The role of inhibitors of hemagglutination in the evaluation of host-virus interactions in the chick embryo-influenza virus system has been analyzed. Comparisons were made between materials (allantoic fluids and membrane suspensions) derived from in vivo (growth curve) experiments at hourly intervals after inoculation, and from in vitro tests in which normal allantoic fluids and membrane suspensions were incubated with virus at 37°C. for various periods of time. In both instances large amounts of virus were added to the systems, resulting in comparable concentrations of the agent. The seeds employed were either fully active or irradiated by ultraviolet light to the extent that the virus lost its capacity to increase but kept its interfering and hemagglutinating properties. The various materials were assayed for (a) the hemagglutinating titers of the virus present in the systems before and after heating to 56°C.; (b) the concentration of inhibitor in the materials at various stages of incubation after heating to 70°C. for 30 minutes as measured by the hemagglutination-inhibition reaction with native or heated test virus (30 minutes 56°C.); and (c) the degree of adsorption of the hemagglutinins present in the materials onto chicken red cells at 0°C. and their subsequent elution at 37°C. The effects of receptor-destroying enzyme (RDE), treatment with sodium periodate, or high speed centrifugation on the inhibitory activities were studied in some of the tests.

The essential results which indicate certain sources of error in the evaluation of host-virus interactions as well as means for studying virus activity at the early stages of the infectious process, were as follows:

1. Though some inhibitory effects on hemagglutination were noticeable in the allantoic fluid during the 1st hour after inoculation they were, as a rule, no longer apparent after this interval, and treatment with RDE did not increase the hemagglutinin titers. Thus, the interpretation of growth curve data concerning allantoic fluids hardly seems to be affected by inhibitor. On the other hand, striking effects were noted with the membrane suspensions of growth curve experiments in that RDE shortened the latent period to 2 hours and the titers in the first few positive samples (4 to 5 hours) increased) whereas in later harvests no such effect was noted. Under these conditions complement-fixation antigens and hemagglutinins made their appearance in the tissues simultaneously and not as previously reported the former prior to the latter. However, the infectivity showed increments only several hours after these two activities had become measurable. Thus the hypothesis of the stagewise development of influenza virus is still supported by these data.

2. Using the inhibition of hemagglutination technic it was found that the inhibitor in allantoic fluid rapidly decreased as a result of the action of active and irradiated virus, but destruction was never complete. In the membranes of the in vivo series only active seed led to loss of inhibitor, again without complete destruction, beginning at the time complement-fixing antigen and hemagglutinins became measurable. Irradiated seed was without effect in vivo whereas, in the in vitro tests it equalled the activity of the active virus. The implications of this difference in the effectiveness of active and irradiated seed in vivo with regard to the understanding of the mode of viral multiplication are discussed.

3. Although many factors may influence the shape of adsorption-elution curves it is felt that at 0°C. the extent of adsorption is directly related to the amount of inhibitor present in the systems. In the early hours after inoculation the degree of adsorption was relatively small but it increased gradually with the time of incubation. The inhibitor of adsorption was destroyed by RDE and NaIO4 and was only partially sedimentable by high speed centrifugation. In every respect studied its properties corresponded with the findings obtained with inhibitors in the hemagglutination-inhibition technic. Although the difference in the rapidity of inhibitor destruction as measured by the various technics might suggest a multiplicity of inhibitors it is felt that it rather denotes a greater sensitivity of the adsorption technic as compared to the others.

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