Structure/function relationship of activating Ly-49D and inhibitory Ly-49G2 NK receptors.

Murine NK cells express Ly-49 family receptors capable of either inhibiting or activating lytic function. The overlapping patterns of expression of the various receptors have complicated their precise biochemical characterization. Here we describe the use of the Jurkat T cell line as the model for the study of Ly-49s. We demonstrate that Ly-49D is capable of delivering activation signals to Jurkat T cells even in the absence of the recently described Ly-49D-associated chain, DAP-12. Ly-49D signaling in Jurkat leads to tyrosine phosphorylation of TCRzeta and requires Syk/Zap70 family kinases and arginine 54 of Ly-49D, suggesting that Ly-49D signals via association with TCRzeta. Coexpression studies in 293-T cells confirmed the ability of Ly-49D to associate with TCRzeta. In addition, we have used this model to study the functional interactions between an inhibitory Ly-49 (Ly-49G2) and an activating Ly-49 (Ly-49D). Ly-49G2 blocks activation mediated by Ly-49D in an immunoreceptor tyrosine-based inhibitory motif (ITIM)-dependent manner. In contrast, Ly-49G2 was incapable of inhibiting activation by the TCR even though human killer cell inhibitory receptor (KIR) (KIR3DL2(GL183)) effectively inhibits TCR. Both the ability of Ly-49G2 to block Ly-49D activation and the failure of Ly-49G2 to inhibit TCR signaling were confirmed in primary murine NK cells and NK/T cells, respectively. These data demonstrate the dominant effects of the inhibitory receptors over those that activate and suggest an inability of the Ly-49 type II inhibitory receptors to efficiently inhibit type I transmembrane receptor signaling in T cells and NK cells.

Multigenic evasion of inflammation by poxviruses.

Analyses of different cowpox virus (Brighton Red strain [CPV-BR]) mutants indicate that there is a minimum of three genes encoded by CPV-BR that are nonessential for virus replication in tissue culture but are involved in inhibiting the generation of an inflammatory response in the chicken embryo chorioallantoic membrane (CAM) model. The CPV-BR-encoded anti-inflammatory genes include the gene encoding the 38-kDa protein (also called 38K, crmA, SPI-2, or VV-WR-ORF-B13R), a tumor necrosis factor receptor homolog, and an unidentified gene that maps to the right end of the CPV genome. The kinetics of triggering of an inflammatory response at the site of virus infection as well as the magnitude of the response is dependent on the virus-encoded inhibitor that is deleted. Virus yields recovered from pocks decreased in proportion to the magnitude of the inflammatory response. The deletion of these identified inhibitors of inflammation was associated with attenuation of the mutant viruses in mice. These data confirm the existence of multiple poxvirus-encoded host defense modifiers whose function is to block the generation of an inflammatory response at the site of virus infection, which allows enhanced virus replication and potentially facilitates virus transmission.

Poxvirus-induced alteration of arachidonate metabolism.

Recent evidence suggests that orthopoxviruses have an obligate requirement for arachidonic acid metabolites during replication in vivo and in vitro. Our report indicates that a virus family (Poxviridae) possesses multiple genes that function to regulate arachidonate metabolism. Analyses of BS-C-1 cells infected with cowpox virus or vaccinia virus detected enhanced arachidonate product formation from both the cyclooxygenase (specifically prostaglandins E2 and F2 alpha) and lipoxygenase (specifically 15-hydroxyeicosatetraenoic acid and 12-hydroxyeicosatetraenoic acid) pathways. In contrast, human parainfluenza type 3 or herpes simplex virus type 1 infections did not increase arachidonate metabolism. Results were consistent with a virus early-gene product either directly mediating or inducing a host factor that mediated the up-regulation of arachidonate metabolism, although vaccinia growth factor was not responsible. In addition, the cowpox virus 38-kDa protein-encoding gene, which is associated with inhibition of an inflammatory response, correlated with inhibition of formation of a product biochemically characteristic of (14R,15S)-dihydroxyeicosatetraenoic acid. We propose that orthopoxvirus-induced up-regulation of arachidonic acid metabolism during infection renders the infected cells susceptible to generation of inflammatory mediators from both the cyclooxygenase and the lipoxygenase pathways, and poxviruses, therefore, possess at least one gene (38K) that can alter the lipoxygenase-metabolite spectrum.

Inhibition of agonist-induced activation of phospholipase C following poxvirus infection.

Recent studies indicate that viruses may influence polyphosphoinositide levels. This study has examined the effects of vaccinia virus infection on phospholipase C activity. Infection of BS-C-1 cells, an African Green Monkey kidney cell line, or A431 cells, a human carcinoma cell line, with vaccinia virus inhibits receptor-mediated phospholipase C activation. As a consequence, agonist-mediated Ca2+ mobilization in BS-C-1 cells also was inhibited by vaccinia virus infection. Alleviation of the inhibition of phospholipase C activation was observed in vaccinia virus-infected cells treated with cycloheximide without influencing uninfected cells. Treatment of infected cells with alpha-amanitin, an inhibitor of host mRNA synthesis but not virus mRNA synthesis, failed to alleviate the inhibition of phospholipase C activation. Together these results suggest that a virus-encoded gene product mediates the inhibition of phospholipase C activation without the need of a virus-induced host factor. Analysis of the processes involved in the formation of inositol (1,4,5)-trisphosphate and mobilization of intracellular Ca2+ indicate that the vaccinia virus gene product exerts its inhibitory effects at the level of phospholipase C activity. This may occur by either directly reducing the amount of phospholipase C, reducing the specific activity of phospholipase C, or by inhibiting the association of phospholipase C with its substrate, phosphatidylinositol 4,5-bisphosphate.

