## 4 Discussion
_IL1b_ is a key pro-inflammatory cytokine, which was also up-regulated at early stages of infection in the gill of rainbow trout [20] and Atlantic cod [27], as well as in the head kidney infected with _A. salmonicida achromogenes_[70]. _IL11_ belongs to the _il6_ cytokine family and plays a major role in hematopoiesis, and it may show pro- and anti-inflammatory responses in fish [70]. In contrast to other interleukins, _il17d_ was repressed in moribund fish and then came back to the control level in the survivors. The decreased expression of _il17d_ is similar to that of the early stage of bacterial infection in _Siberia sturgeon_[71]. In summary, these results suggest that the expression of chemokines and interleukins in response to bacterial invasion depends on the type of tissue and post infection time, which has also been suggested in black rockfish (_Sebastes schlegelii_) infected with _A. salmonicida_[72]. At an early stage of bacterial infection, the inflammation process is activated and the inflammatory cells (neutrophils and monocytes/macrophages) secrete cytokines such as _il1b_, _il8_, _il6_ and tumor necrosis factor (_tnfa_) into the bloodstream, stimulating the production of acute-phase proteins (APPs) [73]. The APPs are involved in many immune processes such as the inactivation of proteolytic enzymes, the control of the distribution of infectious agents (by eliminating pathogens or by modifying surface targets) and in the recovery of damaged tissues [74]. In this study, several genes encoding APPs, such as _saa1_, serum amyloid a5 (_saa5_), haptoglobin (_lup_), _cbln1_, _lysozyme c_ and _lysozyme g_, were significantly elevated after an infection in fish skin. Of these genes, the expression levels of _lysozyme c_, _hump_, _c3_, _c4_ and _steap4_ were higher at 2 dpi and decreased after 6 dpi to the control level, whereas the serum amyloids were highly expressed at both 2 and 6 dpi. _SAAs_ play roles in inflammation, opsonization, cholesterol transport and the degradation of the extracellular matrix, and they might be useful in monitoring and evaluating health in fish [75]. Previous studies on rainbow trout gill, liver and spleen confirmed the up-regulation of _lup_ at 3 dpi [20] and the up-regulation of _saa1_ at 3 and 13 dpi [21]. The RT-qPCR analysis from this study also confirmed the up-regulation of _saa1_ (Figure 8). Otherwise, the ceruloplasmin level in the plasma decreased in both the moribund and survivor fish, which is in accordance with studies on the Nile tilapia (_Oroochromis niloticus_) after an infection with _A. hydroophila_[73]. Previous studies suggested a correlation of complement cascade to acute phase and inflammatory response [29; 76]. This study revealed that in rainbow trout skin at 2 days after infection, the complement and coagulation cascades pathway was activated. The complement components _c3_, _c4_, _c6_ and _c7_, the complement c1q-like protein 2 (_c1q12_) and the complement c5a receptor 1 (_c5 ar1_) were up-regulated in the moribund fish, which is in accordance with previous studies on rainbow trout [20; 21; 23]. Of these, only _c7_ was slightly up-regulated in the survivor fish. _C4_ plays an important role in classical and lectin pathways, _c1q12_ is the initial protein of the classical complement pathway and _c7_ plays an integral role in the formation of the membrane attack complex (MAC) [77]. Further, _c5 ar1_was also raised at 2 dpi, which may promote the development of inflammation through chemotaxis and the degranulation of granulocytes and monocytes [78]. Cell death plays a fundamental role during the homeostasis of the host and the defense against pathogens [79]. Several types of programmed cell death (PCD) such as autophagy, apoptosis, ferrotptosis, necroptosis and pyroptosis have been identified and classified in vertebrates [79]. In this study, apoptosis, autophagy and necroptosis processes were enriched in both the moribund and survivor fish.