The Fas-based mechanism of lymphocytotoxicity

Membrane-bound CD95 ligand modulates CD19-mediated B cell receptor signaling and EBV activation

Post-transplant lymphoproliferative disorders (PTLDs) are associated with Epstein-Barr virus (EBV) infection in transplant recipients. Most of lymphoblastoid cell lines (LCLs) derived from EBV-immortalized B cells or PTLDs are sensitive to CD95-mediated apoptosis and cytotoxic T cell (CTL) killing. CD95 ligand (CD95L) exists as a transmembrane ligand (mCD95L) or a soluble form (sCD95L). Using recombinant mCD95L and sCD95L, we observed that sCD95L does not affect LCLs. While high expression of mCD95L in CTLs promotes apoptosis of LCLs, low expression induces clathrin-dependent CD19 internalization, caspase-dependent CD19 cleavage, and proteasomal/lysosomal-dependent CD19 degradation. The CD95L/CD95-mediated CD19 degradation impairs B cell receptor (BCR) signaling and inhibits BCR-mediated EBV activation. Interestingly, although inhibition of the caspase activity restores CD19 expression and CD19-mediated BCR activation, it fails to rescue BCR-mediated EBV lytic gene expression. EBV-specific CTLs engineered to overexpress mCD95L exhibit a stronger killing activity against LCLs. This study highlights that engineering EBV-specific CTLs to express a higher level of mCD95L could represent an attractive therapeutic approach to improve T cell immunotherapy for PTLDs.

Myocardium and microvessel endothelium apoptosis at day 7 following reperfused acute myocardial infarction

OBJECTIVES

This study was to investigate the salvaged myocardial and microvascular endothelial cells apoptosis at the first week of reperfused acute myocardial infarction (AMI).

METHODS

Sixteen mini swines (20-30 kg) were randomly assigned to the sham-operated group and the AMI group. The acute myocardial infarction and reperfusion model was created, and pathologic myocardial tissue was collected at day 7 following left anterior descending coronary artery reperfusion, and detected by transmission electron microscope, in situ cell apoptosis detection (TUNEL method), Real-time Quantitative Polymerase Chain Reaction and Western blot.

RESULTS

In the AMI group, the infarcted area showed the myolysis, fibroblast and injuried endothelial cells under transmission electron microscope. The infarcted area had higher apoptotic index of microvascular endothelial cells than the marginal area, the normal area, and the sham-operated area (all P<0.05). Fas and Bax mRNA expressions in the infarcted area were higher than those in the marginal area, the normal area, and the sham-operated area (all P<0.05), and both protein overexpressions and Bcl-2 low expression in the infarcted and marginal areas compared with the normal area and the sham-operated area.

CONCLUSIONS

The overexpressions of Fas and Bax or the low expression of Bcl-2 in the infarcted and marginal heart tissue may play an important role in the acceleration of myocardial and endothelial apoptosis at 7th day following reperfused acute myocardial infarction.

Hydrogen peroxide predisposes neonatal rat ventricular myocytes to Fas-mediated apoptosis

Neonatal rat ventricular myocytes (NRVM) grown in normoxic environment are not susceptible to Fas-induced apoptosis. In the present work, we tested the hypothesis that free radical injury represented by transient exposure to H2O2 sensitizes NRVM to Fas-mediated apoptosis. NRVM were treated with H2O2 (0.5 mM) for 2-4 h and thereafter exposed for 7 h to recombinant Fas ligand (rFasL, 10 ng/ml) plus an enhancing antibody (1 microg/ml). Apoptotic cardiomyocytes were counted and apoptosis-related proteins were measured by Western blot. H2O2 alone induced apoptosis (9.4+/-1.0%) that was preceded by activation of caspases-8 and -3, and PARP degradation. Incubation of NRVM with H2O2, followed by exposure to rFasL, increased the apoptotic index to 13.8+/-2.0%, but did not change caspase-8 or PARP activation. To investigate the mechanism underlying the sensitizing affect of H2O2 towards Fas-induced apoptosis, we studied the effects of H2O2 on the expression of key apoptosis signaling proteins. Incubation with H2O2 for 2-4 h decreased Fas expression and the expression of the Fas-related antiapoptotic proteins FLIP(L) and ARC, and increased the expression of the antiapoptotic proteins bcl-2 and xIAP. FADD expression was unchanged. Next, we tested the effect of H2O2 on the apoptosis-inducing, Fas-dependent Daxx-ASK-1-JUN kinase pathway. H2O2 dramatically increased ASK-1 expression and JUN kinase activation, but did not effect Daxx expression. Based on these findings we concluded that H2O2 sensitizes NRVM to Fas-mediated apoptosis by activating the Daxx-ASK-1-JUN kinase pathway, and by shifting the balance between proapoptotic and antiapoptotic proteins towards the former.

