Immune Protective Effect of Chitosan Oligosaccharide on Lipopolysaccharide-Stimulated Coelomocytes of Sea Cucumber Apostichopus japonicus In Vitro

In recent years, chitosan oligosaccharide (COS) has demonstrated promising applications in enhancing the immune protective function of sea cucumbers. However, the immune-protective effect of COS on sea cucumber coelomocytes in vitro remains unclear. This study investigated the effect of COS on lipopolysaccharide (LPS)-stimulated inflammation in sea cucumber coelomocytes. First, we measured the effects of COS and LPS on the viability of coelomocytes. COS exhibited no toxic effects on sea cucumber coelomocytes. Furthermore, pre-incubating the coelomocytes with COS significantly improved coelomocytes' viability after LPS stimulation (<i>p</i> < 0.05). Secondly, the phagocytic activity and respiratory burst of the coelomocytes were assessed to evaluate their immune levels. COS alone significantly increased the respiratory burst and phagocytic activity of the coelomocytes (<i>p</i> < 0.05). However, with LPS stimulated, COS significantly increased both the respiratory burst and phagocytic activity of the coelomocytes. The activities of lysozyme (Lyz), total nitric oxide synthase (T-NOSs), and superoxide dismutase (SOD) in sea cucumber coelomocytes were measured to evaluate their response to LPS stimulation. The results indicated that LPS stimulation significantly increases the activities of Lyz, T-NOSs, and SOD in sea cucumber coelomocytes (<i>p</i> < 0.05). Additionally, it was found that COS could inhibit the LPS-mediated expression of Lyz, T-NOSs, and SOD activities in the coelomocytes (<i>p</i> < 0.05). Furthermore, the relative expression of six immune-related genes-<i>Aj-IL-17</i>, <i>Aj-TNF-α</i>, <i>Aj-i-Lys</i>, <i>Aj-NOS</i>, <i>Aj-Rel</i>, <i>Aj-P105</i>-were analyzed in the coelomocytes stimulated by LPS after being cultured with COS. Finally, through transcriptomic technology analysis, it was determined that COS primarily alleviates LPS-induced inflammation via the tumor necrosis factor signaling pathway and the phagosome signaling pathway. The findings demonstrated that COS inhibited the expression of immune genes in sea cucumber coelomocytes in a dose-dependent manner. In summary, pretreatment with chitosan oligosaccharides appears to confer an immune protective role in LPS-stimulated sea cucumber coelomocytes.