Prolongation of serum half-life of interleukin 2 and augmentation of lymphokine-activated killer cell activity by pepstatin in mice.

We reported previously using a murine model that the kidney is the organ involved in catabolism of exogenous human recombinant interleukin 2 (IL-2) and that cathepsin D, a major renal acid protease, is responsible for the degradation of IL-2. In the present report also using BALB/c mice we have investigated the effect of in vivo pepstatin, an acid protease inhibitor, treatment on serum half-life of IL-2, and generation of lymphokine-activated killer (LAK) cell activity. The in vivo pepstatin treatment by i.p. injection resulted in a significant reduction in the accumulation of 125I-IL-2 by the kidney in a reverse dose-response manner. Pepstatin treatment prolonged the serum half-life of 125I-IL-2, and the increase in serum half-life of 125I-IL-2 was pepstatin dose dependent. A significant reduction in renal cathepsin D activity, as monitored by the degradation of 125I-IL-2, was detected. In vivo pepstatin (0.6 mg/kg) treatment along with IL-2 (300,000 IU/mouse) daily for 3 or 6 days resulted in an augmentation of natural killer activity exhibited by freshly prepared and uncultured splenocytes against YAC-1 cells. An additional culturing of the splenocytes with IL-2 (3,000 IU/ml) in vitro for 1 day significantly enhanced the effect of in vivo pepstatin treatment; i.e., LAK cell activity generated from the splenocytes of animals treated with IL-2 plus pepstatin was greatly augmented in comparison with that treated with IL-2 alone. Phenotypic assessment by cell surface markers (Thy-1.2, Lyt-2, L3T4, and asialo-GM1) on the fresh splenocytes prepared from animals treated in vivo with pepstatin plus IL-2 revealed a decrease in the percentage of cells expressing Thy-1.2 and Lyt-2 and an increase in those carrying asialo-GM1. These results demonstrated that, as a result of in vivo pepstatin treatment, renal cathepsin D activity was greatly inhibited, which in turn reduced the degradation of circulating IL-2, then prolonged serum half-life of IL-2, and subsequently augmented natural killer and LAK cell activity. The in vivo pepstatin and IL-2 treatment decreased the T-cells and increased the natural killer-like LAK precursor cells, possibly also with an increase in its activity, which were further induced by in vitro IL-2 culture to generate an augmented LAK cell activity. This study also suggests the clinical potential of pepstatin in IL-2-related immunotherapy.