BABESIOSIS AS A PARADIGM FOR THE EFFECTS OF AGING

巴贝斯虫病是一种类似疟疾的寄生虫病，在新英格兰、中西部和太平洋沿岸的西北部各州发病率不断上升。在新英格兰，微小巴贝斯虫病菌株通过莱姆病的扁虱媒介从田鼠传播给人类。B.Microti感染通常会在免疫正常的年轻患者中引起轻微的流感样疾病。然而，它会导致脾切除或免疫功能低下的人出现严重症状，临床症状从高烧到成人呼吸窘迫综合征和休克。在50岁及以上的健康个人中也可以观察到严重的症状。因此，衰老是人类临床巴贝斯虫病的易感因素。我们已经建立了一种与年龄相关的人类巴贝斯虫病易感性的小鼠模型，使用了一种适应于小鼠的微小巴贝斯虫病菌株(菌株RM/NS)。将寄生的红细胞分别注射到DBA/2小鼠的幼龄(2月龄)和老年(18月龄)。寄生虫血症的高峰和寄生虫红细胞完全清除的时间是通过Giemsa染色的血涂片的形态分析来确定的。在人类患者中，“老年”小鼠表现出强烈而持续的寄生虫血症，而“年轻”小鼠在暴露于RM/NS后表现出轻微的一过性寄生虫血症。这项研究项目的第一个目的将是以足够的统计能力重申我们的初步研究的结论，即年龄是小鼠感染模型中的一个重要变量。我们的第二个目标将是阐明这种与年龄相关的微小芽孢杆菌易感性的细胞基础。我们将使用来自“年轻”和“老年”供体小鼠的全脾细胞制剂在“年轻”和“老年”辐射受体小鼠中进行细胞转移研究。如果脾细胞是与年龄相关的巴贝斯虫病易感性的基础，那么从幼鼠身上获得的全脾细胞制剂将保护这两组小鼠，而来自“老年”鼠的细胞将无法保护任何一组小鼠。脾T和B细胞的作用将通过负选择和脾细胞群的重建来确定，然后再转移到受照射的小鼠身上。如果年龄影响了脾T细胞的功能，T细胞的耗尽可能会使“年轻”的脾细胞制剂无法保护“老年”受体小鼠。相反，去除T细胞的“年轻”脾细胞制剂，与由B细胞、巨噬细胞和树突状细胞组成的“老”脾细胞制剂重组，将无法保护“老年”受体小鼠。用类似的方法，我们将确定年龄是否影响脾B细胞功能。这些拟议的研究可能对巴贝斯虫病的发病机制和与年龄相关的免疫学变化产生新的见解，并为未来影响这些因素的研究提供有用的动物模型，如营养、抗氧化剂等。

Babesiosis is a malaria-like parasitic disease of increasing incidence in New England, the Upper Midwest and the Northwestern states of the Pacific Coast. In New England, the strain Babesiosis microti is transmitted from field mice to humans via the tick vector for Lyme disease. B. microti infection usually causes a mild, flu-like illness in young, immunologically normal patients. However, it results in severe symptoms in splenectomized or immunocompromised individuals, with clinical symptoms ranging from high fevers to adult respiratory distress syndrome and shock. Severe symptoms are also observed in otherwise healthy individuals aged 50 and above. Thus, aging is a predisposing factor for clinical babesiosis in humans. We have developed a murine model of age-related babesiosis susceptibility in humans using a mouse-adapted strain of B. microti (strain RM/NS). Parasitized red blood cells are injected into DBA/2 mice at "young" (2 month) and "old" (18 month) ages. Times of peak parasitemia and of complete clearance of parasitized red blood cells are determined by morphological analysis of Giemsa-stained blood smears As in human patients, "old" mice display an intense and sustained parasitemia, while "young" mice harbor a modest and transient parasitemia upon exposure to RM/NS. The first aim of this research project will be to reaffirm, with sufficient statistical power, our pilot study's conclusion that age is a significant variable in the murine model of infection. Our second aim will be to elucidate the cellular basis for this age-related susceptibility to B. microti. We will use cell transfer studies of whole spleen cell preparations from both "young" and "old" donor mice in both "young" and "old" irradiated recipient mice. If spleen cells are the basis of the age-related susceptibility to babesiosis, whole spleen cell preparations obtained from young mice will protect both groups of mice, while cells from "old" mice will fail to protect either group. The role of splenic T and B cells will be established using negative selection and reconstitution of spleen cell populations prior to transfer to irradiated-recipient mice. If age affects splenic T cell function, depletion of T cells may render "young" spleen cell preparations unable to protect "old" recipient mice. Conversely, "young" spleen cell preparations depleted of T cells, but reconstituted with "old" spleen cell preparations consisting of B cells, macrophages and dendritic cells will fail to protect 'old" recipient mice. Using a similar approach, we will determine whether age affects splenic B cell function. These proposed studies may yield new insights on the pathogenesis of babesiosis and immunologic changes associated with age and should provide a useful animal model for future studies of factors affecting these, such as nutrition, antioxidants, etc.