Objective: To evaluate the protective efficacy against a lethal dose of E. coli O157:H7 after intranasal, oral and subcutaneous immunization with the outer membrane protein (OMP) extracted from the whole cell of E. coli O157:H7.
Methods: Female BALB/C mice were immunized three times (on days 0, 7 and 14) with OMP and CT or Complete Frund adjuvant. On the 21st day after the last immunization, serum, fecal extracts and vaginal washes were collected for the detection of antigen-specific antibody responses by ELISA before the oral challenge with E. coli O157:H7 933. And the antigens that induced the specific antibody responses were analysed by Western-blotting. Then, the mice were orally challenged with 0.3 ml 4.25 x 10(10)/ml live E. coli O157:H7, and the mortality was recorded. On the 7th day after the challenge, the mice were sacrificed and the heart, liver, lung, kidney, small intestine and colon were collected. Then the histology lesions were observed by light microscopy.
Results: In ELISA, both intranasal and oral immunization, with CT as a mucosal adjuvant, induced strong anti-OMP IgA responses in serum, fecal extract and vaginal washes, and anti-OMP IgG responses in serum. However, the intranasal immunization was much more effective to induce specific IgA and IgG responses than the oral immunization. In contrast to mucosal immunization, subcutaneous immunization only induced high levels of specific IgG antibodies in serum, and did not effectively promote IgA immune response. The results of the protective efficacy after challenge showed that both intranasal and oral immunizations with OMP provided significant protection (86.7% to 40%, P < 0.01 and 73.3% to 40%, P < 0.05) against a lethal dose of E. coli O157:H7 challenge, and intranasal immunization possessed a better protective ability (86.7% to 73.3%, P < 0.05). In contrast, the mice immunized subcutaneously were not protected. They were died more early after oral challenge. Furthermore, the histopathological features of the kidney, colon and other organs also appeared to be well correlated with immunization route. In comparison with the mice immunized subcutaneously and those in control group, the severity of the histopathological lesions in the tissues of the mice immunized intranasally was minimal.
Conclusion: These results suggested that the intranasal immunization should be the best choice of vaccine development against E. coli O157:H7.