Apoptosis prevention as a mechanism of immune evasion

Roles of Non-Coding RNAs in Virus-Host Interaction About Pathogenesis of Hand-Foot-Mouth Disease

Noncoding RNAs (ncRNAs) represent the largest and main transcriptome products and play various roles in the biological activity of cells and pathological processes. Accumulating evidence shows that microRNA (miRNA), long non-coding RNA (lncRNA), and circular RNA (circRNA) are important ncRNAs that play vital regulatory roles during viral infection. Hand-foot-mouth disease (HFMD) virus causes hand-foot-mouth disease, and is also associated with various serious complications and high mortality. However, there is currently no effective treatment. In this review, we focus on advances in the understanding of the modulatory role of ncRNAs during HFMD virus infection. Specifically, we discuss the generation, classification, and regulatory mechanisms of miRNA, lncRNA, and circRNA in the interaction between virus and host, with a particular focus on their influence with viral replication and infection. Analysis of these underlying mechanisms can help provide a foundation for the development of ncRNA-based antiviral therapies.

Engagement of gcFKBP5/TRAF2 by spring viremia of carp virus to promote host cell apoptosis for supporting viral replication in grass carp

Spring viremia of carp virus (SVCV) causes severe morbidity and mortality in grass carp (Ctenopharyngodon idellus) in Europe, America and several Asian countries. We found that FKBP5 (FK506-binding protein 5) is an SVCV infection response factor; however, its role in the innate immune mechanism caused by SVCV infection remains unknown. This study cloned gcFKBP5 (grass carp FKBP5) and made its mimic protein structure for function discussion. We found that gcFKBP5 expression in the primary innate immune organs of grass carp, including intestine, liver and spleen, was highly upregulated by SVCV in 24 h, with a similar result in fish cells by poly(I:C) treatment. gcFKBP overexpression aggravates viral damage to cells and increases viral replication. Furthermore, SVCV engages gcFKBP5 interacting with TRAF2 (tumour necrosis factor receptor-associated factor 2) to promote host cell apoptosis for supporting viral replication. The enhanced viral replication seems not to be due to the repression of IFN and other antiviral factors as expected. For the first time, these data show the pivotal role of gcFKBP5 in the innate immune response of grass carp to SVCV infection.

Host relieves lnc-IRAK3-3-sequestered miR-891b to attenuate apoptosis in Enterovirus 71 infection

Enterovirus 71 (EV71) is an emerging life-threatening pathogen particularly in the Asia-Pacific region. Apoptosis is a major pathogenic feature in EV71 infection. However, which molecular mechanism participating in EV71-induced apoptosis is not completely understood. Long noncoding RNAs (lncRNAs), a newly discovered class of regulatory RNA molecules, govern a wide range of biological functions through multiple regulatory mechanisms. Whether lncRNAs involved in EV71-induced apoptosis was investigated in this study. We conducted an apoptosis-oriented approach by integrating lncRNA and mRNA profilings. lnc-IRAK3-3 is down-regulated in EV71 infection and plays an important role in EV71 infection-induced apoptosis. Compensation of lnc-IRAK3-3 in EV71 infection promoted cell apoptosis wherein GADD45β expression was increased and further triggered caspase3 and PARP cleavage. Using bioinformatics analysis and functional assays, lnc-IRAK3-3 could functionally sequester miR-891b and GADD45β 3'UTR whereas miR-891b showed the inhibitory activity on GADD45β expression. Taken together, lnc-IRAK3-3 has the ability capturing miR-891b to enforce GADD45β expression and eventually promotes apoptosis. On the contrary, host cells suppress lnc-IRAK3-3 to relieve lnc-IRAK3-3-sequestered miR-891b, restrain GADD45β, and attenuate apoptosis in EV71 infection that prevent host cells from severe damages. We discover a new molecular mechanism by which host cells counteract EV71-induced apoptosis through the lnc-IRAK3-3/miR-891b/GADD45β axis partially.

