Identification and functional analyses of CD4-1⁺ cells in grass carp (Ctenopharyngodon idella)

In mammals, CD4 is found to be expressed on T cells and innate immune cells, however, teleost cells bearing CD4 have not been well identified and characterized. In this study, we identified two different CD4-1⁺ cell subsets in grass carp (Ctenopharyngodon idella): CD4-1⁺ lymphocytes (Lym) and CD4-1⁺ myeloid cells (Mye), both of which had the highest proportions in the head kidney. The mRNA expression analysis showed that CD4-1, CD4-2, TCRβ, CD3γ/δ, and LCK1 are highly expressed in CD4-1⁺ Lym and also expressed in CD4-1⁺ Mye. Furthermore, we found that CD4-1⁺ Lym have a Lym morphology and highly express T-cell cytokines, suggesting that they are CD4⁺ T cells equivalent to mammalian Th cells. On the other hand, CD4-1⁺ Mye were found to have a morphology of macrophage and highly express macrophage marker gene MCSFR, indicating that they are macrophages. In addition, functional analysis revealed that CD4-1⁺ Mye possess phagocytic ability and great antigen-processing ability. Taken together, our study sheds further light on the composition and function of CD4⁺ cells in teleost fish.