Systemic immunological tolerance to ocular antigens is mediated by TRAIL-expressing CD8+ T cells

TNF-Related Apoptosis-Inducing Ligand: Non-Apoptotic Signalling

TNF-related apoptosis-inducing ligand (TRAIL or Apo2 or TNFSF10) belongs to the TNF superfamily. When bound to its agonistic receptors, TRAIL can induce apoptosis in tumour cells, while sparing healthy cells. Over the last three decades, this tumour selectivity has prompted many studies aiming at evaluating the anti-tumoral potential of TRAIL or its derivatives. Although most of these attempts have failed, so far, novel formulations are still being evaluated. However, emerging evidence indicates that TRAIL can also trigger a non-canonical signal transduction pathway that is likely to be detrimental for its use in oncology. Likewise, an increasing number of studies suggest that in some circumstances TRAIL can induce, via Death receptor 5 (DR5), tumour cell motility, potentially leading to and contributing to tumour metastasis. While the pro-apoptotic signal transduction machinery of TRAIL is well known from a mechanistic point of view, that of the non-canonical pathway is less understood. In this study, we the current state of knowledge of TRAIL non-canonical signalling.

Local T cell infiltrates are predominantly associated with corneal allograft rejection

BACKGROUND

Corneal transplantations (CTXs) are a vision-saving procedure. Routinely, while CTXs' survival rates remain high, the risk of graft failure increases significantly for repeated CTXs. The reason is an alloimmunization following previous CTXs and development of memory T (Tm) and B (Bm) cells.

METHODS

We characterized populations of cells present in explanted human corneas from patients receiving the first CTX and marked as a primary CTX (PCTX) or the second or more CTXs and marked as a repeated CTX (RCTX). Cells extracted from resected corneas and from peripheral blood mononuclear cells (PBMCs) were analyzed by the flow cytometry method using multiple surface and intracellular markers.

RESULTS

Overall, the number of cells was similar in PCTX and RCTX patients. Extracted infiltrates from PCTXs and RCTXs contained similar numbers of T cell subsets, namely CD4+, CD8+, CD4+ Tm, CD8+ Tm, CD4+Foxp3+ T regulatory (Tregs), CD8+ Treg cells, while very few B cells (all p = NS). However, when compared to peripheral blood, PCTX and RCTX corneas contained significantly higher percentages of effector memory CD4+ and CD8+ T cells (both p < 0,05). In comparison to PCTX, RCTX group had the highest levels of Foxp3 in T CD4+ Tregs (p = 0,04) but decreased percentage of Helios-positive CD4+ Tregs.

CONCLUSION

PCTXs and especially RCTXs are rejected mainly by local T cells. The accumulation of effector CD4+ and CD8+ T cells, as well as CD4+ and CD8+ Tm cells is associated with the final rejection. Furthermore, local CD4+ and CD8+ Tregs expressing Foxp3 and Helios are probably insufficient to impose the acceptance of CTX.

Interactions of Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand (TRAIL) with the Immune System: Implications for Inflammation and Cancer

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF superfamily. TRAIL has historically been distinct from the Fas ligand and TNFα in terms of selective apoptosis induction in tumor cells and has a nearly non-existent systemic toxicity. Consequently, in the search for an ideal drug for tumor therapy, TRAIL rapidly drew interest, promising effective tumor control with minimal side effects. However, euphoria gave way to disillusionment as it turned out that carcinoma cells possess or can acquire resistance to TRAIL-induced apoptosis. Additionally, studies on models of inflammation and autoimmunity revealed that TRAIL can influence immune cells in many different ways. While TRAIL was initially found to be an important player in tumor defense by natural killer cells or cytotoxic T cells, additional effects of TRAIL on regulatory T cells and effector T cells, as well as on neutrophilic granulocytes and antigen-presenting cells, became focuses of interest. The tumor-promoting effects of these interactions become particularly important for consideration in cases where tumors are resistant to TRAIL-induced apoptosis. Consequently, murine models have shown that TRAIL can impair the tumor microenvironment toward a more immunosuppressive type, thereby promoting tumor growth. This review summarizes the current state of knowledge on TRAIL’s interactions with the immune system in the context of cancer.