Activation of Interleukin-1β Release by the Classical Swine Fever Virus Is Dependent on the NLRP3 Inflammasome, Which Affects Virus Growth in Monocytes

Classical swine fever virus (CSFV) is a classic Flavivirus that causes the acute, febrile, and highly contagious disease known as classical swine fever (CSF). Inflammasomes are molecular platforms that trigger the maturation of proinflammatory cytokines to engage innate immune defenses that are induced upon cellular infection or stress. However, the relationship between the inflammasome and CSFV infection has not been thoroughly characterized. To understand the function of the inflammasome response to CSFV infection, we infected porcine peripheral blood monocytes (PBMCs) with CSFV. Our results indicated that CSFV infection induced both the generation of pro-interleukin-1β (pro-IL-1β) and its processing in monocytes, leading to the maturation and secretion of IL-1β through the activation of caspase 1. Moreover, CSFV infection in PBMCs induced the production and cleavage of gasdermin D (GSDMD), which is an inducer of pyroptosis. Additional studies showed that CSFV-induced IL-1β secretion was mediated by NLRP3 and that CSFV infection could sufficiently activate the assembly of the NLRP3 inflammasome in monocytes. These results revealed that CSFV infection inhibited the expression of NLRP3, and knockdown of NLRP3 enhanced the replication of CSFV. In conclusion, these findings demonstrate that the NLRP3 inflammasome plays an important role in the innate immune response to CSFV infection.

Shrimp hemocyte homeostasis-associated protein (PmHHAP) interacts with WSSV134 to control apoptosis in white spot syndrome virus infection

Hemocyte homeostasis-associated protein (PmHHAP) was first identified as a viral-responsive gene, due to a high upregulation in transcription following white spot syndrome virus (WSSV) infection. Functional studies using RNA interference have suggested that PmHHAP is involved in hemocyte homeostasis by controlling apoptosis during WSSV infection. In this study, the role of PmHHAP in host-viral interactions was further investigated. Yeast two-hybrid assay and co-immunoprecipitation revealed that PmHHAP binds to an anti-apoptosis protein, WSSV134. The viral protein WSSV134 is a late protein of WSSV, expressed 24 h post infection (hpi). Gene silencing of WSSV134 in WSSV-infected shrimp resulted in a reduction of the expression level of the viral replication marker genes VP28, wsv477, and ie-1, which suggests that WSSV134 is likely involved in viral propagation. However, co-silencing of PmHHAP and WSSV134 counteracted the effects on WSSV infection, which implies the importance of the host-pathogen interaction between PmHHAP and WSSV134 in WSSV infection. In addition, caspase 3/7 activity was noticeably induced in the PmHHAP and WSSV134 co-silenced shrimp upon WSSV infection. Moreover, PmHHAP and WSSV134 inhibited caspase-induced activation of PmCasp in vitro in a non-competitive manner. Taken together, these results suggest that PmHHAP and WSSV134 play a role in the host-pathogen interaction and work concordantly to control apoptosis in WSSV infection.

MicroRNA and Pathogenesis of Enterovirus Infection

There are no currently available specific antiviral therapies for non-polio Enterovirus infections. Although several vaccines have entered clinical trials, the efficacy requires further evaluation, particularly for cross-strain protective activity. Curing patients with viral infections is a public health problem due to antigen alterations and drug resistance caused by the high genomic mutation rate. To conquer these limits in the development of anti-Enterovirus treatments, a comprehensive understanding of the interactions between Enterovirus and host cells is urgently needed. MicroRNA (miRNA) constitutes the biggest family of gene regulators in mammalian cells and regulates almost a half of all human genes. The roles of miRNAs in Enterovirus pathogenesis have recently begun to be noted. In this review, we shed light on recent advances in the understanding of Enterovirus infection-modulated miRNAs. The impacts of altered host miRNAs on cellular processes, including immune escape, apoptosis, signal transduction, shutdown of host protein synthesis and viral replication, are discussed. Finally, miRNA-based medication provides a promising strategy for the development of antiviral therapy.

