Vaccination of guinea pigs with DNA encoding the mycobacterial antigen MPB83 influences pulmonary pathology but not hematogenous spread following aerogenic infection with Mycobacterium bovis

Abstract

Protection of cattle against bovine tuberculosis by vaccination could be an important control strategy in countries where there is persistent Mycobacterium bovis infection in wildlife and in developing countries where it is not economical to implement a tuberculin test and slaughter control program. The main aim of such a vaccination strategy would be to reduce transmission of infection by reducing the lung pathology caused by infection and preventing seeding of the organism to organs from which M. bovis could be excreted. Recent reports of successful DNA vaccination against Mycobacterium tuberculosis in small-animal models have suggested that DNA vaccines act by reducing lung pathology without sensitizing animals to tuberculin testing. We therefore evaluated the ability of vaccines consisting of DNA encoding the mycobacterial antigens MPB83 and 85A to reduce lung pathology and prevent hematogenous spread in guinea pigs challenged with a low dose of aerosolized M. bovis. Vaccination with MPB83 DNA reduced the severity of pulmonary lesions, as assessed by histopathology, and resembled M. bovis BCG vaccination in this respect. However, unlike BCG vaccination, MPB83 DNA vaccination did not protect challenged guinea pigs from hematogenous spread of organisms to the spleen. In contrast, vaccination with antigen 85A DNA, a promising DNA vaccine for human tuberculosis, had no measurable protective effect against infection with M. bovis.

FIG. 1.

Number of gross lesions on the dorsal surfaces of fixed lungs 10 weeks after challenge with a low-dose aerosol of M. bovis strain AF2122/97. Guinea pigs were vaccinated and then challenged 8 weeks later. Each point represents an individual animal. The bar indicates the mean value for each group.

FIG. 2.

Representative micrographs of guinea pig lungs, showing the reduction in granulomatous inflammation in the BCG-vaccinated (A) and pCMV-83-vaccinated (B) groups compared with the pVR1020-85A-vaccinated group (C) and the PBS control (D). Vaccination with BCG or pCMV-83, but not vaccination with pVR1020-85A, reduced both the number and the size of the granulomas, as reflected in the pathology scores shown in Table 1. The sections were given the following scores for granuloma size: panel A, 1.00; panel B, 1.00; panel C, 2.00; and panel D, 2.00. These photographs also demonstrate the marked reductions in the numbers of lymphocytes observed in the three vaccinated groups compared with the control. The sections were given the following scores for lymphocytic infiltration: panel A, 1.00; panel B, 0.67; panel C, 1.00; and panel D, 1.67. Bar = 400 μm.

FIG. 3.

Ability of vaccines to protect guinea pigs from hematogenous spread following aerogenic challenge with M. bovis. Guinea pigs were vaccinated and then challenged 8 weeks later. The numbers of CFU recovered from the spleens of animals 10 weeks after challenge are shown (limit of detection, 25 CFU). The bars indicate the median values. Statistical significance was demonstrated by using the Mann-Whitney nonparametric test.