Cellular and Molecular Mechanisms of Anterior Chamber-Associated Immune Deviation (ACAID): What We Have Learned from Knockout Mice

Anterior chamber-associated immune deviation (ACAID) is a well-known phenomenon that can occur after an antigen is introduced without any danger signal into the anterior chamber of a murine eye. It is reported to lead to an antigen-specific immune deviation throughout the body. Despite the relatively little evidence of this phenomenon in humans, it has been suggested as a potential prophylactic strategy in allograft rejections and in several autoimmune diseases. Cellular and molecular mechanisms of ACAID have been explored in different murine models mainly as proofs of concept, first by direct analyses of immune components in normal immunocompetent settings and by cell transfer experiments. Later, use of knockout (KO) mice has helped considerably to decipher ACAID mechanisms. However, several factors raise questions about the reliability and validity of studies using KO murine models. This mini-review summarizes results obtained with KO mice and discusses their advantages, their potential weaknesses, and their potential methods for further progress.

TRAIL Promotes Tumor Growth in a Syngeneic Murine Orthotopic Pancreatic Cancer Model and Affects the Host Immune Response

OBJECTIVES

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is currently being evaluated as a possible biological agent for cancer treatment. However, many tumor cells are resistant to TRAIL-induced apoptosis. In these cases, TRAIL may activate different pathways promoting tumor growth as well as showing different interactions with the immunological tumor microenvironment. In this study, the impact of TRAIL on tumor growth and survival in a syngeneic model of TRAIL-resistant pancreatic cancer cells was investigated.

METHODS

Murine 6606PDA pancreatic cancer cells were injected into the pancreatic heads of TRAIL mice and their littermates. To examine a direct effect of TRAIL on tumor cells, cultures of 6606PDA were TRAIL stimulated.

RESULTS

The TRAIL mice displayed significantly decreased tumor volumes and an enhanced overall survival in pancreatic cancer. The decreased tumor growth in TRAIL mice was accompanied by a decrease of regulatory CD4 cells within tumors. Concordantly, TRAIL treatment of wild-type mice enhanced tumor growth and increased the fraction of regulatory CD4 cells. Yet, a direct effect of TRAIL on 6606PDA cells was not detected.

CONCLUSIONS

Thus, TRAIL can promote tumor growth in TRAIL-resistant tumor cells. This may restrict possible future clinical applications of TRAIL in pancreatic cancer.

Therapeutic Use of Soluble Fas Ligand Ameliorates Acute and Recurrent Herpetic Stromal Keratitis in Mice

PURPOSE

The present study was designed to test the therapeutic value of soluble FasL (sFasL) in an acute model of herpetic stromal keratitis (HSK) and, more importantly, a recurrent model of HSK using BALB/c, BALB-lpr, and National Institutes of Health (NIH) mice.

METHODS

Mice were infected either acutely with the KOS strain of herpes simplex virus 1 (HSV-1) or latently with the McKrae strain of HSV-1. Acutely infected mice as well as ultraviolet-B (UV-B) reactivated mice (recurrent infection) were treated with sFasL, or soluble TNF-related apoptosis inducing ligand (sTRAIL), or BSA daily or 3 times/wk by using either a combination of subconjunctival injection and topical ointment, or with topical ointment alone. These mice then were evaluated for corneal opacity and neovascularization for 6 weeks.

RESULTS

Following acute and recurrent HSV-1 infection, wild-type BALB/c mice treated with sFasL displayed significantly reduced incidence of corneal opacity and neovascularization compared to the control animals. However, BALB-lpr mice, which are deficient in Fas+ inflammatory cells, displayed no such differences in ocular disease, as expected. Latently infected NIH mice treated with sFasL displayed similar results. Flow cytometric analysis revealed that the corneal inflammatory infiltrate in those treated with sFasL was significantly less than in sTRAIL- or BSA-treated mice. Furthermore, corneas from sFasL-treated mice displayed relatively more cells undergoing apoptosis.

CONCLUSIONS

This study provides evidence that sFasL treatment has potential therapeutic benefit in reducing inflammatory infiltrate and neovascularization in primary and recurrent forms of HSK, and that it does so by augmenting the restriction of Fas+ inflammatory cells mediated by membrane FasL.

