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Cytokines can be induced by pathogen stimulation for the information exchange between cells, and their expression patterns indicate the response status of cells. In the present study, the expression patterns of ten cytokines (Cg IL17–1, −2, −3, −4, −5, −6, Cg TNF-1/2, Cg TGF-β and Cg IFNLP) in haemocytes were analyzed in the Pacific oyster Crassostrea gigas after Vibrio splendidus stimulation in the indoor and outdoor experiments to compare the induction patterns of different cytokines and evaluate the response status of haemocytes. After the indoor V. splendidus immersion stimulation, the mRNA abundances of six cytokines (Cg IL17–1, −3, −4, −5, Cg TGF-β and Cg IFNLP) in haemocytes were significantly downregulated at 48 h compared with that in the control group, among which Cg IL17–4 was the most down-regulated with 0.009-fold of that in the control group (p < 0.05). The mRNA abundances of Cg IL17–2, Cg IL17–6, Cg TNF-1 and Cg TNF-2 showed no significant change at 48 h compared with that in the control group. The apoptosis rate of haemocytes reached the maximum level of 1.138% at 48 h after the stimulation. In the oysters cultured outdoor from April to September 2021, the mRNA abundances of all ten cytokines in haemocytes were found to decrease in June (0.270-fold on average) and increase in July (5.402-fold on average) compared with those in May. After the in situ V. splendidus immersion stimulation in the outdoor cultured oysters in June and July, seven cytokines (Cg IL17–1, Cg IL17–2, Cg IL17–4, Cg IL17–5, Cg TNF-1, Cg TGF-β and Cg IFNLP) exhibited upregulated mRNA abundance in June, while in July, the mRNA abundances of five cytokines (Cg IL17–1, Cg IL17–3, Cg TNF-2, Cg TGF-β, and Cg IFNLP) remained constant, and four cytokines (Cg IL17–2, Cg IL17–4, Cg IL17–5 and Cg TNF-1) were suppressed after the stimulation. Cg IL17–4 had the highest fold-change in mRNA abundance among all cytokines in June after the stimulation, which was 85.599-fold compared with that in the control group (p < 0.05). The score of haemocyte response status (SH) was evaluated using the mRNA abundance database of all ten cytokines. It decreased to 124.860 at 48 h after the indoor V. splendidus stimulation, and it decreased to 66.793 in June and increased to 451.092 in July in the cultured oysters prior to the stimulation. Collectively, the expression patterns of ten cytokines in haemocytes varied against bacterial stimulation, among which Cg IL17–4 was found to response the most significantly, and SH indicated by the mRNA abundances of cytokines suggested an abnormal response status of haemocytes in the oysters at 48 h after indoor V. splendidus stimulation and the oysters cultured outdoor during June and July. These findings will further our understanding of the cytokine variation and haemocyte response of the Pacific oyster against bacterial stimulation. • Cg IL17-4 exrhibited the most significant response after stimulation in ten cytokines. • A formula was established to evaluate haemocyte response status using cytokine data. • The bacterial stimulation for 48 h resulted in abnormal response status of haemocytes. • Abnormal response status of haemocytes occurred in cultured oysters in June and July. [ABSTRACT FROM AUTHOR]