Chlamydial plasmid-encoded virulence factor Pgp3 neutralizes the antichlamydial activity of human cathelicidin LL-37

Chlamydia trachomatis infection in the lower genital tract can ascend to and cause pathologies in the upper genital tract, potentially leading to severe complications, such as tubal infertility. However, chlamydial organisms depleted of plasmid or deficient in the plasmid-encoded Pgp3 are attenuated in ascending infection and no longer are able to induce the upper genital tract pathologies, indicating a significant role of Pgp3 in chlamydial pathogenesis. We now report that C. trachomatis Pgp3 can neutralize the antichlamydial activity of human cathelicidin LL-37, a host antimicrobial peptide secreted by both genital tract epithelial cells and infiltrating neutrophils. Pgp3 bound to and formed stable complexes with LL-37. We further showed that the middle region of Pgp3 (Pgp3m) was responsible for both the binding to and neutralization of LL-37, suggesting that Pgp3m can be targeted for attenuating chlamydial pathogenicity or developed for blocking LL-37-involved non-genital-tract pathologies, such as rosacea and psoriasis. Thus, the current study has provided significant information for both understanding the mechanisms of chlamydial pathogenesis and developing novel therapeutic agents.

miR-146a and miR-370 coordinate enterovirus 71-induced cell apoptosis through targeting SOS1 and GADD45β

Enterovirus 71 (EV71) is an emerging life-threatening pathogen particularly in the Asia-Pacific region. The major pathogenic feature in EV71-infected cells is apoptosis. However, which molecular mechanism mainly contributes to EV71-induced apoptosis is not investigated thoroughly. MicroRNAs (MiRNAs), the newly discovered molecules, govern a wide range of biological functions through post-transcriptional regulation including viral pathogenesis. Whether miRNAs and messenger RNAs (mRNAs) coordinate to trigger host cell apoptosis in EV71 infection was investigated in this study. We conducted an apoptosis-oriented approach using both mRNA and miRNA profiling and bioinformatic analysis. We identified two major apoptosis-associated signalling pathways, Bcl2 antagonist of cell death (BAD) phosphorylation and p53-dependent apoptosis pathways, in which Son of sevenless homolog 1 (SOS1) and Growth arrest and DNA damage-inducible protein 45β (GADD45β) were predicted as targets of miR-146a and miR-370 respectively. Luciferase reporter assays and Western blots demonstrated the negative regulation between miR-146a and SOS1 and between miR-370 and GADD45β. Silencing of miR-146a restored SOS1 expression and partially attenuated EV71 infection-induced apoptosis. Conversely, ectopic expression of miR-370 decreased virus infection-induced GADD45β expression and also diminished apoptosis. Finally, the transfection of antagomiR-146a and miR-370 contributed to attenuating EV71 infection-induced apoptosis. Herein we clearly demonstrate that EV71-induced cell apoptosis is partly governed by altered miRNAs.

Herpes simplex virus type 1 (HSV-1)-induced apoptosis in human dendritic cells as a result of downregulation of cellular FLICE-inhibitory protein and reduced expression of HSV-1 antiapoptotic latency-associated transcript sequences

Herpes simplex virus type 1 (HSV-1) is one of the most frequent and successful human pathogens. It targets immature dendritic cells (iDCs) to interfere with the antiviral immune response. The mechanisms underlying apoptosis of HSV-1-infected iDCs are not fully understood. Previously, we have shown that HSV-1-induced apoptosis of iDCs is associated with downregulation of the cellular FLICE-inhibitory protein (c-FLIP), a potent inhibitor of caspase-8-mediated apoptosis. In this study, we prove that HSV-1 induces degradation of c-FLIP in a proteasome-independent manner. In addition, by using c-FLIP-specific small interfering RNA (siRNA) we show for the first time that downregulation of c-FLIP expression is sufficient to drive uninfected iDCs into apoptosis, underlining the importance of this molecule for iDC survival. Surprisingly, we also observed virus-induced c-FLIP downregulation in epithelial cells and many other cell types that do not undergo apoptosis after HSV-1 infection. Microarray analyses revealed that HSV-1-encoded latency-associated transcript (LAT) sequences, which can substitute for c-FLIP as an inhibitor of caspase-8-mediated apoptosis, are much less abundant in iDCs as compared to epithelial cells. Finally, iDCs infected with an HSV-1 LAT knockout mutant showed increased apoptosis when compared to iDCs infected with the corresponding wild-type HSV-1. Taken together, our results demonstrate that apoptosis of HSV-1-infected iDCs requires both c-FLIP downregulation and diminished expression of viral LAT.

