Attenuation of aerosolized yellow fever virus after passage in cell culture.

Reports in the literature, recently reviewed by Mussgay (8), contain numerous examples of alterations in the properties of arboviruses as a result of their passage in various host systems. Countless other observations of this type undoubtedly have been made but not reported. Early accounts of alterations in virulence of yellow fever virus (YFV) as a result of propagation in vitro have been described by Lloyd, Theiler, and Ricci (6) and by Theiler and Smith (11). Studies by Theiler and his associates led to the isolation and eventual use of the well-known 17-D strain for human vaccination. A more detailed description of the development of various attenuated strains of YFV may be found in a review by Theiler (10). More recently, Hallauer (1) reported losses in virulence of the 17-D strain for mice anid of the Asibi strain for monkeys after passage in KB cell cultures. Following this, Schindler and Hallauer (9) described additional losses in the viscerotropism of Asibi strain variants obtained after prolonged passage in KB cells. These authors also reported the isolation from human cell lines of a 17-D viral substrain whose virulence for monkeys was reduced to a degree that has apparently not been found in any other 17-D strains. Hardy (2) demonstrated attenuation of the Asibi strain for monkeys with viral isolates obtained after serial passage in HeLa cells. In keeping with the general subject of this symposium, we should like to present some observations on changes in the properties of yellow fever virus that were discernible upon aerosolization of preparations after growth in HeLa cells. A major consideration in determining our experimental approach was the prevailing lack of information on the behavior of airborne virus that previously had undergone routine passages in cell culture in the laboratory. Information of this kind has broad applications not only for the viral geneticist but for those who are engaged in the problems of laboratory safety. To carry out a meaningful study, it was recognized that a comparison of properties of virus when in suspension with those of virus that was aerosolized was needed. The results will show that the dominant characteristic of virus that was serially passed in cell culture was its decline in virulence. Other properties that were also found to change appeared to vary in parallel with the loss in virulence. It seems probable that some of these alterations may have resulted in, or at least contributed to, the decline in capability of the virus to induce lethal infections.