Oral immunization with heat-inactivated Mycobacterium bovis reduces local parasite dissemination and hepatic granuloma development in mice infected with Leishmania amazonensis

Heat-inactivated Mycobacterium bovis and P22PI protein immunocomplex: Two candidates for use as immunostimulants of innate immune response

Tuberculosis (TB), caused by members of the Mycobacterium tuberculosis complex, remains a critical global health challenge, affecting humans and a wide range of domestic and wild animals. Despite the availability of anti-TB drugs, cure rates remain suboptimal, exacerbated by the rise of multidrug-resistant TB strains. The Bacille Calmette-Guérin (BCG) vaccine, the only licensed vaccine against TB, has demonstrated efficacy in reducing lesion severity and bacterial burden in animals, as well as lowering TB-related and all-cause mortality in infants. However, BCG presents several safety concerns inherent to live vaccines. To overcome these limitations, exploring alternative vaccine candidates that do not incorporate live mycobacteria is crucial. This study aimed to evaluate and compare the immunostimulatory potential of two candidates based in mycobacteria inactivated or their derivatives, heat-inactivated Mycobacterium bovis (HIMB) and P22PI protein immunocomplex (P22PI), in bovine foetal lung cells. To assess the expression of innate immune components, including Toll-like receptors (TLRs), cathelicidins, and cytokines, bovine foetal lung were exposed to different concentrations of HIMB and P22PI immunostimulants, starting at 7.8 × 10⁶ CFU/ml and 10 µg/ml, respectively. These initial concentrations were subsequently diluted to 1/2 and 1/10 to evaluate dose-dependent effects. Our findings reveal that both HIMB and P22PI significantly stimulate innate immune mechanisms, as evidenced by the upregulation of TLR2 and TLR4, alongside the induction of BMAP28 cathelicidin, tumour necrosis factor alpha (TNFA) and interferons (IFNs). These results suggest their potential to orchestrate a robust innate immune response providing valuable insights into the immunological mechanisms underlying the protective effects of these immunostimulants. This underscores their potential role in in vivo studies as vaccine candidates. Furthermore, their ability to enhance antigen recognition via TLR and induce pro-inflammatory cytokines also indicates broader applications in immune modulation, potentially extending protection against heterologous pathogens through trained immunity.

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.

Non-specific effects of inactivated Mycobacterium bovis oral and parenteral treatment in a rabbit scabies model

Tuberculosis BCG vaccination induced non-specific protective effects in humans led to postulate the concept of trained immunity (TRAIM) as an innate type of immune mechanism that triggered by a pathogen, protects against others. Killed vaccines have been considered not to be effective. However, field efficacy of a commercial vaccine against paratuberculosis, as well as of a recently developed M. bovis heat-inactivated vaccine (HIMB) prompted to test whether it could also induce TRAIM. To this, we used a sarcoptic mange rabbit model. Twenty-four weaned rabbits were treated orally or subcutaneously with a suspension of either HIMB (107 UFC) or placebo. Eighty-four days later the animals were challenged with approximately 5000 S. scabiei mites on the left hind limb. Skin lesion extension was measured every 2 weeks until 92 days post-infection (dpi). Two animals were killed at 77 dpi because of extensive skin damage. The rest were euthanized and necropsied and the lesion area and the mite burden per squared cm were estimated. Specific humoral immune responses to S. scabiei and to M. bovis were investigated with the corresponding specific ELISA tests. Subcutaneously and orally HIMB vaccinated animals compared with placebo showed reduced lesion scores (up to 74% and 62%, respectively) and mite counts (-170% and 39%, respectively). This, together with a significant positive correlation (r = 0.6276, p = 0.0031) between tuberculosis-specific antibodies and mite count at 92 dpi supported the hypothesis of non-specific effects of killed mycobacterial vaccination. Further research is needed to better understand this mechanism to maximize cross protection.