A novel checkpoint in the Bcl-2-regulated apoptotic pathway revealed by murine cytomegalovirus infection of dendritic cells

Sensitization with vaccinia virus encoding H5N1 hemagglutinin restores immune potential against H5N1 influenza virus

H5N1 highly pathogenic avian influenza (H5N1 HPAI) virus causes elevated mortality compared with seasonal influenza viruses like H1N1 pandemic influenza (H1N1 pdm) virus. We identified a mechanism associated with the severe symptoms seen with H5N1 HPAI virus infection. H5N1 HPAI virus infection induced a decrease of dendritic cell number in the splenic extrafollicular T-cell zone and impaired formation of the outer layers of B-cell follicles, resulting in insufficient levels of antibody production after infection. However, in animals vaccinated with a live recombinant vaccinia virus expressing the H5 hemagglutinin, infection with H5N1 HPAI virus induced parafollicular dendritic cell accumulation and efficient antibody production. These results indicate that a recombinant vaccinia encoding H5 hemagglutinin gene does not impair dendritic cell recruitment and can be a useful vaccine candidate.

Deficient NLRP3 and AIM2 Inflammasome Function in Autoimmune NZB Mice

Inflammasomes are protein complexes that promote caspase activation, resulting in processing of IL-1β and cell death, in response to infection and cellular stresses. Inflammasomes have been anticipated to contribute to autoimmunity. The New Zealand Black (NZB) mouse develops anti-erythrocyte Abs and is a model of autoimmune hemolytic anemia. These mice also develop anti-nuclear Abs typical of lupus. In this article, we show that NZB macrophages have deficient inflammasome responses to a DNA virus and fungal infection. Absent in melanoma 2 (AIM2) inflammasome responses are compromised in NZB by high expression of the AIM 2 antagonist protein p202, and consequently NZB cells had low IL-1β output in response to both transfected DNA and mouse CMV infection. Surprisingly, we also found that a second inflammasome system, mediated by the NLR family, pyrin domain containing 3 (NLRP3) initiating protein, was completely lacking in NZB cells. This was due to a point mutation in an intron of the Nlrp3 gene in NZB mice, which generates a novel splice acceptor site. This leads to incorporation of a pseudoexon with a premature stop codon. The lack of full-length NLRP3 protein results in NZB being effectively null for Nlrp3, with no production of bioactive IL-1β in response to NLRP3 stimuli, including infection with Candida albicans. Thus, this autoimmune strain harbors two inflammasome deficiencies, mediated through quite distinct mechanisms. We hypothesize that the inflammasome deficiencies in NZB alter the interaction of the host with both microflora and pathogens, promoting prolonged production of cytokines that contribute to development of autoantibodies.

BH3 Profiling Reveals Selectivity by Herpesviruses for Specific Bcl-2 Proteins To Mediate Survival of Latently Infected Cells

Herpesviruses, including human cytomegalovirus (HCMV), Epstein-Barr virus (EBV), and Kaposi's sarcoma-associated herpesvirus, establish latency by modulating or mimicking antiapoptotic Bcl-2 proteins to promote survival of carrier cells. BH3 profiling, which assesses the contribution of Bcl-2 proteins towards cellular survival, was able to globally determine the level of dependence on individual cellular and viral Bcl-2 proteins within latently infected cells. Moreover, BH3 profiling predicted the sensitivity of infected cells to small-molecule inhibitors of Bcl-2 proteins.

