DC8 and DC13 var Genes Associated with Severe Malaria Bind Avidly to Diverse Endothelial Cells

During blood stage infection, Plasmodium falciparum infected erythrocytes (IE) bind to host blood vessels. This virulence determinant enables parasites to evade spleen-dependent killing mechanisms, but paradoxically in some cases may reduce parasite fitness by killing the host. Adhesion of infected erythrocytes is mediated by P. falciparum erythrocyte membrane protein 1 (PfEMP1), a family of polymorphic adhesion proteins encoded by var genes. Whereas cerebral binding and severe malaria are associated with parasites expressing DC8 and DC13 var genes, relatively little is known about the non-brain endothelial selection on severe malaria adhesive types. In this study, we selected P. falciparum-IEs on diverse endothelial cell types and demonstrate that DC8 and DC13 var genes were consistently among the major var transcripts selected on non-brain endothelial cells (lung, heart, bone marrow). To investigate the molecular basis for this avid endothelial binding activity, recombinant proteins were expressed from the predominant upregulated DC8 transcript, IT4var19. In-depth binding comparisons revealed that multiple extracellular domains from this protein bound brain and non-brain endothelial cells, and individual domains largely did not discriminate between different endothelial cell types. Additionally, we found that recombinant DC8 and DC13 CIDR1 domains exhibited a widespread endothelial binding activity and could compete for DC8-IE binding to brain endothelial cells, suggesting they may bind the same host receptor. Our findings provide new insights into the interaction of severe malaria adhesive types and host blood vessels and support the hypothesis that parasites causing severe malaria express PfEMP1 variants with a superior ability to adhere to diverse endothelial cell types, and may therefore endow these parasites with a growth and transmission advantage.

在红细胞内期感染阶段，恶性疟原虫（Plasmodium falciparum）感染的红细胞（infected erythrocytes, IE）可黏附于宿主血管的内皮细胞（endothelial cell）表面。这一毒力决定因子能够帮助寄生虫逃避脾脏依赖的杀伤机制，但矛盾的是，在部分情境下，该因子会因宿主死亡而降低寄生虫的适合度。感染红细胞的黏附过程由恶性疟原虫红细胞膜蛋白1（P. falciparum erythrocyte membrane protein 1, PfEMP1）介导，该蛋白是由var基因编码的多态性黏附蛋白家族。尽管已知脑黏附与重症疟疾与表达DC8型和DC13型var基因的寄生虫密切相关，但目前对于重症疟疾黏附表型寄生虫所面临的非脑源性内皮细胞选择过程，仍知之甚少。本研究针对多种内皮细胞类型筛选恶性疟原虫感染红细胞，并证实DC8型与DC13型var基因始终是在非脑源性内皮细胞（肺、心脏、骨髓）上筛选得到的主要var转录本之一。为探究这种高亲和力内皮黏附活性的分子基础，我们从优势上调的DC8型转录本IT4var19中表达了重组蛋白（recombinant proteins）。深入的结合对比实验显示，该蛋白的多个胞外结构域（extracellular domains）均可结合脑源性与非脑源性内皮细胞，且单个结构域在多数情况下无法区分不同内皮细胞类型。此外，我们发现重组DC8型与DC13型CIDR1结构域（CIDR1 domains）展现出广泛的内皮细胞结合活性，且能够竞争性抑制DC8型感染红细胞与脑内皮细胞的结合，提示二者可能结合同一宿主受体。本研究结果为重症疟疾黏附表型寄生虫与宿主血管的相互作用提供了新的见解，并支持以下假说：引发重症疟疾的寄生虫所表达的PfEMP1变体，具备更强的黏附多种内皮细胞类型的能力，因此可能赋予这些寄生虫生长与传播优势。