Synergy between interleukin-2 and a second factor in the long-term growth of human T cells.

It has recently been shown that factors in addition to interleukin-2 (IL-2) are required for the proliferation or differentiation of at least some murine T-cell lines. We have previously shown that conditioned medium from human mononuclear cells stimulated with phorbol ester and staphylococcal enterotoxin A is superior to commercial sources of IL-2 for the long-term growth of human T cells. We have identified in these supernatants a non-IL-2 factor (synergistic factor, SF) which synergizes with JURKAT IL-2 in the long-term growth of human T cells. [3H]TdR incorporation by IL-2-dependent human T cells after growth in IL-2 or SF alone for 14 days was slight, but significant. By contrast, growth in a combination of SF and IL-2 for 14 days stimulated [3H]TdR incorporation 10-20-fold higher, generally equal to the high incorporation measured when cells were grown in the presence of the conditioned medium from which SF was obtained. In a standard 2-day IL-2 assay, there was no correlation between activity and long-term growth-promoting ability. These results suggest that the 14-day assay better discerns the growth-promoting activity of various factors or combinations of factors. The mechanism of this interaction between SF and IL-2 remains to be elucidated. It is clear, however, that T-cell growth factor activity, when assessed by the long-term growth of human T cells, is not due to interleukin-2 alone.

Acid treatment enhances IL-2 activity of conditioned medium.

Although conditioned medium (CM) from human lymphocytes or mononuclear cells is the most readily available source of interleukin-2 (IL-2) for human T cell culture, its IL-2 activity in our experience is inconsistent. It is likely that this is, at least in part, due to the presence of toxic substances in the CM. Using CM from TPA/SEA induced human mononuclear cells, we have found that acid treatment (pH 2.0, greater than 30 min) significantly improves its ability to promote T cell growth. It is postulated that the selection process which occurs in cultures of mitogen stimulated T cells may result in cells which are sensitive to mitogen-induced lymphotoxins and that these are inactivated by the acid treatment. Since acid treatment did not similarly improve the T cell growth promoting ability of PHA induced, lectin-free commercial IL-2, there must be other differences between it and our CM, which play a role in T cell growth.