THE DEMONSTRATION OF LESIONS AND VIRUS IN THE LUNGS OF MICE RECEIVING LARGE INTRA-PERITONEAL INOCULATIONS OF EPIDEMIC INFLUENZA VIRUS

STUDIES ON THE TOXICITY OF INFLUENZA VIRUSES : II. THE EFFECT OF INTRA-ABDOMINAL AND INTRAVENOUS INJECTION OF INFLUENZA VIRUSES

Upon intra-abdominal or intravenous injection of allantoic fluids infected with influenza viruses, mice frequently died within 8 to 96 hours. Similar results were observed upon injection of rabbits, rats, and guinea pigs. Autopsy of the mice revealed widespread necrosis of liver and spleen, hemorrhages into the intestines, pleural exudation, and other occasional findings. Survivors frequently developed pulmonary consolidation or jaundice. The dominant type of lesion depended on the strain of virus used. All attempts to demonstrate propagation of the influenza viruses outside of the respiratory tract failed. It was concluded that the early lesions were the result of toxic activities of the virus and not of virus multiplication in the affected tissues. Injection into chick embryos of highly diluted inocula produced higher titers of virus, hemagglutmin, and toxicity in the allantoic fluids than the use of more concentrated seed culture. Serial passage of various strains in high dilution frequently increased the toxic activity. The infectivity often reached its peak in 24 hours when tests for toxicity were still negative. Maximal toxicity was usually not attained before 48 hours. The toxic activity could not be separated from the infective property by such means as differential centrifugation and adsorption onto and elution from chicken red cells. However, upon heating, formalinization, and irradiation with ultraviolet light, the ability of the agents to propagate was lost at a faster rate than the toxic property. The toxic property remained stable for 2 to 3 months at 4°C. This stability was comparable to that of the infectivity for chick embryos. Specific immune sera neutralized in high dilution the toxic activity of the homologous virus. Non-specific neutralization occurred in low dilutions of normal and heterologous immune sera. Strain differences were indicated by this method of testing. Vaccination of mice by the subcutaneous or intra-abdominal routes protected mice specifically against the toxic effects of intra-abdominally or intravenously injected preparations of virus.

QUANTITATIVE RELATIONSHIPS BETWEEN THE IMMUNIZING DOSE OF EPIDEMIC INFLUENZA VIRUS AND THE RESULTANT IMMUNITY

A direct proportion exists between the concentration of epidemic influenza virus used for intraperitoneal immunization of mice and the degree of immunity to intranasal infection which develops. Mice vaccinated with virus of a given strength resist infection with virus of the same concentration but not more. An irreducible minimum exists since mice vaccinated with less than ten intranasal lethal doses do not develop sufficient immunity to overcome intranasal infection with virus of the same strength. The fact that there exists a limiting threshold for the degree of immunity which a certain strength of virus will induce indicates that the virus does not multiply after intraperitoneal inoculations. In ferrets a state of partial immunity is induced as a result of subcutaneous vaccination with active influenza virus. Vaccination with doses containing 100 or more intranasal infectious units is required for the production of circulating antibodies, protection of the animals from pulmonary involvement, and modification of the severity of the disease. On the other hand, intranasal inoculation with one infectious unit results in a firm, immediate immunity, although the duration of immunity may bear a relation to the severity of the original infection and consequently to the size of the infecting dose. Ferrets in a state of partial immunity resulting from subcutaneous vaccination, or from the waning of a firm immunity following infection, respond to intranasal inoculation of influenza virus with an accelerated production of neutralizing antibodies. The antibody titer under these conditions reaches a much higher level than occurs following a primary infection. Fully immune animals, however, show no further antibody response to a second inoculation.

Depression of rat neutrophil exudation and motility by influenza virus

Purified influenza A and B viruses (3000 HA units) were injected intraperitoneally into rats. 1 h later the animals were given starch solution intraperitoneally, after which the number and composition of the exuded cells were determined at intervals. Both influenza A and B viruses depressed neutrophil exudation into the peritoneal cavity for a period of 10 to 17 h after the starch injection. The exudation of leukocytes other than neutrophils (a mixture of mononuclear cells and 5% to 10% eosinophils) was depressed for 17 h after starch injection by influenza A virus. When tested in vitro by a modification of the Boyden method, the peritoneal exudate neutrophils from virus-treated animals exhibited significantly depressed chemotaxis and consistently, although not always significantly, reduced chemokinesis (migration in the presence of a chemoattractant without a gradient) and random locomotion. Both influenza A and B viruses inhibited in vitro the motility of starch-induced peritoneal exudate neutrophils in Boyden chambers after a virus-cell interaction of 60 min at 37 degrees C. The present findings support the concept that impaired neutrophil exudation caused by depressed motility is one mechanism responsible for the increased susceptibility to bacterial superinfection during influenza.

Cytotoxic T cells in the lungs of mice infected with an influenza A virus

Cytotoxic T cells are present in the lungs and the bronchoalveolar washings of mice infected intravenously (i.v.) or intranasally (i.n.) with live influenza A/WSN virus. After i.v. injection, cytotoxic T cell activity in both spleens and lungs reaches a peak at 6 days when the level of infectious virus recovered from the lungs falls sharply and the mice do not die. If a lethal dose of virus is given intranasally, very high levels of virus appear rapidly in the lungs, and the development of lung consolidation follows slightly behind the appearance of cytotoxic T cells there. When a non-lethal dose of virus is given intranasally, lower levels of virus are found in the lung and the appearance of cytotoxic T cells is delayed. These results suggest that the cytotoxic T cells play a protective role if the level of virus in the lungs does not reach very high levels. After injection of antithymocyte serum, the subsequent level of cytotoxic T cell activity in the lungs was greatly reduced, suggesting that the T cells recovered in lungs had at an earlier stage been circulating cells. However, splenectomized mice develop high levels of cytotoxic T cell activity, after intranasal infection of mice, indicating that the spleen did not contribute substantially to the T cells recovered in the lungs.

Studies on the pathogenesis of influenzal pneumonitis; intranasal vs. intravenous infection of mice

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