The market for veterinary biologicals in the United States and Europe was approximately US$ 2.4 billion in 1998, an estimate that does not include large markets in Asia, such as China and India. Animal vaccines are responsible for preventing diseases and reducing losses, and thus have a major impact on the animal industry; both attenuated and inactivated conventional vaccines have contributed enormously to animal health worldwide. Unfortunately, the two most important properties of vaccines, namely their high antigenicity and absence of adverse side effects, tend to be mutually incompatible. Although attenuated vaccines stimulate the best immune response, they may present hazards of residual and reverted virulence. Conversely, whereas inactivated vaccines may be safer, they are usually poorer immunogens. The main focus of vaccine research today is to replace these conventional vaccines with new more effective and safe technologies, such as DNA vaccines. In this issue, Lorenzen et al.1 present what is to my knowledge the first example of inducing systemic protective immunity using a DNA vaccine to deliver genes encoding a protective antibody.
Unfortunately, the titer of antibodies induced by DNA vaccines in various disease models has been low, compared with that induced by conventional vaccines3, which has prompted the development of alternative strategies to boost their efficacy. One such approach delivers pathogen-specific antibodies, termed “intracellular antibodies,” into the host, instead of directly delivering pathogen-specific antigens. In short, the concept of intracellular antibodies is based on expression of an antibody fragment inside the cell that can bind to and inactivate a pathogen (Fig. 1). In comparison to passive immunization, which involves transferring antibodies from a resistant to a susceptible animal in order to confer immunity, intracellular antibodies offer a more sustained immunity half-life, which allows them to more effectively eliminate the pathogen while avoiding the risk of transferring infectious pathogens from the donor animal, a danger inherent in passive immunization. Indeed, the concept of intracellular antibodies delivered by viral vectors has been shown to have potential use in anti-HIV therapy4.
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