An animal model to evaluate the protective efficacy of Haemophilus influenzae type b conjugate vaccines

An efficacy test of PRP (polyribosylribitol phosphate)-TT (Tetanus toxoid) conjugate vaccines was carried out using BALB/c mice as an animal model by inoculating Haemophilus influenzae type b (Hib) with a virulence enhancement factor (VEF). Three administrations of the conjugate vaccines at 2-week intervals elicited a significantly high level of PRP antibodies (P < 0.0001). The protective activity of the PRP immunization was challenged with. either Hib with iron dextran (Hibi) or with a combination of mucin and hemoglobin (Hibmh) as a VEF. The medium lethal dose (LD50) for Hibmh and Hibi was measured as 10 CFU (Colony Forming Unit) and 2.5 x 10(8) CFU respectively. Each immunized animal was challenged with five or ten times the LD50 level of bacteria with a VEF. A significant difference in mortality between the immunized and control mice (P < 0.01) was observed with the Hibmh challenge inoculation but not with the Hibi challenge inoculation. These results show that a combination of mucin and hemoglobin was able to enhance the virulence of Hib in BALB/c mice to cause a lethal infection, thus suggesting that BALB/c mice introduced to this method can be an effective model animal for testing the protective efficacy of H. influenzae conjugate vaccines.