## 4 Discussion
On the other hand, the up-regulation of lysozyme was detected in the skin tissue of crucial carp (_Carassius auratus_) in response to _Aeromonas hydrophila_[59]. Both up- and down-regulation of lysozyme was observed in the gill tissue of rainbow trout depending on the transcript variant [20]. Thus, the expression profile of lysozyme after an infection with _A. salmonicida_ depends on the fish species, type of tissue and time after infection. The first line of defense against pathogens constitute pattern recognition receptors (PRRs) that are innate immune sensors responding to conserved patterns of microorganisms [60]. In this study, several c-type lectin receptors (CLR) showed up-regulation after _A. salmonicida_ infection, and some of them, such as _c-type lectin 2-1, dec4m_ and _cd209_, showed similar magnitudes in both the moribund and survivor fish. A previous study of the immune defense of rainbow trout against _A. salmonicida_ suggested that _cd209_ is an essential receptor that captures this bacteria [24]. Our results showed a high up-regulation of _cd209_ in rainbow trout skin, which is consistent with previous studies in the spleen and head kidney tissues (after 1 and 7 days post infection) [23; 24]. The expression of the other CLR (_decde_) was significantly higher at 2 dpi compared to 6 dpi (FC = 17.22) and was the highest between all of the CLRs. C-type lectins recognize the carbohydrate patterns on pathogen surfaces, opsonize them or activate complement cascade and may induce signaling cascade, leading to the activation of NF-\(\kappa\)B, and thus, inflammatory responses [61; 62]. After an infection with _A. salmonicida_, antimicrobial peptides (AMPs) were activated, such as _camp_ and hepcidin (_hamp_). _Histone h1_ and _camp_ showed similar magnitudes of expression in the moribund and survivor fish, which is in agreement with the multi-tissue gene expression analysis after 3 and 13 days of _A. salmonicida_ infection in rainbow trout [21]. Otherwise, the expression of _hamp_ was higher at 2 dpi and then decreased after six days. The up-regulation of these AMPs was noticed in previous studies; however, in other tissues, _Hamp_ was previously up-regulated in the liver of rainbow trout using an ELISA test [63], whereas _histone h1_ was detected in Atlantic salmon after _Escherichia coli_ infection [64]. In summary, AMPs are pivotal parts of the first line of host defense against pathogens that disrupt pathogenic bacterial membranes and regulate the innate immune response [65]. The success of the immune response depends on the signaling, communication and migration of immune cells. Chemokines and cytokines such as interleukins are small glycoproteins that play crucial roles in inflammation, hematopoiesis and immune cell activation and they induce the migration of leukocytes from blood vessels to inflamed tissues [66; 67]. In this study, most of the identified chemokines were differentially expressed only at 2 dpi, and thus included down-regulated _ccl2_, _ccl21_ and _cxcl11_ and up-regulated _ccl19_, _ccl25_, _ccl28_, _cxcl8_ and _ccr1_. However, only _ccl19_ showed a statistically significant difference between the groups. In accordance, the expressions of _cxcl8_ (_il-8_) and _ccl19_ were increased in rainbow trout gill tissue after _A. salmonicida_ infection [20]. Previously, studies on the teleost revealed that _ccl19_ promotes anti-viral and anti-bacterial defense and inflammation [68]. Further, in this study, we noticed an up-regulation of genes encoding interleukins and their receptors including _il1b_, _il6_, _il11_, _il17d_, _il21_, _il1r2_ and _il13 ra2_. Contrary to rainbow trout gills, the expression of _il6_ was up-regulated in the skin, and according to a previous study, it might be induced by lipopolysaccharidesaccharides and promote phagocyte proliferation [69]. Further, the expressions of _il1b_ and _il11_ significantly increased at 2 dpi (FC = 6.49 and FC = 8.14, respectively) and decreased after 6 dpi.