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 genes is known to be restricted to a limited number of tissues and cell types. In order to have a better understanding of the function of the equivalent genes in teleost fish, the distribution of MHC class II beta transcripts (Cyca-DAB) in the common carp (Cyprinus carpio L.) was investigated. RNA was isolated from tissues and leucocytes, cDNA was produced, 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 expected different cellular organization, total blood, head kidney, spleen and the second segment of the gut had similar Cyca-DAB expression levels. No class II transcripts were detected in the skeletal muscle. The studies carried out with leucocytes isolated from the lymphoid tissues point 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 this correlation since the highest level of class II expression was found in a thymocyte fraction that contained very low numbers of Ig+ cells. In PBL the Ig+ cells were highly positive whereas the Ig- were weakly positive. Adherent leucocytes shown to be class II positive, although adherent cells from PBL show a lower level of expression compared to those from the spleen and head kidney.