Antiviral activity of an intracellularly expressed single-chain antibody fragment directed against the murine leukemia virus capsid protein

A nucleic-acid hydrolyzing single chain antibody confers resistance to DNA virus infection in hela cells and C57BL/6 mice

Viral protein neutralizing antibodies have been developed but they are limited only to the targeted virus and are often susceptible to antigenic drift. Here, we present an alternative strategy for creating virus-resistant cells and animals by ectopic expression of a nucleic acid hydrolyzing catalytic 3D8 single chain variable fragment (scFv), which has both DNase and RNase activities. HeLa cells (SCH7072) expressing 3D8 scFv acquired significant resistance to DNA viruses. Virus challenging with Herpes simplex virus (HSV) in 3D8 scFv transgenic cells and fluorescence resonance energy transfer (FRET) assay based on direct DNA cleavage analysis revealed that the induced resistance in HeLa cells was acquired by the nucleic acid hydrolyzing catalytic activity of 3D8 scFv. In addition, pseudorabies virus (PRV) infection in WT C57BL/6 mice was lethal, whereas transgenic mice (STG90) that expressed high levels of 3D8 scFv mRNA in liver, muscle, and brain showed a 56% survival rate 5 days after PRV intramuscular infection. The antiviral effects against DNA viruses conferred by 3D8 scFv expression in HeLa cells as well as an in vivo mouse system can be attributed to the nuclease activity that inhibits viral genome DNA replication in the nucleus and/or viral mRNA translation in the cytoplasm. Our results demonstrate that the nucleic-acid hydrolyzing activity of 3D8 scFv confers viral resistance to DNA viruses in vitro in HeLa cells and in an in vivo mouse system.

Most strategies for developing virus-resistant transgenic cells and animals are based on the concept of virus-derived resistance, in which dysfunctional virus-derived products are expressed to interfere with the pathogenic process of the virus in transgenic cells or animals. However, these viral protein targeting approaches are limited because they only target specific viruses and are susceptible to viral mutations. We describe a novel strategy that targets the viral genome itself, rather than viral gene products, to generate virus-resistant transgenic cells and animals. We functionally expressed 3D8 scFv which has both DNase and RNase activities, in HeLa cells and transgenic mice. We found that the transgenic cells and mice acquired complete resistance to two DNA viruses (HSV and PRV) without accumulating the virus, and showed delayed onset of disease symptoms. The antiviral effects against DNA viruses demonstrated in this study were caused by (1) DNase activity of 3D8 scFv in the nucleus, which inhibited DNA replication or RNA transcription and (2) 3D8 scFv RNase activity in the cytoplasm, which blocked protein translation. This strategy may facilitate control of a broad spectrum of viruses, including viruses uncharacterized at the molecular level, regardless of their genome type or variations in gene products.

Immunotherapy of viral infections

Among the microorganisms that cause diseases of medical or veterinary importance, the only group that is entirely dependent on the host, and hence not easily amenable to therapy via pharmaceuticals, is the viruses. Since viruses are obligate intracellular pathogens, and therefore depend a great deal on cellular processes, direct therapy of viral infections is difficult. Thus, modifying or targeting nonspecific or specific immune responses is an important aspect of intervention of ongoing viral infections. However, as a result of the unavailability of effective vaccines and the extended duration of manifestation, chronic viral infections are the most suitable for immunotherapies. We present an overview of various immunological strategies that have been applied for treating viral infections after exposure to the infectious agent.

The production and application of single-chain antibody fragments

This review discusses methods for the single-chain antibody fragment ($cFv) generation and scFv expression systems, and describes potential applications of scFv in the therapy of viral diseases and cancer, with emphasis on intracellularly expressed scFvs (intrabodies), application of scFvs in detection and diagnostics, and their use in proteomics.

Efficient cell infection by Moloney murine leukemia virus-derived particles requires minimal amounts of envelope glycoprotein

Retrovirus entry into cells is mediated by specific interactions between the retrovirally encoded Env envelope glycoprotein and a host cell surface receptor. Though a number of peptide motifs responsible for the structure as well as for the binding and fusion activities of Env have been identified, only a few quantitative data concerning the infection process are available. Using an inducible expression system, we have expressed various amounts of ecotropic and amphotropic Env at the surfaces of Moloney murine leukemia virus-derived vectors and assayed for the infectivity of viral particles. Contrary to the current view that numerous noncooperative Env-viral receptor interactions are required for cell infection, we report here that very small amounts of Env are sufficient for optimal infection. However, increasing Env density clearly accelerates the rate at which infectious attachment to cells occurs. Moreover, our data also show that a surprisingly small number of Env molecules are sufficient to drive infection, albeit at a reduced efficiency, and that, under conditions of low expression, Env molecules act cooperatively. These observations have important consequences for our understanding of natural retroviral infection as well as for the design of cell-targeted infection techniques involving retroviral vectors.

Recombinant antibodies: a novel approach to cancer diagnosis and therapy

Recombinant antibodies and their fragments currently represent over 30% of all biological proteins undergoing clinical trials for diagnosis and therapy. These reagents dominate the cancer-targeting field, as highlighted by the recent approval of the first engineered therapeutic antibodies by the Food and Drugs Administration (FDA). Last year, important advances have been made in the design, selection and production of recombinant antibodies. The natural immune repertoire and somatic cell affinity maturation has been superseded by large antibody display libraries and rapid molecular evolution strategies. These novel libraries and selection methods have enabled the rapid isolation of high-affinity cancer targeting and antiviral antibodies, the latter capable of redirecting viruses for gene therapy applications. In alternative strategies for cancer diagnosis and therapy, recombinant antibody fragments have been fused to radioisotopes, drugs, toxins, enzymes and biosensor surfaces. Antibody-directed cancer pre-targeting followed by prodrug activation (ADEPT) has proved a most promising therapeutic strategy. Multi-specific antibodies have been effective for cytotoxic T-cell recruitment and antibody-fusion proteins have delivered enhanced immunotherapeutic and vaccination strategies. The new millennium is indeed an exciting time for the design, selection and formulation of a range of new antibody-based products for cancer diagnosis and therapy.