The Early Peritoneal Cavity Immune Response to Infection and to Inactivated Bacterium in Olive Flounder ().

The peritoneal cavity plays an important role in the immune response, and intraperitoneal administration is an ideal vaccination route in fish. However, immune responses in the peritoneal cavity of teleost fish are still not completely characterized. This study characterized the morphology of peritoneal cavity cells (PerC cells) and their composition in flounder (). Flow cytometric analysis of the resident PerC cells revealed two populations varying in granularity and size. One population, approximately 15.43% ± 1.8%, was smaller with a lower granularity, designated as lymphocytes. The other population of the cells, about 78.17% ± 3.52%, was larger with higher granularity and was designated as myeloid cells. The results of cytochemical staining and transmission electron microscopy indicated that peritoneal cavity in flounder normally contains a resident population of leukocytes dominated by granulocytes, macrophages, dendritic cells, and lymphocytes. The percentages of IgM, CD4, G-CSFR, MHCII, and CD83 leukocytes among PerC cells determined by flow cytometry were 3.13% ± 0.4%, 2.83% ± 0.53%, 21.12% ± 1.44%, 27.11% ± 3.30%, and 19.64% ± 0.31%, respectively. Further, the changes in IgM, CD4, G-CSFR, MHCII, and CD83 leukocytes in flounder after infection and immunization were compared. The composition changed rapidly after the infection or vaccination treatment and included two stages, a non-specific stage dominated by phagocytes and a specific immune stage dominated by lymphocytes. Due to the virulence effectors of bacteria, the infected group exhibited a more intense and complicated PerC cells immune response than that of the immunization group. Following our previous study, this is the first report on the morphology and composition of PerC cells and the early activation of PerC cells in flounder response to infection and vaccination.