Good Protection but Excessive Pulmonary Inflammation in Balb/C Mice Vaccinated with Mycobacterium Bovis Mce-2A Mutant after Challenge with Homologous Strains

Abstract

Tuberculosis (TB) remains a major threat to public and veterinary health. Zoonotic TB (caused by Mycobacterium bovis) is present in wild animals and cattle in most developing countries, and M. bovis is also able to infect humans on a worldwide basis. Thus, the high incidence of bovine TB is a major economic problem and an additional risk to human health, being the development of new vaccines to prevent both human and bovine TB urgent and a major challenge. The aims of the present study were to characterize the pathogenicity and immunogenicity of M. bovis mce2A mutant in BALB/c mice, and then evaluate its potential as vaccine. Mutant M. bovis mce2A produced limited tissue damage (pneumonia) and lower bacilli burdens than its parental strain when administered in high dose by intratracheal inoculation, and showed limited dissemination when used as subcutaneous vaccine. Challenge experiments using low, middle and highly virulent M. tuberculosis or M. bovis strains showed similar protection conferred by mce-2 mutant than BCG. Interestingly, vaccinated animals showed low bacilli loads but high inflammatory response when were challenged with M. bovis strains, while vaccinated mice challenged with M. tuberculosis exhibited low bacilli burdens and scarce inflammation. Thus, in spite of the high genome homology between M. tuberculosis and M. bovis, it seems that there is higher antigenic recognition and in consequence extensive inflammatory response when the strain used as vaccine is homologous to the challenge strain, in this case M. bovis.