The Fas Receptor-1,4,5-IP3Cascade: A Potential Target for Treating Heart Failure and Arrhythmias

Activation of the Fas receptor in various cell types, including myocytes, triggers apoptotic as well as nonapoptotic effects. Recent studies suggest that Fas activation in the heart participates in the development of major pathologies such as myocarditis and ischemic/reperfusion insults, which are manifested by arrhythmias and mechanical dysfunction. To decipher the contribution of the Fas/FasL pathway to myocardial pathologies, we have investigated the functional consequences of Fas activation in normoxic and hypoxic ventricular myocytes. Our major findings were as follows. (1) Although Fas is constitutively expressed in ventricular myocytes, normoxic myocytes are resistant to Fas-mediated apoptosis. In contrast, hypoxia predisposes myocytes to apoptosis induced by Fas activation. The underlying mechanism is a shift in the balance between proapoptotic proteins (including Fas) and antiapoptotic proteins toward the former. (2) In normoxic myocytes, Fas activation causes a wide range of functional disturbances, which include reduction in resting potential and action potential amplitude, prolonged action potential duration, development of delayed and early after-depolarizations, occasionally culminating into arrhythmias, diastolic [Ca(2+)](i) level increase, decreased I(to) and increased I(Ca,L). (3) The above-mentioned effects in normoxic myocytes (but not Fas-mediated apoptosis in hypoxic myocytes) depend on the phospholipase C --> 1,4,5-IP(3) --> SR [Ca(2+)](i) release cascade. (4) Inhibition of tyrosine kinases with genistein blocks both the apoptotic and nonapoptotic consequences of Fas activation in ventricular myocytes. Based on these studies we propose that tyrosine phosphorylation in ventricular myocytes can serve as a novel potential target for attenuating Fas-mediated dysfunction in normoxic and hypoxic myocardium.

Cytotoxic lymphocytes and cardiac electrophysiology

It is widely recognized that immune effector mechanisms contribute to cardiac dysfunction in major cardiac pathologies, such as myocarditis and the consequent dilated cardiomyopathy, Chagas' disease and heart transplant rejection. Of the wealth of immune mechanisms known to affect cardiac function, this review will deal with the adverse effects caused by cytotoxic T lymphocytes (CTL, CD4(+) and CD8(+) T lymphocytes), which participate in a broad range of heart pathologies. The interaction between cytotoxic lymphocytes and their target cells can set off two different effector mechanisms: (1) The perforin/granzymes, and (2) The Fas/FasL. In this review, I will discuss these mechanisms, and present experimental evidence showing that both can adversely affect cardiac myocytes in vitro, in a way that can contribute to a decline in the overall cardiac function.

Hypoxia predisposes neonatal rat ventricular myocytes to apoptosis induced by activation of the Fas (CD95/Apo-1) receptor: Fas activation and apoptosis in hypoxic myocytes

OBJECTIVE

Since apoptosis is an important contributor to heart diseases in which ischemia and hypoxia are key elements, we tested the hypothesis that hypoxia predisposes neonatal rat ventricular myocytes (NRVM) to Fas-mediated apoptosis, by shifting the balance between antiapoptotic and proapoptotic proteins towards the latter.

METHODS

Normoxic or hypoxic (22 h, 1% O(2)) cultured NRVM were exposed to recombinant Fas L (rFasL) for 7 h, and apoptosis measured thereafter.

