Secreted Respiratory Syncytial Virus G Glycoprotein Induces Interleukin-5 (IL-5), IL-13, and Eosinophilia by an IL-4-Independent Mechanism

The attachment glycoprotein G of respiratory syncytial virus (RSV) is produced as both membrane-anchored and secreted forms by infected cells. Immunization with secreted RSV G (Gs) or formalin-inactivated alumprecipitated RSV (FI-RSV) predisposes mice to immune responses involving a Th2 cell phenotype which results in more severe illness and pathology, decreased viral clearance, and increased pulmonary eosinophilia upon subsequent RSV challenge. These responses are associated with increased interleukin-4 (IL-4) production in FI-RSV-primed mice, and the responses are IL-4 dependent. RNase protection assays demonstrated that similar levels of IL-4 mRNA were induced after RSV challenge in mice primed with vaccinia virus expressing Gs (vvGs) or a construct expressing only membrane-anchored G (vvGr). However, upon RSV challenge, vvGs-primed mice produced significantly greater levels of IL-5 and IL-13 mRNA and protein than vvGr-primed mice. Administration of neutralizing anti-IL-4 antibody 11.B11 during vaccinia virus priming did not alter the levels of vvGs-induced IL-5, IL-13, pulmonary eosinophilia, illness, or RSV titers upon RSV challenge, although immunoglobulin G (IgG) isotype profiles revealed that more IgG2a was produced. vvGs-priming of IL-4-deficient mice demonstrated that G-induced airway eosinophilia was not dependent on IL-4. In contrast, airway eosinophilia induced by FI-RSV priming was significantly reduced in IL-4-deficient mice. Thus we conclude that, in contrast to FI-RSV, the secreted form of RSV G can directly induce IL-5 and IL-13, producing pulmonary eosinophilia and enhanced illness in RSV-challenged mice by an IL-4-independent mechanism.

呼吸道合胞病毒（respiratory syncytial virus, RSV）的黏附糖蛋白G可由感染细胞表达为膜锚定型与分泌型两种形式。以分泌型RSV G（Gs）或甲醛灭活铝佐剂沉淀RSV（FI-RSV）进行免疫接种，可使小鼠倾向于发生以Th2细胞表型为特征的免疫应答，该应答会在后续RSV攻毒实验中引发更严重的疾病与病理损伤、降低病毒清除效率，并加剧肺嗜酸性粒细胞浸润。此类应答与FI-RSV预免疫小鼠体内白细胞介素4（IL-4）生成增多密切相关，且该应答依赖IL-4。
核糖核酸酶保护实验结果显示，在分别经表达Gs的痘苗病毒（vvGs）或仅表达膜锚定型G的重组载体（vvGr）预免疫的小鼠中，RSV攻毒后诱导的IL-4 mRNA水平无显著差异。但在RSV攻毒后，vvGs预免疫小鼠体内白细胞介素5（IL-5）与白细胞介素13（IL-13）的mRNA及蛋白表达水平显著高于vvGr预免疫小鼠。
在痘苗病毒预免疫阶段施加中和性抗IL-4抗体11.B11，虽可使免疫球蛋白G（IgG）同种型谱向更多IgG2a生成的方向偏移，但并未改变vvGs诱导的IL-5、IL-13表达水平、肺嗜酸性粒细胞浸润程度、疾病严重程度以及RSV攻毒后的病毒滴度。对IL-4缺陷小鼠进行vvGs预免疫的实验证实，Gs诱导的气道嗜酸性粒细胞增多并不依赖IL-4。与之相反，FI-RSV预免疫引发的气道嗜酸性粒细胞增多在IL-4缺陷小鼠中显著降低。
综上，本研究表明：与FI-RSV不同，RSV的分泌型G可通过IL-4非依赖的通路直接诱导IL-5与IL-13的产生，进而在RSV攻毒小鼠中引发肺嗜酸性粒细胞增多与疾病加重。