Quantitative Studies on Prophylactic Effectiveness of Western Equine Encephalitis Antiserum in Mice. ∗

Abstract

Summary

(1) Hyperimmune, WEE antiserum, prepared in rabbits by a series of 5 fairly closely spaced inoculations, yielded a 50% serum dilution titer of 1:1,800 against 40 LD₅₀ of virus in the routine intracerebral neutralization test. When the intra-abdominal neutralization test was used, the amount of virus neutralized by various dilutions of the serum was always greater, but the limiting effective dilution of the serum was about the same as in the intracerebral assay against 40 LD₅₀ of virus. (2) After intravenous injection of this antiserum in mice, the mouse serum acquired a neutralizing activity which indicated that the antibody underwent a dilution roughly in proportion to the size of the animal. (3) Prophylactic effectiveness was tested by injecting intravenously various dilutions of antiserum into 2-week-old mice, and 4 hours later inoculating these mice intra-abdominally with varying dilutions of the virus. Passive transfer of just enough antibody to impart to the undiluted serum an intracerebral neutralization index of approximately 50, protected against 400-4,000 intraabdominal encephalitogenic doses of virus; the actual amount of virus contained in this dose was 2 million (mouse) intracerebral LD_50. The amount of antibody required for passive protection varied with the size of the infecting dose, so that 0.02 ml, 0.2 ml and 2 ml of the antiserum per kilo of body weight yielded protective indexes of 400, 16,000 and 100,000 respectively. The significance of these data for the problem of the prophylactic use in human beings of concentrated antisera against the arthropod-borne encephalitis viruses is discussed.