Immunology. Ways around rejection

Suppression of T cell responses in the tumor microenvironment

The immune system recognizes protein antigens expressed in transformed cells evidenced by accumulation of antigen-specific T cells in tumor and tumor draining lymph nodes. However, despite demonstrable immune response, cancers grow progressively suggesting that priming of antitumor immunity is insufficiently vigorous or that antitumor immunity is suppressed, or both. Compared to virus infection, antitumor T cells are low abundance that likely contributes to tumor escape and enhancement of priming is a long-sought goal of experimental vaccination therapy. Furthermore, patient treatment with antigen-specific T cells can in some cases overcome deficient priming and cause tumor regression supporting the notion that low numbers of T cells permits tumor outgrowth. However, tumor-induced suppression of antitumor immune response is now recognized as a significant factor contributing to cancer growth and reversal of the inhibitory influences within the tumor microenvironment is a major research objective. Multiple cell types and factors can inhibit T cell functions in tumors and may be grouped in two general classes: T cell intrinsic and T cell extrinsic. T cell intrinsic factors are exemplified by T cell expression of cell surface inhibitory signaling receptors that, after contact with cells expressing a cognate ligand, inactivate proximal T Cell Receptor-mediated signal transduction therein rendering T cells dysfunctional. T cell extrinsic factors are more diverse in nature and are produced by tumors and various non-tumor cells in the tumor microenvironment. These include proteins secreted by tumor or stromal cells, highly reactive soluble oxygen and nitrogen species, cytokines, chemokines, gangliosides, and toxic metabolites. These factors may restrict T cell entrance into the tumor parenchyma, cause inactivation of effector phase T cell functions, or induce T cell apoptosis ultimately causing diminished cancer elimination. Here, we review the contributions of inhibitory factors to tumor T cell dysfunction leading to tumor escape.

Negativity towards negative results: a discussion of the disconnect between scientific worth and scientific culture

Science is often romanticised as a flawless system of knowledge building, where scientists work together to systematically find answers. In reality, this is not always the case. Dissemination of results are straightforward when the findings are positive, but what happens when you obtain results that support the null hypothesis, or do not fit with the current scientific thinking? In this Editorial, we discuss the issues surrounding publication bias and the difficulty in communicating negative results. Negative findings are a valuable component of the scientific literature because they force us to critically evaluate and validate our current thinking, and fundamentally move us towards unabridged science.

Effect of reactive oxygen species generation in rabbit corneal epithelial cells on inflammatory and apoptotic signaling pathways in the presence of high osmotic pressure

It is generally accepted that high osmotic pressure (HOP) of lacrimal fluid is the core mechanism causing ocular inflammation and injury. However, the association between HOP and the regulation of cell inflammatory response and apoptotic pathways remains unclear. In the present study, we used HOP to interfere with in vitro cultured rabbit corneal epithelial cells, and found that HOP increased the generation of reactive oxygen species (ROS) in rabbit corneal epithelial cells, and increased ROS in turn induced the activation of JNK inflammatory signaling pathway, which further promoted the expression of pro-inflammatory factor NF-κβ and induced the generation of inflammatory factor IL-1β and TNF-α. In addition, HOP-induced ROS in rabbit corneal epithelial cells regulated the CD95/CD95L-mediated cell apoptotic signaling pathway by activating JNK inflammatory signaling pathway. These findings may serve as new theoretical basis and a new way of thinking about the treatment of ocular diseases, especially dry eye.

Fas/CD95 deficiency in ApcMin/+ mice increases intestinal tumor burden

BACKGROUND

Fas, a member of the tumor necrosis family, is responsible for initiating the apoptotic pathway when bound to its ligand, Fas-L. Defects in the Fas-mediated apoptotic pathway have been reported in colorectal cancer.