Identification of cellular proteins that interact with human cytomegalovirus immediate-early protein 1 by protein array assay

Human cytomegalovirus (HCMV) gene expression during infection is characterized as a sequential process including immediate-early (IE), early (E), and late (L)-stage gene expression. The most abundantly expressed gene at the IE stage of infection is the major IE (MIE) gene that produces IE1 and IE2. IE1 has been the focus of study because it is an important protein, not only for viral gene expression but also for viral replication. It is believed that IE1 plays important roles in viral gene regulation by interacting with cellular proteins. In the current study, we performed protein array assays and identified 83 cellular proteins that interact with IE1. Among them, seven are RNA-binding proteins that are important in RNA processing; more than half are nuclear proteins that are involved in gene regulations. Tumorigenesis-related proteins are also found to interact with IE1, implying that the role of IE1 in tumorigenesis might need to be reevaluated. Unexpectedly, cytoplasmic proteins, such as Golgi autoantigen and GGA1 (both related to the Golgi trafficking protein), are also found to be associated with IE1. We also employed a coimmunoprecipitation assay to test the interactions of IE1 and some of the proteins identified in the protein array assays and confirmed that the results from the protein array assays are reliable. Many of the proteins identified by the protein array assay have not been previously reported. Therefore, the functions of the IE1-protein interactions need to be further explored in the future.

MCMV-mediated inhibition of the pro-apoptotic Bak protein is required for optimal in vivo replication

Successful replication and transmission of large DNA viruses such as the cytomegaloviruses (CMV) family of viruses depends on the ability to interfere with multiple aspects of the host immune response. Apoptosis functions as a host innate defence mechanism against viral infection, and the capacity to interfere with this process is essential for the replication of many viruses. The Bcl-2 family of proteins are the principle regulators of apoptosis, with two pro-apoptotic members, Bax and Bak, essential for apoptosis to proceed. The m38.5 protein encoded by murine CMV (MCMV) has been identified as Bax-specific inhibitor of apoptosis. Recently, m41.1, a protein product encoded by the m41 open reading frame (ORF) of MCMV, has been shown to inhibit Bak activity in vitro. Here we show that m41.1 is critical for optimal MCMV replication in vivo. Growth of a m41.1 mutant was attenuated in multiple organs, a defect that was not apparent in Bak^−/− mice. Thus, m41.1 promotes MCMV replication by inhibiting Bak-dependent apoptosis during in vivo infection. The results show that Bax and Bak mediate non-redundant functions during MCMV infection and that the virus produces distinct inhibitors for each protein to counter the activity of these proteins.

The cytomegaloviruses (CMV) are a family of viruses that establish a latent infection that lasts for the life of the host, with the virus able to reactivate when the host is immunosuppressed. We have used murine CMV (MCMV) as a model to understand how CMV interferes with the anti-viral immune response. Apoptosis, or programmed cell death, is one of the defence mechanisms used by multicellular organisms to impair viral infection. In order for viral replication to proceed, many viruses have evolved mechanisms to prevent the apoptosis of infected host cells. Under most circumstances the activation of Bax, or the closely related protein Bak, is required for apoptosis to proceed. The m41.1 protein was recently identified as a candidate Bak inhibitor during in vitro infection. We have generated a mutant virus which is unable to produce the m41.1 protein and found that growth of this virus was attenuated in wild-type mice. Importantly, growth of the mutant virus was equivalent to that of the wild-type virus in mice lacking the Bak protein. These studies establish that m41.1 is an inhibitor of Bak and that the capacity to prevent apoptosis triggered by Bak is required for efficient replication of MCMV in vivo.

Prevention of cellular suicide by cytomegaloviruses

As intracellular parasites, viruses rely on many host cell functions to ensure their replication. The early induction of programmed cell death (PCD) in infected cells constitutes an effective antiviral host mechanism to restrict viral spread within an organism. As a countermeasure, viruses have evolved numerous strategies to interfere with the induction or execution of PCD. Slowly replicating viruses such as the cytomegaloviruses (CMVs) are particularly dependent on sustained cell viability. To preserve viability, the CMVs encode several viral cell death inhibitors that target different key regulators of the extrinsic and intrinsic apoptosis pathways. The best-characterized CMV-encoded inhibitors are the viral inhibitor of caspase-8-induced apoptosis (vICA), viral mitochondrial inhibitor of apoptosis (vMIA), and viral inhibitor of Bak oligomerization (vIBO). Moreover, a viral inhibitor of RIP-mediated signaling (vIRS) that blocks programmed necrosis has been identified in the genome of murine CMV (MCMV), indicating that this cell death mode is a particularly important part of the antiviral host response. This review provides an overview of the known cell death suppressors encoded by CMVs and their mechanisms of action.