Killing me softly: chlamydial use of proteolysis for evading host defenses

Chlamydial infections in humans cause severe health problems, including blinding trachoma and sexually transmitted diseases. Although the involved pathogenic mechanisms remain unclear, the ability to replicate and maintain long-term residence in the infected cells seems to significantly contribute to chlamydial pathogenicity. These obligate intracellular parasites maintain a delicate balance between exploiting and protecting their host: they occupy intracellular space and acquire nutrients from the infected cells, but at the same time they have to maintain the integrity of the host cells for the completion of their intracellular growth. For this purpose, chlamydiae hijack certain signaling pathways that prevent the host cells from undergoing apoptosis induced by intracellular stress and protect the infected cells from recognition and attack by host defenses. Interestingly, one of the strategies that chlamydiae use for these purposes is the induction of limited proteolysis of host proteins, which is the main focus of this article.

Antiviral immunity in crustaceans

Viral diseases of shrimp have caused negative effects on the economy in several countries in Asia, South America and America, where they have numerous shrimp culture industries. The studies on the immunity of shrimp and other crustaceans have mainly focused on general aspects of immunity and as a consequence little is known about the antiviral responses in crustaceans. The aim of this review is to update recent knowledge of innate immunity against viral infections in crustaceans. Several antiviral molecules have been isolated and characterized recently from decapods. Characterization and identification of these molecules might provide a promising strategy for protection and treatment of these viral diseases. In addition dsRNA-induced antiviral immunity is also included.

Functional impairment of PRRSV-specific peripheral CD3+CD8high cells

The replication of porcine reproductive and respiratory syndrome virus (PRRSV) in lungs and lymphoid tissues of PRRSV-infected pigs is already strongly reduced before the appearance of neutralizing antibodies, indicating that other immune mechanisms are involved in eliminating PRRSV at those sites. This study aimed to determine whether PRRSV Lelystad virus (LV)-specific cytotoxic T-lymphocytes (CTL) can efficiently eliminate PRRSV-infected alveolar macrophages. Therefore, CTL assays were performed with PRRSV-infected alveolar macrophages as target cells and autologous peripheral blood mononuclear cells (PBMC) from PRRSV-infected pigs as a source of PRRSV-specific CTL. PBMC of 3 PRRSV-infected pigs were used either directly in CTL assays, or following restimulation in vitro. CTL assays with pseudorabies virus (PRV) Begonia-infected alveolar macrophages and autologous PBMC, from 2 PRV Begonia-inoculated pigs, were performed for validation of the assays. In freshly isolated PBMC, derived from PRRSV-infected pigs, CTL activity towards PRRSV-infected macrophages was not detected until the end of the experiment (56 days post infection-dpi). Restimulating the PBMC with PRRSV in vitro resulted in proliferation of CD3+CD8high cells starting from 14 dpi. Although CD+CD8high cells are generally considered to be CTL, CTL activity was not detected in PRRSV-restimulated PBMC of the 3 pigs until 49 dpi. A weak PRRSV-specific CTL activity was observed only at 56 dpi in PRRSV-restimulated PBMC of one pig. In contrast, a clear CTL activity was observed in PRV Begonia-restimulated PBMC, derived from PRV Begonia-infected pigs, starting from 21 dpi. This study indicates that PBMC of PRRSV-infected pigs contain proliferating CD3+CD8high cells upon restimulation in vitro, but these PBMC fail to exert CTL activity towards PRRSV-infected alveolar macrophages.

