Immune functions of tumor necrosis factor. I. Tumor necrosis factor induces apoptosis of mouse thymocytes and can also stimulate or inhibit IL-6-induced proliferation depending on the concentration of mitogenic costimulation.

Murine rTNF produces at least three effects on mouse thymocytes in vitro: 1) Is a modest co-stimulator of proliferation with low PHA-P doses. 2) Has a bi-directional interaction with rIL-6-depending on PHA concentration: at low PHA (5 to 10 micrograms/ml) TNF augments and at high PHA (20 to 30 micrograms/ml) inhibits IL-6-induced proliferation. A comparable bidirectional PHA dose-dependent TNF interaction was seen with IL-1 beta, whereas only inhibition at high PHA with IL-2 and only augmentation at low PHA with IL-4 were seen. 3) TNF induces direct thymocyte apoptosis (a property not shared by IL-1 beta, IL-2, IL-4, IL-6 and IL-7). Of the cytokines studied, only IL-7 reduced TNF apoptosis. Thymocyte apoptosis by TNF showed the same species specificity as costimulation (i.e., human TNF had no effect) and was not inhibited by CY. The thymocyte CD4-CD8 phenotype after 72-h cultures showed that TNF decreased mainly double negative (DN) and single positive (SP) subsets, whereas IL-6 with low or high PHA increased DN and SP, especially the SP CD8+ subset. The regulatory and apoptotic effects of TNF were seen only with thymocytes and not with peripheral splenic or lymph node T cells. Four mAb to mouse TNF (2E2, XT22, 1C6, and 1OD9) could abrogate TNF costimulation and the TNF effects on IL-6-induced thymocyte proliferation, at both augmenting and inhibitory PHA conditions. However, only the two antibodies that also neutralize TNF lytic activity (2E2, XT22) could inhibit TNF-mediated apoptosis, implying two different but neighboring functional domains in the TNF molecule mediating apoptosis/lysis and costimulation. Our studies show that TNF might have unique and complex regulatory effects on growth and death of thymocyte populations in adult mice quite different from its effects on T cells in periphery.