Natural and genetically modified oncolytic viruses have been systematically tested as anticancer therapeutics. Among this group, conditionally replicative adenoviruses have been developed for a broad range of tumors with a rapid transition to clinical settings. Unfortunately, clinical trials have shown limited antitumor efficacy partly due to insufficient viral delivery to tumor sites. We investigated the possibility of using mesenchymal progenitor cells (MPC) as virus carriers based on the documented tumorhoming abilities of this cell population. We confirmed preferential tumor homing of MPCs in an animal model of ovarian carcinoma and evaluated the capacity of MPCs to be loaded with oncolytic adenoviruses. We showed that MPCs were efficiently infected with an adenovirus genetically modified for coxsackie and adenovirus receptorindependent infection (Ad5/3), which replicated in the cell carriers. MPCs loaded with Ad5/3 caused total cell killing when cocultured with a cancer cell line. In an animal model of ovarian cancer, MPC-based delivery of the Ad5/3 increased the survival of tumor-bearing mice compared with direct viral injection. Further, tumor imaging confirmed a decrease in tumor burden in animals treated with oncolytic virus delivered by MPC carriers compared with the direct injection of the adenovirus. These data show that MPCs can serve as intermediate carriers for replicative adenoviruses and suggest that the natural homing properties of specific cell types can be used for targeted delivery of these virions. [Mol Cancer Ther 2006;5(3):755 -66]

Formation of virus-specific replicative complexes (RCs) in infected cells is one of the most intriguing and important processes that determine virus replication and ultimately their pathogenesis on the molecular and cellular levels. Alphavirus replication was known to lead to formation of so-called type 1 cytopathic vacuoles (CPV1s), whose distinguishing feature is the presence of numerous membrane invaginations (spherules) and accumulation of viral nonstructural proteins (nsPs) at the cytoplasmic necks of these spherules. These CPV1s, modified endosomes and lysosomes, were proposed as the sites of viral RNA synthesis. However The Alphavirus genus of the Togaviridae family contains almost 30 currently known members, which are distributed all over the world and are grouped into eight serocomplexes (57). Alphaviruses cause a variety of diseases ranging from mild rash and arthritis to a serious febrile illness and encephalitis (27) that may result in death and severe neurological disorders (7,21). In natural conditions, alphaviruses are transmitted between vertebrate hosts by mosquito vectors. In mosquitoes, they develop a persistent, life-long infection (4) characterized by the presence of infectious virus in the salivary glands, facilitating the infection of avian or mammalian hosts during the mosquito's blood meal. In infected vertebrate hosts, alphaviruses induce an acute infection, which leads to the high-titer viremia required for infection of new mosquitoes during blood ingestion. Replication in cultured cells mirrors the natural transmission cycle: alphaviruses develop a highly productive, cytopathic infection in cells of vertebrate origin, characterized by numerous modifications of the intracellular environment and cell death within 24 to 48 h postinfection (55). In mosquito cells, they develop a noncytopathic, persistent or chronic infection that also results in high titer virus release.Thus, alphaviruses are capable of efficient replication in fundamentally different types of cells (of insect and vertebrate origin) and appear to utilize a number of diverse host protein factors required for numerous processes in viral replication. Most importantly, in order to develop efficient spreading of infection in both cell types, they have to interfere with two different antiviral systems. In insect cells, the antiviral effect is determined mostly by double-stranded RNA (dsRNA)-mediated RNA interference (RNAi) (47). On the other hand, in vertebrate cells, the antiviral response is characterized by the induction of hundreds of cellular genes and is activated by pattern recognition receptors (PRRs), which detect virus-specific dsRNA molecules, synthesized during virus replication, and other virus-specific products and processes (29).The alphavirus genome is represented by a single-stranded RNA of positive polarity that is almost 11.5 kb in length (53,54). It mimics the structure of cellular mRNAs, in that it contains a cap at the 5Ј terminus and a poly(A) tail at the 3Ј terminus. This RNA contains two open reading fr...

An approach combining virology with light and electron microscopy was used to study the organs of guinea pigs during nine serial passages of Ebola virus, strain Zaire. It was observed that the wild type of Ebola virus causes severe granulomatous inflammation in the liver and reproduces in the cells of the mononuclear phagocyte system (MPS). Based on morphological characterization, two types of virus-cell interactions were demonstrated. The obtained data evidenced for heterogeneity of the population of wild type of Ebola virus. The virus accumulated in the liver of the infected animals, and the lesions became more pronounced with passage. Degenerative changes appeared, and their severity was increased with passage in the other organs as well. The set of target cells diversified and, as a result, not only the MPS cells, but also hepatocytes, spongiocytes, endotheliocytes and fibroblasts became involved in the reproduction of Ebola virus. The possible role of granulomatous inflammation in the development of the adaptive mechanism of Ebola virus to guinea pigs is discussed.

ABSTRACT

Use of cells as therapeutic carriers has increased in the past few years and has developed as a distinct concept and delivery method. Cell-based vehicles are particularly attractive for delivery of biotherapeutic agents that are difficult to synthesize, have reduced half-lives, limited tissue penetrance or are rapidly inactivated upon direct in vivo introduction. Initial studies using cell-based approaches served to identify some of the key factors for the success of this type of therapeutic delivery. These factors include the efficiency of cell loading with a therapeutic payload, the means of cell loading and the nature of therapeutics that cells can carry. However, one important aspect of cell-based delivery yet to be fully investigated is the process of actual delivery of the cell payload in vivo. In this regard, the potential ability of cell carriers to provide site-specific or targeted delivery of therapeutics deserves special attention. The present review focuses on a variety of targeting approaches that may be utilized to improve cell-based therapeutic delivery strategies. The different aspects of targeting that can be applied to cell vehicles will be discussed, including physical methods for directing cell distribution, intrinsic cell-mediated homing mechanisms and the feasibility of engineering cells with novel targeting mechanisms. Development of cell targeting strategies will further advance cell vehicle applications, broaden the applicability of this delivery approach and potentiate therapeutic outcomes.