RESULTS

Whereas in normoxic NRVM, rFasL did not cause apoptosis measured by the TUNEL assay (4.8+/-0.5% in control versus 4.5+/-0.9% in rFasL), in hypoxic cultures rFasL increased the background apoptosis level by 100%. That Fas was functional in normoxic NRVM, despite its inability to mediate apoptosis, was evidenced by the finding that Fas activation increased the diastolic [Ca(2+)](i) levels measured by Fura 2 fluorescence, and caused arrhythmias. In support of our working hypothesis, hypoxia increased Fas expression by 200% (measured by quantitative Western blot), and the expression of the proapoptotic proteins ARTS and FADD by 323 and 250%, respectively, and decreased the expression of the antiapoptotic proteins ARC and FLIP by 90 and 60%, respectively.

CONCLUSION

By upregulating Fas expression and key proapoptotic proteins, and by downregulating antiapoptotic proteins, hypoxia predisposes ventricular myocytes to Fas-induced apoptosis.

The relationship of natural killer cell counts, perforin mRNA and CD2 expression to post-exercise natural killer cell activity in humans

The aim of this study was to further investigate the mechanism of suppression of natural killer (NK) cell cytotoxic activity in peripheral blood following strenuous exercise. Blood was collected for analysis of NK cell concentration, cytotoxic activity, CD2 surface expression and perforin gene expression from runners (RUN, n=6) and resting controls (CONTROL, n=4) pre-exercise, 0, 1.5, 5, and 24 h following a 60-min treadmill run at 80% of VO2 peak. Natural killer cytotoxic activity, measured using a whole blood chromium release assay, fluctuated minimally in the CONTROL group and increased by 63% and decreased by 43% 0 and 1.5 h post-exercise, respectively, in the RUN group (group x time, P < 0.001). Lytic index (cytotoxic activity per cell) did not change. Perforin mRNA, measured using quantitative real-time polymerase chain reaction (QRT-PCR) decreased from pre- to post-exercise and remained decreased through 24 h. The decrease from pre- to 0 h post-exercise was seen predominately in the RUN group and was inversely correlated (r=- 0.95) to pre-exercise perforin mRNA. The NK cell surface expression of CD2 (lymphocyte function-associated antigen-2) was determined using fluorescent antibodies and flow cytometry. There was no change in the proportion of NK cells expressing CD2 or CD2 density. We conclude that (1) numerical redistribution accounted for most of the change in NK cytotoxic activity following a strenuous run, (2) decrease in perforin gene expression during the run was inversely related to pre-exercise levels but did not parallel changes in cytotoxic activity, and (3) CD2 surface expression was not affected by exercise.

Pharmacologic modulation of the immune interaction between cytotoxic lymphocytes and ventricular myocytes

Numerous studies have demonstrated that immune effector mechanisms cause serious heart diseases, among which are heart transplant rejection, myocarditis, and the resulting dilated cardiomyopathy, as well as Chagas' disease. Whereas different effectors of the immune system can affect cardiac function, this review primarily focuses on the immune damage caused by cytotoxic T lymphocytes. The immune attack staged by cytotoxic T lymphocytes is carried out by one of two distinct modes of lymphocytotoxicity: (a) secretion of lytic granules containing the pore-forming protein perforin and a family of serine proteases (i.e., granzymes) and (b) interaction between the lymphocyte Fas ligand and the target cell Fas receptor. Ventricular myocytes challenged by the immune system sustain diverse intracellular changes, among which the rise in intracellular calcium ([Ca2+]i) constitutes an important contributor to myocyte dysfunction. Hence, this [Ca2+]i rise, which does not necessarily result in apoptosis, can affect cardiac function directly and indirectly. Importantly, the final outcomes of these perturbations vary markedly and depend on intracellular circumstances such as the magnitude of the absolute rise in [Ca2+]i and its temporal and spatial determinants, the metabolic status of the myocyte, as well as a fine balance between pro-apoptotic and anti-apoptotic factors. In view of the central role of [Ca2+]i rise in immune-mediated myocyte dysfunction and possibly cell death, this review addresses three topics related to the immune assault on the heart: (a) [Ca2+]i rise in affected myocytes; (b) the source for the [Ca2+]i rise; and (c) pharmacologic modification of the immune-mediated [Ca2+]i rise.

