SUMMARY

Various deletions were introduced into a cloned subgenomic fragment (BamHI-7), located in the unique short (Us) region of the DNA from the virulent Northern Ireland Aujeszky-3 (NIA-3) strain of pseudorabies virus (PRV). In the cloned HindIII-B fragment, he MluI-BglII fragment was replaced by different MluI-BgllI fragments of the deleted BamHI-7 clones. Transfection of the deleted HindIII-B fragments together with the HindlII-A fragment of either the NIA-3 or the non-virulent NIA-4 strain yielded replication-competent deletion mutants. The region in Us in which sequences were deleted specified several mRNAs. Some of the mRNAs present in cells infected with NIA-3 were absent from cells infected with the deletion mutants, whereas other differently sized mRNAs were generated. The mutants were examined with respect to their biological properties in cell culture, mice and pigs. The results howed that (i) the type of cytopathic effect induced in cell culture seemed to be determined by the UL region, (ii) using the mean time to death in mice as a parameter, markers for virulence were present in the Us and UL regions and (ii) the introduction of deletions in Us strongly reduced the virulence of PRV for pigs. Despite the impaired capacity of the deletion mutants to induce high titres of neutralizing antibodies in the serum, inoculation with mutants derived from NIA-3 prevented clinical disease in pigs upon challenge with the virulent parent strain. These deletion mutants provide a good basis for the production of bioengineered live PRV vaccines