ABSTRACT: Purpose: Since the peripheral blood (PB) is the most available physiological fluid for the detection of biomarkers, in the current we examine whether it recapitulates, at least in part, the transcriptome of the ileocecal valve (ICV), the primary site of MAP colonization. For this purpose, RNA-Seq was used to identify host genes differentially expressed (DE) in ICV and PB samples collected from PTB-infected animals with focal or diffuse lesions in gut tissues versus control animals. Methods: RNA-Seq libraries were single-end sequenced in a 1x75 format using an Illumina NextSeq 500 sequencer. The raw reads were filtered by their length (minimum size 60 bp long) and percentage of ambiguous base N less than 5 % using Prinseq-lite. Trimmed reads were subsequently mapped to the Bos Taurus reference genome (Bos_taurus.UMD3.1. version 87) with TopHat mapper. The resulting alignment files were provided to Cufflinks to generate a transcriptome assembly for each condition. These assemblies were then merged together using Cuffmerge, which is included in the Cufflinks package. This merged assembly provided a uniform basis for calculating gene and transcript expression levels in each condition. The reads and merged assembly were fed to Cuffdiff which calculates expression levels and tested the statistical significance of each observed change in expression. The fold change (in log2 scale), P-values and false discovery rates (FDR) for each gene were obtained. The genes with a FDR-adjusted threshold ˂ 0.05 were considered differentially expressed (DE) when compared to the control group. Results: Our results demonstrated both shared, and PB and ICV-specific gene expression in response to a natural Map infection. As expected, the number of differentially expressed (DE) genes was larger in the ICV than in the PB samples. Among the DE genes in the PB and ICV samples, there were some common genes irrespective of the type of lesion including the C-X-C motif chemokine ligand 8 (CXCL8/IL8), apolipoprotein L (APOLD1), and the interferon inducible protein 27 (IFI27). The biological processes (BP) enriched in the ICV gene expression profiles from the cows with focal lesions included the killing of cells of other organism, defense response, immune response and the regulation of neutrophil chemotaxis. Two of these BP, the defense and immune response, were also enriched in the ICV and PB from the cows with diffuse lesions. Metabolic analysis of the DE genes revealed that the N-glycan biosynthesis, bile secretion, one-carbon pool by folate and purine metabolism were significantly enriched in the ICV from the cows with focal lesions. In the ICV from cows with diffuse lesions; the valine, leucine and isoleucine degradation route, purine metabolism, vitamin digestion and absorption and the cholesterol routes were enriched Conclusions: This study is the first to simultaneously describe transcriptomic changes in PB and ICV samples of cattle naturally infected with MAP. Several important observations were made. First, by comparing the transcriptomic profiles of two MAP-targeted tissues we described both unique and overlapping changes in the transcriptome of the infected cows versus the control group. Second, our study highlighted the ability of the RNA-Seq technology to reveal roles for genes that have not been previously implicated in the host response to MAP infection. Third, our transcriptomic analysis provided potential biomarkers for the development of future diagnostic tools, vaccines and therapeutics.
