Bovine CD2-/NKp46+ cells are fully functional natural killer cells with a high activation status

Natural killer (NK) cells in the cow have been elusive due to the lack of specific NK cell markers, and various criteria including a CD3-/CD2+ phenotype have been used to identify such cells. The recent characterization of the NK-specific NKp46 receptor has allowed a more precise definition of bovine NK cells. NK cells are known as a heterogeneous cell group, and we here report the first functional study of bovine NK cell subsets, based on the expression of CD2.

Bovine CD2- NK cells, a minor subset in blood, proliferated more rapidly in the presence of IL-2, dominating the cultures after a few days. Grown separately with IL-2, CD2- and CD2+ NK cell subsets did not change CD2 expression for at least two weeks. In blood, CD2- NK cells showed a higher expression of CD44 and CD25, consistent with a high activation status. A higher proportion of CD2- NK cells had intracellular interferon-gamma in the cytoplasm in response to IL-2 and IL-12 stimulation, and the CD2- subset secreted more interferon-gamma when cultured separately. Cytotoxic capacity was similar in both subsets, and both carried transcripts for the NK cell receptors KIR, CD16, CD94 and KLRJ. Ligation by one out of two tested anti-CD2 monoclonal antibodies could trigger interferon-gamma production from NK cells, but neither of them could alter cytotoxicity.

These results provide evidence that bovine CD2- as well as CD2+ cells of the NKp46+ phenotype are fully functional NK cells, the CD2- subset showing signs of being more activated in the circulation.