Antigen-mediated T cell expansion regulated by parallel pathways of death

T cells enigmatically require caspase-8, an inducer of apoptosis, for antigen-driven expansion and effective antiviral responses, and yet the pathways responsible for this effect have been elusive. A defect in caspase-8 expression does not affect progression through the cell cycle but causes an abnormally high rate of cell death that is distinct from apoptosis and does not involve a loss of NFkappaB activation. Instead, antigen or mitogen activated Casp8-deficient T cells exhibit an alternative type of cell death similar to programmed necrosis that depends on receptor interacting protein (Ripk1). The selective genetic ablation of caspase-8, NFkappaB, and Ripk1, reveals two forms of cell death that can regulate virus-specific T cell expansion.

Involvement of gD/HVEM interaction in NF-kB-dependent inhibition of apoptosis by HSV-1 gD

In the present paper, we aimed to verify whether the interaction of the glycoprotein D (gD) of herpes simplex 1 (HSV-1) with the HSV-1 receptor HVEM is involved in NF-kappaB-dependent protection against apoptosis by gD. To this purpose, first we utilized MAbs that interfere with gD/HVEM interaction and U937 cells that naturally express human HVEM on their surface. Pre-incubation with these MAbs, but not with a control antibody, partially reverted the protection of infectious HSV-1 towards anti-Fas induced apoptosis in U937 cells. Similarly, pre-incubation of UV-inactivated HSV-1 (UV-HSV-1) or recombinant gD with the same MAbs, significantly reduced the inhibition of Fas-mediated apoptosis by UV-HSV-1 or gD, respectively, in U937 cells. Moreover, coculture with stable transfectants expressing at surface level wild type gD protected U937 cells against Fas-induced apoptosis, while coculture with transfectants expressing a mutated form of gD, incapable to bind HVEM, did not protect. Finally, UV-HSV-1 protected against staurosporine-induced apoptosis in U937 cells as well as in the CHO transfectants expressing human HVEM on their surface, but not in the control CHO transfectants, which did not express HVEM. These results suggest that signaling triggered by binding of gD to HVEM could represent an additional mechanism of evasion from premature apoptotic death exerted by HSV-1-gD in HVEM-expressing cells, disclosing new opportunities of cell death manipulation by using gD preparations.

Flow cytometric detection of activated caspase-3

Apoptosis (programmed cell death) is an active process that plays a critical role in multiple biologic processes from embryologic development, to lymphocyte development and selection, and homeostasis. The two major mechanisms of cell death are referred to as the intrinsic and extrinsic pathways. These pathways lead to a cascade of events that ultimately converge to the activation of an effector enzyme, caspase-3. Caspase-3 is a cysteine protease with aspartic specificity and a well-characterized effector of apoptosis or programmed cell death signaling. The pro-form of caspase-3 (p32 caspase-3) is sequestered as a zymogen, where upon proteolysis at a conserved DEVD sequence, is converted to the active (p17 caspase-3) enzyme capable of disassembling the cell. Cell death can become disregulated under various conditions and multiple disease states (e.g., viral infection, carcinogenesis, and metastasis). Sensitive and reproducible detection of active caspase-3 is critical to advance the understanding of cellular functions and multiple pathologies of various etiologies. Here, we provide two simple and reproducible methods to measure active caspase-3 in multiple cell types and conditions using a flow cytometric-based analysis.

