An in vitro study was undertaken to characterize the mechanisms by which the coronavirus, mouse hepatitis virus (MHV), mediates persistent infection. MHV persists in cultures of mouse fibroblast LM cells (designated LMTK(\u27-)), but produces a lytic infection in L-2 cells. Persistence in LMTK(\u27-) cells was not accompanied by the production of virus mutants nor soluble antiviral factors. Rather, LMTK(\u27-) cells possess two characteristics which allow cultures to support MHV persistence: a reduced level of initial infectability, and a resistance to virus-induced cell fusion. Similar host-cell determinants were found to be operative in persistent MHV infection of another mouse LM cell line, LM-ATCC. A semi-permissive category of host-cells was defined on the basis of reduced infectability by MHV. However, within this category, cell fusion-resistant (LMTK(\u27-) and LM-ATCC) and permissive (mouse neuroblastoma; C-1300) hosts were identified. Evidence is presented that cell fusion by MHV is activated by a cellular chymotrypsin-like enzyme resulting in cleavage of the viral E(,2) glycoprotein (180K MW) to a 90K MW form. Induction of cell fusion by proteolytically-activated E(,2) was not observed in fusion-resistant host cells, due to some inherent property of the plasma membrane. Ammonium chloride, an agent reported to inhibit both virus penetration and cell fusion, was employed in an attempt to convert the acute MHV infection of L-2 cells to a state of persistence. Evidence for an endosomal uncoating mechanism by MHV was found on the basis of uncoating inhibition by ammonium chloride. However, all other parameters of MHV replication, while chronologically displaced due to inhibition of virus uncoating were not otherwise inhibited by ammonium chloride, and the infection followed a lytic, fusogenic course. The expression of cell fusion in permissive L-2 cells was found to be associated with increased cellular permeability to sodium ions. The results of cell-free translation studies indicated that preferential synthesis of MHV proteins occurs in the presence of elevated sodium ion concentration
