Johnes disease bovine monocyte-derived macrophages are insensitive to ex vivo infection by Mycobacterium avium ssp. paratuberculosis

Bovine paratuberculosis (PTB), also known as Johne’s disease, is a worldwide insidious disease of ruminants that is induced by an intracellular pathogen called Mycobacterium avium ssp. paratuberculosis (MAP). There is no cure for PTB, and vaccine therapy does not prevent new infection. The host’s genetic susceptibility, tolerance to the intracellular pathogen during the early stage of the infection, and inefficient immune response are predisposing factors to the development of PTB. The pathogen invades the bovine intestinal tract, where it is phagocytosed by macrophages, the primary reservoir of MAP. It is therefore interesting to study this specialized immune cell type. Six PTB-positive cows identified by bacterial culture of fecal samples and six herdmates found to be negative by both serum ELISA and MAP culture were selected. Monocyte-derived macrophages from these cows were infected ex vivo for 4 and 24 h with live MAP. RNA was harvested, and the whole transcriptome was sequenced with 100 bp-end sequencing using Illumina HiSeq 2000 technology. A minimum of a 2-fold change in gene expression and a P value ≤ 0.05 after false discovery rate correction were used to select significant genes. In total, 243 and 364 genes were differentially expressed in primary bovine macrophages between the negative and positive cows at the 4- or 24 h post-infection time points, respectively. Common to both time points, the 37 upregulated and 33 downregulated genes were associated with the immune system, biological regulation, developmental processes, or the response to stimulus. The impact of ex vivo infection by MAP on the macrophages was observed mainly in the negative cows. In total, 712 and 2662 genes were identified as being differentially expressed between the control and the 4- and 24 h post-infection time points, respectively. Interestingly, no significant difference was observed for the positive cows. These findings suggest that monocyte-derived macrophages could be metabolically predisposed to not respond to MAP infection. These results give new insight into the possible effect of the predisposition of PTB cows on the ability of macrophages to achieve an efficient immune response to MAP infection, on putative outcomes in tissues, and on disease progression. Further research is required to determine whether stably inherited traits or epigenetic modifications affect gene regulation.