Evidence for two points of restriction in the expression of adenovirus type 2 in cultured epidermal keratinocytes

Cultures of epidermal keratinocytes contain two populations of cells, a basal undifferentiated population and a suprabasal terminally differentiated population. When exposed to wild-type adenovirus type 2 (wtAd2), the suprabasal cells are positive by immunofluorescence for capsid antigen and exhibit cytopathic effects (CPE) (R.F. LaPorta, and L.B. Taichman, Virology 110:137-146, 1981). The basal cells, although infected, are not positive for capsid antigen and do not display CPE. Despite CPE and capsid antigens in suprabasal cells, yields of virus from the entire culture are very low (10 PFU per cell). These observations suggest that Ad2 expression is restricted at different times in the viral life cycle in basal and suprabasal cells. To test this hypothesis, we isolated host range (hr) mutants of Ad2 on two lines of squamous cell carcinoma (SCC) keratinocytes which were shown to be restrictive for wtAd2 replication. The hrAd2 mutants produced high yields of progeny virus in epidermal cell cultures (500 to 600 PFU per cell). However, the pattern of CPE induction in these cultures was like that produced by wtAd2, i.e., basal cells were CPE negative and suprabasal cells were CPE positive. The high yield of hrAd2 progeny indicated that the restriction present in suprabasal cells was overcome. However, the failure of hrAd2 mutants to induce CPE in basal cells indicated that the hrAd2 mutants remain restricted in the basal population and supported our hypothesis that a second and distinct restriction exists in basal keratinocytes.