NK cells recover early and mediate cytotoxicity via perforin/granzyme and Fas/FasL pathways in umbilical cord blood recipients

Umbilical cord blood (UCB) is now widely accepted as a source of stem cells in patients with malignant hematologic and genetic disorders. We have recently reported that in a series of 30 pediatric UCB transplant recipients comparable outcome to that anticipated with other unrelated stem cell sources. In our series, however, the probability of GVHD for grade III-IV was 9% and no UCB recipient developed chronic GVHD. The reason for the low incidence of GVHD after UCB transplantation is not fully understood. Because functional NK cells are among the first population of lymphocytes to be detected in UCB transplant recipients, 2 months post-transplant on average, we wanted to establish whether NK cells could be implicated in reducing the risk of GVHD. Here, we confirm that early NK cells detected in UCB transplant recipients activate the granzyme/perforin lytic pathway and, in addition, they can mediate Fas/Fas ligand (FasL) activity, a finding not previously reported. Both pathways develop simultaneously and are detectable months before the other lymphocytes, notably CD8 are fully functional. Our contention, therefore, is that the low GVHD observed in UCB recipients may be partially due to early NK cells.

T cell-mediated tumor rejection displays diverse dependence upon perforin and IFN-gamma mechanisms that cannot be predicted from in vitro T cell characteristics

Experimental pulmonary metastases have been successfully treated by adoptive transfer of tumor-sensitized T cells from perforin knockout (KO) or Fas/APO-1 ligand(KO) mice, suggesting a prominent role for secretion of cytokines such as IFN-gamma. In the present study we confirmed that rejection of established methylcholanthrene-205 (MCA-205) pulmonary metastases displayed a requirement for T cell IFN-gamma expression. However, this requirement could be obviated by transferring larger numbers of tumor-sensitized IFN-gamma (KO) T cells or by immunosensitizing sublethal irradiation (500 rad) of the host before adoptive therapy. Extrapulmonary tumors (MCA-205 s.c. and intracranial) that required adjunct sublethal irradiation for treatment efficacy also displayed no requirement for host or T cell expression of IFN-gamma. Nonetheless, rejection of MCA-205 s.c. tumors and i.p. EL-4 tumors, but not MCA-205 pulmonary or intracranial tumors, displayed a significant requirement for T cell perforin expression (i.e., CTL participation). The capacity of T cells to lyse tumor targets and secrete IFN-gamma in vitro before adoptive transfer was nonpredictive of the roles of these activities in subsequent tumor rejection. Adoptive therapy studies employing KO mice are therefore indispensable for revealing a diversity of tumor rejection mechanisms that may lack in vitro correlation due to delays in their induction. Seemingly contradictory KO data from different studies are reconciled by the capacity of anti-tumor T cells to rely on alternative mechanisms when treated in larger numbers, the variable participation of CTL at different anatomic locations of tumor, and the apparent capacity of sublethal irradiation to provide a therapeutic alternative to host or T cell IFN-gamma production.

Death the Fas way: regulation and pathophysiology of CD95 and its ligand

Apoptotic cell death mediated by the members of the tumor necrosis factor receptor family is an essential process involved in the regulation of cellular homeostasis during development, differentiation, and pathophysiological conditions. Among the cell death receptors comprising the tumor necrosis factor receptor superfamily, CD95/APO-1 (Fas) is the best characterized. The specific interaction of Fas with its cognate ligand, Fas ligand (FasL), elicits the activation of a death-inducing caspase (cysteine aspartic acid proteases) cascade, occurring in a transcription-independent manner. Caspase activation executes the apoptosis process by cleaving various intracellular substrates, leading to genomic DNA fragmentation, cell membrane blebbing, and the exposure of phagocytosis signaling molecules on the cell surface. Recent studies have shown that the Fas/FasL pathway plays an important role in regulating the life and death of the immune system through activation-induced cell death. In addition, these molecules have been implicated in aging, human immunodeficiency virus infection, drug abuse, stress, and cancer development. In this review, we will focus on the mechanisms that regulate Fas and FasL expression, and how their deregulation leads to diseases.

