過敏性氣喘主要為過敏原所引起的呼吸道慢性發炎及氣道過度反應而導致支氣管過度收縮，氣流阻塞和呼吸不順的一種常見疾病。其免疫病理機制為過敏原活化了免疫系統中的第二型輔助型T淋巴球（CD4+Th2 lymphocytes）。然而，CD4+ T 淋巴球中有一群由胸腺製造維持免疫反應平衡相當重要的先天性CD4+CD25highFOXP3+調節性T細胞。如果減少小鼠的這群細胞則會造成各種不同的發炎反應與自體免疫疾病。所以我們推測或許這群先天性CD4+CD25highFOXP3+調節性T細胞在過敏性氣喘中扮演關鍵角色。我們研究發現患有家塵蹣所引起的過敏性氣喘的孩童，其專一性標記轉錄因子FOXP3在CD4+CD25highT細胞的表現較正常孩童低且其抑制CD4+CD25-T細胞活化增生的能力也顯著降低。因此我們假設FOXP3表現的下降使得CD4+CD25high T細胞喪失其調節免疫反應的功能。我們將家塵蹣所引起的過敏性氣喘孩童週邊血中的單核球與過敏原家塵蹣萃取物培養六天，發現轉錄因子FOXP3在CD4+CD25high T細胞的表現大量下降，同時CD4+CD25high T細胞無法抑制單核球的活化增生。相反地，接受過減敏治療(allergen-specific immunotherapy)的過敏性氣喘孩童，其單核球在家塵蹣萃取物的刺激下，轉錄因子FOXP3在CD4+CD25high T細胞的表現不僅回復，且這群細胞抑制單核球活化增生的功能也跟著回復。但如果以家塵蹣萃取物直接刺激CD4+CD25high T細胞，其轉錄因子FOXP3的表現並不會受到影響。過去許多研究和我們的實驗都發現來自過敏性氣喘兒童週邊血中的單核球，在家塵蹣萃取物的刺激下，腫瘤壞死因子(TNF-)會大量被製造，但減敏治療可改善這種現象。我們發現腫瘤壞死因子可以直接降低CD4+CD25high T細胞中FOXP3的表現，藉此抑制它們調節免疫反應的功能。這樣的調節可能是透過表現在CD4+CD25highFOXP3+調節性T細胞膜上的第二型腫瘤壞死因子接受器進行的訊息傳遞所導致。因此我們在單核球與過敏原家塵蹣萃取物培養時加入游離態腫瘤壞死因子接受器融合蛋白etanercept，它可以跟第二型腫瘤壞死因子接受器競爭腫瘤壞死因子，因而減弱腫瘤壞死因子所傳遞的訊號。結果發現透過etanercept可回復CD4+CD25high T細胞中FOXP3的表現和其抑制單核球活化增生的功能。此數據再次證明腫瘤壞死因子(TNF-)藉著降低CD4+CD25high T細胞中FOXP3的表現而抑制它們的功能。這或許可以解釋一部分過敏性氣喘兒童中，先天性CD4+CD25high FOXP3+調節性T細胞功能缺失的原因。

Allergic asthma is characterized by chronic airway inflammation and airway hyperresponsiveness mediated by activated CD4+Th2 lymphocytes. However, one of the subset of CD4+ T lymphocytes, thymus-derived (natural) CD4+CD25highFOXP3+ regulatory T cells are important in the maintenance of homeostasis of the immune system. Depletion of this regulatory population of T lymphocytes leads to several different autoimmune and inflammation conditions in mice. It is suggested that natural CD4+CD25highFOXP3+ regulatory T cells play the key role in diseases characterized by dysregulated peripheral tolerance, including allergic asthma. In this study, we found that although the percentage of CD4+CD25high T cells in peripheral blood was increased, FOXP3 expression of CD4+CD25high T cells was significantly lower in house dust mite(Dermatophagoides pteronyssinus, Der p)-allergic asthmatic individuals as compared to normal controls. The inhibitory function of isolated CD4+CD25highFOXP3+ regulatory T cells on the proliferation of CD4+CD25- T cells was also significantly suppressed in Der p-sensitive asthmatic individuals. It was hypothesized that the suppressed regulatory T cell function was due to decreased FOXP3 expression. In further study, we discovered that, peripheral blood mononuclear cells, collected from allergic asthmatic individuals, incubated with the specific allergen Der p extract for six days, FOXP3 expression of CD4+CD25high T cells was significantly decreased and CD4+CD25high T cells lost their inhibitory function on the proliferation of PBMCs. In contrast, when PBMCs from allergen-specific immunotherapy (SIT)-treated allergic asthmatic individuals were ex vivo cultured with Der p extract for six days, we detected that FOXP3 expression was reversed and the suppressive function of CD4+CD25high T cells was also restored. However, FOXP3 expression was not changed when isolated CD4+CD25high T cells were directly stimulated with Der p extract. Moreover, several reports have shown that increased TNF-production in Der p extract-incubated PBMCs from allergic asthmatic individuals, but not from SIT-treated allergic asthmatic individuals. In the following study, we found that TNF-coulddirectly downregulate FOXP3 expression of CD4+CD25high T cells and abrogate their suppressive function.The downregulation might be mediated through TNF- receptor 2 (TNFR2) signaling, commonly expressed on CD4+CD25highFOXP3+ regulatory T cells. Therefore, we added the decoy soluble TNF receptor fused to human IgG1 (Etanercept), which blocks TNF-signaling via the competition with TNFR2, in Der p-incubated PBMCs from allergic asthmatic individuals, and it reversed above phenomena. This may explain partly the mechanism of functional insufficiency of natural CD4+CD25high FOXP3+ regulatory T cells in the disease of allergic asthma.

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