a selective pressure for their outgrowth; and 3) identified a large set of differentially expressed genes distinguishing the primary from metastatic malignant phenotypes. Thus, Fas-based interactions may represent a novel mechanism for the biologic or immunologic selection of certain types of Fas-resistant neoplastic clones with enhanced metastatic ability.

Pathophysiology of acute graft-versus-host disease

Graft-versus-host disease (GVHD) has been the primary limitation to the wider application of allogeneic bone marrow transplantation (BMT). The pathophysiology of acute GVHD is complex and can be conceptualized to be a three-step process based on murine studies. In step 1, the conditioning regimen leads to the damage and activation of host tissues and induces the secretion of inflammatory cytokines. As a consequence, the expression of MHC antigens and adhesion molecules is increased enhancing the recognition of host alloantigens by donor T cells. Donor T-cell activation in step 2 is characterized by donor T cell interaction with host APCs and subsequent proliferation, differentiation and secretion of cytokines. Cytokines such as IL-2 and IFN-gamma enhance T-cell expansion, induce cytotoxic T cells (CTL) and natural killer (NK) cell responses and prime additional mononuclear phagocytes to produce TNF-alpha and IL-1. These inflammatory cytokines in turn stimulate production of inflammatory chemokines, thus recruiting effector cells into target organs. In step 3, effector functions of mononuclear phagocytes are triggered via a secondary signal provided by lipopolysaccharide (LPS) that leaks through the intestinal mucosa damaged during step 1. This mechanism may result in the amplification of local tissue injury and further promotion of an inflammatory response, which, together with the CTL and NK components, leads to target tissue destruction in the transplant host. The following review discusses the three-step process of the pathophysiology of experimental acute GVHD.

The Granzyme B ELISPOT assay: an alternative to the 51Cr-release assay for monitoring cell-mediated cytotoxicity

Background

The interferon-γ (IFN-γ) ELISPOT assay is one of the most useful techniques for immunological monitoring of cancer vaccine trials and has gained increased application as a measure of specific T cell activation. However, it does not assess cell-mediated cytotoxicity directly as IFN-γ secretion is not limited to only cytolytic cells. Granzyme B (GrB) is a key mediator of target cell death via the granule-mediated pathway. Therefore, the release of GrB by cytolytic lymphocytes upon effector-target interaction may be a more specific indicator of CTL and NK cytotoxic ability than IFN-γ secretion.

Methods

We assessed whether the GrB ELISPOT assay is a viable alternative to the ⁵¹Cr-release and IFN-γ ELISPOT assays for measuring antigen-specific CTL cytotoxicity. Direct comparisons between the three assays were made using human CTL cell lines (αEN-EBV and αJY) and an in vitro stimulated anti-Flu matrix peptide (FMP)-specific CTL.

Results

When the GrB ELISPOT was directly compared to the IFN-γ ELISPOT and ⁵¹Cr-release assays, excellent cross-correlation between all three assays was shown. However, measurable IFN-γ secretion in the ELISPOT assay was observed only after 1 hour of incubation and cytotoxicity assessed via the ⁵¹Cr-release assay after 4 hours, whereas GrB secretion was detectable within 10 min of effector-target contact with significant secretion observed after 1 h. Titration studies demonstrated a strong correlation between the number of effector cells and GrB spots per well. Irrelevant targets or antigens did not induce significant GrB secretion. Additionally, GrB secretion was abrogated when CTL cultures were depleted of CD8+ cells.

Conclusion

Our findings demonstrate that the GrB ELISPOT assay is a superior alternative to the ⁵¹Cr-release assay since it is significantly more sensitive and provides an estimation of cytotoxic effector cell frequency. Additionally, unlike the IFN-γ ELISPOT assay, the GrB ELISPOT directly measures the release of a cytotolytic protein. Detection of low frequency tumor-specific CTL and their specific effector functions can provide valuable insight with regards to immunological responses.

