Prostaglandin E2 (PGE2) is generally accepted to be an immunosuppressant produced by cancer cells and their surrounding macrophages. Although several investigators have reported detecting high concentrations of PGE2 in the portal veins of patients with colorectal cancer, the relationship between these high concentrations of PGE2 in the portal vein and liver-associated immunity remains unclear. In this study, we attempted to determine if the portal administration of PGE2 suppresses the immune function of the liver in a rat model. Donryu rats were administered PGE2 via the portal vein for 7 days, following which the cytotoxic activity of hepatic sinusoidal lymphocytes (HSL) against natural killer (NK)-sensitive YAC-1 and rat syngeneic AH60C tumor cells was assessed. Purified HSL are spontaneously cytolytic; however, the continuous administration of PGE2 dramatically suppressed the cytotoxic activity of HSLs in a dose-dependent fashion. Flow cytometric analysis showed that the large granular lymphocyte (LGL) fraction, hepatic natural killer (pit) cells, and CD4-8+ killer/suppressor T cells were mainly reduced in number in the HSLs following PGE2 infusion. In this rat AH60C metastasis model, the continuous administration of PGE2 increased the number and size of metastatic tumor nodules in the liver, suggesting that high concentrations of PGE2 in the portal vein suppress liver-associated immunity and promote the formation of hepatic metastasis.

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A pilot study we conducted on hepatic infusion chemotherapy combined with interleukin-2 (IL-2) for metastatic liver malignancies revealed very encouraging results indicating that this treatment modality is more effective than either of the anticancer drugs used alone. To clarify the mechanisms underlying the synergism of these modalities, the pharmacokinetics of anticancer drugs were examined in a rat model. Adult rats were given 5-fluorouracil (5-FU) or mitomycin C (MMC) combined with various doses of IL-2 up to 7500 JRU/kg per minute for the measurement of hepatic extraction rates (HER). The HER of 5-FU was significantly increased (P < 0.01) in combination with IL-2 in a dose-dependent fashion while that of MMC also showed a tendency to increase. Thus, it is conceivable that the increase of vascular permeability caused by IL-2 results in augmentation of the HER of associated anticancer drugs. This effect may improve the delivery of anticancer drugs to the liver and alleviate general toxicity by reducing the amount of circulating anticancer agents.

The antitumor activity of Interleukin-1 (IL-1), was assessed against the murine adenocarcinoma colon 26 tumor model in combination with Interleukin-2 (IL-2). Colon 26 tumor cells were inoculated on the back of syngeneic BALB/c mice. Fourteen days after inoculation, when the tumor nodule reached approximately 10 mm in diameter, tumor nodules were resected and Hank's solution, IL-2, IL-1, or IL-2 plus IL-1 were injected directly into the mouse spleen. One week after treatment, potent natural killer (NK) and enhanced lymphokine activated killer (LAK) cell activities were seen in the splenocytes treated by the combination of IL-2 plus IL-1. Furthermore the combination treatment by IL-2 plus IL-1 resulted in a significantly prolonged survival. Phenotypic analysis showed an increased number of percent positive cells expressing asialo GM 1 and IL-2 receptor after treatment with IL-2 plus IL-1. A possible role of IL-1 in augmentation of IL-2 dependent antitumor activity in vivo is discussed.

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