Cathepsin C limits acute viral infection independently of NK cell and CD8+ T-cell cytolytic function

Destruction of target cells by cytotoxic T lymphocytes (CTLs) or natural killer (NK) cells requires the coordinated action of the pore forming protein perforin (Pfp) and the granzyme (Gzm) family of serine proteases. The activation of a number of serine proteases, including GzmA and B, is predominately mediated by cathepsin C (CatC). Deficiencies in CatC‐null mice were therefore expected to replicate the defects observed in GzmAB‐deficient mice. We have previously determined that GzmAB‐deficient mice exhibit increased susceptibility to murine cytomegalovirus (MCMV) infection. Here, we have compared the ability of CatC^−/− mice to control MCMV infection with that of GzmAB‐deficient animals. We found that CatC^−/− mice have organ‐specific defects in the ability to control MCMV replication, a phenotype that is distinct to that observed in GzmAB^−/− mice. Significantly, the cytolytic function of CatC‐deficient NK cells and CTLs elicited during infection was indistinguishable from that of wild‐type cells. Hence, CatC is involved in limiting MCMV replication; however, this effect is independent of its role in promoting effector cytolytic activity. These data provide evidence for a novel and unexpected role of CatC during viral infection.

Suicide watch: how cytomegalovirus interferes with the cell-death pathways of infected cells

The cytomegaloviruses (CMVs) are a family of species-specific viruses that have evolved sophisticated methods to interfere with the host's ability to generate innate and adaptive immune responses. In addition, CMVs must guard against another host defence mechanism, namely the induction of apoptosis that results in the elimination of infected cells. The importance of inhibiting cell death to the evolutionary survival of CMVs is underlined by the fact that these viruses encode an array of molecules devoted to interfering with host apoptotic pathways. CMVs have also been recognised for their ability to inhibit non-apoptotic forms of cells death. Recent publications have provided important insights into how some of these CMV-encoded molecules mediate their pro-survival effects, and this review will compare the mechanisms used by various members of the CMV family to prevent the premature death of the host cell. The capacity for some of the virally encoded cell-death inhibitors to mediate effects unrelated to the suppression of cell death will also be discussed.

Virally mediated inhibition of Bax in leukocytes promotes dissemination of murine cytomegalovirus

The evolutionary survival of viruses relies on their ability to disseminate infectious progeny to sites of transmission. The capacity to subvert apoptosis is thought to be crucial for ensuring efficient viral replication in permissive cells, but its role in viral dissemination in vivo has not been considered. We show here that the murine cytomegalovirus (MCMV) m38.5 protein specifically counters the action of Bax. As predicted from our biochemical data, the capacity of m38.5 to inhibit apoptosis is only apparent in cells unable to activate Bak. Deletion of m38.5 resulted in an attenuated growth of MCMV in vitro. In vivo replication of the Deltam38.5 virus was not significantly impaired in visceral organs. However, m38.5 played a central role in protecting leukocytes from Bax-mediated apoptosis, thereby promoting viral dissemination to the salivary glands, the principal site of transmission. These results establish that in vivo MCMV replication induces the activation of Bax in leukocytes, but not other permissive cells, and that MCMV interferes with this process to attain maximum dissemination.

