Modified vaccinia virus Ankara as antigen delivery system: how can we best use its potential?

Abstract

Safety-tested modified vaccinia virus Ankara (MVA) has been established as a potent vector system for the development of candidate recombinant vaccines. The versatility of the vector system was recently demonstrated by the rapid production of experimental MVA vaccines for immunization against severe acute respiratory syndrome associated coronavirus. Promising results were also obtained in the delivery of Epstein-Barr virus or human cytomegalovirus antigens and from the clinical testing of MVA vectors for vaccination against immunodeficiency virus, papilloma virus, Plasmodium falciparum or melanoma. Moreover, MVA is considered to be a prime candidate vaccine for safer protection against orthopoxvirus infections. Thus, vector development to challenge dilemmas in vaccinology or immunization against poxvirus bio-threat seems possible, yet the right choice should be made for a most beneficial use.

Figure 1

The generation of recombinant MVA. (a) Schematic representation of an MVA particle on the left and the MVA transfer plasmid on the right. MVA DNA sequences adjacent to the deletion site (MVA-flank1, MVA-flank2) were cloned into the plasmid and target genes are inserted between these sequences and placed under transcriptional control of vaccinia virus-specific promoters (VV-P). Recombinant MVA are generated by infection of chicken embryo fibroblast (CEF) or baby hamster kidney (BHK-21) cells with MVA and concurrent transfection with transfer vectors, resulting in recombination between homologous DNA sequences of vector and virus sequences. (b) An MVA-infected, transfected cell. Schematic map of the MVA genome and plasmids designed for the insertion of foreign DNA. Sites of the restriction endonuclease Hind III within the genome of MVA are indicated at the top. The positions of the naturally occurring deletions II, III, and VI are marked by arrows. MVA DNA sequences adjacent to the deletion sites (flank1, flank2) were cloned into plasmids to generate the pII, pIII and pVI transfer vectors.

Figure 2

Comparative monitoring of epitope-specific CD8⁺ T-cell responses directed against vaccinia virus and recombinant antigens. Quantitation of epitope-specific T cells was performed following one immunization of humanized HLA-A*0201-transgenic mice with 10⁸ IU of either wild-type MVA (MVA-wt), recombinant MVA-TYR or MVA-H2N expressing the human tumor antigens tyrosinase and Her-2/neu, respectively. Ten days post vaccination, peptide-specific intracellular cytokine release of splenocytes was determined after stimulation with vaccinia epitope VP35#1 (VV), tyrosinase (TYR) or Her-2/neu peptides (H2N). Cells were analyzed by flow cytometry for the presence of peptide-specific, activated (CD62L^low) CD8⁺ T cells. The magnitude of the specifically induced T-cell response is depicted as the cells shifted to the lower right and indicated as a percentage (numbers in blue) of interferon-γ-secreting CD8⁺ T cells within the CD8⁺ cell population.