Modified vaccinia virus Ankara protein F1L is a novel BH3-domain-binding protein and acts together with the early viral protein E3L to block virus-associated apoptosis

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.

Vaccinia virus B1R kinase interacts with JIP1 and modulates c-Jun-dependent signaling

Viruses have to adjust to the host cell to guarantee their life cycle and survival. This aspect of the virus-host cell interaction is probably performed by viral proteins, such as serine-threonine kinases, that are present early during infection. Vaccinia virus has an early Ser-Thr kinase, B1R, which, although required for successful viral infection, is poorly characterized regarding its effects on cellular proteins, and thus, its potential contribution to pathogenesis is not known. Signaling by mitogen-activated protein kinase (MAPK) is mediated by the assembly of complexes between these kinases and the JIP scaffold proteins. To understand how vaccinia virus B1R can affect the host, its roles in the cellular signaling by MAPK complexes and c-Jun activation have been studied. Independently of its kinase activity, B1R can interact with the central region of the JIP1 scaffold protein. The B1R-JIP1 complex increases the amount of MAPK bound to JIP1; thus, MKK7 and TAK1 either bind with higher affinity or bind more stably to JIP1, while there is an increase in the phosphorylation state of JNK bound to JIP1. The functional consequence of these more stable interactions is an increase in the activity of transcription factors, such as c-Jun, that respond to these complexes. Furthermore, B1R is also able to directly phosphorylate c-Jun in residues different from those targeted by JNK and, thus, B1R can also cooperate by an independent route in c-Jun activation. Vaccinia virus B1R can thus modulate the signaling of pathways that respond to cellular stress.

Mechanisms of apoptosis regulation by viral oncogenes in infection and tumorigenesis

Apoptosis mediated by the proapoptotic BCL-2 family members BCL-2-associated X-protein (BAX) and BCL-2 antagonist/killer (BAK) is part of the antiviral response at the cellular level to limit virus replication. Viruses, in turn, have evolved to encode antiapoptotic BCL-2 homologs (v-BCL-2s) to prevent the premature death of the infected host cell to sustain virus replication. These same v-BCL-2 proteins cooperate with loss of retinoblastoma protein and p53 tumor suppressor function, by inactivating the BAX and BAK apoptotic pathway to promote epithelial solid tumor growth and resistance to chemotherapy. Analogously to infected cells, failure of apoptosis in tumors permits the survival of abnormal, damaged cells displaying chromosome instability that may further promote tumor progression. Thus, both infected cells and tumor cells require inhibition of the apoptotic host defense mechanism, the insights from which can be exploited for therapy development.

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.

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.

The vaccinia virus F1L protein interacts with the proapoptotic protein Bak and inhibits Bak activation

Many viruses have evolved strategies to counteract cellular immune responses, including apoptosis. Vaccinia virus, a member of the poxvirus family, encodes an antiapoptotic protein, F1L. F1L localizes to mitochondria and inhibits apoptosis by preventing the release of cytochrome c by an undetermined mechanism (S. T. Wasilenko, T. L. Stewart, A. F. Meyers, and M. Barry, Proc. Natl. Acad. Sci. USA 100:14345-14350, 2003; T. L. Stewart, S. T. Wasilenko, and M. Barry, J. Virol. 79:1084-1098, 2005). Here, we show that in the absence of an apoptotic stimulus, F1L associates with Bak, a proapoptotic member of the Bcl-2 family that plays a pivotal role in the release of cytochrome c. Cells infected with vaccinia virus were resistant to Bak oligomerization and the initial N-terminal exposure of Bak following the induction of apoptosis with staurosporine. A mutant vaccinia virus missing F1L was no longer able to inhibit apoptosis or Bak activation. In addition, the expression of F1L was essential to inhibit tBid-induced cytochrome c release in both wild-type murine embryonic fibroblasts (MEFs) and Bax-deficient MEFs, indicating that F1L could inhibit apoptosis in the presence and absence of Bax. tBid-induced Bak oligomerization and N-terminal exposure of Bak in Bax-deficient MEFs were inhibited during virus infection, as assessed by cross-linking and limited trypsin proteolysis. Infection with the F1L deletion virus no longer provided protection from tBid-induced Bak activation and apoptosis. Additionally, infection of Jurkat cells with the F1L deletion virus resulted in cellular apoptosis, as measured by loss of the inner mitochondrial membrane potential, caspase 3 activation, and cytochrome c release, indicating that the presence of F1L was pivotal for inhibiting vaccinia virus-induced apoptosis. Our data indicate that F1L expression during infection inhibits apoptosis and interferes with the activation of Bak.