Targeting and amplification of immune killing of tumor cells by pro-Smac

Overexpression of inhibitors of apoptosis (IAP) is one potential mechanism for tumor cells to evade immune surveillance. To determine whether immune-mediated killing of tumor cells can be enhanced by neutralization of IAP proteins, 2 novel eGFP-Smac fusion proteins (pro-Smac) were introduced into the poorly immunogenic mouse melanoma cell line, B16BL6-D5 (D5). Each fusion protein contained Smac and a cleavage site specific for granzyme B (GrB) or caspase 8, thereby targeting the 2 major killing mechanisms of cytotoxic T-lymphocyte (CTL) and NK cells. Expression of a pro-Smac fusion protein by D5 tumor cells greatly enhanced the susceptibility to killing by lymphokine-activated killer (LAK) cells or purified GrB. GrB-mediated killing was increased to a much greater extent when tumor cells expressed the eGFP-Smac fusion protein with a GrB cleavage site compared to a caspase 8 cleavage site. In contrast, perforin-deficient LAK cells, which lack GrB-mediated cytotoxicity but process normal ligands for death receptors, killed D5 tumor cells expressed pro-Smac with caspase 8 cleavage site more efficiently. Enhanced killing by GrB was also accompanied by processing of the fusion protein and increased caspase-3-like activity. These results indicate that killing of tumor cells can be amplified by targeting cell-mediated cytotoxic mechanisms via expression of pro-Smac fusion proteins.

NOD/LtSz-Rag1nullPfpnull mice: a new model system with increased levels of human peripheral leukocyte and hematopoietic stem-cell engraftment

BACKGROUND

A critical need exists for effective small-animal models that accept engraftment of human hematopoietic progenitor cells and mature lymphocytes. The purpose of this study was to determine the phenotypic effects of perforin (Pfp) deficiency on nonobese diabetic (NOD)-Rag1null mice and to evaluate the ability of NOD/LtSz-Rag1nullPfpnull recipients to support engraftment with human hematolymphoid cells.

METHODS

A new genetic stock of NOD mice doubly homozygous for targeted mutations at the recombination activating gene (Rag)-1 and Pfp genes was developed. NOD/LtSz-Rag1nullPfpnull mice were studied for immunopathologic and hematologic abnormalities. The ability of these mice to support engraftment with human peripheral blood mononuclear cells (PBMC) and umbilical-cord blood hematopoietic progenitor cells was assessed.

RESULTS

NOD/LtSz-Rag1nullPfpnull mice lacked mature B cells, T cells, natural killer (NK) cell cytotoxic activity and were devoid of serum immunoglobulin (Ig) throughout a 37-week lifespan. These mice supported heightened engraftment with human PBMC as compared with NOD/LtSz-Rag1null controls as evidenced by a 4- to 5-fold increase in percentages of human lymphocytes and a 7- to 13-fold increase in percentages of CD4+ T cells in the peripheral blood and spleen. Total numbers of human CD4+ T cells were increased approximately 20-fold in the spleens of NOD/LtSz-Rag1nullPfpnull mice. These mice also showed approximately 12-fold higher levels of engraftment with human umbilical-cord blood cells compared with NOD/LtSz-Rag1null mice.

CONCLUSIONS

NOD/LtSz-Rag1nullPfpnull mice are devoid of mature B cell, T cell, and NK cell cytotoxic activity, engraft at high levels with human PBMC, and hematopoietic progenitor cells and provide a new NK cell-deficient model for human hematolymphoid cell engraftment.

The immunobiology of natural killer cells and bone marrow allograft rejection

Natural killer (NK) cells mediate the acute rejection of bone marrow cell (BMC) allografts, but not solid tissue grafts, in lethally irradiated mice. However, the mechanisms underlying this capability for rejecting BMC remain unclear. NK cells express (1) inhibitory receptors specific for major histocompatibility complex (MHC) class I molecules and (2) activating receptors with diverse specificities. Inhibitory NK receptors confer to NK cells the ability to discriminate between MHC class I-positive and -negative target cells and are therefore involved in the control of NK cell tolerance to self, as well as in the elimination of cells that have downregulation of MHC class I molecules. Preclinical studies in mice have provided good evidence that subsets of NK cells that bear different combinations of both inhibitory and activating Ly49 receptors can interact with each other and target specific BMC rejection, as well as NK cell responses toward tumor cells. Recent clinical studies have also shown that the use of killer cell immunoglobulin-like receptor ligand incompatibility in patients with leukemia who received hematopoietic stem cell transplants correlated not only with the elimination of graft rejection, but also with eradication of tumor and prevention of graft-versus-host disease; this offers a significant advantage for survival. In this review, we attempt to bring together literature regarding the biology of NK cells and discuss the current issues in bone marrow transplantation and the potential clinical role of NK cell alloreactivity in the efficacy of this procedure for immunotherapy of cancer and infectious states.