Anti-third party CD8+ CTLs as potent veto cells: coexpression of CD8 and FasL is a prerequisite

Several bone marrow cells and lymphocyte subpopulations, known as "veto cells," were shown to induce transplantation tolerance across major histocompatibility antigens. Recently, it has been suggested that anti-third party CTLs depleted of alloreactivity are endowed with marked veto activity and therefore might potentially facilitate bone marrow allografting without graft versus host disease (GVHD). The veto mechanism is still obscure. While early studies emphasized the role of CD8-mediated apoptosis, more recent evidence indicates a role for Fas-FasL. In the present study we show, by using blocking anti-CD8 antibody, by generating CTLs from FasL or perforin mutated mice, and by gene transfer of FasL, that the veto activity of anti-third party CD8+ CTLs is dependent upon the simultaneous expression of both CD8 and FasL.

Impaired binding of perforin on the surface of tumor cells is a cause of target cell resistance against cytotoxic effector cells

Exocytosis of perforin, subsequent binding of perforin to the target cell membrane, and formation of lytic pores form an important pathway involved in the induction of tumor cell death by cytotoxic effector cells. Here we describe a novel escape mechanism employed by tumor cells to protect themselves from granule-mediated cell death: We were able to demonstrate that the resistance of the human leukemia cell line ML-2 to natural killer (NK)-cell–mediated killing is not caused by impaired NK-cell activation but by resistance against effector molecules contained in the granules of cytotoxic cells. No resistance was observed against other pore-forming agents like complement and streptolysin O. By using the NK-susceptible leukemia cell line K562, we could show that the induction of cell death by cytotoxic granules can be blocked completely by anti-perforin antibodies, indicating that perforin is essentially involved in this process. Flow cytometric data revealed that an impaired binding of perforin on the tumor cell membrane is mainly responsible for target cell resistance, because perforin turned out to bind well on K562 cells but is not able to attach to the surface of ML-2 cells. After impaired binding of perforin was identified as a potential mechanism of tumor cell resistance, leukemia cells from 6 patients with acute myeloid leukemia (AML) were examined. As predicted, AML cells that failed to bind perforin on their surface demonstrated complete resistance toward NK-cell–mediated cytotoxicity. Thus, perforin resistance could represent an important tumor escape mechanism that should be considered when cytotoxic effector cells are used for cellular immunotherapy.

Impaired binding of perforin on the surface of tumor cells is a cause of target cell resistance against cytotoxic effector cells

Exocytosis of perforin, subsequent binding of perforin to the target cell membrane, and formation of lytic pores form an important pathway involved in the induction of tumor cell death by cytotoxic effector cells. Here we describe a novel escape mechanism employed by tumor cells to protect themselves from granule-mediated cell death: We were able to demonstrate that the resistance of the human leukemia cell line ML-2 to natural killer (NK)-cell–mediated killing is not caused by impaired NK-cell activation but by resistance against effector molecules contained in the granules of cytotoxic cells. No resistance was observed against other pore-forming agents like complement and streptolysin O. By using the NK-susceptible leukemia cell line K562, we could show that the induction of cell death by cytotoxic granules can be blocked completely by anti-perforin antibodies, indicating that perforin is essentially involved in this process. Flow cytometric data revealed that an impaired binding of perforin on the tumor cell membrane is mainly responsible for target cell resistance, because perforin turned out to bind well on K562 cells but is not able to attach to the surface of ML-2 cells. After impaired binding of perforin was identified as a potential mechanism of tumor cell resistance, leukemia cells from 6 patients with acute myeloid leukemia (AML) were examined. As predicted, AML cells that failed to bind perforin on their surface demonstrated complete resistance toward NK-cell–mediated cytotoxicity. Thus, perforin resistance could represent an important tumor escape mechanism that should be considered when cytotoxic effector cells are used for cellular immunotherapy.

