Biological and biochemical studies on the inactivation of avian oncoviruses by ultraviolet irradiation

Predicted inactivation of viruses of relevance to biodefense by solar radiation

UV radiation from the sun is the primary germicide in the environment. The goal of this study was to estimate inactivation of viruses by solar exposure. We reviewed published reports on 254-nm UV inactivation and tabulated the sensitivities of a wide variety of viruses, including those with double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA genomes. We calculated D(37) values (fluence producing on average one lethal hit per virion and reducing viable virus to 37%) from all available data. We defined "size-normalized sensitivity" (SnS) by multiplying UV(254) sensitivities (D(37) values) by the genome size, and SnS values were relatively constant for viruses with similar genetic composition. In addition, SnS values were similar for complete virions and their defective particles, even when the corresponding D(37) values were significantly different. We used SnS to estimate the UV(254) sensitivities of viruses for which the genome composition and size were known but no UV inactivation data were available, including smallpox virus, Ebola, Marburg, Crimean-Congo, Junin, and other hemorrhagic viruses, and Venezuelan equine encephalitis and other encephalitis viruses. We compiled available data on virus inactivation as a function of wavelength and calculated a composite action spectrum that allowed extrapolation from the 254-nm data to solar UV. We combined our estimates of virus sensitivity with solar measurements at different geographical locations to predict virus inactivation. Our predictions agreed with the available experimental data. This work should be a useful step to understanding and eventually predicting the survival of viruses after their release in the environment.

Cell killing by ultraviolet-inactivated human immunodeficiency virus

Extensive cell killing and cytopathology were observed within 24 hr after exposure of a clonal cell line of human T-4 lymphocytes (RH9) to culture supernatants containing human immunodeficiency virus (HIV). Ultraviolet-irradiated HIV-containing culture fluids were also capable of killing RH9 cells and of inducing specific cytopathic effects which were indistinguishable from those induced by unirradiated virus-containing preparations. The uv-irradiated HIV was incapable of forming proviral DNA using the endogenous virion genomic RNA as a template. The RH9 cells persistently infected with HIV did not release soluble cytotoxic factors to account for the cell killing observed when culture supernatants were added to uninfected RH9 cells. The fraction involved in cell killing had the hydrodynamic properties of a retrovirus. These results suggest that a virion component is responsible for cell killing by HIV.

Infectious entry of murine retroviruses into mouse cells: evidence of a postadsorption step inhibited by acidic pH

The entry into cells by many enveloped RNA viruses is accomplished by endocytosis and subsequent penetration of the endosomal membrane by an acidic pH-dependent fusion event. In the current study, we examined early events in the infectious entry of mouse retroviruses, using as a framework the observation that infection of a mouse tail skin cell line by the ecotropic virus Friend murine leukemia virus was inhibited at mildly acidic pH (pH 6). This inhibition operated on a postadsorption step, since binding of virus was unaffected at this pH. The rate of penetration of preadsorbed virus, which displayed first-order kinetics, was markedly affected by changes in the pH of the medium. The half-time for disappearance of infectious cell surface virus at 37 degrees C was approximately 10 min at pH 7.6. At pH 6.0, however, greater than 98% of the adsorbed infectivity remained at the cell surface after 45 min. This cell surface virus, though not infecting the cell at pH 6.0, retained its capacity to enter and infect the cell when the pH of the medium was raised. Acidic pH had little effect on the rate of fluid uptake by the cells, as measured by internalization of [3H]sucrose, indicating that global inhibition of endocytosis had not occurred. In contrast, cell fusion induced by Friend murine leukemia virus was optimal at pH 7.6 but markedly inhibited at a pH of less than 6.4. This inhibitory effect of acidic pH on membrane fusion is unique among the enveloped viruses which have been studied and would preclude entry of Friend murine leukemia virus from within acidified endocytic vesicles. Entry of other members of the ecotropic, mink cell focus-forming, and xenotropic host range groups displayed similar pH sensitivity. However, one xenotropic virus was relatively resistant to the effect of acidic pH, suggesting that differences might exist in the requirements for entry of different retroviruses.

Early syncytium formation by bovine leukemia virus

Bovine leukemia virus (BLV) from either persistently infected bat cells or fetal lamb kidney cells induced rapid syncytium formation in F81 indicator cells. Distinct syncytia were seen within 2 h after inoculation of cells with highly concentrated (500-fold) cell-free BLV preparations and within 4 to 8 h when unconcentrated cell-free BLV preparations were used. Indicator cell densities of 1 x 10(5) to 2 x 10(5) were optimal for rapid and maximal syncytium formation. Pretreatment of BLV with reference BLV leukemic serum and antiserum prepared against purified BLV significantly inhibited (95%) syncytium formation. Reference bovine viral diarrhea virus serum, foamy-like bovine syncytial virus serum, and control serum had little effect (17% inhibition). Antiserum to BLV gp51 inhibited syncytium formation by greater than 96%, whereas antiserum to BLV p24 reduced syncytium activity to a much lesser extent (38% inhibition). Treatment of BLV with beta-propiolactone (0.005 to 0.05%) had little or no effect upon syncytium-forming activity, whereas UV irradiation (15 ergs/mm(2) per s for 30 min) reduced, but did not completely destroy, the fusion activity. However, both beta-propiolactone and UV irradiation drastically reduced the replication potential of BLV, as demonstrated by the lack of p24 expression in the inoculated cells. Concentrations of cycloheximide, cytosine arabinoside, tunicamycin, and 2-deoxy-D-glucose which effectively blocked cellular macromolecular synthesis did not significantly inhibit syncytium formation. These latter results suggested that de novo protein and DNA synthesis as well as protein glycosylation were not required for early syncytium formation. Thus, these experiments demonstrated that replication of BLV by the indicator cells was not essential for cell fusion.