Vaccination with the Surface Proteins MUL_2232 and MUL_3720 of Mycobacterium ulcerans Induces Antibodies but Fails to Provide Protection against Buruli Ulcer

BackgroundBuruli ulcer, caused by infection with Mycobacterium ulcerans, is a chronic ulcerative neglected tropical disease of the skin and subcutaneous tissue that is most prevalent in West African countries. M. ulcerans produces a cytotoxic macrolide exotoxin called mycolactone, which causes extensive necrosis of infected subcutaneous tissue and the development of characteristic ulcerative lesions with undermined edges. While cellular immune responses are expected to play a key role against early intracellular stages of M. ulcerans in macrophages, antibody mediated protection might be of major relevance against advanced stages, where bacilli are predominantly found as extracellular clusters.Methodology/Principal FindingsTo assess whether vaccine induced antibodies against surface antigens of M. ulcerans can protect against Buruli ulcer we formulated two surface vaccine candidate antigens, MUL_2232 and MUL_3720, as recombinant proteins with the synthetic Toll-like receptor 4 agonist glucopyranosyl lipid adjuvant-stable emulsion. The candidate vaccines elicited strong antibody responses without a strong bias towards a TH1 type cellular response, as indicated by the IgG2a to IgG1 ratio. Despite the cross-reactivity of the induced antibodies with the native antigens, no significant protection was observed against progression of an experimental M. ulcerans infection in a mouse footpad challenge model.ConclusionsEven though vaccine-induced antibodies have the potential to opsonise the extracellular bacilli they do not have a protective effect since infiltrating phagocytes might be killed by mycolactone before reaching the bacteria, as indicated by lack of viable infiltrates in the necrotic infection foci.

背景：布鲁里溃疡（Buruli ulcer）是由溃疡分枝杆菌（Mycobacterium ulcerans）感染引发的慢性溃疡性被忽视热带皮肤病，在西非国家最为流行。溃疡分枝杆菌可分泌一种具有细胞毒性的大环内酯类外毒素——分枝菌内酯（mycolactone），该毒素会导致受感染的皮下组织广泛坏死，并形成具有潜行性边缘的特征性溃疡性皮损。尽管细胞免疫应答被认为在对抗巨噬细胞内早期溃疡分枝杆菌感染阶段发挥关键作用，但抗体介导的保护作用或许在疾病晚期更为关键——此时杆菌主要以细胞外簇的形式存在。
方法/主要发现：为评估疫苗诱导的抗溃疡分枝杆菌表面抗原抗体能否抵御布鲁里溃疡，我们将两种表面疫苗候选抗原MUL_2232与MUL_3720制备为重组蛋白，并搭配合成Toll样受体4（Toll-like receptor 4, TLR4）激动剂葡糖基脂质佐剂-稳定乳液（glucopyranosyl lipid adjuvant-stable emulsion, GLA-SE）。候选疫苗可诱导强烈的抗体应答，且从IgG2a与IgG1的比值来看，并未出现向TH1型细胞免疫应答的显著偏向。尽管诱导产生的抗体与天然抗原存在交叉反应，但在小鼠足垫攻击模型中，未观察到其对实验性溃疡分枝杆菌感染进展的显著保护作用。
结论：尽管疫苗诱导的抗体具备调理细胞外杆菌的潜力，但它们并未发挥保护作用。坏死感染灶中缺乏存活的浸润细胞这一现象提示，浸润的吞噬细胞或许在抵达细菌前就已被分枝菌内酯杀死。