Mouse NK1.1 + cytotoxic T cells can be generated by IL-2 exposure from lymphocytes which express an intermediate level of T cell receptor

Abstract

NK-like T cells which express the NK1.1 molecule and CD3 (or TCR) of intermediate level (CD3 int or TCR int cells) were recently demonstrated to be present in various immune organs, and to have NK-like cytotoxic activity against NK target cells. In this study, we investigated whether NK1.1 − T cells could express NK1.1. We found that NK1.1 + TCR int cells were much more abundant in the liver (20%) than in the spleen (2%). When hepatic and splenic mononuclear cells (MNCs) were cultured either in the absence of IL-2 or in the presence of CD3/TCR cross-linking, the original NK1.1 + TCR int cells disappeared. However, when they were cultured in the presence of a high dose of IL-2 for 4 days, a new type of NK1.1 + T cell was formed to the extent of approximately 15–20%, and the liver and spleen contained similar percentages of this new type of NK1.1 + T cells. The phenotypes of the original and the new type of NK1.1 + T cells were clearly distinct. The freshly obtained NK1.1 + TCR int cells consisted of double-negative (DN) CD4 −CD8 − cells and single-positive (SP) CD4 + cells, whereas the new type of NK1.1 + T cells predominantly consisted of DN CD4 −CD8 − cells and SP CD8 + cells and expressed a high level of CD3 (CD3 high or TCR high cells). When NK1.1 − cells or IL-2 receptor β-chain (IL-2R β) − cells were isolated from the liver and spleen, and cultured in the presence of IL-2 for 4 days, NK1.1 + T cells were generated from NK1.1 − cells, but not from IL-2R β − cells. Our results suggested that the NK1.1 − cells, but not IL-2R β − cells, contained the precursor of IL-2-stimulated NK1.1 + TCR high cells. When purified NK1.1 − IL-2R β + TCR int cells were cultured in the presence of IL-2 for 4 days, approximately 10% of the cells became NK1.1 + TCR high cells. Approximately 60% of the purified NK1.1 + TCR int cells lost NK1.1 expression. The IL-2-stimulated NK1.1 + TCR high cells that had arisen from NK1.1 − TCR int cells exerted an NK cell-like cytotoxic activity similar to that of the original NK1.1 + T cells. Thus, NK1.1 − TCR int cells could express NK1.1 and exert NK-like cytotoxic activity regardless of their origin. It appears that NK1.1 + TCR high cells can only be induced through an IL-2-stimulation pathway but not via CD3/TCR cross-linking.