Murine cytomegalovirus m38.5 protein inhibits Bax-mediated cell death

Many viruses encode proteins that inhibit the induction of programmed cell death at the mitochondrial checkpoint. Murine cytomegalovirus (MCMV) encodes the m38.5 protein, which localizes to mitochondria and protects human HeLa cells and fibroblasts from apoptosis triggered by proteasome inhibitors but not from Fas-induced apoptosis. However, the ability of this protein to suppress the apoptosis of murine cells and its role during MCMV infection have not been investigated previously. Here we show that m38.5 is expressed at early time points during MCMV infection. Cells infected with MCMVs lacking m38.5 showed increased sensitivity to cell death induced by staurosporine, MG132, or the viral infection itself compared to the sensitivity of cells infected with wild-type MCMV. This defect was eliminated when an m38.5 or Bcl-X(L) gene was inserted into the genome of a deletion mutant. Using fibroblasts deficient in the proapoptotic Bcl-2 family proteins Bak and/or Bax, we further demonstrated that m38.5 protected from Bax- but not Bak-mediated apoptosis and interacted with Bax in infected cells. These results consolidate the role of m38.5 as a viral mitochondrion-localized inhibitor of apoptosis and its functional similarity to the human cytomegalovirus UL37x1 gene product. Although the m38.5 gene is not homologous to the UL37x1 gene at the sequence level, m38.5 is conserved among rodent cytomegaloviruses. Moreover, the fact that MCMV-infected cells are protected from both Bak- and Bax-mediated cell death suggests that MCMV possesses an additional, as-yet-unidentified mechanism to block Bak-mediated apoptosis.

Cytomegalovirus proteins vMIA and m38.5 link mitochondrial morphogenesis to Bcl-2 family proteins

Apoptosis is a host defense mechanism against viruses that can be subverted by viral gene products. Human cytomegalovirus encodes viral mitochondria-localized inhibitor of apoptosis (vMIA; also known as pUL37x1), which is targeted to mitochondria and functions as a potent cell death suppressor by binding to and inhibiting proapoptotic Bcl-2 family members Bax and Bak. vMIA expression also dramatically alters mitochondrial morphology, causing the fragmentation of these organelles. A potential ortholog of vMIA, m38.5, which was identified in murine cytomegalovirus, has been shown to localize to mitochondria and protect against chemically induced apoptosis by unknown mechanisms. Despite sharing negligible homology with vMIA and no region detectably corresponding to the vMIA Bax-binding domain, we find that m38.5, like vMIA, binds to Bax and recruits Bax to mitochondria. Interestingly, m38.5 and vMIA appear to block Bax downstream of translocation to mitochondria and after an initial stage of Bax conformational change. In contrast to vMIA, m38.5 neither binds to Bak nor causes mitochondrial fragmentation. Consistently with Bax-selective inactivation by m38.5, m38.5 fragments mitochondria in Bak knockout (KO) cells and protects Bak KO cells from apoptosis better than Bax KO cells. Thus, vMIA and m38.5 share some, but not all, features of apoptosis regulation through Bcl-2 family interaction and allow the dissection of Bax translocation into discrete steps.

Cellular mechanisms of neuronal damage from hyperthermia

Hyperthermia can cause brain damage and also exacerbate the brain damage produced by stroke and amphetamines. The developing brain is especially sensitive to hyperthermia. The severity of, and mechanisms underlying, hyperthermia-induced neuronal death depend on both temperature and duration of exposure. Severe hyperthermia can produce necrotic neuronal death. For a window of less severe heat stresses, cultured neurons exhibit a delayed death with apoptotic characteristics including cytochrome c release and caspase activation. Little is known about mechanisms of hyperthermia-induced damage upstream of these late apoptotic effects. This chapter considers several possible upstream mechanisms, drawing on both in vivo and in vitro studies of the nervous system and other tissues. Hyperthermia-induced damage in some non-neuronal cells includes endoplasmic reticular stress due to denaturing of nascent polypeptide chains, as well as nuclear and cytoskeletal damage. Evidence is presented that hyperthermia produces mitochondrial damage, including depolarization, in cultured mammalian neurons.