Herpes simplex virus blocks Fas-mediated apoptosis independent of viral activation of NF-kappaB in human epithelial HEp-2 cells

The goal of our study was to characterize the apoptotic response of herpes simplex virus (HSV)-infected, human epithelial HEp-2 cells to extrinsic treatments through the Fas receptor. Initially, we defined the Fas response of these cells. We found the following: (1) Treatment of HEp-2 cells with anti-Fas antibody or Fas ligand (FasL) alone did not induce apoptosis. (2) In addition, these inducers did not activate NF-kappaB in these cells. (3) The addition of cycloheximide (CHX) during these treatments caused a dramatic increase in programmed cell death. (4) HEp-2 cells infected with HSV for 6 h prior to anti-Fas plus CHX treatment were nonapoptotic, and (5) these cells possessed nuclear NFkappaB. (6) HSV blocked anti-Fas or FasL plus CHX-induced apoptosis in HEp-2 cells that stably expressed a dominant-negative form of IkappaBalpha. These results indicate that HSV infection can block the process of Fas-mediated apoptosis through a mechanism that is independent of viral activation of NFkappaB. Our findings help define the molecular mechanisms involved in HSV evasion of the cytokine-driven, innate immune response in human epithelial cells.

Interferon-mediated immunopathological events are associated with atypical innate and adaptive immune responses in patients with severe acute respiratory syndrome

It is not understood how immune inflammation influences the pathogenesis of severe acute respiratory syndrome (SARS). One area of strong controversy is the role of interferon (IFN) responses in the natural history of SARS. The fact that the majority of SARS patients recover after relatively moderate illness suggests that the prevailing notion of deficient type I IFN-mediated immunity, with hypercytokinemia driving a poor clinical course, is oversimplified. We used proteomic and genomic technology to systematically analyze host innate and adaptive immune responses of 40 clinically well-described patients with SARS during discrete phases of illness from the onset of symptoms to discharge or a fatal outcome. A novel signature of high IFN-alpha, IFN-gamma, and IFN-stimulated chemokine levels, plus robust antiviral IFN-stimulated gene (ISG) expression, accompanied early SARS sequelae. As acute illness progressed, SARS patients entered a crisis phase linked to oxygen saturation profiles. The majority of SARS patients resolved IFN responses at crisis and expressed adaptive immune genes. In contrast, patients with poor outcomes showed deviated ISG and immunoglobulin gene expression levels, persistent chemokine levels, and deficient anti-SARS spike antibody production. We contend that unregulated IFN responses during acute-phase SARS may culminate in a malfunction of the switch from innate immunity to adaptive immunity. The potential for the use of the gene signatures we describe in this study to better assess the immunopathology and clinical management of severe viral infections, such as SARS and avian influenza (H5N1), is therefore worth careful examination.

Apoptosis and antigen receptor function in T and B cells following exposure to herpes simplex virus

T cells are an essential component of the immune response against herpes simplex virus (HSV) infection. We previously reported that incubation of T cells with HSV-infected fibroblasts inhibits subsequent T cell antigen receptor signal transduction. In the current study, we found that incubation of T cells with HSV-infected fibroblasts also leads to apoptosis in exposed T cells. Apoptosis was observed in Jurkat cells, a T cell leukemia line, and also in CD4(+) cells isolated from human peripheral blood mononuclear cells. Direct infection of these cells with HSV also resulted in apoptosis. Clinical isolates of both HSV type 1 and 2 induced apoptosis in infected T cells at comparable levels to cells infected with laboratory strains of HSV, suggesting an immune evasion mechanism that may be clinically relevant. Further understanding of these viral immune evasion mechanisms could be exploited for better management of HSV infection.

Differences and similarities in viral life cycle progression and host cell physiology after infection of human dendritic cells with modified vaccinia virus Ankara and vaccinia virus

