Effect of L-ornithine on proliferative and cytotoxic T-cell responses in allogeneic and syngeneic mixed leukocyte cultures.

The effect of L-ornithine on cytotoxic and proliferative responses in mixed leukocyte cultures has been analyzed. The activation of cytotoxic T lymphocytes (CTL) was strongly inhibited when 9 X 10(-3) M L-ornithine was added at the initiation of the cultures. The CTL precursor cells were not completely and irreversibly inactivated, however, since the cells generated normal cytotoxic activity if resuspended after 6 days in fresh culture medium together with a fresh set of stimulator cells. Experiments in microcultures with nylon-wool-nonadherent T-cell-enriched spleen cells as responder cells and "plastic adherent cells" as stimulator cells revealed that the cytotoxic responses were almost completely suppressed if ornithine was added within the first 20 hr but were only partially suppressed if ornithine was added after 48 hr. Also, ornithine had only a mild suppressive effect on proliferative responses in allogeneic and syngeneic mixed leukocyte cultures. The strong suppressive effect of the cytotoxic response was therefore not explained by a general toxic effect of L-ornithine on the responding cells in the culture. The addition of interleukin 2 (IL-2)-containing EL-4 supernatants did not prevent but rather enhanced the suppressive effect of L-ornithine. This indicated that the inhibitory effect was not (exclusively) expressed at the level of the IL-2-producing helper T cells. Since activated macrophages have been reported to secrete arginase, it appears that L-ornithine may be part of a regulatory circuit that selectively regulates the development of cytotoxic effector T cells.