Electron microscopy studies on proliferation of foot-and-mouth disease virus in cell cultures. I. Morphologic changes in cell nucleus]

The first morphological indication of FMD infection of a cell culture was in the nucleus. Components of nucleoli became segregated and were finally present only as remnants. It was not possible to distinguish different stages of segregation, as in the case of entero-virus infections, because of the rapidity of FMD virus proliferation. Following changes in nucleoli there was margination of chromatin. Particularly striking was an increase in interchromatin granules. Changes in the nuclear membrane seemed to facilitate the transfer of nuclear material to the cytoplasm. Strongly pronounced dilatation of the peri-nuclear cleft, like that seen in aphthae and other tissues, were rarely visible in infected cell cultures.     

Electron microscopy studies on the proliferation of foot-and-mouth disease virus in cell cultures. III. Morphogenesis in cytoplasm]

The previous parts have been concerned with the participation of the cell nucleus in the formation of the RNA of FMD virus. However, the actual morphogenesis of the virus takes place in cytoplasm. In BHK cells, changes attributable to virus infection were visible by the second hour, with the formation of threads and large polysome complexes near the nucleus. Viral particles soon appeared between these structures. There were no pronounced foci of viroplasma, and it seemed that they were not necessary. Simultaneously new membranes formed in the cell. Clumps of viral particles were next visible in the cxtoplasma. The clumps became enveloped and were transported in this way to the periphery of the cell. Elsewhere there was uptake of particles in autophagic vacuoles, an expression of cellular defensive processes. In ultra-thin sections the virions measured 21-25 nm. Within vacuoles the inner part of the virus, the nucleoid, showed greater contrast than the periphery, the capsid. At first there were only slight changes in mitochondria. Liberation of virus by cell rupture occurred only after severe damage to the cell, particularly the lysosome membranes.     

Sensitivity of seven different types of cell cultures to three serotypes of foot-and-mouth disease virus.

The ability of bovine tongue origin foot-and-mouth disease virus serotypes A, O and C to replicate in seven different types of cell cultures was studied. Primary and secondary calf thyroid cells were equivalent in susceptibility to bovine kidney cell cultures passaged up to five times. Calf thyroid cells lost their susceptibility after two passages. Cryopreserved bovine kidney cell cultures passaged three and four times were equivalent in susceptibility to sensitive calf thyroid and bovine kidney cells. Susceptibility to foot-and-mouth disease virus serotype C was most variable among the cells tested. Lamb testicle and porcine kidney cells were susceptible to foot-and-mouth disease virus while goat and calf testicle and calf lung cells were refractory.     

Electron microscopy studies on the proliferation of foot-and-mouth disease virus in cell cultures. II. Changes in nucleic acid metabolism of the cell nucleus]

BHK cells were infected with FMD virus and treated with tritium-labelled thymidine and uridine for examination by autoradiography under the electron microscope. Labelling of the DNA, examined by autoradiography under the optical microscope, showed inhibition of 3H-thymidine incorporation. For demonstrating RNA labelling of nuclei, some cells were treated with actinomycin D and others were left untreated. Under the lectron microscope there was no evidence of increased 3H-uridine incorporation in the untreated cells after virus infection, but actinomycin treatment increased RNA labelling in extranucleolar parts of the nucleus, evidently RNA synthesis independent of DNA. There was evidence of some synthesis of virus-specific RNA in the nuclei. The extent of virus-specific RNA synthesis in the cytoplasm was less extensive than in the nucleus.