Effect of adjuvants on immunization with dengue virus-induced cytotoxic factor

Specific active immunization with dengue type 2 virus (DV)-induced cytokine, cytotoxic factor (CF), prevents CF-mediated pathology in mice. The present study was undertaken to determine the optimum dose of CF and the effect of different adjuvants on the immune response as assessed by the study of anti-CF antibody titre by ELISA and protection against increase in capillary permeability to challenging dose of 3 micrograms CF. The maximum protection of 94 +/- 4% against increase in capillary permeability was observed at week 4 after immunization with 5 micrograms dose of CF mixed with Freund's incomplete adjuvant (FIA), which gradually decreased to 21 +/- 10% on week 24. With a dose of 10 micrograms the protection obtained was 79 +/- 5%, but persisted for a longer time at a higher level. The response was poor with 1 microgram dose of CF. The mean anti-CF antibody titres gradually decreased after reaching the peak at week 4 after immunization. Mice immunized with different adjuvants emulsified with 5 micrograms CF were challenged at different intervals with 3 micrograms CF. Maximum protection observed with CF + tetanus toxoid (TT) and 84/246 was about 93 +/- 2% and 97 +/- 2%, while that with alhydrogel was 33 +/- 12% and with bacille Calmette-Guérin (BCG) was 67 +/- 4%. At week 24 after immunization, however, the best response was obtained with 10 micrograms of adjuvant 84/246. Intracerebral challenge with 10 or 100 LD50 dose of dengue type 2 virus showed significantly prolonged mean survival time and delayed onset of signs of sickness in immunized mice compared with normal mice. The maximum survival time was with adjuvant 84/246 even at week 24. The findings thus show that the optimum dose of CF is 5 micrograms and the adjuvant of choice is 84/246.