Immunostimulant effect of heat-inactivated Mycobacterium bovis in mice challenged with vector-borne pathogens

Trained immunity is defined as an enhanced state of the innate system which leads to an improved immune response against related or non-related pathogens. Bacillus Calmette-Guérin (BCG) vaccine, a live attenuated Mycobacterium bovis strain, is currently one of the main inductors of trained immunity. The objective of the present study was to evaluate the protective effects of heat-inactivated M. bovis (HIMB) against Plasmodium berghei and Borrelia burgdorferi and characterize the immunological mechanisms involved. BALB/c and C3H/HeN mice were randomly assigned in similar number to either immunized group receiving two oral doses of HIMB with a 4-week interval, or control group treated with PBS. All the BALB/c mice were intraperitoneally infected with P. berghei while the C3H/HeN mice were subcutaneously infected with B. burgdorferi. Pathogen burden was significantly reduced in both immunized groups when compared to controls. The number of macrophages significantly decreased in the liver or in the spleen of the mice that had been immunized prior to the challenge with P. berghei or B. burgdorferi, respectively. Furthermore, the immunized groups showed an apparent upregulation of IFN-γ, TNF-α and IL-1α in the liver (P. berghei challenge) or a significant increase in IL-1α producing cells in the spleen (B. burgdorferi challenge). Our findings suggest that oral immunization with heat-inactivated mycobacteria limits pathogen burden through stimulation of the innate immune response in two vector-borne diseases in mice.