Segregation of Mechanisms for Cytotoxic T Lymphocyte Killing between Lungs and Regional Lymph Nodes Subsequent to Intratracheal Delivery of Adenovirus 2 Vector

Recombinant adenovirus (Ad) vectors, highly effective for targeting the respiratory epithelium, have been investigated for proposed application in mucosal immunization. For rendering successful use of Ad vectors, it is imperative to understand the host immune responses in affected organs. We investigated the mechanisms of cytotoxic T lymphocyte (CTL) killing following intratracheal instillation with recombinant Ad2/betagal-2 vector. From the analysis of CTL responses, it became apparent that the lung CTLs were more Fas-dependent, whereas pulmonary lymph nodes (LN) and splenic CTLs were more perforin-dependent. Although there was a segregation in the mode of CTL killing, both mechanisms of cytolysis were used in the described tissues, and the observed dominance in CTL killing was maintained irrespective of the target evaluated. Restimulation of LN and spleen cells did not change dominance in the CTL mechanism utilized. Absence or blockage of perforin or Fas did not result in reciprocal compensation by the other effector mechanism except in Fas ligand-deficient LN and spleens. In vitro restimulation of immune lymphocytes from each mouse group tested showed segregation in the types of cytokines generated. Ad2-restimulated cells showed bias toward IL-2 and IFN-gamma, while betagal-restimulated cells showed bias toward IL-4 and IL-10.

CD95 ligand--death factor and costimulatory molecule?

The CD95 ligand is involved as a death factor in the regulation of activation-induced cell death, establishment of immune privilege and tumor cell survival. In addition, CD95L may serve as a costimulatory molecule for T-cell activation. Alterations in expression or shedding of membrane and soluble CD95L are associated with numerous diseases, and underscore the pathophysiological relevance of the CD95/CD95L system. In most cases, the causal link between altered CD95L expression and pathophysiology is unknown. Given the potency of the molecule to regulate death and survival of many different cell types, the control of CD95L production, transport, storage, shedding and inactivation is of tremendous biological and clinical interest. This review summarizes the current knowledge, hypotheses and controversies about CD95L as a multifunctional ligand and receptor. It considers the different roles of membrane and soluble forms of CD95L and the complex networks of intracellular dynamics of protein trafficking, as well as the potential bidirectional signal transduction capacity of CD95L, with a focus on molecular interactions that have been worked out over the past years.

Slowly getting a clue on CD95 ligand biology

Since the ligand for the death factor CD95 (CD95L) was identified almost a decade ago, it has been established that this molecule (CD95L, FasL, Apo-1L, CD178, TNFSF6, APT1LG1) has multiple immunoregulatory and pathophysiologically relevant functions. CD95L does not only act as a death factor when externalized with secretory lysosomes on cytotoxic T and NK cells or when expressed on CD4(+) T cells in the course of activation-induced cell death, it is also a key molecule for the establishment of immune privilege or tumor cell survival and may serve as a costimulatory molecule during T cell activation. Moreover, alterations of expression or shedding of different forms of CD95L are associated with many diseases including various malignancies, HIV infection, autoimmune disorders (systemic lupus erythematodes, rheumatoid arthritis), acute myocardial infarction, traumatic injury and many others. In most cases, however, the physiological link between altered CD95L expression and pathophysiology is unknown. Given the potency of the molecule to regulate death and survival of many different cell types, the control of CD95L production, transport, storage, shedding and release is of tremendous biological and clinical interest. This commentary aims at briefly summarizing the current knowledge, hypotheses and controversies about CD95L as a multifunctional ligand and receptor. It touches upon the complex networks of intracellular dynamics of protein transport and trafficking and the potential bidirectional signal transduction capacity of CD95L with a focus on molecular interactions that have been worked out over the past years.

The JAM-assay: optimized conditions to determine death-receptor-mediated apoptosis

Apoptosis induced by interaction of members of the TNF-/TNF-receptor superfamily has been considered as a major mechanism of cell-mediated cytotoxicity. For functional analysis, the 51Cr release assay has been widely used, which requires loss of membrane integrity in the apoptotic target cell. However, loss of membrane integrity is a late event during apoptosis and therefore only late apoptotic cells will be detected by this method. In contrast, the JAM-assay first described by Polly Matzinger has been demonstrated to be more sensitive than the 51Cr release assay, since this method is dependent on DNA-fragmentation which precedes loss of membrane integrity in most apoptotic cells. The JAM-assay is easier to perform, less expansive, and safer than the current standard (51)Cr release assay. Therefore, this article will focus on optimized conditions of the JAM-assay to detect and quantitate Fas (CD95/Apo-1)-induced apoptosis as an example of death-receptor-mediated cytotoxicity.

