Mitogenic effect of interleukin-2 on unstimulated human T cells: An editorial review

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Abstract
The most salient feature of the lymphokine interleukin‐2 (IL‐2), a hormone‐like protein, is its ability to sustain the proliferation of immunocompetent T cells. Results of initial studies characterizing IL‐2 in vitro led investigators to conclude that IL‐2 had no known effects on lymphocytes that had not been previously activated by exposure to a mitogen or antigen. Several groups postulated that T cell growth required two signals. The first signal, delivered by a mitogen or antigen, induced T cell activation. Resting T cells, which were thought to lack the membrane receptor for interleukin‐2 (IL‐2R), progressed from the G0‐G1 phase to the S phase, during which time they converted from IL‐2R− to IL‐2R+ cells. Thereafter, the second signal, served by IL‐2, induced T cell proliferation of the tL‐2R+ cells. Recently, a number of investigators have demonstrated that highly purified preparations of both natural and recombinant IL‐2 induced high levels of T cell proliferation in the absence of any known mitogens or antigens. Presented herein is a review of these studies and an overview of the hypotheses of the mechanisms whereby IL‐2 alone induces T cell activation and proliferation.