Differing levels of Mhc class II β chain expression in a range of tissues from vaccinated and non-vaccinated Atlantic salmon (Salmo salarL.)

The expression of major histocompatibility complex class II β chain (Mhc class II β chain) was investigated in the tissues from vaccinated and non-vaccinated Atlantic salmon (Salmo salarL.). A reverse transcriptase-polymerase chain reaction (RT-PCR) technique was used to detect messenger ribonucleic acid (mRNA) in brain, skeletal muscle, heart, liver, foregut, head kidney, spleen, hindgut and gills. A 313 nucleotide RT-PCR product was produced and sequence analysis demonstrated that the amplified product was from the βchain of Mhc-Sasa class II. After 20 cycles of PCR, expression of Mhc class II β chain was only detected in the foregut, spleen, hindgut and gills of non-vaccinated fish. The same number of cycles of PCR detected expression in a broader range of tissues from vaccinated fish, including heart, liver, foregut, head kidney, spleen, hindgut and gills. There was low or negligible expression in brain and skeletal muscle. In the tissues from both groups there was variation in the level of expression of Mhc class II β chain between fish. Computer-assisted morphometric analysis of agarose gels scanned on a UV-light table allowed a semi-quantitative estimate of the level of Mhc class II β chain expression in the different tissues from the two groups, each of which contained five fish. Of those tissues showing expression in both vaccinated and non-vaccinated fish, there were significant increases in expression in the spleen and gills but not the foregut and hindgut of vaccinated fish (P<0·05). Further, the different tissues in the vaccinated fish were separated into three groups on the basis of significantly different levels of expression: low or negligible expression in brain and skeletal muscle; an intermediate level of expression in the heart, liver and foregut; and a high level of expression in the head kidney, spleen, hindgut and gills. These findings are consistent with the studies of others in carp and rainbow trout in that Mhc class II β chain expression can be related to the lymphoid/myeloid cell content of tissues. However, the responsiveness of Mhc expression in tissues following vaccination has not been previously reported in teleosts. The relatively high level of expression of Mhc class II β chain in the mucosal tissues of both vaccinated and non-vaccinated fish focuses attention on the interaction between the mucosal and systemic immune systems in fish, particularly in the regulation of Mhc class II expression.