Therapeutic applications of TRAIL receptor agonists in cancer and beyond

TRAIL/Apo-2L is a member of the TNF superfamily first described as an apoptosis-inducing cytokine in 1995. Similar to TNF and Fas ligand, TRAIL induces apoptosis in caspase-dependent manner following TRAIL death receptor trimerization. Because tumor cells were shown to be particularly sensitive to this cytokine while normal cells/tissues proved to be resistant along with being able to synthesize and release TRAIL, it was rapidly appreciated that TRAIL likely served as one of our major physiologic weapons against cancer. In line with this, a number of research laboratories and pharmaceutical companies have attempted to exploit the ability of TRAIL to kill cancer cells by developing recombinant forms of TRAIL or TRAIL receptor agonists (e.g., receptor-specific mAb) for therapeutic purposes. In this review article we will describe the biochemical pathways used by TRAIL to induce different cell death programs. We will also summarize the clinical trials related to this pathway and discuss possible novel uses of TRAIL-related therapies. In recent years, the physiological importance of TRAIL has expanded beyond being a tumoricidal molecule to one critical for a number of clinical settings - ranging from infectious disease and autoimmunity to cardiovascular anomalies. We will also highlight some of these conditions where modulation of the TRAIL/TRAIL receptor system may be targeted in the future.

Impact of sepsis on CD4 T cell immunity

Sepsis remains the primary cause of death from infection in hospital patients, despite improvements in antibiotics and intensive-care practices. Patients who survive severe sepsis can display suppressed immune function, often manifested as an increased susceptibility to (and mortality from) nosocomial infections. Not only is there a significant reduction in the number of various immune cell populations during sepsis, but there is also decreased function in the remaining lymphocytes. Within the immune system, CD4 T cells are important players in the proper development of numerous cellular and humoral immune responses. Despite sufficient clinical evidence of CD4 T cell loss in septic patients of all ages, the impact of sepsis on CD4 T cell responses is not well understood. Recent findings suggest that CD4 T cell impairment is a multipronged problem that results from initial sepsis-induced cell loss. However, the subsequent lymphopenia-induced numerical recovery of the CD4 T cell compartment leads to intrinsic alterations in phenotype and effector function, reduced repertoire diversity, changes in the composition of naive antigen-specific CD4 T cell pools, and changes in the representation of different CD4 T cell subpopulations (e.g., increases in Treg frequency). This review focuses on sepsis-induced alterations within the CD4 T cell compartment that influence the ability of the immune system to control secondary heterologous infections. The understanding of how sepsis affects CD4 T cells through their numerical loss and recovery, as well as function, is important in the development of future treatments designed to restore CD4 T cells to their presepsis state.

Ex-vivo tolerogenic F4/80⁺ antigen-presenting cells (APC) induce efferent CD8⁺ regulatory T cell-dependent suppression of experimental autoimmune uveitis

It is known that inoculation of antigen into the anterior chamber (a.c.) of a mouse eye induces a.c.-associated immune deviation (ACAID), which is mediated in part by antigen-specific local and peripheral tolerance to the inciting antigen. ACAID can also be induced in vivo by intravenous (i.v.) inoculation of ex-vivo-generated tolerogenic antigen-presenting cells (TolAPC). The purpose of this study was to test if in-vitro-generated retinal antigen-pulsed TolAPC suppressed established experimental autoimmune uveitis (EAU). Retinal antigen-pulsed TolAPC were injected i.v. into mice 7 days post-induction of EAU. We observed that retinal antigen-pulsed TolAPC suppressed the incidence and severity of the clinical expression of EAU and reduced the expression of associated inflammatory cytokines. Moreover, extract of whole retina efficiently replaced interphotoreceptor retinoid-binding protein (IRBP) in the preparation of TolAPC used to induce tolerance in EAU mice. Finally, the suppression of EAU could be transferred to a new set of EAU mice with CD8⁺ but not with CD4⁺regulatory T cells (T_reg). Retinal antigen-pulsed TolAPC suppressed ongoing EAU by inducing CD8⁺ T_reg cells that, in turn, suppressed the effector activity of the IRBP-specific T cells and altered the clinical symptoms of autoimmune inflammation in the eye. The ability to use retinal extract for the antigen raises the possibility that retinal extract could be used to produce autologous TolAPC and then used as therapy in human uveitis.

