Clinical and Parasitological Protection in a Leishmania infantum-Macaque Model Vaccinated with Adenovirus and the Recombinant A2 Antigen

BackgroundVisceral leishmaniasis (VL) is a severe vector-born disease of humans and dogs caused by Leishmania donovani complex parasites. Approximately 0.2 to 0.4 million new human VL cases occur annually worldwide. In the new world, these alarming numbers are primarily due to the impracticality of current control methods based on vector reduction and dog euthanasia. Thus, a prophylactic vaccine appears to be essential for VL control. The current efforts to develop an efficacious vaccine include the use of animal models that are as close to human VL. We have previously reported a L. infantum-macaque infection model that is reliable to determine which vaccine candidates are most worthy for further development. Among the few amastigote antigens tested so far, one of specific interest is the recombinant A2 (rA2) protein that protects against experimental L. infantum infections in mice and dogs.Methodology/Principal FindingsPrimates were vaccinated using three rA2-based prime-boost immunization regimes: three doses of rA2 plus recombinant human interleukin-12 (rhIL-12) adsorbed in alum (rA2/rhIL-12/alum); two doses of non-replicative adenovirus recombinant vector encoding A2 (Ad5-A2) followed by two boosts with rA2/rhIL-12/alum (Ad5-A2+rA2/rhIL12/alum); and plasmid DNA encoding A2 gene (DNA-A2) boosted with two doses of Ad5-A2 (DNA-A2+Ad5-A2). Primates received a subsequent infectious challenge with L. infantum. Vaccines, apart from being safe, were immunogenic as animals responded with increased pre-challenge production of anti-A2-specific IgG antibodies, though with some variability in the response, depending on the vaccine formulation/protocol. The relative parasite load in the liver was significantly lower in immunized macaques as compared to controls. Protection correlated with hepatic granuloma resolution, and reduction of clinical symptoms, particularly when primates were vaccinated with the Ad5-A2+rA2/rhIL12/alum protocol.Conclusions/SignificanceThe remarkable clinical protection induced by A2 in an animal model that is evolutionary close to humans qualifies this antigen as a suitable vaccine candidate against human VL.背景 内脏利什曼病（Visceral leishmaniasis, VL）是一类由杜氏利什曼虫复合体（Leishmania donovani complex）寄生虫引发的严重媒介传播性人畜共患病，可感染人类与犬类。全球每年新增人类VL病例约20万至40万例。在新大陆，该发病数字居高不下主要源于当前以媒介防控和犬只安乐死为核心的防控方法实用性不足。因此，开发预防性疫苗似乎是VL防控的必要手段。当前开发有效疫苗的研究中，多采用尽可能贴近人类VL发病特征的动物模型。我们此前曾报道过一种婴儿利什曼原虫（L. infantum）-猕猴感染模型，该模型可可靠筛选出最值得进一步开发的疫苗候选株。在目前已测试的少数无鞭毛体抗原中，重组A2（rA2）蛋白备受关注，该蛋白可在小鼠和犬类中抵御实验性L. infantum感染。 方法与主要结果 研究人员采用三种基于rA2的初免-加强免疫方案对灵长类动物进行免疫接种：其一为三次接种吸附于明矾（alum）的rA2联合重组人白细胞介素-12（rhIL-12）（rA2/rhIL-12/alum）；其二为先接种两剂编码A2的非复制型腺病毒重组载体（Ad5-A2），随后以rA2/rhIL-12/alum进行两次加强免疫（Ad5-A2+rA2/rhIL12/alum）；其三为先接种编码A2基因的质粒DNA（DNA-A2），再以两剂Ad5-A2进行加强免疫（DNA-A2+Ad5-A2）。随后，对这些灵长类动物实施L. infantum感染攻毒。结果显示，各疫苗不仅安全性良好，且均具有免疫原性：免疫动物在攻毒前的抗A2特异性IgG抗体水平均有所升高，但免疫应答存在一定个体差异，差异取决于疫苗配方与免疫方案。与对照组相比，免疫猕猴的肝脏相对寄生虫载量显著降低。保护效果与肝脏肉芽肿消退及临床症状减轻呈正相关，尤其当灵长类动物采用Ad5-A2+rA2/rhIL12/alum方案免疫时，保护效果最为显著。 结论与意义 A2抗原在进化上贴近人类的动物模型中诱导了显著的临床保护效果，这使得该抗原成为对抗人类VL的合适疫苗候选物。