Distinct T cell lineages marked by expression of cd4 paralogues in medaka

T lymphocytes are classified into CD4⁺ and CD8⁺ T cells based on the glycoproteins expressed on their cell surface. Teleosts possess two CD4 paralogs, CD4-1 and CD4-2; however, their functional roles in T cell differentiation remain unclear. We generated fish mutant lines lacking the two cd4 paralog genes and genes encoding the two components of the CD8 co-receptor. The resulting phenotypes were analyzed using polyclonal antisera specific to medaka (Oryzias latipes) CD4-1 and CD8α, raised using a DNA immunization strategy. Our results indicate that cd4-2 deficiency does not affect the development of CD4-1⁺ T cells, providing genetic evidence for their distinct functions. Interestingly, in cd4-1 deficient mutants, the smaller compartment of CD4-1⁺ T cells was accompanied by a relative increase of CD8α⁺ T cells. Reciprocally, the lack or reduction of CD8α⁺ T cells caused by the deletion of cd8 genes was accompanied by a greater preponderance of CD4-1⁺ T cells. For the first time, our study provides direct genetic evidence for the interconnected differentiation programs of CD4⁺ and CD8⁺ T cells in teleost.