Expression and Immune Characterization of Major Histocompatibility Complex in Paralichthys olivaceus after Antigen Stimulation

The Major histocompatibility complex (Mhc) plays an essential role in antigen presentation as part of the adaptive immune system. To investigate the role of MhcII in adaptive immunity, this study cloned the mhcII alpha and mhcII beta of flounder (Paralichthys olivaceus) and examined their distribution and expression patterns, which varied by antigen stimulation or pathogens infection. The analyses showed that they were significantly expressed in gills, spleen, and peripheral blood leukocytes (PBLs), and MhcII molecules were co-localized with CD83 and IgM on leukocytes, respectively. The expression of both mhcII alpha and mhcII beta were significantly upregulated in flounder after infection. The percentages of MhcII(+) cells, MhcII(+)/CD83(+), and MhcII(+)/IgM(+) cells increased significantly after PHA and ConA stimulation, respectively; they varied significantly in PBLs after polyI:C stimulation and no obvious variations were found after LPS treatment. These results suggested that MhcII, mainly expressed in B cells and dendritic cells, responded significantly to exogenous antigens and T cell-dependent antigens. In conclusion, MhcII was associated with cellular immunity in teleosts.The Major histocompatibility complex (Mhc) is an important molecule for antigen presenting and binds to T cell receptors, activating T lymphocytes and triggering specific immune responses. To investigate the role of MhcII in adaptive immunity, in this study, mhcII alpha and mhcII beta of flounder (Paralichthys olivaceus) were cloned, polyclonal antibodies (Abs) against their extracellular regions were produced, respectively, and their distribution on cells and tissues and expression patterns, which varied by antigen stimulation or pathogen infection, were investigated. The results showed that the open reading frame (ORF) of mhcII alpha is 708 bp, including 235 amino acids (aa); and the ORF of mhcII beta is 741 bp, encoding 246aa. The mhcII alpha and mhcII beta were significantly expressed in gills, spleen, and peripheral blood leukocytes (PBLs). Their antibodies could specifically recognize eukaryotic expressed MhcII alpha and MhcII beta. MhcII alpha(+) and MhcII beta(+) cells were 30.2 +/- 2.9% of the percentage in peripheral blood leukocytes. MhcII molecules were co-localized with CD83 and IgM on leukocytes, respectively, but not on CD4(+) or CD8(+) T lymphocyte subpopulations. The expression of both mhcII alpha and mhcII beta were significantly upregulated in flounder after bacteria and virus challenges. The percentages of MhcII(+) cells, MhcII(+)/CD83(+), and MhcII(+)/IgM(+) double-positive cells increased significantly after PHA and ConA stimulation, respectively; they varied significantly in PBLs after polyI:C stimulation, and no variations were found after LPS treatment. In the meantime, variations in MhcII(+) cells were consistent with that of CD4(+) T lymphocytes. These results suggest that MhcII, mainly expressed in B cells and dendritic cells, play an essential role in antigen presentation, and respond significantly to exogenous antigens and T cell-dependent antigens. These results may provide an important reference for the study of cellular immunity in teleosts.