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A comparative transcriptome analysis of human and porcine choroid plexus cells in response to Streptococcus suis Serotype 2 infection

Streptococcus suis is an important opportunistic zoonotic pathogen, of which serotype 2 (ST2) is considered to be the most virulent for its natural host, the pig, as well as for humans. The most common clinical manifestations for pigs and humans are septicemia and meningitis. Previous in vivo observations in S. suis-infected pigs, revealed lesions at the choroid plexus (CP), which forms the interface at the blood-cerebrospinal fluid barrier to the central nervous system. In vitro experiments with primary porcine CP epithelial cells (PCPEC) and immortal human CP epithelial (HIBCPP) cells demonstrated that S. suis can invade and traverse the CP epithelial cells, and that the CP contributes to the immunological response via cytokine expression and secretion. In this study, a comparative approach to investigate the global transcriptome profile via RNA-seq of in vitro and in vivo S. suis ST2 challenged PCPEC/HIBCPP cells and pigs, respectively, was performed. 
In a first step to process the RNA-seq generated data, significant differentially expressed genes (DEGs) were identified. The highest amount of DEGs was observed for the dataset obtained from the CP isolated from in vivo S. suis ST2 infected pigs which suffered from meningitis. The primary PCPEC, which were infected in vitro with S. suis ST2, exhibited a strong inflammatory response 6 h post-infection, with a maximal differential regulation of 134-fold, whereas HIBCPP cells exhibited a predominant differential gene regulation of genes related to hypoxia, where a maximum of an 8-fold change was observed. In the Gene Set Enrichment Analysis (GSEA), which interpreted the data based on gene groups performing a common biological function, a significant enrichment of 18, 28, and 21 hallmark gene sets (GSs) for infected HIBCPP cells, PCPEC, and in the CP of pigs suffering from S. suis ST2 meningitis, respectively, were identified. Of these significantly enriched GSs, 8 GSs overlapped between the three different sample sets. The majority of these GSs are involved in in cellular signaling (e.g. TNFα via NFκB) and pathways (e.g. hypoxia), immune response (e.g. inflammatory response), and development (e.g. epithelial-mesenchymal transition). In contrast, suppressed GSs observed during in vitro and in vivo S. suis ST2 infections included those, which were involved in cellular proliferation and metabolic processes. Furthermore, the RNA-seq method was validated via quantitative PCR. 
In a further comparative analysis using the CP samples obtained from the in vivo infection experiments, the conventional RNA-sequencing method was compared to the Massive Analysis of cDNA Ends (MACE) sequencing method. A concordance of approximately 50 % was observed for the significant DEGs, and the GSEA revealed an enrichment of the same 17 GSs. 
This study suggests that similar cellular processes occur in human and porcine infected CP (epithelial) cells, especially in terms of inflammatory response. However, further follow-up investigations need to be performed to gain specific insights to the cellular response at the CP during a S. suis infection.