Bioassays for the detection of a cytokine that enhances the development of cytolytic T lymphocytes.

Interleukin 2 (IL-2) is an important growth factor for cytolytic T lymphocytes (CTL). Other factors here termed cytolytic differentiation factor(s) (CDF) may be required for CTL responses, but it has been difficult to identify suitable bioassays. We report a polyclonal assay in which lytic activity develops after 2 days of culture with lectin and lymphokines. CDF is required for the development of Thy-1+, CD8+, CD4- CTL in this assay. The responsive T cells are thymocytes from certain strains of mice (A, Swiss NCS, B6.H-2k) or peripheral T cells cultured in the presence of high doses of hydrocortisone acetate. If these precautions are taken, little or no lytic activity develops in the presence of rIL-2 alone. CDF is present in the media of mitogen-stimulated mouse spleen or human blood mononuclear cells, but not in the conditioned media of a number of mouse and human cell lines. It is immunologically distinct from IL-2, but only acts in concert with IL-2 to induce CTL. A large panel of available mouse and human cytokines do not synergize with Il-2 in the bioassay. These include interferons, colony-stimulating factors, lymphotoxin/tumor necrosis factor, and IL-1, -3, and -4. Therefore a distinct CDF(s) seems essential for the induction of at least some CTL. The assays described here should be useful in purifying the molecule responsible for this biological activity.