Lymphoma cell apoptosis in the liver induced by distant murine cytomegalovirus infection

Cytomegalovirus (CMV) poses a threat to the therapy of hematopoietic malignancies by hematopoietic stem cell transplantation, but efficient reconstitution of antiviral immunity prevents CMV organ disease. Tumor relapse originating from a minimal residual leukemia poses another threat. Although a combination of risk factors was supposed to enhance the incidence and severity of transplantation-associated disease, a murine model of a liver-adapted B-cell lymphoma has previously shown a survival benefit and tumor growth inhibition by nonlethal subcutaneous infection with murine CMV. Here we have investigated the underlying antitumoral mechanism. Virus replication proved to be required, since inactivated virions or the highly attenuated enhancerless mutant mCMV-DeltaMIEenh did not impact the lymphoma in the liver. Surprisingly, the dissemination-deficient mutant mCMV-DeltaM36 inhibited tumor growth, even though this virus fails to infect the liver. On the other hand, various strains of herpes simplex viruses consistently failed to control the lymphoma, even though they infect the liver. A quantitative analysis of the tumor growth kinetics identified a transient tumor remission by apoptosis as the antitumoral effector mechanism. Tumor cell colonies with cells surviving the CMV-induced "apoptotic crisis" lead to tumor relapse even in the presence of full-blown tissue infection. Serial transfer of surviving tumor cells did not indicate a selection of apoptosis-resistant genetic variants. NK cell activity of CD49b-expressing cells failed to control the lymphoma upon adoptive transfer. We propose the existence of an innate antitumoral mechanism that is triggered by CMV infection and involves an apoptotic signal effective at a distant site of tumor growth.

Induction of apoptosis limits cytomegalovirus cross-species infection

Cross-species infections are responsible for the majority of emerging and re-emerging viral diseases. However, little is known about the mechanisms that restrict viruses to a certain host species, and the factors viruses need to cross the species barrier and replicate in a different host. Cytomegaloviruses (CMVs) are representatives of the beta-herpesviruses that are highly species specific. They replicate only in cells of their own or a closely related species. In this study, the molecular mechanism underlying the cytomegalovirus species specificity was investigated. We show that infection of human cells with the murine cytomegalovirus (MCMV) triggers the intrinsic apoptosis pathway involving caspase-9 activation. MCMV can break the species barrier and replicate in human cells if apoptosis is blocked by Bcl-2 or a functionally analogous protein. A single gene of the human cytomegalovirus encoding a mitochondrial inhibitor of apoptosis is sufficient to allow MCMV replication in human cells. Moreover, the same principle facilitates replication of the rat cytomegalovirus in human cells. Thus, induction of apoptosis serves as an innate immune defense to inhibit cross-species infections of rodent CMVs.

Myxoma virus M11L blocks apoptosis through inhibition of conformational activation of Bax at the mitochondria

Many viruses inhibit or retard apoptosis, a strategy that subverts one of the most ancient antiviral mechanisms. M11L, a myxoma virus-encoded antiapoptotic protein, has been previously shown to localize to mitochondria and block apoptosis of virus-infected cells (H. Everett, M. Barry, S. F. Lee, X. J. Sun, K. Graham, J. Stone, R. C. Bleackley, and G. McFadden, J. Exp. Med. 191:1487-1498, 2000; H. Everett, M. Barry, X. Sun, S. F. Lee, C. Frantz, L. G. Berthiaume, G. McFadden, and R. C. Bleackley, J. Exp. Med. 196:1127-1139, 2002; and G. Wang, J. W. Barrett, S. H. Nazarian, H. Everett, X. Gao, C. Bleackley, K. Colwill, M. F. Moran, and G. McFadden, J. Virol. 78:7097-7111, 2004). This protection from apoptosis involves constitutive-forming inhibitory complexes with the peripheral benzodiazepine receptor and Bak on the outer mitochondrial membrane. Here, we extend the study to investigate the interference of M11L with Bax activation during the process of apoptosis. Myxoma virus infection triggers an early apoptotic signal that induces rapid Bax translocation from cytoplasm to mitochondria, despite the existence of various viral antiapoptotic proteins. However, in the presence of M11L, the structural activation of Bax at the mitochondrial membrane, which is characterized by the occurrence of a Bax conformational change, is blocked in both M11L-expressing myxoma-infected cells and M11L-transfected cells under apoptotic stimulation. In addition, inducible binding of M11L to the mitochondrially localized Bax is detected in myxoma virus-infected cells and in M11L/Bax-cotransfected cells as measured by immunoprecipitation and tandem affinity purification analysis, respectively. Importantly, this inducible Bax/M11L interaction is independent of Bak, demonstrated by the complete block of Bax-mediated apoptosis in myxoma-infected cells that lack Bak expression. Our findings reveal that myxoma M11L modulates apoptosis by multiple independent strategies which all contribute to the blockade of apoptosis at the mitochondrial checkpoint.

