The influence of stress on the immune system of the common carp (Cyprinus carpio) was studied by measuring leukocytes levels using flow cytometry and mRNA immune components by real time qPCR. Acute and chronic oxidative stresses were generated by different regimes of exposure of carp to environmental air. In acute stress, induced by single air exposure, the pro-inflammatory cytokines (IL1β, IL6 and TNFα) and the down-regulatory ones (IL10 and TGFβ) showed significant simultaneous elevations (515, 147, 373, 300 and 198% respectively). Following chronic stress (multiple air exposures) however, a drastic decline of 80%, in macrophages/monocytes, B-cells likes and plasma-cells like, occurred in peripheral blood. No statistical changes in IL6 and TNFα, as well as in IgM and C3s mRNA levels could be shown during this experiment. CD4 mRNA decreased up to 6% in the 2nd week of chronic stress and elevated only to 55% at the 3rd week Vs a temporal decline of up to 22% in CD8a mRNA at the 2nd week. The regulatory cytokines (IL10, FoxP3 and TGFβ) as well as the pro-inflammatory ones (IL1β and IL17) decreased significantly up to 0.06, 0.2, 5, 6 and 4% respectively, at the second week before being restored to normal at the 3rd week. Moreover, a persistent decrease, up to null levels, in the cytokines IFNγ2b, IL12b and IL8 was also revealed. These downregulations were suggested as a result of the impaired Th1 and/or cytotoxic cell function and, to a certain degree, the leukocytes mobilization. The above findings show that in contrast to the detrimental effects of chronic stress, in which cells and functions of acquired immunity were partially or completely impaired, the acute stress was found rather beneficial and in line with the known ephemeral "fight and flight" response.

The purpose of this study was to verify whether NCCRP-1 is a marker of a unique type of cells in teleost, so-called NCC, or an ubiquitin, like in cytotoxic cells. Therefore, common carp peripheral blood leukocytes were isolated and tested for the binding of fluorescent NCCRP-1 antibody to stained MAIT, γδ T and T cells following stress treatments. The results were analyzed by a confocal microscope. The results revealed the presence of NCCRP-1 in γδ T, MAIT and T cells in more than one type of leukocytes. γδ T cells were the dominant population in carp leukocytes. Therefore, it was concluded that there might be no presence of NCC cells in the common carp leukocytes and that the NCC marker, NCCRP-1, acts probably as an ubiquitin in cytotoxic cells such as γδ T and MAIT cells, which were abundant in peripheral blood leukocytes of common carp.

Aquaculture conditions expose fish to internal and environmental stressors that increase their susceptibility to morbidity and mortality. The brain accumulates stress signals and processes them according to the intensity, frequency duration and type of stress, recruiting several brain functions to activate the autonomic or limbic system. Triggering the autonomic system causes the rapid release of catecholamines, such as adrenaline and noradrenaline, into circulation from chromaffin cells in the head kidney. Catecholamines trigger blood cells to release proinflammatory and regulatory cytokines to cope with acute stress. Activation of the limbic axis stimulates the dorsolateral and dorsomedial pallium to process emotions, memory, behaviour and the activation of preoptic nucleus-pituitary gland-interrenal cells in the head kidney, releasing glucocorticoids, such as cortisol to the bloodstream. Glucocorticoids cause downregulation of various immune system functions depending on the duration, intensity and type of chronic stress. As stress persists, most immune functions, with the exception of cytotoxic functions, overcome these effects and return to homeostasis. The deterioration of cytotoxic functions during chronic stress appears to be responsible for increased morbidity and mortality.