Increased accuracy of renal allograft rejection diagnosis using combined perforin, granzyme B, and Fas ligand fine-needle aspiration immunocytology

BACKGROUND

Two major routes by which cytotoxic T lymphocytes induce apoptosis in target cells are the perforin-granzyme and the Fas ligand/Fas pathways. Intragraft expression of message for these immune activation genes has been shown to correlate very closely with clinical rejection. We have immunolabeled fine-needle aspiration biopsy samples using a panel of cytotoxic T-cell activation markers to evaluate the immunocytochemical identification of the protein products of these genes in the verification of human renal allograft rejection.

METHODS

In this retrospective pilot study, 140 fine-needle aspiration biopsy samples from 50 human renal allografts were labeled using alkaline phosphatase/ anti-alkaline phosphatase immunocytochemistry incorporating monoclonal antibodies to perforin, granzyme B, and Fas ligand. Levels of positive labeling for these markers were compared with the original clinical diagnosis of rejection.

RESULTS

An excellent correlation with clinical rejection was obtained when all three antibodies were positive. The false positive rate for each antibody was sufficient to make any one alone or in combination with one other unreliable for diagnosing rejection. When all three antibodies gave positive labeling, agreement with clinical rejection status was superior to using conventional morphological cytology.

CONCLUSIONS

In addition to providing valuable morphological information regarding the composition of inflammatory leukocyte populations and the preservation status of renal parenchymal cells, fine-needle aspiration biopsy samples may be labeled using combined perforin, granzyme B, and Fas ligand immunocytochemistry to offer a safe and reliable method for diagnosing rejection with an excellent level of accuracy.

Adenoviral vector which delivers FasL-GFP fusion protein regulated by the tet-inducible expression system

Fas ligand (FasL) is a member of the tumor necrosis family and when bound to its receptor, Fas, induces apoptosis. It plays important roles in immune response, degenerative and lymphoproliferative diseases, development and tumorigenesis. It is also involved in generation of immune privilege sites in the eye and testis. Harnessing the power of this molecule is expected to lead to a powerful chemotherapeutic. We describe the construction and characterization of replication-deficient adenoviral vectors that express a fusion of murine FasL and green fluorescent protein (GFP). FasL-GFP retains full activity of wild-type FasL, at the same time allowing for easy visualization and quantification in both living and fixed cells. The fusion protein is under the control of a tetracycline-regulated gene expression system. Tight control of expression is achieved by creating a novel 'double recombinant' Ad vector, in which the tet-responsive element and the transactivator element are built into the opposite ends of the same vector to avoid enhancer interference. Expression can be conveniently regulated by tetracycline or its derivatives in a dose-dependent manner. The vector was able to deliver FasL-GFP gene to cells in vitro efficiently, and the expression level and function of the fusion protein was modulated by the concentration of doxycycline. This regulation allows us to produce high titers of the vector by inhibiting FasL expression in an apoptosis-resistant cell line. Induction of apoptosis was demonstrated in all cell lines tested. These results indicate that our vector is a potentially valuable tool for FasL-based gene therapy of cancer and for the study of FasL/Fas-mediated apoptosis and immune privilege.

Tumor immunity in perforin-deficient mice: a role for CD95 (Fas/APO-1)

CTL and NK cells use two distinct cytocidal pathways: 1) perforin and granzyme based and 2) CD95L/CD95 mediated. The former requires perforin expression by the effectors (CTL or NK), whereas the latter requires CD95 (Fas/APO-1) expression by the target. We have investigated how these two factors contribute to tumor immune surveillance by studying the immunity of perforin-deficient mice against the progressor C57BL/6 Lewis lung carcinoma 3LL, which expresses no CD95 when cultured in vitro. Unexpectedly, the results indicated that the perforin-independent CD95L/CD95 pathway of CTL/NK plays a role in acting against D122 and Kb39.5 (39.5) high and low metastatic sublines, respectively, derived from the 3LL tumor. Although no membrane-bound CD95 was detected on cultured D122 and 39. 5 cells, surface CD95 expression on both D122 and 39.5 was considerably up-regulated when the tumors were grown in vivo. A similarly enhanced expression of CD95 was observed with three additional tumors; LF-, BW, and P815, injected into syngeneic and allogeneic mice. The finding of up-regulated CD95 expression on tumor cells placed in vivo suggests that a CD95-based mechanism plays a role in tumor immunity at early stages of tumor growth. Consequently, the progressive down-regulation of CD95 expression during tumor progression may indeed be an escape mechanism as previously reported. Together, these results suggest a role for CD95-dependent, perforin-independent immunity against certain tumors.