Acute inflammatory response to cowpox virus infection of the chorioallantoic membrane of the chick embryo.

The chick embryo chorioallantoic membrane was used to study the acute inflammatory response in the absence of contributions from the immune system. In preliminary experiments, lesions of wild-type cowpox virus strain Brighton (CPV-BR) and a 38K gene deletion mutant of CPV-BR (CPV-BR.D1) were compared with vaccinia virus (strains WR and Copenhagen), fowlpox virus, laryngotracheitis virus, and infectious tenosynovitis virus, and were ranked for degree of induced inflammation. The maximal and minimal inflammatory responses were observed with CPV-BR.D1 and CPV-BR viruses, respectively. CPV-BR.D1 lacks a 38K gene which encodes an anti-inflammatory 38-kDa protein that has homology to SERPINs. The kinetics and character of the inflammatory response were examined further in the wild-type CPV-BR and mutant CPV-BR.D1 infections using cell counts, electron microscopy, and assays for inflammatory cell activation. CPV-BR virus infection rapidly spread through the ectoderm, uniformly infecting all cells with the production of large amounts of virions and viral-induced cytopathic effect, but evoking little or no inflammatory response until 144 hr p.i. The CPV-BR.D1 infection, on the other hand, was rapidly contained by a dexamethasone-sensitive inflammatory response mainly of activated heterophils which was advanced by 36 hr p.i. Both infections resulted in disseminated disease with similar numbers of liver lesions and only a slight difference in the LD50, with the CPV-BR.D1 values being higher than that for CPV-BR virus. In this model, the acute inflammatory response alone is unable to prevent disseminated disease and associated mortality.

Poxvirus pathogenesis.

Poxviruses are a highly successful family of pathogens, with variola virus, the causative agent of smallpox, being the most notable member. Poxviruses are unique among animal viruses in several respects. First, owing to the cytoplasmic site of virus replication, the virus encodes many enzymes required either for macromolecular precursor pool regulation or for biosynthetic processes. Second, these viruses have a very complex morphogenesis, which involves the de novo synthesis of virus-specific membranes and inclusion bodies. Third, and perhaps most surprising of all, the genomes of these viruses encode many proteins which interact with host processes at both the cellular and systemic levels. For example, a viral homolog of epidermal growth factor is active in vaccinia virus infections of cultured cells, rabbits, and mice. At least five virus proteins with homology to the serine protease inhibitor family have been identified and one, a 38-kDa protein encoded by cowpox virus, is thought to block a host pathway for generating a chemotactic substance. Finally, a protein which has homology with complement components interferes with the activation of the classical complement pathway. Poxviruses infect their hosts by all possible routes: through the skin by mechanical means (e.g., molluscum contagiosum infections of humans), via the respiratory tract (e.g., variola virus infections of humans), or by the oral route (e.g., ectromelia virus infection of the mouse). Poxvirus infections, in general, are acute, with no strong evidence for latent, persistent, or chronic infections. They can be localized or systemic. Ectromelia virus infection of the laboratory mouse can be systemic but inapparent with no mortality and little morbidity, or highly lethal with death in 10 days. On the other hand, molluscum contagiosum virus replicates only in the stratum spinosum of the human epidermis, with little or no involvement of the dermis, and does not spread systemically from the site of infection. The host response to infection is progressive and multifactorial. Early in the infection process, interferons, the alternative pathway of complement activation, inflammatory cells, and natural killer cells may contribute to slowing the spread of the infection. The cell-mediated response involving learned cytotoxic T lymphocytes and delayed-type hypersensitivity components appears to be the most important in recovery from infection. A significant role for specific antiviral antibody and antibody-dependent cell-mediated cytotoxicity has yet to be demonstrated in recovery from a primary infection, but these responses are thought to be important in preventing reinfection.

Inhibitors of the lipoxygenase pathway specifically block orthopoxvirus replication.

Inhibitors of arachidonic acid metabolism, 5,8,11,14-eicosatetraynoic acid (ETYA), BW755c, and nordihydroguaiaretic acid were found to specifically interfere with the replication of cowpox virus (an orthopoxvirus) both in vivo and in vitro. Further studies in vitro showed that the drugs ETYA and BW755c were effective in inhibiting the replication of two additional orthopoxviruses, ectromelia and vaccinia viruses, but not human parainfluenza virus-3. In ETYA-treated and cowpox virus-infected cells, early and late gene expression were near normal levels, whereas the assembly of virus-specific membranes was severely reduced. These results are compatible with a model of orthopoxvirus replication that has an obligate requirement for arachidonic acid or one of its metabolic forms, possibly in the assembly of virus-specific membranes.

