Hemophagocytosis induced by Leishmania donovani infection is beneficial to parasite survival within macrophages

Visceral leishmaniasis (VL) is caused by parasitic protozoa of the genus Leishmania and is characterized by clinical manifestations such as fever, hepatosplenomegaly and anemia. Hemophagocytosis, the phenomenon of phagocytosis of blood cells by macrophages, is found in VL patients. In a previous study we established an experimental model of VL, reproducing anemia in mice for the first time, and identified hemophagocytosis by heavily infected macrophages in the spleen as a possible cause of anemia. However, the mechanism for parasite-induced hemophagocytosis or its role in parasite survival remained unclear. Here, we established an in vitro model of Leishmania-induced hemophagocytosis to explore the molecules involved in this process. In contrast to naïve RAW264.7 cells (mouse macrophage cell line) which did not uptake freshly isolated erythrocytes, RAW264.7 cells infected with L. donovani showed enhanced phagocytosis of erythrocytes. Additionally, for hemophagocytes found both in vitro and in vivo, the expression of signal regulatory protein α (SIRPα), one of the receptors responsible for the ‘don’t-eat-me’ signal was suppressed by post-transcriptional control. Furthermore, the overlapped phagocytosis of erythrocytes and Leishmania parasites within a given macrophage appeared to be beneficial to the parasites; the in vitro experiments showed a higher number of parasites within macrophages that had been induced to engulf erythrocytes. Together, these results suggest that Leishmania parasites may actively induce hemophagocytosis by manipulating the expression of SIRPα in macrophages/hemophagocytes, in order to secure their parasitism.

内脏利什曼病（Visceral leishmaniasis, VL）由利什曼原虫属的寄生性原生动物引发，临床表现为发热、肝脾肿大与贫血。血细胞吞噬作用（hemophagocytosis）指巨噬细胞吞噬血细胞的现象，该现象可在VL患者体内检出。既往研究中，我们首次建立了可在小鼠中重现贫血症状的VL实验模型，并证实脾脏内重度感染寄生虫的巨噬细胞发生的血细胞吞噬作用，可能是贫血的致病诱因。然而，寄生虫诱导血细胞吞噬的具体分子机制，及其在寄生虫存活过程中发挥的作用仍不明确。
本研究中，我们构建了利什曼原虫诱导血细胞吞噬的体外模型，以探究该过程中涉及的相关分子。与未致敏的RAW264.7细胞（小鼠巨噬细胞系）无法摄取新鲜分离的红细胞不同，感染杜氏利什曼原虫（Leishmania donovani, L. donovani）的RAW264.7细胞对红细胞的吞噬能力显著增强。此外，无论在体外还是体内环境中观测到的血细胞吞噬细胞，其表面负责介导“别吃我”信号的受体之一——信号调节蛋白α（signal regulatory protein α, SIRPα）的表达，均通过转录后调控受到抑制。进一步研究发现，单个巨噬细胞同时吞噬红细胞与利什曼原虫的现象似乎对寄生虫存活更为有利：体外实验显示，被诱导吞噬红细胞的巨噬细胞内，寄生虫的载量更高。
综上，上述结果表明，利什曼原虫可通过调控巨噬细胞/血细胞吞噬细胞中SIRPα的表达，主动诱导血细胞吞噬作用，以此保障自身的寄生生存。