Insights into the mechanisms of CMV‐mediated interference with cellular apoptosis

Apoptosis has the potential to function as a defence mechanism during viral infection. Identification of CMV mutants that cause the apoptotic death of infected cells confirmed that viral infection activates apoptotic pathways and that this process is counteracted by CMV to ensure efficient viral replication. The recent identification of CMV-encoded proteins that suppress cell death has greatly enhanced our understanding of the mechanisms used by this family of viruses to prevent apoptosis. CMV do not encode homologues of known death-suppressing proteins, suggesting that the CMV family has evolved novel, more sophisticated strategies for the inhibition of apoptosis. The identification and characterization of the human CMV (HCMV)-encoded antiapoptotic proteins UL36 (viral inhibitor of caspase-8 activation [vICA]) and UL37 (viral mitochondria-localized inhibitor of apoptosis [vMIA]) have confirmed that CMV target unique apoptotic control points. For example, vMIA inhibits apoptosis by binding Bax and sequestering it at the mitochondrial membrane as an inactive oligomer. This knowledge not only provides a more complete understanding of the CMV replication process but also allows the identification of previously unrecognized apoptotic checkpoints. Because HCMV is an important cause of birth defects and an increasingly important opportunistic pathogen, a firm grasp of the mechanisms by which it affects cellular apoptosis may provide avenues for the design of improved therapeutic strategies. Here, we review the recent progress made in understanding the role of CMV-encoded proteins in the inhibition of apoptosis.

Interaction of F1L with the BH3 domain of Bak is responsible for inhibiting vaccinia-induced apoptosis

Apoptosis represents an important cellular defence mechanism against viral pathogens by virtue of its ability to remove infected cells. Consequently, many viruses have developed numerous strategies to prevent or delay host cell apoptosis in order to achieve productive replication. Here we report that deletion of the F1L gene from the vaccinia genome results in increased apoptosis during infection. We demonstrate that F1L, which has no sequence homology to Bcl-2 family members, inhibits apoptosis at the level of mitochondria by binding to Bak. As a consequence, F1L prevents Bak activation, oligomerization and interaction with active Bax, all critical steps in the induction of apoptosis. We demonstrate that residues 64-84 of F1L interact directly with the Bcl-2 homology domain 3 (BH3) domain of Bak. This region of F1L has limited sequence similarity to known Bak-interacting BH3 domains. We also find that such additional BH3-like domains exist in the vaccinia genome. We conclude that F1L uses this specific, BH3-like domain to bind and inhibit Bak at the mitochondria.

Mitochondrial cell death suppressors carried by human and murine cytomegalovirus confer resistance to proteasome inhibitor-induced apoptosis

Human cytomegalovirus carries a mitochondria-localized inhibitor of apoptosis (vMIA) that is conserved in primate cytomegaloviruses. We find that inactivating mutations within UL37x1, which encodes vMIA, do not substantially affect replication in TownevarATCC (Towne-BAC), a virus that carries a functional copy of the betaherpesvirus-conserved viral inhibitor of caspase 8 activation, the UL36 gene product. In Towne-BAC infection, vMIA reduces susceptibility of infected cells to intrinsic death induced by proteasome inhibition. vMIA is sufficient to confer resistance to proteasome inhibition when expressed independent of viral infection. Murine cytomegalovirus m38.5, whose position in the viral genome is analogous to UL37x1, exhibits mitochondrial association and functions in much the same manner as vMIA in inhibiting intrinsic cell death. This work suggests a common role for vMIA in rodent and primate cytomegaloviruses, modulating the threshold of virus-infected cells to intrinsic cell death.