Modified vaccinia virus Ankara (MVA) is an attenuated strain of vaccinia virus (VV) that has attracted significant attention as a candidate viral vector vaccine for immunization against infectious diseases and treatment of malignancies. Although MVA is unable to replicate in most nonavian cells, vaccination with MVA elicits immune responses that approximate those seen after the administration of replication-competent strains of VV. However, the mechanisms by which these viruses elicit immune responses and the determinants of their relative immunogenicity are incompletely understood. Studying the interactions of VV and MVA with cells of the human immune system may elucidate these mechanisms, as well as provide a rational basis for the further enhancement of the immunogenicity of recombinant MVA vectors. Toward this end, we investigated the consequences of MVA or VV infection of human dendritic cells (DCs), key professional antigen-presenting cells essential for the generation of immune responses. We determined that a block to the formation of intracellular viral replication centers results in abortive infection of DCs with both VV and MVA. MVA inhibited cellular protein synthesis more rapidly than VV and displayed a distinct pattern of viral protein expression in infected DCs. MVA also induced apoptosis in DCs more rapidly than VV, and DC apoptosis after MVA infection was associated with an accelerated decline in the levels of intracellular Bcl-2 and Bcl-X(L). These findings suggest that antigen presentation pathways may contribute differentially to the immunogenicity of VV and MVA and that targeted modifications of virus-induced DC apoptosis may further increase the immunogenicity of MVA-vectored vaccines.

Caspase inhibition in apoptotic T cells triggers necrotic cell death depending on the cell type and the proapoptotic stimulus

CD95 (Fas/Apo-1) triggers apoptotic cell death via a caspase-dependent pathway. Inhibition of caspase activation blocks proapoptotic signaling and thus, prevents execution of apoptosis. Besides induction of apoptotic cell death, CD95 has been reported to trigger necrotic cell death in susceptible cells. In this study, we investigated the interplay between apoptotic and necrotic cell death signaling in T cells. Using the agonistic CD95 antibody, 7C11, we found that caspase inhibition mediated by the pancaspase inhibitor, zVAD-fmk, prevented CD95-triggered cell death in Jurkat T cells but not in A3.01 T cells, although typical hallmarks of apoptosis, such as DNA fragmentation or caspase activation were blocked. Moreover, the caspase-independent cell death in A3.01 cells exhibited typical signs of necrosis as detected by a rapid loss of cell membrane integrity and could be prevented by treatment with the radical scavenger butylated hydroxyanisole (BHA). Similar to CD95-induced cell death, apoptosis triggered by the DNA topoisomerase inhibitors, camptothecin or etoposide was shifted to necrosis when capsase activation was inhibited. In contrast to this, ZVAD was fully protective when apoptosis was triggered by the serpase inhibitor, Nalpha-tosyl-phenyl-chloromethyl ketone (TPCK). TPCK was not protective when administered to anti-CD95/ZVAD-treated A3.01 cells, indicating that TPCK does not possess anti-necrotic activity but fails to activate the necrotic death pathway. Our findings show (a) that caspase inhibition does not always protect apoptotic T cells from dying but merely activates a caspase-independent mode of cell death that results in necrosis and (b) that the caspase-inhibitor-induced shift from apoptotic to necrotic cell death is dependent on the cell type and the proapoptotic stimulus.

The evolution of immune mechanisms

From early on in evolution, organisms have had to protect themselves from pathogens. Mechanisms for discriminating "self" from "non-self" evolved to accomplish this task, launching a long history of host-pathogen co-evolution. Evolution of mechanisms of immune defense has resulted in a variety of strategies. Even unicellular organisms have rich arsenals of mechanisms for protection, such as restriction endonucleases, antimicrobial peptides, and RNA interference. In multicellular organisms, specialized immune cells have evolved, capable of recognition, phagocytosis, and killing of foreign cells as well as removing their own cells changed by damage, senescence, infection, or cancer. Additional humoral factors, such as the complement cascade, have developed that co-operate with cellular immunity in fighting infection and maintaining homeostasis. Defensive mechanisms based on germline-encoded receptors constitute a system known as innate immunity. In jaw vertebrates, this system is supplemented with a second system, adaptive immunity, which in contrast to innate immunity is based on diversification of immune receptors and on immunological memory in each individual.Usually, each newly evolved defense mechanism did not replace the previous one, but supplemented it, resulting in a layered structure of the immune system. The immune system is not one system but rather a sophisticated network of various defensive mechanisms operating on different levels, ranging from mechanisms common for every cell in the body to specialized immune cells and responses at the level of the whole organism. Adaptive changes in pathogens have shaped the evolution of the immune system at all levels.