Cattaneo, Roberto

AbstractPurpose: Measles virus (MV) causes the regression of human lymphoma xenografts. The purpose of this study was to determine if canine lymphoid cells could be infected in vitro with MV or canine distemper virus (CDV, the canine Morbillivirus equivalent of MV) and determine if in vitro viral infection leads to apoptotic cell death.Experimental Design: Reverse transcriptase-PCR was used to examine the expression of both signal lymphocyte activation molecule (CD150) and membrane cofactor molecule (CD46) mRNA. An attenuated CDV expressing enhanced green fluorescent protein was used to infect canine cells in vitro. Both flow cytometry and reverse transcriptase-PCR was used to document CDV infection. Cell death was examined using a propidium iodide staining assay and Annexin V binding.Results: Canine lymphoid cell lines and neoplastic B and T lymphocytes collected from dogs with spontaneous lymphoma expressed the Morbillivirus receptor CD150 mRNA. In contrast, only neoplastic lymphocytes expressed detectable levels of CD46 mRNA. Although MV did not infect canine cells, CDV efficiently infected between 40% and 70% of all three canine lymphoid lines tested. More importantly, CDV infected 50% to 90% of neoplastic lymphocytes isolated from dogs with both B and T cell lymphoma. Apoptosis of CDV-infected cell lines was documented.Conclusions: Attenuated CDV may be a useful treatment for canine lymphoma. As such, dogs with lymphoma may represent a biologically relevant large animal model to investigate the feasibility, safety, and efficacy of Morbillivirus therapy in a clinical setting with findings that may have direct applicability in the treatment of human non-Hodgkin's lymphoma.

A recombinant measles virus (MV) expressing red fluorescent protein (MVDsRed1) was used to produce a persistently infected cell line (piNT2-MVDsRed1) from human neural precursor (NT2) cells. A similar cell line (piNT2-MVeGFP) was generated using a virus that expresses enhanced green fluorescent protein. Intracytoplasmic inclusions containing the viral nucleocapsid protein were evident in all cells and viral glycoproteins were present at the cell surface. Nevertheless, the cells did not release infectious virus nor did they fuse to generate syncytia. Uninfected NT2 cells express the MV receptor CD46 uniformly over their surface, whereas CD46 was present in cell surface aggregates in the piNT2 cells. There was no decrease in the overall amount of CD46 in piNT2 compared to NT2 cells. Cell-to-cell fusion was observed when piNT2 cells were overlaid onto confluent monolayers of MV receptor-positive cells, indicating that the viral glycoproteins were correctly folded and processed. Infectious virus was released from the underlying cells, indicating that persistence was not due to gross mutations in the virus genome. Persistently infected cells were superinfected with MV or canine distemper virus and cytopathic effects were not observed. However, mumps virus could readily infect the cells, indicating that superinfection immunity is not caused by general soluble antiviral factors. As MVeGFP and MVDsRed1 are antigenically indistinguishable but phenotypically distinct it was possible to use them to measure the degree of superinfection immunity in the absence of any cytopathic effect. Only small numbers of non-fusing green fluorescent piNT2-MVDsRed1 cells (1 : 300 000) were identified in which superinfecting MVeGFP entered, replicated and expressed its genes.

ABSTRACTThe human regulator of complement activation membrane cofactor protein (CD46) has recently been identified as an attachment receptor for most species B adenoviruses (Ads), including Ad type 3 (Ad3), Ad11, and Ad35, as well as species D Ad37. To characterize the interaction between Ad35 and CD46, hybrid receptors composed of different CD46 short consensus repeat (SCR) domains fused to immunoglobulin-like domains of CD4 and a set of 36 CD46 mutants containing semiconservative changes of single amino acids within SCR domains I and II were tested in binding and in Ad35-mediated luciferase transduction assays. In addition, anti-CD46 antibodies and soluble polypeptides constituting various CD46 domains were used in binding inhibition studies. Our data indicate that (i) CD46 SCR I or SCR II alone confers low but significant Ad35 binding; (ii) the presence of SCR I and II is required for optimal binding and transgene expression; (iii) transduction efficiencies equivalent to that of full-length CD46 are obtained if SCR I and II are at an appropriate distance from the cell membrane; (iv) ablation of the N-glycan attached to SCR I has no influence on receptor function, whereas ablation of the SCR II N-glycan results in about a two- to threefold reduction of binding and transgene expression; (v) most putative Ad35 binding residues are located on the same solvent-exposed face of the SCR I or SCR II domain, which are twisted by about 90°; and (vi) the putative Ad35 binding sites partly overlap with the measles virus binding surface.

ABSTRACTSignaling lymphocytic activation molecule (SLAM, CD150) is the universal morbillivirus receptor. Based on the identification of measles virus (MV) hemagglutinin (H) amino acids supporting human SLAM-dependent cell entry, we mutated canine distemper virus (CDV) H and identified residues necessary for efficient canine SLAM-dependent membrane fusion. These residues are located in two nearby clusters in a new CDV H structural model. To completely abolish SLAM-dependent fusion, combinations of mutations were necessary. We rescued a SLAM-blind recombinant CDV with six mutations that did not infect ferret peripheral blood mononuclear cells while retaining full infectivity in epithelial cells.

