STUDIES ON THE MECHANISM OF ACTION OF RILEY VIRUS III. REPLICATIOI~ OF RILEY'S PLASlA ENZYM~.-ELEvATING VIRUS IN VITRO

The first report of replication of Riley's plasma enzyme-elevating virus (Riley virus) in tissue culture was by Yaffe (1), 2 years after the discovery of the virus in 1960 (2). Yaffe found that Riley virus multiplied in primary mouse embryo cultures until the death of the cultures through overcrowding (3 to 4 weeks), without producing cytopathic or other detectable changes, and could be serially passaged indefinitely by transference of supernatant fluid at weekly intervals to fresh primary cultures. However, when an infected culture was serially trypsinized and replated, infective Riley virus was lost after the 2nd passage, and the passaged cultures were insusceptible to a 2nd infection. Similarly, passaged uninfected cultures also became insusceptible to infection. Subsequently , preliminary experiments were reported by Plagemann et al. (3) which suggested that replication of Riley virus occurred in cultures of mouse embryo, spleen, lung, and kidney. No detailed account of this work has been published. Georgii et al. (4, 5) failed to propagate Riley virus in primary mouse embryo cultures for longer than 14 days, and concluded that Riley virus merely survived without replicating in such cultures. Their results were in marked contrast to those of Yaffe (1). As they explained, two possible reasons for this might be considered: differences of method, or the existence of various plasma enzyme-elevating viruses differing in their biological properties. Anderson et al. (6) have recently reported propagation of Riley virus in primary mouse embryo cultures for as long as 88 days. In this laboratory evidence has been obtained of true replication of Riley virus in primary mouse embryo cultures, but only for 8 to 12 days. Yaffe's observation that replated cultures would not support replication was confirmed. Similar temporary growth occurred in primary mouse embryo liver, and adult spleen, but not in Hela, Rhesus monkey kidney, or rat peritoneal macrophage cultures. An hypothesis which would explain the limitation of growth of the virus in 993