Virion uncoating is a critical step in the life cycle of mammalian orthoreoviruses. In cell culture, and probably in extraintestinal tissues in vivo, reovirus virions undergo partial proteolysis within endosomal or/or lysosomal compartments. This process converts the virion into a form referred to as an intermediate subvirion particle (ISVP). In natural enteric reovirus infections, proteolytic uncoating takes place extracellularly within the intestinal lumen. The resultant proteolyzed particles, unlike intact virions, have the capacity to penetrate cell membranes and thereby gain access to cytoplasmic components required for viral gene expression. We hypothesized that the capacity of reovirus outer capsid proteins to be proteolyzed is a determinant of cellular host range. To investigate this hypothesis, we asked if the addition of protease to cell culture medium would expand the range of cultured mammalian cell lines that can be productively infected by reoviruses. We identified many transformed and nontransformed cell lines, as well as primary cells, that restrict viral infection. In several of these restrictive cells, virion uncoating is inefficient or blocked. Addition of proteases to the cell culture medium generates ISVP-like particles and promotes viral growth in nearly all cell lines tested. Interestingly, we found that some cell lines that restrict reovirus uncoating still express mature cathepsin L, a lysosomal protease required for virion disassembly in murine L929 cells. This finding suggests that factors in addition to cathepsin L are required for efficient intracellular proteolysis of reovirus virions. Our results demonstrate that virion uncoating is a critical determinant of reovirus cellular host range and that many cells which otherwise support productive reovirus infection cannot efficiently mediate this essential early step in the virus life cycle.Mammalian reoviruses (reoviruses) are prototypic members of the Reoviridae family, which includes the pathogenic rotaviruses, coltiviruses, and orbiviruses. Whereas reovirus causes infections that are generally asymptomatic in humans, it can induce respiratory, enteric, and nervous system diseases in animal models (reviewed in reference 67). Reovirus virions comprise a multilayered protein capsid that surrounds a segmented, double-stranded RNA genome (reviewed in reference 51). The outermost capsid layer consists of protein 3. The presence of 3 imparts environmental stability to the virion (54) but also appears to negatively regulate critical virion functions such as membrane penetration. The ability of reovirus to establish a productive infection requires proteolysis of the outer capsid (13,62,65). More recent data point toward 3 as the critical target for degradation (19,20).The first step in reovirus infection is attachment to cellular receptors through interactions with the viral receptor protein 1 (46, 75). Following attachment, virions are internalized by receptor-mediated endocytosis and delivered to endosomal compartments (12,13,65). In cell c...