Recombinant viruses as vaccines against viral diseases

Vaccine approaches to infectious diseases are widely applied and appreciated. Amongst them, vectors based on recombinant viruses have shown great promise and play an important role in the development of new vaccines. Many viruses have been investigated for their ability to express proteins from foreign pathogens and induce specific immunological responses against these antigens in vivo. Generally, gene-based vaccines can stimulate potent humoral and cellular immune responses and viral vectors might be an effective strategy for both the delivery of antigen-encoding genes and the facilitation and enhancement of antigen presentation. In order to be utilized as a vaccine carrier, the ideal viral vector should be safe and enable efficient presentation of required pathogen-specific antigens to the immune system. It should also exhibit low intrinsic immunogenicity to allow for its re-administration in order to boost relevant specific immune responses. Furthermore, the vector system must meet criteria that enable its production on a large-scale basis. Several viral vaccine vectors have thus emerged to date, all of them having relative advantages and limits depending on the proposed application, and thus far none of them have proven to be ideal vaccine carriers. In this review we describe the potential, as well as some of the foreseeable obstacles associated with viral vaccine vectors and their use in preventive medicine.

Isolation and characterization of soluble antigens of sheep poxvirus.

A soluble antigen fraction of sheep poxvirus (SPV) isolated from infectious virus particles by ultracentrifugation and purified by subtractive immunoaffinity chromatography was characterized. Exclusion chromatography studies revealed 10 proteins of molecular weight (MW) 220, 168, 87.3, 71.5, 52.5, 36.7, 31.0, 23.4, 18.3 and 14.2 kDa. Nine of them were found to be precipitinogens and 5 were identified as structural components of the virus particles. SDS-PAGE analysis revealed a polypeptide profile of 10 bands with 2 prominent polypeptides of 64 and 42 kDa. Western blotting, however, detected 2 immunogenic polypeptides of MW 100 and 64 kDa. Moreover, crossed immunoelectrophoresis showed the presence of proteins of varied electrophoretic mobility and sharing of antigenic determinants among a few soluble antigens. Physico-chemical characterization further revealed that these precipitinogens can withstand ambient temperatures, but were sensitive to trypsin and ether whereas, chloroform had no effect on immunoprecipitation pattern of soluble antigens.

Polyclonal B-lymphocyte activation of sheep by Mycobacterium phlei against sheep pox virus.

A trypsinized preparation of Mycobacterium phlei, non specific stimulator of immunity (NSI), and Sheep Pox Virus (SPV) were inoculated in different groups of sheep to activate B-lymphocytes and induce SPV neutralizing substance(s). NSI sensitized sheep B-lymphocytes in the presence of NSI or lymphokine elaborated SPV neutralizing substance(s). The SPV sensitized B-lymphocytes also mediated such neutralizing substance(s). Healthy control sheep B-lymphocytes failed to show any appreciable amount of viral neutralizing substance. However, a significant virus neutralizing substance(s) was detected when healthy sheep B-lymphocytes were cultured in presence of NSI antigen along with lymphokines.

Enhanced production of antibody with specific antigen.

On inoculation of nonspecific stimulator of immunity (NSI), prepared from Mycobacterium phlei (M. phlei), simultaneously along with sheep pox virus (SPV) in sheep, the recipient has exhibited appreciable level of SPV specific antibody as early as on 10th day which reached at peak level on 20th day and remained unaltered on 30th day of postimmunisation as evinced by serum neutralisation test (SNT), enzyme linked immunosorbant assay (ELISA) indirect, fluorescent antibody technique (FAT) indirect, counter immunoelectrophoresis (CIEP) and finally by virulent SPV challenge. On the contrary, sheep, when immunised with SPV only could not produce appreciable level of antibody on 10th day but did so on 20th day of inoculation. SPV and NSI immunised sheep produced enhanced protection against virulent SPV challenge in comparison with sheep immunised with SPV only. Healthy control sheep, however, could not resist challenge.

Effects of cyclophosphamide on pigs infected and vaccinated with swinepox virus.

Piglets treated with cyclophosphamide (50 mg/kg body wt, iv) before infection/vaccination with swinepox virus showed a strong suppression of humoral immune response. Cell mediated immune response was also affected to some extent and the cyclophosphamide treated piglets did not resist the challenge on 21 day post infection/vaccination and developed mild lesions.

Immune response induced by Vero cell culture adapted buffalo pox virus in rabbits and buffaloes.

The Vero cell culture adapted buffalo pox virus was found to be completely attenuated at 40th passage for rabbits as well as buffaloes since it did not produce any thermal reaction or skin lesions. It induced high level of humoral and cell mediated immune response in rabbits as well as buffaloes. The antibody titres obtained were 80-160 for SN antibody, 32 for complement fixing and 640-1280 for enzyme immunoassay antibodies. The percent migration inhibition (MI) of leukocytes was 65.3% in rabbits and 69.50% in buffaloes, MI of macrophages was 62.15% in rabbits and 63.02% in buffaloes with a high skin reactive factor value. In protection tests conducted in rabbits and buffaloes, all the vaccinated animals were immune as compared to controls which showed severe disease.