NKG2D regulates production of soluble TRAIL by ex vivo expanded human γδ T cells

Soluble TRAIL (sTRAIL) can be produced by myeloid-derived cells to kill cancer cells. Whether this mechanism is used by T cells, and if so, how sTRAIL production is regulated, remains unclear. Our previous studies showed that ex vivo expanded human γδ T cells express TRAIL and NK receptor group 2 (R2), member D (NKG2D), and possess potent anticancer activities both in vitro and in vivo. Here, we investigated in greater detail the mechanisms by which γδ T cells utilize TRAIL and NKG2D to kill lung cancer cells. We demonstrate that human lung cancer cells express TRAIL R2 and NKG2D ligands. Blocking TRAIL or NKG2D during γδ T-cell-lung cancer cell co-cultures significantly reduced γδ T-cell-mediated cytotoxicity. Cross-linking NKG2D with anti-NKG2D antibody to mimic ligand binding promoted γδ T cells to produce sTRAIL, which induced apoptosis in lung cancer cells through TRAIL R2. Either neutralizing sTRAIL or blocking lung cancer cell TRAIL R2 significantly reduced γδ T-cell-mediated cytotoxicity to lung cancer cells. This study demonstrates that γδ T cells can mediate anticancer immunity via NKG2D-regulated production of sTRAIL.

T-cell-mediated immunity and the role of TRAIL in sepsis-induced immunosuppression

Sepsis is the leading cause of death in most intensive care units, and the death of septic patients usually does not result from the initial septic event but rather from subsequent nosocomial infections. Patients who survive severe sepsis often display severely compromised immune function. Not only is there significant apoptosis of lymphoid and myeloid cells that depletes critical components of the immune system during sepsis, there is also decreased function of the remaining immune cells. Studies of animals and humans suggest the immune defects that occur during sepsis may be critical to pathogenesis and subsequent mortality. This review focuses on sepsis-induced alterations with the cluster differentiation (CD) 8 T-cell compartment that can affect the control of secondary heterologous infections. Understanding how a septic event directly influences CD8 T-cell populations through apoptotic death and homeostatic proliferation and indirectly by immune-mediated suppression will provide valuable starting points for developing new treatment options.

Good news-bad news: the Yin and Yang of immune privilege in the eye

The eye and the brain are prototypical tissues manifesting immune privilege (IP) in which immune responses to foreign antigens, particularly alloantigens are suppressed, and even completely inhibited. Explanations for this phenomenon are numerous and mostly reflect our evolving understanding of the molecular and cellular processes underpinning immunological responses generally. IP is now viewed as a property of many tissues and the level of expression of IP varies not only with the tissue but with the nature of the foreign antigen and changes in the limited conditions under which privilege can operate as a mechanism of immunological tolerance. As a result, IP functions normally as a homeostatic mechanism preserving normal function in tissues, particularly those with highly specialized function and limited capacity for renewal such as the eye and brain. However, IP is relatively easily bypassed in the face of a sufficiently strong immunological response, and the privileged tissues may be at greater risk of collateral damage because its natural defenses are more easily breached than in a fully immunocompetent tissue which rapidly rejects foreign antigen and restores integrity. This two-edged sword cuts its swathe through the eye: under most circumstances, IP mechanisms such as blood-ocular barriers, intraocular immune modulators, induction of T regulatory cells, lack of lymphatics, and other properties maintain tissue integrity; however, when these are breached, various degrees of tissue damage occur from severe tissue destruction in retinal viral infections and other forms of uveoretinal inflammation, to less severe inflammatory responses in conditions such as macular degeneration. Conversely, ocular IP and tumor-related IP can combine to permit extensive tumor growth and increased risk of metastasis thus threatening the survival of the host.

