Nonreciprocal and host-dependent interference between two variants of influenza virus

Two variants of the WSN strain of influenza virus, designated F and C because they produce distinct plaque types on bovine kidney (MDBK) cells, have been found to exhibit nonreciprocal and host-dependent inteference. The interference is mediated by C particles obtained from either MDBK cells or chick embryo fibroblasts (CEF). It does not operate in CEF, and in MDBK cells the C variant is always the interference-inducer regardless of which virus enters the cell first. Standard virus prepared in CEF contains at least 20 interfering units per plaque-forming unit. Increasing the particle to infectivity ratio of the C preparation increases the number of interfering particles per plaque-forming unit. UV-inactivated C virions retain the capacity to interfere with F synthesis. Strains of virus with limited capacity to grow in MDBK cells also interfere with the synthesis of F but not of C. Preinfection of MDBK cells with F results in enhancement of early yields of C. This is not observed in CEF. Enhancement can be induced by F nonplaque formers but not by uv-inactivated virus or by other strains of influenza virus. The experiments suggest that C virions interfere with an early step in the replication of F by usurping components which are essential for virus replication and which are presented in limiting amounts in infected MDBK cells