RNA-seq analysis of local tissue of Carassius auratus gibelio with pharyngeal myxobolosis: Insights into the pharyngeal mucosal immune response in a fish-parasite dialogue

The lack of practical control measures for pharyngeal myxobolosis is becoming an important limiting factor for the sustainable development of the gibel carp (Carassius auratus gibelio) culture industry in China. Myxobolus honghuensis has been identified as the causative agent of this pandemic disease, which exclusively infects the pharynx of gibel carp, a potential important mucosal lymphoid-associated tissue (MLAT). Myxozoa generally initiate invasion through the mucosal tissues of fish, where some of them also complete their sporogonial stages. However, the pharynx-associated immune responses of teleost against myxosporeans infection remain unknown. Here, a de navo transcriptome assembly of the pharynx of gibel carp naturally infected with M honghuensis was performed for the first time, using RNA-seq. Comparative analysis of severely infected and mildly infected pharyngeal tissues (SI group and MI group) from the same fish individuals and control pharyngeal tissues (C group) from the uninfected fish was carried out to investigate the potential mucosal immune function of the fish pharynx, and characterize the panoramic picture of pharynx local mucosal immune responses of gibel carp against the M. honghuensis infection. A total of 242,341 unigenes were obtained and pairwise comparison resulted in 13,009 differentially-expressed genes (DEGs) in the SI/C group comparison, 6014 DEGs in the MI/C group comparison, and 9031 DEGs in the SI/MI group comparison. Comprehensive analysis showed that M. honghuensis infection elicited a significant parasite load-dependent alteration of the expression of numerous innate and adaptive immune-related genes in the local lesion tissue. Innate immune molecules, including mucins, toll-like receptors, C-type lectin, serum amyloid A, cathepsins and complement components were significantly up-regulated in the SI group compared with the C group. Up-regulation of genes involved in apoptosis signaling pathway and the IFN-mediated immune system were found in the SI group, suggesting these two pathways played a crucial role in innate immune response to M. honghuensis infection. Up-regulation of chemokines and chemokine receptors and the induction of the leukocyte trans-endothelial migration pathways in the severely and mildly infected pharynx suggested that many leucocytes were recruited to the local infected sites to mount a strong mucosal immune responses against the myxosporean infection. Up-regulation of CD3D, CD22, CD276, IL4/13A, GATA3, arginase 2, IgM, IgT and pIgR transcripts provided strong evidences for the presence of T/B cells and specific mucosal immune responses at local sites with M. honghuensis infection. Our results firstly demonstrated the mucosal function of the teleost pharynx and provided evidences of intensive local immune defense responses against this mucosa-infecting myxosporean in the gibel carp pharynx. Pharyngeal myxobolosis was shaped by a prevailing anti-inflammatory response pattern during the advanced infection stages. Further understanding of the functional roles of fish immune molecules involved in the initial invasion and/or final sporogony site may facilitate future development of control strategies for this myxobolosis