TNF-related apoptosis-inducing ligand (TRAIL) exerts therapeutic efficacy for the treatment of pneumococcal pneumonia in mice

Apoptotic death of alveolar macrophages observed during lung infection with Streptococcus pneumoniae is thought to limit overwhelming lung inflammation in response to bacterial challenge. However, the underlying apoptotic death mechanism has not been defined. Here, we examined the role of the TNF superfamily member TNF-related apoptosis-inducing ligand (TRAIL) in S. pneumoniae-induced macrophage apoptosis, and investigated the potential benefit of TRAIL-based therapy during pneumococcal pneumonia in mice. Compared with WT mice, Trail(-/-) mice demonstrated significantly decreased lung bacterial clearance and survival in response to S. pneumoniae, which was accompanied by significantly reduced apoptosis and caspase 3 cleavage but rather increased necrosis in alveolar macrophages. In WT mice, neutrophils were identified as a major source of intraalveolar released TRAIL, and their depletion led to a shift from apoptosis toward necrosis as the dominant mechanism of alveolar macrophage cell death in pneumococcal pneumonia. Therapeutic application of TRAIL or agonistic anti-DR5 mAb (MD5-1) dramatically improved survival of S. pneumoniae-infected WT mice. Most importantly, neutropenic mice lacking neutrophil-derived TRAIL were protected from lethal pneumonia by MD5-1 therapy. We have identified a previously unrecognized mechanism by which neutrophil-derived TRAIL induces apoptosis of DR5-expressing macrophages, thus promoting early bacterial killing in pneumococcal pneumonia. TRAIL-based therapy in neutropenic hosts may represent a novel antibacterial treatment option.

Influence of CD8+ T regulatory cells on intraocular tumor development

The interior of the eye, or uvea, is a site of immune privilege where certain immune responses are attenuated or completely excluded to protect non-regenerating tissues essential for vision. One consequence of this immunoregulation is compromised immune mediated elimination of intraocular tumors. For example, certain murine tumor cell lines which are rejected by host immune responses when transplanted in the skin grow progressively when placed in the anterior chamber (a.c.) of the eye. Progressive ocular tumor growth occurs despite induction of tumor-specific CD8+ T cell responses capable of eliminating a subsequent tumor challenge in the skin or opposite eye. Why these CD8+ T effectors fail to eliminate established ocular tumors is not known. It is well appreciated that growth of tumors in the a.c. induces the generation of immunosuppressive CD8+ T regulatory (Treg) cells. However, the contribution of CD8+ Treg in ocular tumor progression remains unclear. Several studies indicate that these CD8+ Treg target responding CD4+ T cells to inhibit their induction of macrophage-dependent delayed type hypersensitivity (DTH) responses to tumor antigens (Ags). However, induction of tumor-specific CD4+ T cell responses does not assure intraocular tumor elimination. This review is focused on how CD8+ Treg could influence the tumoricidal activity of ocular tumor-specific CD8+ T effector cells.

Trauma patients' elevated tumor necrosis related apoptosis inducing ligand (TRAIL) contributes to increased T cell apoptosis

Immunosuppression resulting from excessive post-trauma apoptosis of hyperactivated T cells is controversial. TRAIL mediated T cell apoptosis decreases highly activated T cells' responses. Caspase-10, a particular TRAIL target, was increased in trauma patients' T cells with concomitantly elevated plasma TRAIL levels. These patients' T cells developed anergy, implicating increased TRAIL-mediated T cell apoptosis in post-trauma T cell anergy. Control T cells cultured with patients' sera containing high TRAIL levels increased their caspase-10 activity and apoptosis. Stimulated primary T cells are TRAIL apoptosis resistant. Increased plasma thrombospondin-1 and T cell expression of CD47, a thrombospondin-1 receptor, preceded patients' T cell anergy. CD47 triggering of T cells increased their sensitivity to TRAIL-induced apoptosis. Augmentation of T cell TRAIL-induced apoptosis was secondary to CD47 triggered activation of the Src homology-containing phosphatase-1 (SHP-1) and was partially blocked by a SHP-1 inhibitor. We suggest that combined post-trauma CD47 triggering, SHP-1 mediated NFκB suppression, and elevated TRAIL levels increase patients' CD47 expressing T cell apoptosis, thus contributing to subsequent T cell anergy.

