Deletion analysis of two tandemly arranged virulence genes in myxoma virus, M11L and myxoma growth factor

Myxoma virus T2 protein, a tumor necrosis factor (TNF) receptor homolog, is secreted as a monomer and dimer that each bind rabbit TNFalpha, but the dimer is a more potent TNF inhibitor

The myxoma virus T2 (M-T2) gene expresses a secreted protein that contains significant sequence similarity to the ligand binding domains of the cellular tumor necrosis factor (TNF) receptors, specifically inhibits the cytolytic activity of rabbit TNFalpha and is an important virulence factor for myxoma virus infection in rabbits. M-T2 protein was overexpressed from vaccinia virus vectors, purified to apparent homogeneity, and found to specifically protect mouse and rabbit cells from lysis by rabbit TNFalpha at molar ratios comparable with the soluble versions of the host tumor necrosis factor receptors. M-T2 secreted from virus-infected cells is detected as both a monomer and a disulfide-linked dimer, both of which were shown by Scatchard analysis to bind rabbit TNFalpha (Kd values of 170 pM and 195 pM, respectively), values that are comparable with the affinities of mammalian TNFs with their receptors. In contrast to the rabbit ligand, M-T2 interacts with mouse TNFalpha with a much lower affinity, Kd of 1.7 nM, and was unable to inhibit the cytolytic activity of this ligand on mouse cells. Although both monomeric and dimeric forms bound rabbit TNFalpha with comparable affinity, the dimeric M-T2 protein was a far more potent inhibitor of rabbit TNFalpha, presumably because it can more effectively prevent dimerization of TNF receptors than can the M-T2 monomer.

One step ahead of the game: viral immunomodulatory molecules

For decades cell biologists have relied on viruses to facilitate the study of complex cellular function. More recently, the tragedy of the AIDS epidemic has focused considerable human and financial resources on both virology and immunology, resulting in the generation of new information relating these disciplines. As the miracle of the mammalian immune system unfolds in the laboratory, the elegance of the mechanisms used by co-evolving viruses to circumvent detection and destruction by the host becomes inescapably obvious. Although many observation of virus-induced phenomena that likely contribute to the virus's escape of immune surveillance are still empirical, many other such phenomena have now been defined at the molecular level and confirmed in in vivo models. Immune modulators encoded within viral genomes include proteins that regulate antigen presentation, function as cytokines or cytokine antagonists, inhibit apoptosis, and interrupt the complement cascade. The identification of such gene products and the elucidation of their function have substantially strengthened our understanding of specific virus-host interactions and, unexpectedly, have contributed to the recognition of potent synergy between viruses, which can result in an unpredictable exacerbation of disease in co-infected individuals. Because many viral immune modulators clearly have host counterparts, viruses provide a valuable method for studying normal immune mechanisms. It is conceivable that an improved understanding of virus-encoded immunomodulators will enhance our ability to design reagents for use in therapeutic intervention in disease and in vaccine development.

Myxoma virus as a vaccine vector for rabbits: antibody levels to influenza virus haemagglutinin presented by a recombinant myxoma virus

To determine whether myxoma virus (MV) could be used as a vector for antigen presentation to European rabbits (Oryctolagus cuniculus), despite its immunosuppressive properties, a recombinant MV expressing the influenza virus haemagglutinin (HA) was constructed. Domestic rabbits, inoculated with the recombinant virus, developed high plasma antibody titres to the HA and IgG to HA was also detected in vaginal secretions. Antibody titres to HA obtained with recombinant MV were similar to titres obtained in rabbits inoculated with vaccinia virus expressing the HA. These results indicate that MV is an efficient vector for antigen presentation in rabbits.