ABSTRACT The efficiency of viruses in cancer therapy is enhanced by proteins that mediate the fusion of infected cells with their neighbors. It was reported that replication-competent adenovirus particles can spread between nuclei within fusion-generated syncytia. To assess this conjecture, we generated fusogenic adenoviruses that express a balanced ratio of the F and H glycoproteins of measles virus. The viruses displayed enhanced cytotoxicity but largely unchanged replication efficiencies compared to a nonfusogenic virus. Most notably, the virus genomes did not spread through fusion-generated multinuclear cells. Hence, adenovirus replication in syncytia remains largely restricted to initially transduced nuclei.

ABSTRACT The measles virus (MV) P gene codes for three proteins: P, an essential polymerase cofactor, and V and C, which have multiple functions but are not strictly required for viral propagation in cultured cells. V shares the amino-terminal domain with P but has a zinc-binding carboxyl-terminal domain, whereas C is translated from an overlapping reading frame. During replication, the P protein binds incoming monomeric nucleocapsid (N) proteins with its amino-terminal domain and positions them for assembly into the nascent ribonucleocapsid. The P protein amino-terminal domain is natively unfolded; to probe its conformational flexibility, we fused it to the green fluorescent protein (GFP), thereby also silencing C protein expression. A recombinant virus (MV-GFP/P) expressing hybrid GFP/P and GFP/V proteins in place of standard P and V proteins and not expressing the C protein was rescued and produced normal ratios of mono-, bi-, and tricistronic RNAs, but its replication was slower than that of the parental virus. Thus, the P protein retained nearly intact polymerase cofactor function, even with a large domain added to its amino terminus. Having noted that titers of cell-associated and especially released MV-GFP/P were reduced and knowing that the C protein of the related Sendai virus has particle assembly and infectivity factor functions, we produced an MV-GFP/P derivative expressing C. Intracellular titers of this virus were almost completely restored, and those of released virus were partially restored. Thus, the MV C protein is an infectivity factor.

The immunosuppressive properties of morbilliviruses including measles and canine distemper virus (CDV) are well known, but the host cells supporting infection are poorly characterized. To identify these cells, a recombinant CDV expressing green fluorescent protein was produced by reverse genetics based on a wild-type strain lethal for ferrets. This recombinant virus fully retained virulence and blazed three lymphocyte-based pathways through the immune system of its host: first, it infected rapidly and massively circulating B and T cells; second, it took over and damaged secondary lymphatic organs including spleen, lymph nodes, and gut-associated and mucosal lymphoid tissues; third, it infected most thymocytes. In contrast, replication in epithelial cells was initially not detectable, but substantial before host death. Thus, CDV initially infects lymphocytes and massively replicates therein, thereby causing immunosuppression and preparing systemic invasion and host escape.

ABSTRACT The trimeric fusion (F) glycoproteins of morbilliviruses are activated by furin cleavage of the precursor F 0 into the F 1 and F 2 subunits. Here we show that an additional membrane-proximal cleavage occurs and modulates F protein function. We initially observed that the ectodomain of approximately one in three measles virus (MV) F proteins is cleaved proximal to the membrane. Processing occurs after cleavage activation of the precursor F 0 into the F 1 and F 2 subunits, producing F 1a and F 1b fragments that are incorporated in viral particles. We also detected the F 1b fragment, including the transmembrane domain and cytoplasmic tail, in cells expressing the canine distemper virus (CDV) or mumps virus F protein. Six membrane-proximal amino acids are necessary for efficient CDV F 1a/b cleavage. These six amino acids can be exchanged with the corresponding MV F protein residues of different sequence without compromising function. Thus, structural elements of different sequence are functionally exchangeable. Finally, we showed that the alteration of a block of membrane-proximal amino acids results in diminished fusion activity in the context of a recombinant CDV. We envisage that selective loss of the membrane anchor in the external subunits of circularly arranged F protein trimers may disengage them from pulling the membrane centrifugally, thereby facilitating fusion pore formation.

ABSTRACTMeasles virus (MV) enters cells either through the signaling lymphocyte activation molecule SLAM (CD150) expressed only in immune cells or through the ubiquitously expressed regulator of complement activation, CD46. To identify residues on the attachment protein hemagglutinin (H) essential for fusion support through either receptor, we devised a strategy based on analysis of morbillivirus H-protein sequences, iterative cycles of mutant protein production followed by receptor-based functional assays, and a novel MV H three-dimensional model. This model uses the Newcastle disease virus hemagglutinin-neuraminidase protein structure as a template. We identified seven amino acids important for SLAM- and nine for CD46 (Vero cell receptor)-induced fusion. The MV H three-dimensional model suggests (i) that SLAM- and CD46-relevant residues are located in contiguous areas in propeller β-sheets 5 and 4, respectively; (ii) that two clusters of SLAM-relevant residues exist and that they are accessible for receptor contact; and (iii) that several CD46-relevant amino acids may be shielded from direct receptor contacts. It appears likely that certain residues support receptor-specific H-protein conformational changes. To verify the importance of the H residues identified with the cell-cell fusion assays for virus entry into cells, we transferred the relevant mutations into genomic MV cDNAs. Indeed, we were able to recover recombinant viruses, and we showed that these replicate selectively in cells expressing SLAM or CD46. Selectively receptor-blind viruses will be used to study MV pathogenesis and may have applications for the production of novel vaccines and therapeutics.