Cytotoxic granule-mediated apoptosis: unraveling the complex mechanism

The molecular details of cytotoxic granule-mediated apoptosis have been gleaned from the study of the effects of isolated granzymes and perforin on target cells. Recent evidence indicates that the physiological apoptosis-inducing form is a multi-component macro-complex consisting of cationic granule proteins non-covalently linked to the chondroitin-sulfate proteoglycan, serglycin.

The orphan granzymes of humans and mice

The granzyme/perforin pathway is a central pathway for lymphocyte-mediated killing in both the innate and adaptive immune systems. This pathway is important in a variety of host defenses, including viral clearance and tumor cell killing, and its dysregulation results in several human and rodent diseases. To date, the majority of reports in this field have concentrated on the functions of granzymes A and B. Recent reports, however, suggest that the non-A/non-B 'orphan' granzymes found in both humans and mice are potentially significant. Although the functions of these orphan granzymes have yet to be fully established, initial data suggests their importance in both immune and nonimmune cells.

Measuring T cell-mediated cytotoxicity using fluorogenic caspase substrates

Cytotoxic T lymphocytes (CTLs) play a major role in the immune response against viruses and other intracellular pathogens. In addition, CTLs are implicated in the control of tumor cells in certain settings. Accurate measures of CTL function are of critical importance to study the pathogenesis of infectious diseases and to evaluate the efficacy of new vaccines and immunotherapies. To this end, we have recently developed a flow cytometry-based CTL (FCC) assay that measures the CTL-induced caspase activation within target cells using cell permeable fluorogenic caspase substrates. This novel assay reliably detects, by flow cytometry or fluorescence/confocal microscopy, antigen-specific CTLs in a wide variety of human and murine systems, and is safer and more informative than the standard 51Cr-release assay. In addition, the flow cytometric CTL (FCC) assay provides an alternative method that is often more sensitive and physiologically informative when compared to previously described FCC assays, as it measures a biological indicator of apoptosis within the target cell. The FCC assay may thus represent a useful tool to further understand the molecular and cellular mechanisms that underlie CTL-mediated killing during tumorigenesis or following infection with viruses or other intracellular pathogens.

The Fas/Fas ligand pathway is important for optimal tumor regression in a mouse model of CTL adoptive immunotherapy of experimental CMS4 lung metastases

The mechanisms of CTL-mediated tumor regression in vivo remain to be fully understood. If CTL do mediate tumor regression in vivo by direct cytotoxicity, this may occur via two major effector mechanisms involving the secretion of perforin/granzymes and/or engagement of Fas by Fas ligand (FasL) expressed by the activated CTL. Although the perforin pathway has been considered the dominant player, it is unclear whether Fas-mediated cytotoxicity is additionally required for optimal tumor rejection. Previously, we produced H-2L(d)-restricted CTL reactive against the CMS4 sarcoma, which expresses a naturally occurring rejection Ag recognized by these CTL and harbors a cytokine (IFN-gamma plus TNF)-inducible, Fas-responsive phenotype. The adoptive transfer of these CTL to syngeneic BALB/c mice with minimal (day 3 established) or extensive (day 10 established) experimental pulmonary metastases resulted in strong antitumor responses. Here we investigated whether a FasL-dependent CTL effector mechanism was important for optimal tumor regression in this adoptive immunotherapy model. The approach taken was to compare the therapeutic efficacy of wild-type to FasL-deficient (gld) CTL clones by adoptive transfer. In comparison with wild-type CTL, gld-CTL efficiently mediated tumor cytolysis and produced comparable amounts of IFN-gamma, after tumor-specific stimulation, as in vitro assessments of Ag recognition. Moreover, gld-CTL mediated comparably potent antitumor effects in a minimal disease setting, but were significantly less effective under conditions of an extensive tumor burden. Overall, under conditions of extensive lung metastases, these data revealed for the first time an important role for a FasL-dependent CTL effector mechanism in optimal tumor regression.