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.