The expression of CD8α discriminates distinct T cell subsets in teleost fish

CD8, belonging to the TCR complex, is the main marker molecule of CTLs. Although CD8 genes have been detected in many fish species, the analysis of teleost CD8⁺ cells has been limited because of the lack of antibodies. Using newly established mAbs against rainbow trout CD8α, we found high ratios of CD8α⁺ cells in trout thymus, gill and intestine, but relatively low abundance in pronephros, spleen and blood. Accordingly, tissue sections revealed many CD8α⁺ cells in thymus, numerous intra- and subepithelial CD8α⁺ cells in intestine and gill and few scattered CD8α⁺ cells in spleen and pronephros. In secondary lymphoid tissues, CD8α⁺ lymphocytes, which did not react with anti-thrombocyte or anti-IgM mAbs, expressed CD8α, CD8β and TCRα, while Ig and CD4 transcripts were found in CD8α^- lymphocytes. In contrast, considerable CD4 expression in CD8α⁺ thymocytes suggests the presence of double-positive early T cells. Highly expressed TCRγ, LAG3 and CTLA4 in CD8α⁺ lymphocytes imply that they constitute a heterogeneous population different from found in non-mucosal tissues. PHA stimulation resulted in an up-regulation of CTL effector genes (perforin, granulysin and IFN-γ) in CD8α⁺ pronephrocytes, while both Th1 (IFN-γ) and Th2 (IL-4/13A) cytokines were up-regulated in CD8α^- pronephrocytes. Although the basic characteristics of CD8α⁺ lymphocytes seem similar in teleost and mammals, features such as the low proportion of teleost CD8α⁺ lymphocytes in blood and their high abundance in respiratory tissue reveal a unique dynamics and distribution.