Affinity thresholds for membrane fusion triggering by viral glycoproteins

Reduced ability of hemagglutinin of the CAM-70 measles virus vaccine strain to use receptors CD46 and SLAM

The CAM-70 measles virus (MV) vaccine strain is currently used for vaccination against measles. We examined the fusion-inducing ability of the CAM-70 hemagglutinin (H) protein and found that it was impaired in both CD46- and signaling lymphocyte activation molecule (SLAM)-expressing cells. We also generated recombinant MVs possessing H genes derived from the CAM-70 strain. The CAM-70 H protein impaired viral growth in both CD46- and SLAM-expressing cells. In peripheral blood lymphocytes (PBL) and monocyte-derived dendritic cells (Mo-DC), the CAM-70 strain did not grow efficiently. Infection with recombinant MVs revealed that impaired growth of the CAM-70 strain was attributed to the H gene only partly in PBL and largely in Mo-DC. Thus, impaired fusion-inducing ability of the H protein may be one of the underlying molecular mechanisms resulting in the attenuation of the CAM-70 strain.

Intravascularly administered RGD-displaying measles viruses bind to and infect neovessel endothelial cells in vivo

Systemically administered vectors must cross the endothelial lining of tumor blood vessels to access cancer cells. Vectors that interact with markers on the lumenal surface of these endothelial cells might have enhanced tumor localization. Here, we generated oncolytic measles viruses (MVs) displaying alpha(v)beta(3) integrin-binding peptides, cyclic arginine-glycine-aspartate (RGD) or echistatin, on the measles hemagglutinin protein. Both viruses had expanded tropisms, and efficiently entered target cells via binding to integrins, but also retained their native tropisms for CD46 and signaling lymphocyte activation molecule (SLAM). When fluorescently labeled and injected intravascularly into chick chorioallantoic membranes (CAMs), in contrast to unmodified viruses, the integrin-binding viral particles bound to the lumenal surface of the developing chick neovessels and infected the CAM vascular endothelial cells. In a mouse model of VEGF-induced angiogenesis in the ear pinna, the integrin-binding viruses, but not the parental virus, infected cells at sites of new blood vessel formation. When given intravenously to mice bearing tumor xenografts, the integrin-binding virus infected endothelial cells of tumor neovessels in addition to tumor parenchyma. To our knowledge, this is the first report demonstrating that oncolytic MVs can be engineered to target the lumenal endothelial surface of newly formed blood vessels when administered intravenously in living animals.

Measles virus receptors

Measles virus (MV) has two envelope glycoproteins, the hemagglutinin (H) and fusion protein, which are responsible for attachment and membrane fusion, respectively. Signaling lymphocyte activation molecule (SLAM, also called CD150), a membrane glycoprotein expressed on immune cells, acts as the principal cellular receptor for MV, accounting for its lymphotropism and immunosuppressive nature. MV also infects polarized epithelial cells via an as yet unknown receptor molecule, thereby presumably facilitating transmission via aerosol droplets. Vaccine and laboratory-adapted strains of MV use ubiquitously expressed CD46 as an alternate receptor through amino acid substitutions in the H protein. The crystal structure of the H protein indicates that the putative binding sites for SLAM, CD46, and the epithelial cell receptor are strategically located in different positions of the H protein. Other molecules have also been implicated in MV infection, although their relevance remains to be determined. The identification of MV receptors has advanced our understanding of MV tropism and pathogenesis.

Measles virus interaction with host cells and impact on innate immunity

Because viruses are obligate parasites, numerous partnerships between measles virus and cellular molecules can be expected. At the entry level, measles virus uses at least two cellular receptors, CD150 and a yet to be identified epithelial receptor to which the virus H protein binds. This dual receptor strategy illuminates the natural infection and inter-human propagation of this lymphotropic virus. The attenuated vaccine strains use CD46 as an additional receptor, which results in a tropism alteration. Surprisingly, the intracellular viral and cellular protein partnership leading to optimal virus life cycle remains mostly a black box, while the interactions between viral proteins that sustain the RNA-dependant RNA polymerase activity (i.e., transcription and replication), the particle assembly and the polarised virus budding are documented. Hsp72 is the only cellular protein that is known to regulate the virus transcription and replication through its interaction with the viral N protein. The viral P protein is phosphorylated by the casein kinase II with undetermined functional consequences. The cellular partnership that controls the intracellular trafficking of viral components, the assembly and/or the budding of measles virus, remains unknown. The virus to cell innate immunity war is better documented. The 5' triphosphate-ended virus leader transcript is recognised by RIG-I, a cellular helicase, and induces the interferon response. Measles virus V protein binds to the MDAS helicase and prevents the MDA5-mediated activation of interferon. By interacting with STAT1 and Jak1, the viral P and V proteins prevent the type I interferon receptor (IFNAR) signalling. The virus N protein interacts with eIF3-p40 to inhibit the translation of cellular mRNA. The H protein binds to TLR2, which then transduces an activation signal and CD150 expression in monocytes. The P protein activates the expression of the ubiquitin modifier A20, thus blocking the TLR4-mediated signalling. Few other partnerships between measles virus components and cellular proteins have been postulated or demonstrated, and they need further investigations to understand their physiopathological outcome.

Measles virus for cancer therapy

Measles virus offers an ideal platform from which to build a new generation of safe, effective oncolytic viruses. Occasional so-called spontaneous tumor regressions have occurred during natural measles infections, but common tumors do not express SLAM, the wild-type MV receptor, and are therefore not susceptible to the virus. Serendipitously, attenuated vaccine strains of measles virus have adapted to use CD46, a regulator of complement activation that is expressed in higher abundance on human tumor cells than on their nontransformed counterparts. For this reason, attenuated measles viruses are potent and selective oncolytic agents showing impressive antitumor activity in mouse xenograft models. The viruses can be engineered to enhance their tumor specificity, increase their antitumor potency, and facilitate noninvasive in vivo monitoring of their spread. A major impediment to the successful deployment of oncolytic measles viruses as anticancer agents is the high prevalence of preexisting anti-measles immunity, which impedes bloodstream delivery and curtails intratumoral virus spread. It is hoped that these problems can be addressed by delivering the virus inside measles-infected cell carriers and/or by concomitant administration of immunosuppressive drugs. From a safety perspective, population immunity provides an excellent defense against measles spread from patient to carers and, in 50 years of human experience, reversion of attenuated measles to a wild-type pathogenic phenotype has not been observed. Clinical trials testing oncolytic measles viruses as an experimental cancer therapy are currently underway.

In vitro and in vivo properties of adenovirus vectors with increased affinity to CD46

Gene transfer vectors containing adenovirus (Ad) serotype 35 (Ad35) fibers have shown promise for cancer and stem cell gene therapy. In this study, we attempted to improve the in vitro and in vivo infection properties of these vectors by increasing their affinity to the Ad35 fiber receptor CD46. We constructed Ad vectors containing either the wild-type Ad35 fiber knob (Ad5/35) or Ad35 knob mutants with 4-fold- and 60-fold-higher affinity to CD46 (Ad5/35+ and Ad5/35++, respectively). In in vitro studies with cell lines, the higher affinities of Ad5/35+ and Ad5/35++ to CD46 did not translate into correspondingly higher transduction efficiencies, regardless of the CD46 receptor density present on cells. However, in vivo, in a mouse model with preestablished CD46(high) liver metastases, intravenous injection of Ad5/35++ resulted in more-efficient tumor cell transduction. We conclude that Ad5/35 vectors with increased affinity to CD46 have an advantage in competing with non-CD46-mediated sequestration of vector particles after intravenous injection.