Myxoma virus induces extensive CD4 downregulation and dissociation of p56lck in infected rabbit CD4+ T lymphocytes

Myxoma virus is a pathogenic poxvirus that induces extensive dysregulation of cellular immunity in infected European rabbits. Infection of a rabbit CD4+ T-cell line (RL-5) with myxoma virus results in dramatic reductions of cell surface levels of CD4 as monitored by flow cytometry. The virus-induced downregulation of CD4 requires early but not late viral gene expression and could not be inhibited by staurosporine, an inhibitor of protein kinase C, which effectively blocks phorbol 12-myristate-13-acetate-induced downregulation of CD4. The decrease in total cellular levels of CD4 during myxoma virus infection could be inhibited by the lysosomotrophic agent NH4Cl, suggesting a lysosomal fate for CD4 during myxoma virus infection. Steady-state levels of the CD4-associated protein tyrosine kinase p56lck remained unchanged during myxoma virus infection, suggesting that p56lck dissociates from CD4 prior to CD4 degradation in virus infected cells. Total p56lck kinase activity was unaffected during myxoma virus infection, although the amount of p56lck physically associated with CD4 declined in parallel with the loss of CD4. Thus, myxoma virus infection of CD4+ T lymphocytes triggers CD4 downregulation via a protein kinase C-independent pathway, causing the dissociation of p56lck and the degradation of CD4 in lysosomal vesicles.

The myxoma virus-soluble interferon-gamma receptor homolog, M-T7, inhibits interferon-gamma in a species-specific manner

The myxoma virus M-T7 protein contains significant sequence similarity to the ligand binding domain of the mammalian interferon-gamma receptors, and functions as a soluble homolog which can bind and inhibit the biological activities of rabbit interferon-gamma (Upton, C., Mossman, K., and McFadden, G. (1992) Science 258:1369-1372). M-T7, the most abundantly secreted protein from myxoma virus-infected cells, was shown to be expressed in significant biological amounts as a typical poxvirus early gene product, efficiently secreted at early times of infection to levels that exceed 5 x 10(7) molecules/cell, and function as a stable inhibitory protein in infected cell supernatants until late times of infection. M-T7 was specific in binding and inhibiting rabbit interferon-gamma, and did not bind either human or murine interferon-gamma. Scatchard analysis of rabbit interferon-gamma binding curves yielded a single high affinity binding site on M-T7, with a Kd of 1.2 x 10(-9) M, which is comparable to the affinity between soluble forms of cellular interferon-gamma receptors and their cognate ligands. In comparison, rabbit interferon-gamma was shown to bind its cellular receptor with a Kd of 5.9 x 10(-10) M, again comparable to the affinity of membrane bound forms of other mammalian interferon-gamma receptors for interferon-gamma. Thus, the myxoma virus M-T7 protein is a functional soluble interferon-gamma receptor homolog which binds and inhibits interferon-gamma with high affinity in a species-specific manner.

Mapping and investigation of the role in pathogenesis of the major unique secreted 35-kDa protein of rabbitpox virus

Following infection, many secreted poxvirus proteins are able to modulate the host immune response through interactions with cytokines or components of the complement pathway. A comparison of the secreted protein profiles from cells infected with vaccinia Western Reserve (VV-WR), cowpox virus Brighton strain, or rabbitpox virus (RPV) showed an abundant 35-kDa protein present only in the supernatants from RPV-infected cells. The gene encoding this protein was identified and mapped by N-terminal sequencing of the protein. Examination of the predicted amino acid sequence showed it to be identical to the 35-kDa secreted protein of the Lister strain of vaccinia virus described by Patel et al. (1990, J. Gen. Virol. 71, 2013-2021). The counterpart of this gene in the commonly studied VV-WR strain is truncated and encodes a 7.5-kDa protein under control of the well-characterized p7.5 promoter. While nonessential for replication in cell culture, conservation of this gene in at least two orthopoxvirus strains suggested that this protein might play an important role in vivo. Following intranasal inoculation of Balb/c mice at several doses (10(3), 10(4), or 10(5) PFU), a mutant of RPV lacking a functional 35-kDa gene (RPV delta 35) appeared to induce an earlier onset and more severe illness at low, sublethal doses (10(3) PFU) than was observed with wild-type (wt) RPV. At higher doses (10(4) or 10(5) PFU), the behavior of wt RPV and RPV delta 35 became indistinguishable and the overall LD50 values were similar. Intradermal infection of rabbits simultaneously, at separate sites, with RPV and RPV delta 35 showed no gross or microscopic differences between either primary skin lesions or viremic extension of each virus into the lungs. Therefore, this abundant secreted protein does not appear to play a major role in the virulence of the virus.