METHODOLOGY/PRINCIPAL FINDINGS

In the present study, a variant of the Apc(Min/+) mouse, a model for the human condition, Familial Adenomatous Polyposis (FAP), was generated with an additional deficiency of Fas (Apc(Min/+)/Fas(lpr)) by cross-breeding Apc(Min/+) mice with Fas deficient (Fas(lpr)) mice. One of the main limitations of the Apc(Min/+) mouse model is that it only develops benign polyps. However, Apc(Min/+)/Fas(lpr) mice presented with a dramatic increase in tumor burden relative to Apc(Min/+) mice and invasive lesions at advanced ages. Proliferation and apoptosis markers revealed an increase in cellular proliferation, but negligible changes in apoptosis, while p53 increased at early ages. Fas-L was lower in Apc(Min/+)/Fas(lpr) mice relative to Apc(Min/+) cohorts, which resulted in enhanced inflammation.

CONCLUSIONS/SIGNIFICANCE

This study demonstrated that imposition of a Fas deletion in an Apc(Min/+) background results in a more aggressive phenotype of the Apc(Min/+) mouse model, with more rapid development of invasive intestinal tumors and a decrease in Fas-L levels.

Corneal graft rejection occurs despite Fas ligand expression and apoptosis of infiltrating cells

BACKGROUND/AIMS

Constitutive expression of Fas ligand (CD95L) protects the eye against cell mediated immune responses by inducing apoptosis in infiltrating Fas bearing T cells. This study was designed to examine Fas ligand expression on acutely rejecting rat corneal grafts and to investigate the kinetics of induction of apoptosis in infiltrating leucocytes.

METHODS

Orthotopic penetrating corneal transplantation was performed between genetically disparate inbred rats. Fas ligand expression and the phenotype of infiltrating leucocytes were examined by immunohistochemistry. Apoptotic nuclei were visualised in sections of normal rat cornea, rejecting allografts, and time matched isografts by terminal deoxynucleotidyl transferase mediated dUTP biotin nick end labelling (TUNEL) and quantified by video image analysis. Staining with Hoechst dye 33258 was used to confirm the presence of apoptotic nuclei.

RESULTS

Fas ligand was expressed on corneal endothelial and epithelial cells during acute corneal graft rejection. At all time points examined, including as early as the fifth postoperative day, the cells infiltrating both corneal isografts and allografts were TUNEL positive. By the 15th postoperative day, over 90% of all nuclei, many of which were T cells, were apoptotic.

CONCLUSION

Expression of Fas ligand is not downregulated on the cornea during allograft rejection and infiltrating leucocytes in both isografts and allografts die rapidly in situ. Despite the death of the cells believed to be responsible for rejection, isografts survive indefinitely whereas allografts are irreparably damaged.

Dendritic cells overexpressing CD95 (Fas) ligand elicit vigorous allospecific T-cell responses in vivo

Dendritic cells (DCs) genetically engineered to overexpress CD95 (Fas) ligand (CD95L-DC) were proposed as tools to induce peripheral tolerance to alloantigens. Herein, we observed that CD95L-DC obtained after retroviral gene transfer in bone marrow (BM) precursors derived from CD95-deficient (lpr/lpr) mice elicit much stronger allospecific type 1 helper T-cell and cytotoxic T-cell activities than control DCs upon injection in vivo, although they induce lower T-cell responses in vitro. Indeed, a single injection of CD95L-DC prepared from C57BL/6 mice was sufficient to prime bm13 recipients for acute rejection of C57BL/6 skin allografts that were otherwise tolerated in the context of this single weak major histocompatibility complex (MHC) class I incompatibility. Massive neutrophil infiltrates depending on interleukin (IL)-1 signaling were observed at sites of CD95L-DC injection. Experiments in IL-1 receptor-deficient mice or in animals injected with depleting anti-Gr1 monoclonal antibody (mAb) established that neutrophil recruitment is required for the development of vigorous T-cell responses after injection of CD95L-DC in vivo.