Cell death in the maintenance and abrogation of tolerance: the five Ws of dying cells

The mammalian immune system continually faces death in the form of its own dead and dying cells that arise during normal tissue turnover, infections, cellular damage, and cancer. Complex decisions must then be made that will permit a protective response to pathogens, while at the same time destroying tumors but not attacking vital systems of the host that could lead to autoimmunity. By using an investigative technique termed the five Ws (who, what, when, where, and why), we will examine how the immune system responds to antigens generated via cell death. This analysis will give us a better understanding of the molecular differences fundamental to tolerogenic or immunogenic cell death, the cells that sense and react to the dead cells, and the consequences of these fundamental elements on the maintenance or abrogation of tolerance.

The plasticity of regulatory T cell function

Regulatory T cells (T(regs)) can suppress a wide variety of cell types, in diverse organ sites and inflammatory conditions. Whereas T(regs) possess multiple suppressive mechanisms, the number required for maximal function is unclear. Furthermore, whether any interrelationship or cross-regulatory mechanisms exist to orchestrate and control their utilization is unknown. In this study, we assessed the functional capacity of T(regs) lacking the ability to secrete both IL-10 and IL-35, which individually are required for maximal T(reg) activity. Surprisingly, IL-10/IL-35 double-deficient T(regs) were fully functional in vitro and in vivo. Loss of IL-10 and IL-35 was compensated for by a concurrent increase in cathepsin E (Ctse) expression, enhanced TRAIL (Tnfsf10) expression, and soluble TRAIL release, rendering IL-10/IL-35 double-deficient T(regs) functionally dependent on TRAIL in vitro and in vivo. Lastly, whereas C57BL/6 T(regs) are normally IL-10/IL-35 dependent, BALB/c T(regs), which express high levels of cathepsin E and enhanced TRAIL expression, are partially TRAIL dependent by default. These data reveal that cross-regulatory pathways exist that control the utilization of suppressive mechanisms, thereby providing T(reg) functional plasticity.

Armed response: how dying cells influence T-cell functions

Immune responses during infection, injury, and cancer proceed in the presence of tissue injury and cell death. Consequently, the system must deal with its own dead cells while it determines the appropriate response to the invader. As apoptotic cells are known to induce immune tolerance and necrotic cells can be potent stimulators of immunity, this decision becomes more complex. The key to understanding the immunologic choices made during cell death is to examine the mechanisms of tolerance induction by dying cells and then relate them to the mechanisms of immunity. Ideally, immunogenic cell death should be directed toward tumor cells and infected cells, whereas tolerogenic cell death should be associated with preventing unwanted immune responses to self. In this review, we discuss how the decision is made by focusing on the biochemical process of cell death and how its key components can influence both tolerance and immunity.

Immune unresponsiveness to secondary heterologous bacterial infection after sepsis induction is TRAIL dependent

Sepsis is the leading cause of death in most intensive care units, and patients who survive the hyperinflammation that develops early during sepsis later display severely compromised immunity. Not only is there apoptosis of lymphoid and myeloid cells during sepsis that depletes these critical cellular components of the immune system, but also the remaining immune cells show decreased function. Using a cecal-ligation and puncture (CLP) model to induce intra-abdominal polymicrobial peritonitis, we recently established a link between the apoptotic cells generated during sepsis and induction of sepsis-induced suppression of delayed-type hypersensitivity. The present study extends this earlier work to include a secondary heterologous bacterial infection (OVA(257)-expressing Listeria monocytogenes [LM-OVA]) subsequent to sepsis initiation to investigate sepsis-induced alterations in the control of this secondary infection and the associated naive Ag-specific CD8 T cell response. We found that CLP-treated wild-type (WT) mice had a reduced ability to control the LM-OVA infection, which was paralleled by suppressed T cell responses, versus sham-treated WT mice. In contrast, CLP-treated Trail(-/-) and Dr5(-/-) mice were better able to control the secondary bacterial infection, and the Ag-specific CD8 T cell response was similar to that seen in sham-treated mice. Importantly, administration of a blocking anti-TRAIL mAb to CLP-treated WT mice was able to restore the ability to control the LM-OVA infection and generate Ag-specific CD8 T cell responses like those seen in sham-treated mice. These data further implicate TRAIL-dependent immune suppression during sepsis and suggest TRAIL neutralization may be a potential therapeutic target to restore cellular immunity in septic patients.