Mice with a Targeted Mutation in Lymphotoxin-α Exhibit Enhanced Tumor Growth and Metastasis: Impaired NK Cell Development and Recruitment

Mice deficient in lymphotoxin (LT)-α lack peripheral lymph nodes and Peyer’s patches and have profound defects in development of follicular dendritic cell networks, germinal center formation, and T/B cell segregation in the spleen. Although LTα is known to be expressed by NK cells as well as T and B lymphocytes, the requirement of LTα for NK cell functions is largely unknown. To address this issue, we have assessed NK cell functions in LTα-deficient mice by evaluating tumor models with known requirements for NK cells to control their growth and metastasis. Syngeneic B16F10 melanoma cells inoculated s.c. grew more rapidly in LTα−/− mice than in the wild-type littermates, and the formation of experimental pulmonary metastases was significantly enhanced in LTα−/− mice. Although LTα−/− mice exhibited almost a normal total number of NK cells in spleen, they showed an impaired recruitment of NK cells to lung and liver. Additionally, lytic NK cells were not efficiently produced from LTα−/− bone marrow cells in vitro in the presence of IL-2 and IL-15. These data suggest that LTα signaling may be involved in the maturation and recruitment of NK cells and may play an important role in antitumor surveillance.

The levels of soluble granzyme A and B are elevated in plasma and synovial fluid of patients with rheumatoid arthritis (RA)

Cytotoxic cells possess specialized granules which contain perforin and a group of serine proteinases termed granzymes. Granzyme-positive cells have been identified in synovial fluid and tissue of patients with RA, where they may play an important role as mediators of granule-mediated apoptosis, extracellular proteolysis, and cytokine induction. The aim here was to define further the involvement of cytotoxic cells in RA. Plasma and synovial fluid samples from the knee joint were obtained from 31 RA patients. The disease controls included 20 osteoarthritis (OA) patients and 10 reactive arthritis (ReA) patients. A recently developed capture ELISA was used to detect soluble granzymes A and B in all patients. Compared with OA and ReA disease controls, markedly increased levels of soluble granzymes A and B were detected in both plasma and synovial fluid of RA patients (P < 0.00001). When values for soluble granzymes A and B in plasma and synovial fluid were used simultaneously as independent variables, logistic regression analysis indicated that a diagnosis of RA could be predicted correctly in 84% of the RA patients and a diagnosis of non-RA in 90% of the controls. The markedly elevated levels of soluble granzymes A and B in plasma and synovial fluid of RA patients strongly suggest that cytotoxic cells are active participants in the pathogenesis of RA. Moreover, the results suggest that measurement of granzymes may assist the laboratory evaluation of patients with arthritis. Larger studies in patients with early disease may clarify the role of this test system in differential diagnosis.

Molecular Mechanisms of Immune-Mediated Lysis of Murine Renal Cancer: Differential Contributions of Perforin-Dependent Versus Fas-Mediated Pathways in Lysis by NK and T Cells

Mice bearing the experimental murine renal cancer Renca can be successfully treated with some forms of immunotherapy. In the present study, we have investigated the molecular pathways used by NK and T cells to lyse Renca cells. Renca cells normally express low levels of Fas that can be substantially enhanced by either IFN-γ or TNF-α, and the combination of IFN-γ + TNF-α synergistically enhances cell-surface Fas expression. In addition, cells pretreated with IFN-γ and TNF-α are sensitive to lysis mediated by Fas ligand (FasL)-expressing hybridomas (dllS), cross-linking of anti-Fas Abs or soluble Fas (FasL). Lysis via Fas occurs by apoptosis, since Renca shows all the typical characteristics of apoptosis. No changes in levels of bcl-2 were observed after cytokine treatments. We also examined cell-mediated cytotoxic effects using activated NK cells and T cells from gld FasL-deficient mice, and perforin-deficient mice, as well as wild-type C57BL/6 and BALB/c mice. Interestingly, the granule-mediated pathway predominated in killing of Renca by activated NK cells, while the Fas/FasL pathway contributed significantly to cell-mediated killing of Renca by activated T cells. These results suggest that killing of Renca tumor cells by immune effector cells can occur by both granule and Fas-mediated cytotoxicity. However, for the Fas-mediated pathway to function, cell surface levels of Fas need to be increased beyond a critical threshold level by proinflammatory cytokines such as IFN-γ and TNF-α.

