DETECTION OF MHC CLASS-II TRANSCRIPTS IN LYMPHOID-TISSUES OF THE COMMON CARP (CYPRINUS-CARPIO L)

In all vertebrates studied to date, the expression of MHC class II gen es is known to be restricted to a limited number of tissues and cell t ypes. In order to have a better understanding of the function of the e quivalent genes in teleost fish, the distribution of MHC class II beta transcripts (Cyca-DAB) in the common carp (Cyprinus carpio L.) was in vestigated. RNA was isolated from tissues and leucocytes, cDNA was pro duced, and amplification of the Cyca-DAB genes was carried out by PCR. Of the organs with known immunological function, the highest level of Cyca-DAB transcription was found in the thymus. Despite their expecte d different cellular organization, total blood, head kidney, spleen an d the second segment of the gut had similar Cyca-DAB expression levels . No class II transcripts were detected in the skeletal muscle. The st udies carried out with leucocytes isolated from the lymphoid tissues p oint to a direct correlation between the levels of expression and the amounts of surface immunoglobulin positive (sIg+) cells present in the different cell fractions. However, thymus leucocytes did not follow t his correlation since the highest level of class II expression was fou nd in a thymocyte fraction that contained very low numbers of Ig+ cell s. In PBL the Ig+ cells were highly positive whereas the Ig - were wea kly positive. Adherent leucocytes were shown to be class II positive, although adherent cells from PBL show a lower level of expression comp ared to those from the spleen and head kidney.