水飛薊賓對T細胞免疫反應之調節作用

Abstract

水飛蓟賓 (silibinin) 和水飛蓟 (silymarin) 用於治療肝臟疾病已經有相當久的歷史 (Gazak at al, 2007)，近年來，更廣泛作為保健食品或是醫療用途的藥品製劑，包含臺灣 。截至目前為止， 科學文獻的研究報告證實水飛蓟賓和水飛蓟具有許多有潛力的生物活性作用，其中包含對免疫的 調節作用，例如影響 T 細胞的功能及其細胞激素基因的表現。然而，有關水飛蓟賓和水飛蓟對 Th1/Th2免疫平衡的影響，現有的文獻報告之結果並不ㄧ致，仍有待進一步釐清。因此，本論文 的主旨在研究水飛蓟賓對Th1/Th2 免疫反應的作用，採用包含離體細胞培養和活體動物模式，探 討水飛蓟賓對 T 細胞參與的抗體生成、呼吸道過敏性免疫反應和細胞激素基因表現的影響。實 驗結果顯示，在卵白蛋白(ovalbumin；OVA)免疫的 BALB/c 小鼠，於免疫前連續三天口服投予 水飛蓟賓，具有促進脾臟細胞表現 IFN-γ和血清中OVA 專一性IgG2a 抗體生成的作用；而且，脾 臟細胞表現IL-4 和血清中OVA 專一性抗體IgE及total IgE 則受到水飛蓟賓的抑制。這些證據顯 示口服水飛蓟賓會使OVA 免疫小鼠的 T 細胞免疫反應，傾向Th1 的方向。本論文進一步採用以 OVA氣霧刺激OVA 免疫小鼠之呼吸道過敏的動物模式，結果發現在免疫前以及氣霧刺激前口服 投予水飛蓟賓，對呼吸道所引起的過敏性發炎反應沒有影響，但有促進肺臟組織中IFN-γ mRNA 表現的效果。在離體細胞培養的實驗模式，以水飛蓟賓處理由 OVA 免疫小鼠分離的脾臟細胞， 發現對 IFN-γ和 IL-4 皆有抑制作用，這些結果顯示在離體細胞直接暴露於水飛蓟賓的實驗條件 下，水飛蓟賓對於脾臟細胞表現細胞激素基因的作用和活體給藥的結果不同。 本論文採用BALB/c 鼠，嘗試了二種動物模式活體投予水飛蓟賓，和一種離體細胞模式處理水飛蓟賓，所得實驗結果 仍然欠缺一致的結論，分析水飛蓟賓在現有文獻報告中，對 Th1/Th2 的影響亦有許多分歧的實驗 證據，顯示水飛蓟賓對 T 細胞免疫反應作用之複雜，仍有待進一步加以釐清。值得一提的是， 在本論文的離體試驗模式下，水飛蓟賓在較低的濃度(5-10 μM)下，即對Th1 (IFN-γ)和Th2 (IL-4) 細胞激素有明顯的影響，合併活體給藥的實驗結果，明顯指出 T 細胞是水飛蓟賓的作用標的之 一，其影響細胞激素基因表現的機轉值得未來加以深入探討。

Silibinin and silymarin have been used as herbal medicines to treat liver disease for a long time. Recent years, silibinin and silymarin has been commonly used as dietary supplements, as well as medicinal products in many countries, including Taiwan. To date, scientific evidence indicates that silibinin and silymarin possess potential biological activities, including immune modulation (i.e. effects on T cell reactivity and cytokine expression). Although the immunomodulatory activity of silibinin on T cell functionality has been documented, contrasting effects on the balance of T helper (Th)1/Th2 cell-mediated immunity were reported. Hence, the objective of the present study was to investigate the effect of silibinin on Th1/Th2 immune balance. Both animal models and cell culture experiments were employed to examine the influence of silibinin on T-dependent antibody production, T cell-mediated allergic airway responses, and cytokine expression. The results demonstrated that daily oral administration of silibinin for 3 days prior to ovalbumin (OVA) sensitization markedly enhanced the production of IFN-γ by splenocytes and the serum level of OVA-specific IgG2a in OVA-sensitized BALB/c mice. In contrast, the production of IL-4 and OVA-specific IgE and total IgE was attenuated. These finding indicates that silibinin administration polarizes the Th1/Th2 balance toward the Th1 direction. In the murine model of allergic airway responses induced by the challenge of OVA aerosol to OVA-sensitized mice, no significant effect on the airway allergic immune response by silibinin administration was observed; however, the steady state mRNA expression of IFN-γ in the lung tissues of OVA-sensitized and challenged mice was markedly enhanced by silibinin administration. Furthermore, the direct effect of silibinin on T cell-derived cytokine expression was also studied in vitro. Splenocytes obtained from OVA-sensitized mice were exposed to silibinin in culture and stimulated with OVA to induce cytokine production. The results showed that silibinin exposure significantly attenuated the production of both IFN-γ and IL-4 by OVA-stimulated splenocytes. These results demonstrated that the effects of silibinin on T cell cytokine expression between the employed in vivo and in vitro experimental settings are inconsistent. The present study utilized 3 experimental systems, including animal models and cell culture experiments, to examine the immunomodulatory effect of silibinin on T cells; however, the results on T cell cytokine expression are contrasting. The inconsistency of silibinin-mediated effects on the Th1/Th2 immune balance has been reported in the literature, suggesting a very complicated profile of silibinin influence on the immune system. It is noticed that, under the employed condition of cell culture studies, the effective concentration range of IV silibinin (5-10 μM) to affect Th1 (IFN-γ) and Th2 (IL-4) cytokine expression is lower than the concentrations used by many reports in the literature. Together with the demonstrated effects of silibinin on Th1/Th2 cytokine expression in murine models, it is apparent that T cells are a sensitive target for silibinin. Further studies to elucidate the mechanism of silibinin-mediated effects on cytokine expression are warranted.