STUDIES ON HOST-VIRUS INTERACTIONS IN THE CHICK .: EMBRYO-INFLUENZA VIRUS SYSTEM* IlL DEVELOPMENT OF INFECTIVITY, HEMAGGLUTINATION, AND COMPLEMENT FIXATION ACTIVITIES DURING THE FIRST INFECTIOUS CYCLE

Certain of the experiments reported in the preceding paper suggested that propagation of influenza virus in or on the cells lining the aUantoic sac of the chick embryo can be divided into two major phases. According to this suggestion, the first phase would correspond to the production of some incomplete non-infectious forms or building blocks of the virus which, in the second phase, are converted or combined into fully active virus (1). It seemed possible thai the material produced in the earlier phase might be endowed with some attributes of influenza virus other than infectivity. Consequently, attempts were made to study the development of hemagglutinins and of complement-fixing antigens. In order to be able to measure these properties during the early period after infection, i. e. during the first cycle, it was necessary to employ infective doses in the order of 109 ID60 of active virus. It has been found in previous experiments that the selection of the seed is of prime importance (2). If the inocula were prepared under adverse conditions such as permitted the accumulation of inactive virus in the allantoic fluids, passage of these without dilution was found to result in partial interference with the propagation of the active virus by the non-infective material. On the other hand, if the allantoic fluids intended for seed were harvested when the infectivity just reached its peak, or shortly before that time, they could be used undiluted for passage to new chick embryos without the apparent intervention of interference. It is obvious that the present study required seed prepared under the latter conditions. The experiments to be reported in this paper were conducted exclusively with the PR8 strain of influenza A virus. They show that production of both the virus (600S) and soluble (30S) complement fixation antigens (3) in the tissues can be demonstrated prior to the development of hemagglutinins which, in turn, precede the increase in infectivity. It will also be shown that the titer of hemagglutinins may increase in the allantoic fluid without a concomitant rise in infectivity and that under certain conditions suspensions of aUantoic mem-