Multiple steps in the immortalization of cells of the B lineage.

The ability of A-MuLV to transform bone marrow cells on in vitro culture in agarose is enhanced by inclusion of conditioned media during infection and culture. The conditioned medium of a non-virus producing A-MuLV transformed fibroblast cell line was synergistic with medium from Whitlock-Witte long-term bone marrow cultures, while conditioned medium from modified Dexter-type cultures was not active. These media all contained growth promoting activity for bone marrow cells. There are two types of transformed colonies produced, and transformation of only one type was enhanced by inclusion of conditioned media. Analysis of this type of transformed cell showed them to be pre-B cells. Limiting dilution analysis suggests the transformation process to be dependent on two types of cells, one presumably the target and the second an "accessory cell". Models are presented to account for the factor-dependent in vitro transformation of pre-B cells.

Multistage progression of Abelson virus-infected murine pre-B-cells to the tumorigenic state.

The tumorigenic potential of pre-B-cells at different stages of Abelson murine leukemia virus-induced transformation was determined. Cell lines with low growth potential in liquid culture were found (a) to have a dose-dependent growth requirement for conditioned medium obtained from bone marrow cultures, (b) to have low colony-forming ability in semisolid medium in the absence of conditioned medium, and (c) to be nontumorigenic when inoculated into syngeneic mice. Culture of the factor-dependent cells in vitro leads to the emergence of factor-independent variants, which eventually dominate the population by overgrowth. Cell lines that acquired a factor-independent phenotype were able to form colonies in semisolid medium and form tumors when inoculated into syngeneic mice. These results suggest that Abelson murine leukemia virus is sufficient to initiate transformation in the infected cell but that an additional genetic alteration is needed to confer tumorigenicity.

Insertional inactivation of the large subunit of ribonucleotide reductase encoded by vaccinia virus is associated with reduced virulence in vivo.

To assess whether a fully functional VV ribonucleotide reductase enzyme is required during both in vitro and in vivo replication of VV, three mutant viruses were constructed by marker transfer techniques: M1 lambda, an M1 insertion mutant; TK-, an insertion mutant of the VV thymidine kinase (tk) gene; and M1 lambda/TK-, a double mutant. Extracts of cells infected with the M1 lambda or M1 lambda/TK- mutant viruses were assayed for ribonucleotide reductase activity and it was found that insertional inactivation of the M1 gene abolished the induction of viral enzyme activity in VV-infected cells. Each of the three mutant viruses replicated to levels comparable to the wild-type (WT) virus in BSC40 (monkey), growing A549 (human lung carcinoma) cells, and serum-starved A549 cells, indicating that a functional M1 gene was not required for viral replication in tissue culture. In contrast, in vivo studies indicate that the loss of viral ribonucleotide reductase activity leads to a mild attenuation of VV. By the intracranial route of inoculation, approximately 10-fold more of the M1 lambda recombinant than the WT virus was required to produce the average lethal dose for 50% of the population of injected mice.

Inhibition of an inflammatory response is mediated by a 38-kDa protein of cowpox virus.

The Brighton Red (BR) strain of cowpox virus induces a flat, bright red pock on the chorioallantoic membrane (CAM) of the 12-day-old chick embryo. In contrast, mutants with a deleted 38K gene (which is located 31 to 32 kb from the right-hand end of the virus genome) induced a raised, white, and opaque pock. During the first 24-hr p.i., both CPV-BR and the 38K deletion mutants replicated similarly in the CAM of the chick embryo, as indicated by immunocytochemical detection of similar amounts of virus antigen. By 48 hr p.i., the pocks induced by the mutant and CPV-BR are strikingly different. The pocks induced by the 38K deletion mutants were infiltrated by large numbers of heterophils and macrophages, which correlated with a reduction in the levels of virus antigen and virus infectivity. The CPV-BR pock had an absence of inflammatory cells and increased levels of virus antigen and infectivity. By 72 hr p.i., many of the pocks induced by the mutant were undergoing resolution of the virus infection, as indicated by further decrease of virus antigen and visible signs of healing, whereas CPV-BR pocks continued to be a site of active viral replication. These data are consistent with a model where this 38-kDa protein directly or indirectly inhibits the generation of chemotactic molecules which are elicited during virus replication in the CAM or, alternatively, blocks the interaction of these molecules with cells of the host inflammatory response.

The effect of secretagogue selection on potency determinations for gastric acid antisecretory agents.

The most common treatment for gastric and duodenal ulcers is by suppressing the secretion of gastric acid by the parietal cells of the stomach. Two major categories of drugs can accomplish this specifically: histamine H2-receptor antagonist and (H+ + K+)ATPase inhibitors. Using a canine model of gastric acid secretion and either histamine or food as secretagogues, the effects of two drugs from each of these classes were investigated. The two H2-antagonists (ranitidine, Wy-45,727) demonstrated a significantly greater antisecretory potency when acid secretion was stimulated by histamine as compared to a food stimulus. Conversely, the (H+ + K+)ATPase inhibitors (omeprazole, timoprazole) were equipotent regardless of stimulus.