Inhibition of IgG2ab production by Ig allotype-specific T cells can Be mediated without T-B cell contact

The growth of IgG2ab-producing CB101 myeloma cells, subcutaneously or intraperitoneally inoculated into histocompatible BALB/c Igha mice sensitized against this Ig allotype, was delayed by 2-4 weeks compared to normal mice. While IgG2ab production was detected in the sera of 75-100% of normal mice, it was irreversibly inhibited in 100% of sensitized mice. IgG2ab suppression (IgG2ab sup) was also systematically obtained in sensitized but not normal recipients, implanted ip with a 0.1-micrometer-pore diffusion chamber (DC) containing CB101 cells. This time, the specific IgG2ab sup was reversible in vitro in the absence of anti-IgG2ab T cells. Adoptive transfer, of unfractionated or T but not B splenocytes from their sensitized counterparts into normal mice, 1 day before DC implantation, induced IgG2ab sup as well. These results indicate that, in these experimental circumstances, IgG2ab sup can also be mediated by diffusible suppressive factors produced by the effector T cells, without direct T-B-cell contact.

Fas (CD95/Apo-1)-mediated damage to ventricular myocytes induced by cytotoxic T lymphocytes from perforin-deficient mice: a major role for inositol 1,4,5-trisphosphate

Cytotoxic T lymphocytes (CTLs) that infiltrate the heart are important immune effectors implicated in heart transplant rejection, myocarditis, and other cardiomyopathies. To investigate the mechanism(s) underlying CTL damage to the myocardium through activation of the Fas receptor (Fas/CD95/Apo-1) by the Fas ligand, we explored the interaction between peritoneal exudate CTLs (PELs), derived from perforin gene-knockout (P-/-) mice, and murine ventricular myocytes. Fas expression on isolated ventricular myocytes was demonstrated immunohistochemically. Action potentials, [Ca2+]i transients, and contractions of myocytes conjugated to P-/- PELs or treated with the apoptosis-inducing anti-Fas monoclonal antibody Jo2 were recorded. Action potential characteristics of nonconjugated myocytes and myocytes conjugated with P-/- PELs were, respectively, as follows: Vm, -73.2+/-1.5 and -53.6+/-6.4 mV (mean+/-SEM); action potential amplitude, 117.9+/-3.9 and 74.3+/-21.2 mV; and action potential duration at 80% repolarization, 17+/-6 and 42+/-13 milliseconds (all P<.05). P-/- PELs also induced early and delayed afterdepolarizations as well as arrhythmogenic activity. Diastolic [Ca2+]i increased during the cytocidal interaction with P-/- PELs, from a fluorescence ratio of 0.82+/-0.05 (n=7) to 1.98+/-0.09 (n=13) (P<.05). All of the effects caused by P-/- PELs were reproduced by incubating the myocytes with Jo2. Heparin (50 microg/mL), an antagonist of inositol trisphosphate (IP3)-operated sarcoplasmic reticulum Ca2+ channels, or U-73122 (2 micromol/L), a phospholipase C inhibitor, but not the inactive agonist U-73343, prevented Fas-mediated myocyte dysfunction. Additionally, intracellular application (through the patch pipette) of the active IP3 analogue, inositol 1,4,5-trisphosphate, but not the inactive analogue, inositol 1,3,4-trisphosphate, caused electrophysiological changes resembling those resulting from P-/- PELs and Jo2, suggesting that CTL-induced Fas-based myocyte dysfunction is mediated by IP3. We conclude that a Fas-based perforin-independent mechanism of CTL action can account for the immunopathology seen in the allotransplanted heart, myocarditis, and dilated cardiomyopathy.