Myxoma virus and Shope fibroma virus encode dual-specificity tyrosine/serine phosphatases which are essential for virus viability

Sequence analysis of the genomes of the Leporipoxviruses myxoma virus and Shope fibroma virus (SFV) led to the discovery of open reading frames homologous to the vaccinia H1L gene encoding a soluble protein phosphatase with dual tyrosine/serine specificity. These viral phosphatase genes were subsequently localized to the myxoma BamHI-I fragment and the SFV BamHI-M fragment, and the resulting encoded proteins were designated I1L and M1L, respectively. The localization and orientation of the myxoma I1L and SFV M1L open reading frames within the well conserved central core of the viral genomes closely mirror that of the Orthopoxviruses vaccinia virus and variola virus. The myxoma I1L and SFV M1L phosphatases each contain the conserved tyrosine phosphatase signature sequence motif, (I/V)HCXAGXXR(S/T)G, including the active site cysteine, found previously to be essential for phosphotyrosine dephosphorylation. The vaccinia H1L phosphatase was originally shown to have the ability to dephosphorylate phosphotyrosyl and phosphoseryl residues in vitro. To assess whether this is a common feature of poxvirus phosphatases, myxoma I1L was expressed as a GST-fusion protein, purified, and shown to dephosphorylate substrates containing tyrosine and serine phosphorylated residues, in a similar fashion to vaccinia H1L. A myxoma I1L variant, in which the active site cysteine 110 was mutated to serine, was expressed in a parallel fashion to the wild-type I1L protein and found to be completely deficient in its ability to dephosphorylate both phosphotyrosine and phosphoserine amino acids. In an attempt to ascertain the biological requirement for the myxoma I1L phosphatase, we constructed a recombinant myxoma virus containing a disrupted I1L open reading frame. This I1L mutant virus was able to successfully propagate in tissue culture only in the presence of a wild-type complementing gene, and pure virus clones containing only the disrupted allele were not viable. Thus, we conclude that the myxoma I1L dual specificity phosphatase is an essential factor for virus viability.

Myxoma virus M11L ORF encodes a protein for which cell surface localization is critical in manifestation of viral virulence

Myxoma virus (MYX) induces extensive immunosuppression in infected rabbits and is associated with high levels of mortality. The virus encodes multiple gene products designed to circumvent the cellular immune response to the viral infection. Deletion analysis has shown that the M11L open reading frame (ORF) is an important virulence factor which downregulates leukocyte infiltration of MYX-induced tumors. To investigate the role of the M11L protein in viral pathogenesis, we sequenced the MYX M11L ORF and showed that the sequence has motifs consistent with a 166-aa class III membrane-spanning molecule possessing a single transmembrane helix near the C-terminus and a 142-aa N-terminal extracellular domain that has six cysteine residues plus two consensus N-glycosylation sites. Transcription analysis indicates that M11L is expressed as an early gene, and surface immunofluorescence studies with anti-M11L antibodies reveal that M11L protein is transported to the infected cell surface. Immunoprecipitation analysis of an attenuated viral recombinant, vMYX-GF-delta M11L, indicates that an M11L variant protein with an altered C-terminus is synthesized at about 45% of wild type levels; however, it is not detectable on the cell surface, suggesting that proper M11L function requires localization at the infected cell membrane. We propose that M11L is a virulence factor whose function is to recognize an extracellular ligand essential for the cellular inflammatory response.