DIFFERING LEVELS OF MHC CLASS-II BETA-CHAIN EXPRESSION IN A RANGE OF TISSUES FROM VACCINATED AND NON-VACCINATED ATLANTIC SALMON (SALMO-SALAR L.)

The expression of major histocompatibility complex class II beta chain (Mhc class II beta chain) was investigated in the tissues from vaccin ated and non-vaccinated Atlantic salmon (Salmo salar L.). A reverse tr anscriptase-polymerase chain reaction (RT-PCR) technique was used to d etect messenger ribonucleic acid (mRNA) in brain, skeletal muscle, hea rt, liver, foregut, head kidney, spleen, hindgut and gills. A 313 nucl eotide RT-PCR product was produced and sequence analysis demonstrated that the amplified product was from the beta chain of Mhc-Sasa class I I. After 20 cycles of PCR, expression of Mhc class II beta chain was o nly detected in the foregut, spleen, hindgut and gills of non-vaccinat ed fish. The same number of cycles of PCR detected expression in a bro ader range of tissues from vaccinated fish, including heart, liver, fo regut, head kidney, spleen, hindgut and gills. There was low or neglig ible expression in brain and skeletal muscle. In the tissues from both groups there was variation in the level of expression of Mhc class II beta chain between fish. Computer-assisted morphometric analysis of a garose gels scanned on a UV-light table allowed a semi-quantitative es timate of the level of Mhc class II beta chain expression in the diffe rent tissues from the two groups, each of which contained five fish. O f those tissues showing expression in both vaccinated and non-vaccinat ed 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 ex pression: low or negligible expression in brain and skeletal muscle; a n intermediate level of expression in the heart, liver and foregut; an d a high level of expression in the head kidney, spleen, hindgut and g ills. These findings are consistent with the studies of others in carp and rainbow trout in that Mhc class II beta chain expression can be r elated to the lymphoid/myeloid cell content of tissues. However, the r esponsiveness of Mhc expression in tissues following vaccination has n ot been previously reported in teleosts. The relatively high level of expression of Mhc class II beta chain in the mucosal tissues of both v accinated 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. (C) 1998 Academic Press Limited.