Cytomegalovirus infection inhibits the expression of N-methyl-D-aspartate receptors in the developing mouse hippocampus and primary neuronal cultures

Cytomegalovirus (CMV) is the most significant infectious cause of developmental brain disorders in humans. The infection occasionally persists and causes neurological disorders. The N-methyl-D-aspartate (NMDA) subtype of glutamate receptors is essential for the development and plasticity of synapses, but also is involved in neuronal excitotoxicity during viral infection. Here we investigated the effects of murine CMV (MCMV) infection on the expression of NMDA receptors in the hippocampal neurons of neonatal mice and primary neuronal cultures. Viral antigen was mostly found in hippocampal pyramidal neurons from the CA1 to CA3. Image analysis of immunohistochemistry demonstrated that the expression of NMDA receptor subunit 1 (NMDA-R1) protein in CA1 neurons of MCMV-infected brain was reduced to 40% of that in uninfected brain. The signal of in situ hybridization for NMDA-R1 mRNA was also decreased in CA1 neurons of MCMV-infected brain. In primary neuronal cultures, reduction of NMDA-R1 expression in MCMV-infected neurons was also detected by immunocytochemistry and Western blotting. These results suggest that reduction of NMDA receptor expression by MCMV infection may cause a decrease in the susceptibility of the neurons to excitotoxic cell death, and may be related to the establishment of viral persistence and functional disturbances in MCMV-infected neurons.

Close encounters of different kinds: dendritic cells and NK cells take centre stage

Immune responses are generally divided into innate and adaptive responses, and the efficacy of one is thought to be independent of the other. The regulation of immune responses, however, is complex, and accumulating evidence indicates that multiple interactions between immune effector cells are common and are crucial for the initiation, as well as the outcome, of these responses. Dendritic cells, long recognized as key initiators of primary adaptive immunity, are now also seen as crucial regulators of aspects of innate immunity, in particular natural-killer-cell function. Reciprocally, natural killer cells can influence the activity of dendritic cells. Here, we review recent exciting progress in this field, and we highlight the impact of this cellular crosstalk on the design of immune-based therapies for control of infection and cancer.

Cell death suppression by cytomegaloviruses

Cytomegaloviruses (CMVs), a subset of betaherpesviruses, employ multiple strategies to suppress apoptosis in infected cells and thus to delay their death. Human cytomegalovirus (HCMV) encodes at least two proteins that directly interfere with the apoptotic signaling pathways, viral inhibitor of caspase-8-induced apoptosis vICA (pUL36), and mitochondria-localized inhibitor of apoptosis vMIA (pUL37 x 1). vICA associates with pro-caspase-8 and appears to block its recruitment to the death-inducing signaling complex (DISC), a step preceding caspase-8 activation. vMIA binds and sequesters Bax at mitochondria, and interferes with BH3-only-death-factor/Bax-complex-mediated permeabilization of mitochondria. vMIA does not seem to either interact with Bak, a close structural and functional homologue of Bax, or to suppress Bak-mediated permeabilization of mitochondria and Bak-mediated apoptosis. All sequenced betaherpesviruses, including CMVs, encode close homologues of vICA, and those vICA homologues that have been tested, were found to be functional cell death suppressors. Overt sequence homologues of vMIA were found only in the genomes of primate CMVs, but recent observations made with murine CMV (MCMV) indicate that non-primate CMVs may also encode a cell death suppressor functionally resembling vMIA. The exact physiological roles and relative contributions of vMIA and vICA in suppressing death of CMV-infected cells in vivo have not been elucidated. There is strong evidence that the cell death suppressing function of vMIA is indispensable, and that vICA is dispensable for replication of HCMV. In addition to suppressed caspase-8 activation and sequestered Bax, CMV-infected cells display several other phenomena, less well characterized, that may diminish, directly or indirectly the extent of cell death.