Evaluation of Mycobacterium avium subsp. paratuberculosis isocitrate lyase (IcL) and ABC transporter (BacA) knockout mutants as vaccine candidates

Oral paratuberculosis vaccine efficacy and mucosal immunity in cattle

Mycobacterium avium subsp. paratuberculosis (MAP) primarily invades ruminants' small intestine via the Peyer's patches in the ileum and jejunum. Despite ongoing efforts to develop effective MAP vaccines, the effects of live-attenuated vaccines on mucosal immunity remain poorly understood. Previous studies indicate that the BacA oral vaccine confers localized protection against MAP in the ileum and ileocecal valve of calves, but not in the jejunum. This protection correlates with heightened levels of peripheral blood immune cells exhibiting pro-inflammatory and memory traits. This study aimed to evaluate immune responses induced by oral BacA vaccination in the ileum and jejunum Peyer's patches, comparing protection at both sites through mucosal immune cell profiling and RNA-seq transcriptome analyses. It represents the first exploration of mucosal immune responses in Peyer's patches following oral MAP vaccination. Oral BacA immunization increased CD4 + IFNγ+ and CD4 + TNFα+ cell frequencies, along with the T effector memory to T central memory cell ratio, in the ileum and jejunum of BacA-vaccinated animals challenged with wildtype MAP, compared to the infection control group challenged solely with wildtype MAP. Immune cells isolated from the ileum of vaccinated-challenged animals exhibited significant upregulation in IFNγ, IP-10, TNFα, IL-2, IL-15, and IL-17 expression upon restimulation compared to the uninfected control group, whereas minimal differences were observed in the jejunum under similar conditions. RNA-seq data further indicated a more robust host response in the ileum across all experimental groups. Gene ontology analyses revealed genes associated with increased phagocytic and apoptotic activities in the vaccinated-challenged group. Overall, the BacA oral vaccine's effectiveness appears to vary primarily due to differences in antigen-specific gene expression between the ileum and jejunum, with the ileum showing a more robust host response. Understanding these effects on young calves' mucosal immunity and how live vaccines modulate immune responses is crucial for advancing mucosal vaccine development against MAP.

Immunogenicity and efficacy of an oral live-attenuated vaccine for bovine Johne's disease

Mycobacterium avium subsp. paratuberculosis (MAP), the etiological agent of Johne's disease (JD) in ruminants, establishes a prolonged and often lifelong enteric infection. The implementation of control measures for bovine JD has faced obstacles due to the considerable expenses involved in disease surveillance and hindered by unreliable and inadequate diagnostic tests, emphasizing the need for an effective vaccine that can stimulate mucosal immunity in the gastrointestinal tract. Previous investigations have demonstrated that deletion of the BacA gene in MAP produces an attenuated strain that can transiently colonize the calf small intestine while retaining its capacity to stimulate systemic immune responses similar to wildtype MAP strains. This study assessed the efficacy of the BacA gene deletion MAP strain, referred to as the BacA vaccine, when administered orally to young calves. The research aimed to evaluate its effectiveness in controlling MAP intestinal infection and to investigate the immune responses elicited by mucosal vaccination. The study represents the first evaluation of an enteric modified live MAP vaccine in the context of an oral MAP challenge in young calves. Oral immunization with BacA reduced MAP colonization specifically in the ileum and ileocecal valve. This partially protective immune response was associated with an increased frequency of CD4+ and CD8+ T cells with a pro-inflammatory phenotype (IFNγ+/TNFα+) in vaccinated animals. Moreover, re-stimulated PBMCs from vaccinated animals showed increased expression of IFNγ, IP-10, IL-2, and IL-17 at 10- and 12-weeks post challenge. Furthermore, immunophenotyping of blood leukocytes revealed that vaccinated calves had increased levels of T cells expressing cell-surface markers consistent with long-term central memory. Overall, our findings provide new insights into the development and immunogenicity of a modified live MAP vaccine against bovine JD, demonstrating oral vaccination can stimulate host immune responses that can be protective against enteric MAP infection.

Modeling Paratuberculosis in Laboratory Animals, Cells, or Tissues: A Focus on Their Applications for Pathogenesis, Diagnosis, Vaccines, and Therapy Studies

Paratuberculosis is a chronic granulomatous enteritis caused by Mycobacterium avium subsp. Paratuberculosis that affects a wide variety of domestic and wild animals. It is considered one of the diseases with the highest economic impact on the ruminant industry. Despite many efforts and intensive research, paratuberculosis control still remains controversial, and the existing diagnostic and immunoprophylactic tools have great limitations. Thus, models play a crucial role in understanding the pathogenesis of infection and disease, and in testing novel vaccine candidates. Ruminant animal models can be restricted by several reasons, related to space requirements, the cost of the animals, and the maintenance of the facilities. Therefore, we review the potential and limitations of the different experimental approaches currently used in paratuberculosis research, focusing on laboratory animals and cell-based models. The aim of this review is to offer a vision of the models that have been used, and what has been achieved or discovered with each one, so that the reader can choose the best model to answer their scientific questions and prove their hypotheses. Also, we bring forward new approaches that we consider worth exploring in the near future.

Mycobacterium avium subsp. paratuberculosis Candidate Vaccine Strains Are Pro-apoptotic in RAW 264.7 Murine Macrophages

Mycobacterium avium subsp. paratuberculosis (MAP) is the etiological agent of Johne's disease, a severe gastroenteritis of ruminants. This study developed a model cell culture system to rapidly screen MAP mutants with vaccine potential for apoptosis. Two wild-type strains, a transposon mutant, and two deletion mutant MAP strains (MOI of 10 with 1.2 × 10⁶ CFU) were tested in murine RAW 264.7 macrophages to determine if they induce apoptosis and/or necrosis. Both deletion mutants were previously shown to be attenuated and immunogenic in primary bovine macrophages. All strains had similar growth rates, but cell morphology indicated that both deletion mutants were elongated with cell wall bulging. Cell death kinetics were followed by a real-time cellular assay to measure luminescence (apoptosis) and fluorescence (necrosis). A 6 h infection period was the appropriate time to assess apoptosis that was followed by secondary necrosis. Apoptosis was also quantified via DAPI-stained nuclear morphology and validated via flow cytometry. The combined analysis confirmed the hypothesis that candidate vaccine deletion mutants are pro-apoptotic in RAW 264.7 cells. In conclusion, the increased apoptosis seen in the deletion mutants correlates with the attenuated phenotype and immunogenicity observed in bovine macrophages, a property associated with good vaccine candidates.