Identifying Bacterial and Host Factors Involved in the Interaction of Mycobacterium bovis with the Bovine Innate Immune Cells

Abstract

Bovine tuberculosis is an important animal and zoonotic disease caused by Mycobacterium bovis. The innate immune response is the first line of defense against pathogens and is also crucial for the development of an efficient adaptive immune response. In this study we used an in vitro co-culture model of antigen presenting cells (APC) and autologous lymphocytes derived from peripheral blood mononuclear cells to identify the cell populations and immune mediators that participate in the development of an efficient innate response capable of controlling the intracellular replication of M. bovis. After M. bovis infection, bovine immune cell cultures displayed upregulated levels of iNOS, IL-22 and IFN-γ and the induction of the innate immune response was dependent on the presence of differentiated APC. Among the analyzed M. bovis isolates, only a live virulent M. bovis isolate induced an efficient innate immune response, which was increased upon stimulation of cell co-cultures with the M. bovis culture supernatant. Moreover, we demonstrated that an allelic variation of the early secreted protein ESAT-6 (ESAT6 T63A) expressed in the virulent strain is involved in this increased innate immune response. These results highlight the relevance of the compounds secreted by live M. bovis as well as the variability among the assessed M. bovis strains to induce an efficient innate immune response.

A Transcriptional Analysis of Cattle Immune Cells Reveals a Central Role of Type 1 Interferon in the In Vitro Innate Immune Response against Mycobacterium bovis

“…In a previous study, we had demonstrated that, unlike the attenuated Mb534 isolate, the virulent Mb04-303 strain induced the proliferation of interferon-gamma producer-natural killer (NK) cells when naive bovine PBMC was infected in vitro [ 4 ]. In addition, this innate immune response effectively controlled the intracellular replication of Mb04-303 in macrophages.…”

“…For the RNA-seq analysis, we selected samples from three different animals infected with Mb04-303 or Mb534 that met this criterion. We have previously demonstrated that in the absence of non-adherent cells, such as lymphocytes, infected macrophages failed to control the intracellular replication of Mb04-303 [ 4 ]. For this reason, the transcriptional analysis of infected macrophages cultured alone was excluded from this study.…”

“…Furthermore, natural killer (NK) cells serve as a bridge connecting the innate and adaptive immune responses. Within draining lymph nodes, NK cells significantly contribute to the early innate response against bacterial infections by producing interferon-gamma (IFN-γ) [ 4 ]. These effector responses rely heavily on interactions with antigen-presenting cells (APCs), particularly dendritic cells (DCs).…”

“…A noteworthy aspect of NK cells is their direct cytotoxic effect against macrophages infected with mycobacteria, which underscores their multifaceted role in the immune defence against these pathogens [ 4 ].…”

“…As in human tuberculosis, the outcome of M. bovis infections depends on the immune background of the individual host and the genomics and phenomics variability of the M. bovis strains. In a previous study, we found that the magnitude of the innate immune response induction depends on the secretome of M. bovis strains with different virulence levels [ 4 ]. In the case of the highly virulent strain Mb04-303, either the bacterium itself or its secreted compounds induce a powerful innate immune response capable of controlling M. bovis replication inside bovine macrophages [ 4 ].…”