Ultrastructural changes in cells induced by temperature-sensitive mutants of fowl plague virus at permissive and non-permissive temperature.

Ultrastructural changes developing in chick embryo fibroblast cultures infected with a wild-type strain of fowl plague virus (FPV) or one of six FPV temperature-sensitive (ts) mutants belonging to different complementation groups were studied. Cells infected with wild-type FPV and incubated at optimal (36 degrees C) or nonpermissive temperature (42 degrees C) displayed changes similar to those described for orthomyxoviruses. The same patterns of changes were observed at 36 degrees C in cells infected with ts mutants belonging to five of the complementation groups. Mutant ts 303, possessing mutation-altered haemagglutinin, induced at 36 degrees C the formation of virions carrying a considerably reduced number of spikes on their surfaces. At 42 degrees C, cells infected with ts mutant 131, with a defective primary transcription stage, showed no morphological changes and no formation of electron-dense inclusions. Cells infected with ts mutants with defective secondary transcription or replication displayed nuclear inclusions but no formation of filamentous cytoplasmic structures or virions. Mutant ts 5 with defective late morphogenesis induced formation of considerably enhanced numbers of nuclear inclusions.