The A59 strain of murine coronavirus mouse hepatitis virus (MHV) can cause persistent infection of 17C1-1 cells and other murine cell lines. Persistently infected cultures released large amounts of virus (10 7 to 10 8 PFU/ml) and were resistant to superinfection with MHV but not to infection with unrelated Semliki Forest and vesicular stomatitis viruses. The culture medium from persistently infected cultures did not contain a soluble inhibitor such as interferon that protected uninfected cells from infection by MHV or vesicular stomatitis virus. The persistent infection was cured if fewer than 100 cells were transferred during subculturing, and such cured cultures were susceptible to reinfection and the reestablishment of persistent infection.
Cultures of 17C1-1 cells that had been newly cloned from single cells consisted of a mixture of MHV-resistant and -susceptible cells. 17C1-1/#97 cells, which were cured by subcloning after 97 passages of a persistently infected culture over a 1-year period, contained 5 to 10% of their population as susceptible cells, while 17C1-1/#402 cells, which were cured by subcloning after 402 passages over a 3-year period, had less than 1% susceptible cells. Susceptibility to infection correlated with the expression of MHV receptor glycoprotein (MHVR [Bgp1 a ]). Fluorescenceactivated cell sorter analysis with antibody to MHVR showed that 17C1-1/#97 cells contained a small fraction of MHVR-expressing cells. These MHVR-expressing cells were selectively eliminated within 24 h after challenge with MHV-A59, and pretreatment of 17C1-1/#97 cells with monoclonal antibody CC1, which binds to the N-terminal domain of MHVR, blocked infection. We conclude that the subpopulation of MHVR-expressing cells were infected and killed in acutely or persistently infected cultures, while the subpopulation of MHVRnonexpressing cells survived and proliferated. The subpopulation of MHVR-negative cells produced a small proportion of progeny cells that expressed MHVR and became infected, thereby maintaining the persistent infection as a steady-state carrier culture. Thus, in 17C1-1 cell cultures, the unstable or epigenetic expression of MHVR permitted the establishment of a persistent, chronic infection.
MATERIALS AND METHODSCells and viruses. The spontaneously transformed mouse BALB/c 3T3 cells, 17C1-1 cells (28), and the A59 strain of MHV were kindly provided by L. Sturman. The 17C1-1 cells used in this study were cloned from a single cell and grown at 37ЊC and 7.5% CO 2 in Dulbecco's modified Eagle medium containing 6% fetal bovine serum (FBS), 5% tryptose phosphate broth (TPB), 500 U of penicillin per ml, and 100 U of streptomycin per ml (neutral pH growth medium). The cultures were routinely passaged every 3 days at a dilution of 1:60. The cells were detached from the monolayer by incubation in Hanks' balanced salt * Corresponding author.
