Ultrastructural comparison of Oncovirinae (type C), Spumavirinae, and Lentivirinae: three subfamilies of Retroviridae found in farm animals.

The successive steps of maturation of seven retroviruses from five species of farm animals and one retrovirus from a mouse were compared in cell cultures. The viruses included three type C oncoviruses, one spumavirus, and three lentiviruses. Although members of the 3 subfamilies shared some gross morphologic features such as budding on plasma membranes, core, and surface projections, differences were noted in the ultrastructural detail of these features. Type C oncoviruses did not show any structural differentiation in identifiable form in the cytoplasm as opposed to characteristic features observed in the spumavirus and lentivirus subfamilies, respectively. Budding viruses were distinct among the 3 subfamilies. The type C bovine leukemia virus budding on vacuole membranes differed from the two other type C viruses by lacking an electron-lucent intermediate layer as did the lentiviruses. Differentiation between type C oncoviruses and lentiviruses could be confusing because of the similarity of the fully mature virions appearing in the intercellular space. However, each subfamily of retroviruses can be readily differentiated from one another when each morphologic stage of virus replication is examined by electron microscopy.