EB病毒膜蛋白LMP2A抑制人類第二型主要組織相容性複合體基因表現之探討

Abstract

藉由第二型主要組織相容性複合體(MHC class II)呈現外來抗原讓CD4+ T 細胞辨識，為免疫系統產生專一性免疫反應之重要機制，MHC class II 之表現量直接關係著T 淋巴細胞的活化，活化的CD4+ T 細胞能分泌抑制病毒感染之細胞激素並且協調免疫系統對抗病毒入侵。然而病毒可透過至少兩種方式逃離CD4+ T 細胞之偵測，一是抑制MHC class II 基因群之表現，另一是阻擾第二型抗原呈現之機轉。 EB 病毒感染全球超過92%的人口，為具有致癌性的疱疹病毒，可使B 細胞轉形為不朽化之淋巴母細胞株 (Lymphoblastoid cell lines)。目前研究發現，EB 病毒感染宿主細胞後，表現多種基因產物能干擾第一型抗原呈現的機制，以利長期潛伏宿主細胞中。以此作為模式系統發現，EB 病毒存在的情況下，第二型複合體(MHC class II)的蛋白質及RNA 表現量都受到抑制，已知第二型複合體也能呈現內生性病毒抗原，引發輔助T 細胞的抗病毒免疫功能，故選作進一步研究的對象，釐清EB 病毒干擾第二型抗原呈現的機制。結果顯示EB 病毒潛伏期膜蛋白LMP2A 具有抑制MHC class II 與CIITA 表現的能力，同時也觀察到CIITA 之轉錄因子PU.1 也受LMP2A 抑制之現象。利用LMP2A 之突變株，我們發現LMP2A 之ITAM motif 對此抑制功能是重要的，另一方面，也以單股核酸序列剔除淋巴母細胞株中LMP2A 之表現，發現MHC class II、CD74、CIITA 以及PU.1之表現量都有回升之情形，顯示LMP2A 抑制MHC class II 基因表現之路徑可能透過PU.1 與CIITA 之轉錄調控。

The presentation of peptides to CD4+ T cells by MHC class II molecules is of critical importance in specific recognition to a pathogen by the immune system. The level of MHC class II directly influences T lymphocyte activation. Activated CD4+ T cells can produce antiviral cytokines or co-ordinate antiviral immune responses. Viruses escape detection by CD4+ T cells by at least two mechanisms, inhibiting the expression of MHC class II genes and the antigen presentation pathway. Epstein-Barr virus (EBV) is an oncogenic gamma-herpesvirus that persistently infects over 92% of the human population. The previous studies have shown that several EBV-encoded gene products can disrupt MHC class II function. Using EBV- immortalized B cells (lymphoblastoid cell line) as model system, we found that the expression of MHC class II is repressed in the present of EBV. The aim of this study was to identify the possible mechanisms of the down-regulation of MHC class II expression by EBV. The data in the present study demonstrated that ectopic expression of LMP2A can inhibit the constitutive expression of MHC class II and CIITA in B cell lines. The down-regulated mRNA level of MHC class II and CIITA was correlated to the suppressed level of PU.1, one of the transcription factors of CIITA, in the present of LMP2A. With LMP2A mutant constructs, we displayed that the ITAM motif could be responsible for the repression effect on MHC class II expression. On the other hand, by si-LMP2A expression in LCLs, we found the expression levels of MHC class II, CD74, CIITA and PU.1 were reversed. Taking together, LMP2A might suppress MHC class II expression through PU.1-CIITA pathway, and the ITAM motif on N-terminal tail of LMP2A could be responsible for it.