Expression of functional Fas ligand in choriocarcinoma

PROBLEM

In the course of pregnancy, fetal trophoblast cells and in that of choriocarcinoma-etiology, trophoblast derived tumor cells, invade the uterine mucosa without causing rejection by decidual leukocytes. Fas ligand (FasL, CD95L, APO-IL), a central regulator of the immune system, has been implicated in the maintenance of immune privileged sites, such as the eye, the testis and the pregnant uterus by inducing apoptosis in activated infiltrating leukocytes. In normal pregnancy FasL, which is expressed by trophoblast cells, appears to contribute to the immune privilege of the pregnant uterus. As choriocarcinoma derives from trophoblast we wanted to assess the expression of FasL in this tissue.

METHOD OF STUDY

Immunohistochemistry, immunofluorescence, TUNEL-assay, Western blotting, coculture experiments and flourescence-associated cell sorter-analysis were the techniques used.

RESULTS

Expression of FasL was found on cells of choriocarcinoma in paraffin sections in situ and on three choriocarcinoma cell lines such as JEG-3, JAR and BeWo. These results were confirmed by Western blotting. In coculture experiments choriocarcinoma cells induced apoptosis in a Jurkat cell line - sensitive to FasL mediated killing. However, when the Jurkat cells were pre-incubated with a Fas-blocking monoclonal antibody, apoptosis was abolished to a great extent.

CONCLUSION

Our findings show that choriocarcinoma cells express FasL and this aforementioned molecule is biologically active. We assume that FasL expression on choriocarcinoma cells may contribute to control of anti-tumor responses by inducing apoptosis in activated Fas bearing leukocytes.

Apoptosis-related proteins, BCL-2, BAX, FAS, FAS-L and PCNA in liver biopsies of patients with chronic hepatitis B virus infection

While the elimination of hepatitis B virus (HBV) is a common phenomenon at the end of the acute phase of disease, the persistence of HBV is characteristic for chronic hepatitis (CHB). Recent evidence indicates that the elimination of HBV is achieved by FAS/FAS-L induced apoptosis of infected hepatocytes. The aim of this study was to test the hypothesis that HBV persistence in the hepatocytes of CHB patients is due to the delayed onset of apoptosis caused by altered FAS/FAS-L interactions between lymphocytes and hepatocytes. The expression of FAS, FAS-L, BAX, BCL-2, ICE and PCNA in the liver biopsies of 55 patients (14 HBsAg positive, 20 patients with alcoholic hepatopathy, 21 patients with other hepatopathies) was tested by immunohistochemistry. Apoptosis of hepatocytes was evaluated by morphological as well as by TUNEL method. The results were correlated with a grading/staging score and analysed statistically using a one way analysis of variance and the Duncan test. Significantly highernumbers of BAX positive hepatocytes were observed in HBsAg positive patients when compared to control groups. Similarly, both BAX and FAS positive lymphocytes were more frequent in HBsAg positive patients. FAS-L positive lymphocytes and hepatocytes were numerous in all patient groups. Increased numbers of BAX positive hepatocytes in CHB may reflect the increased readiness of these cells to undergo apoptosis. However, the increased numbers of both BAX and FAS positive lymphocytes in CHB suggest that these cells may be particularly sensitive to FAS-L mediated apoptosis potentially resulting in lowered viability of these lymphocytes. This may explain, at least in part, the defective removal of virus-infected cells in chronic hepatitis. However, we cannot rule out the possibility that survival of hepatocytes during CHB may be due to other mechanisms such as defects in apoptosis activation triggered by CD40, defects involving DNase and/or other caspases downstream in the apoptotic cascade within these cells, or to defects in CTL function.

Selective upregulation of fibroblast Fas ligand expression, and prolongation of Fas/Fas ligand-mediated skin allograft survival, by retinoic acid: the skin as a retinoide-inducible immune privilege site

