Immune Reactions to a Murine Leukemia Virus. I. Induction of Immunity to Infection With Virus in the Natural Host

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Abstract
A formalin-treated extract of spleens of mice infected with Rauscher leukemia virus emulsified with Freund's adjuvant was used to prepare a vaccine. One injection of this vaccine completely protected against a challenge inoculum of virus sufficiently virulent to kill all the control animals. The serums of immunized mice collected immediately before challenge contained a large quantity of neutralizing antibody. Positive evidence was obtained to support the thesis that the observed immunity to infection with this murine leukemia virus is due to the circulating, neutralizing antibody. Negative evidence supports the contention that a cell-associated, delayed-type hypersensitivity antibody is not involved. The findings were as follows: 1) There is a direct correlation between the relative amount of antibody and the state of immunity observed after challenge with virulent virus. 2) Mice may be protected against infection by passive immunization with homologous antiserum. 3) No positive skin reaction attributable to delayed-type hypersensitivity to virus was observed in demonstrably immune mice. 4) In experiments designed to prevent the formation of delayed hypersensitivity, there was no detectable weakening of the immune status of the animal. Under the conditions of these experiments, the emulsifications of the vaccine with Freund's adjuvant appeared necessary for the induction of immunity; emulsification of the adjuvant with saline stimulated a low-grade immunity and a low titer of circulating, neutralizing antibody. These findings may be due to enhancement by the use of adjuvant of the response to a latent form of an antigenically related virus.

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