Mechanisms of Stage-Transcending Protection Following Immunization of Mice with Late Liver Stage-Arresting Genetically Attenuated Malaria Parasites

Malaria, caused by Plasmodium parasite infection, continues to be one of the leading causes of worldwide morbidity and mortality. Development of an effective vaccine has been encumbered by the complex life cycle of the parasite that has distinct pre-erythrocytic and erythrocytic stages of infection in the mammalian host. Historically, malaria vaccine development efforts have targeted each stage in isolation. An ideal vaccine, however, would target multiple life cycle stages with multiple arms of the immune system and be capable of eliminating initial infection in the liver, the subsequent blood stage infection, and would prevent further parasite transmission. We have previously shown that immunization of mice with Plasmodium yoelii genetically attenuated parasites (GAP) that arrest late in liver stage development elicits stage-transcending protection against both a sporozoite challenge and a direct blood stage challenge. Here, we show that this immunization strategy engenders both T- and B-cell responses that are essential for stage-transcending protection, but the relative importance of each is determined by the host genetic background. Furthermore, potent anti-blood stage antibodies elicited after GAP immunization rely heavily on FC-mediated functions including complement fixation and FC receptor binding. These protective antibodies recognize the merozoite surface but do not appear to recognize the immunodominant merozoite surface protein-1. The antigen(s) targeted by stage-transcending immunity are present in both the late liver stages and blood stage parasites. The data clearly show that GAP-engendered protective immune responses can target shared antigens of pre-erythrocytic and erythrocytic parasite life cycle stages. As such, this model constitutes a powerful tool to identify novel, protective and stage-transcending T and B cell targets for incorporation into a multi-stage subunit vaccine.

由疟原虫感染引发的疟疾，仍是全球范围内导致发病与死亡的主要病因之一。有效疟疾疫苗的开发进程长期受限于疟原虫复杂的生活周期：该寄生虫在哺乳类宿主体内存在特征鲜明的红细胞前期与红细胞内期两个感染阶段。长期以来，疟疾疫苗研发均采用单一靶向单一生活周期阶段的策略。然而，理想的疫苗应通过激活免疫系统的多个分支，靶向多个生活周期阶段，既能清除肝脏内的初始感染、后续的红细胞内期感染，还能阻断寄生虫的进一步传播。此前我们的研究证实，使用在肝脏发育晚期停滞的约氏疟原虫基因减毒虫株（genetically attenuated parasites, GAP）对小鼠进行免疫，可诱导出跨阶段保护效力，既能抵御子孢子攻毒感染，也能抵御直接的红细胞内期攻毒感染。本研究显示，该免疫策略可同时激活T细胞与B细胞应答，二者均为跨阶段保护所必需，但二者的相对重要性取决于宿主的遗传背景。此外，经GAP免疫后诱导产生的强效抗红细胞内期抗体，其功能高度依赖Fc介导的效应机制，包括补体结合与Fc受体结合。这些保护性抗体可识别裂殖子表面，但似乎无法识别免疫显性的裂殖子表面蛋白1（merozoite surface protein-1）。跨阶段免疫所靶向的抗原，同时存在于晚期肝脏阶段寄生虫与红细胞内期寄生虫中。实验数据清晰表明，GAP诱导的保护性免疫应答，可靶向疟原虫红细胞前期与红细胞内期生活周期阶段共有的抗原。因此，该模型可作为一款强大的研究工具，用于鉴定新型的保护性跨阶段T细胞与B细胞靶点，进而将其整合至多阶段亚单位疫苗中。