Fas/Fas ligand-mediated lymphocyte apoptosis has been implicated in the suppression of immune responses and may cause immune privilege. Human corneas exhibit immune privilege and can be transplanted across allogeneic barriers without immunosuppressive therapy, perhaps, because corneal keratinocytes express Fas ligand. To characterize Fas and Fas ligand expression in skin, we examined expression by murine keratinocytes, dermal fibroblasts, melanocytes, and human umbilical endothelial cells. We also studied the regulation of Fas and Fas ligand in skin cells by retinoic acid, vitamin D3, and dexamethasone as well as various cytokines. Among the molecules and cells tested, retinoic acid selectively upregulated the expression of Fas ligand molecule by fibroblasts. Retinoic acid-induced Fas ligand+ fibroblasts killed Fas+ target cells, and this killing was blocked by anti-Fas ligand antibody. The function of Fas ligand on dermal fibroblasts in vivo was tested in a cutaneous allograft system. Histoincompatible BALB/C mouse (H-2d) donor skin was grafted on to allogeneic C57BL/6 mice (H-2b). Daily local injection of retinoic acid blocked inflammation and extended graft survival for more than 10 d. Injection of retinoic acid into Fas ligand mutated gld/gld donor skin did not prevent leukocyte infiltration into the allograft or prolong graft survival. These experiments indicate that, in skin, retinoic acid selectively increases Fas ligand expression by fibroblasts and that retinoic acid has potent Fas/Fas ligand-dependent immunosuppressive activity.

Not so Fas: Re-evaluating the mechanisms of immune privilege and tumor escape

Based on early studies, it was hypothesized that expression of Fas ligand (FasL) by tumor cells enabled them to counterattack the immune system, and that transplant rejection could be prevented by expressing FasL on transplanted organs. More recent studies have indicated that the notion of FasL as a mediator of immune privilege needed to be reconsidered, and taught a valuable lesson about making broad conclusions based on small amounts of data.

Programmed cell death

Programmed cell death (PCD) is currently one of the most intensively studied areas in cell biology. Substantial evidence now exists demonstrating the integral role of PCD in many fundamental immunologic processes; therefore, understanding the mechanisms of PCD may provide advances with broad implications in immunobiology. This Overview provides a definition of PCD, a description of known PCD biochemical pathways, and finally a discussion of the implications of PCD in transplantation.

Prolonged allograft survival in cynomolgus monkeys treated with a monoclonal antibody to the human type I interferon receptor and low doses of cyclosporine

A monoclonal antibody (mAb) directed against the extracellular domain of the IFNAR1 chain of the human interferon-alpha (IFN-alpha) receptor (IFN-alphaR), which inhibits activation of the Jak-Stat signal transduction pathway, administered together with a subeffective dose of cyclosporine induced prolonged survival of skin allografts in major histocompatibility complex (MHC) divergent cynomolgus monkeys. Skin biopsies from animals treated with anti-IFN-alphaR mAb and cyclosporine revealed very low levels of MHC class I and class II antigen expression and the absence of histologic signs of rejection. Monkey antibodies (IgG) to the mouse antihuman IFN-alphaR mAb were not detected in the serum of any of the animals treated with the anti-IFN-alphaR mAb either alone or together with cyclosporine. The anti-IFN-alphaR mAb abrogated activation of the Jak-Stat signal transduction pathway in IFN-treated cells. These results, which show that selective and long-lasting immunosuppression can be obtained by short-term administration of an IFN-alpha antagonist together with a subeffective dose of cyclosporine, may have important implications for the therapy of human allograft rejection.

Transgenic expression of CD95 ligand on islet beta cells induces a granulocytic infiltration but does not confer immune privilege upon islet allografts

Binding of CD95 (Fas/APO-1) by its ligand (CD95L) commonly induces apoptosis. Apoptosis of activated T cells, induced by CD95L expressed in the rodent testis, has been proposed to be the mechanism of immune privilege [Bellgrau, D., Gold, D., Selawry, H., Moore, J., Franzusoff, A. & Duke, R. C. (1995) Nature (London) 377, 630-632]. To test whether CD95L could protect pancreatic islet grafts from rejection, we made transgenic mice expressing murine CD95L on their islet beta cells and transplanted fetal pancreata under the kidney capsules of allogeneic animals. Expression of CD95L failed to protect the grafts from rejection. However, transgenic mice developed a granulocytic infiltration in their pancreata. These results demonstrate a pro-inflammatory function of CD95L and suggest that expression of CD95L may not be sufficient to protect organ allografts.