INDUCED RESISTANCE OF THE CENTRAL NERVOUS SYSTEM TO EXPERIMENTAL INFECTION WITH EQUINE ENCEPHALOMYELITIS VIRUS : III. ABORTIVE INFECTION WITH WESTERN VIRUS AND SUBSEQUENT INTERFERENCE WITH THE ACTION OF HETEROLOGOUS VIRUSES

The mechanism of active cerebral immunity to equine encephalomyelitis virus; the local antigenic booster effect of the challenge inoculum

The fate of W.E.E. virus has been followed in the brains of mice vaccinated to such an extent that they failed to resist a large intracerebral challenge dose of a viral variant with a rapid rate of multiplication but were fully protected against a similar amount of a "slow" strain. The growth rate of the "fast" variant in vaccinated animals paralleled that in non-vaccinated ones at a slightly lower level. The "slow" strain also multiplied, but its rate of growth was depressed. Nevertheless, it persisted for 4 days at a level 100-fold higher than its initial titer. After the 4th day the virus was no longer demonstrable and was replaced by neutralizing antibody which rose so high that the serum antibody/brain antibody ratio was reduced from a "physiological" value of about 200/1 to less than 10/1. Antibody persisted in brain tissue in high titer until at least 127 days after challenge inoculation. The shift in the serum/brain ratio of neutralizing antibody was paralleled by a similar shift in the ratio of complement-fixing antibody. The neutralizing antibody in brain tissue, like that in serum, followed the "percentage law" on dilution of underneutralized mixtures. In mice immunized with small doses of vaccine, the intracerebral challenge inoculum induced a significantly greater local immune response than in those immunized to a higher degree. Mice with very low grade immunity were found more resistant to large amounts of virus than to small amounts. This "paradoxical" response to challenge was explained as due to the antigenic booster effect exerted by amounts of virus in excess of that utilized to initiate infection which were present in large inocula but absent in small doses. The broader relation of these findings to the problem of antiviral immunity has been discussed.

Immunosuppression and experimental virus infection of the nervous system

This chapter describes the current views of the pathogenesis of virus infections of the nervous system, with particular attention to certain aspects of virus-host interactions. Following invasion of the central nervous system, infection can follow a variety of patterns, as to number and distribution of neuronal and non-neuronal cells involved. There is a corresponding diversity in the pathological lesions of the central nervous system (CNS) produced by acute virus infection. Infection can be pictured as a race between virus and host defenses, where many factors, acting through different mechanisms, can influence the outcome. Outcome is always determined by multiple virus and host variables, although single variables can be independently studied under experimentally controlled conditions in the laboratory. The chapter demonstrates that in many virus-host combinations, the immune response plays an important role in recovery from primary infections. It mentions that an immunopathological process mediates the disease which follows certain CNS virus infections.

Studies on resistance to bacterial infections in animals infected with Rickettsiae

Data are presented, demonstrating that infection with Rickettsia typhi brings about a reduction of the death rates and a prolongation of the time of survival in animals subsequently challenged with Pasteurella pestis or Pasteurella tularensis. This interference with bacterial infection by previous rickettsial infection does not appear immediately after injection of the rickettsiae; it begins to appear around 16 hours after this injection and becomes more marked during the first 96 hours; later it decreases. The phenomenon is essentially a local tissue reaction with weaker systemic effects. The phenomenon of interference can be overcome by challenge with too large a number or too virulent a strain of bacteria. A strain of P. pestis subjected to 10 passages through mice infected with Rickettsia typhi failed to develop resistance to the interfering activity of the latter microbial species. This does not rule out, but might be evidence against, an assumption that the interfering action is due to antibiotic effects from the rickettsiae.

INTERFERENCE BETWEEN VIRUSES IN TISSUE CULTURE

The influence of one virus on the growth of another in tissue culture was investigated. The 17DD High strain of yellow fever virus was found capable of completely suppressing the growth of both the Asibi strain of the same virus and of the heterologous West Nile virus, even when these were added to the cultures in large amounts. The 17DD High strain of yellow fever virus and the West Nile virus produced either partial or complete suppression of growth of the Venezuelan equine encephalomyelitis virus, depending upon the quantity of the latter inoculated into the cultures. Owing to lack of methods for the detection of interference except in a single direction, reciprocal interference with these viruses could not be investigated. The 17DD High strain of yellow fever virus and the West Nile virus were able to suppress completely, or almost completely, the growth of influenza A virus added to the infected cultures in maximal amounts. Interference in the reverse direction, even with the use of small amounts of the neurotropic viruses, was not demonstrable. Cultures infected with the 17DD High strain of yellow fever virus were examined for the presence of neutralizing antibodies and non-specific antiviral substances; neither was found present.

ENDURING IMMUNITY FOLLOWING VACCINATION OF MICE WITH FORMALIN-INACTIVATED VIRUS OF RUSSIAN SPRING-SUMMER (FAR EASTERN, TICK-BORNE) ENCEPHALITIS : CORRELATION WITH SERUM-NEUTRALIZING AND COMPLEMENT-FIXING ANTIBODIES

A single course of two intraperitoneal injections of formalin-inactivated virus of Russian spring-summer encephalitis induced in albino mice a solidly immune state which endured almost throughout life. Active virus is therefore not essential for the production of a high degree of lasting immunity. The immune response to vaccination consists of resistance to peripherally introduced active virus and development of circulating antibody. A correlation has been found to exist throughout the long period of the immune state between the titer of neutralizing antibody, as determined by the intraperitoneal method described, and the degree of immunity to peripherally introduced active virus. Thus laboratory tests for the immunizing power of a vaccine suggest themselves, to be carried out by an estimation in vaccinated mice of (a) immunity to peripherally inoculated active virus, and (b) serum virus-neutralizing antibody determined by the intraperitoneal method. The rôles as indicators of immunity in vaccinated mice of complement-fixing antibody in the serum, of the intracerebral challenge dose of virus, and of the intracerebral method for testing neutralizing antibody are discussed. Finally, if the immune response of man to vaccination with formalin-inactivated virus of Russian spring-summer encephalitis follows the pattern of the response of mice as here described, and if the correlation of neutralizing antibody with immunity to peripherally introduced virus applies to man as to mice, then possibly the degree of immunity in human beings following vaccination can be appraised by a peripheral test for neutralizing antibody in the serum.