Babesia microti Primarily Invades Mature Erythrocytes in Mice

Abstract

Babesia microti is a tick-borne red blood cell parasite that causes babesiosis in people. Its most common vertebrate reservoir is the white-footed mouse. To determine whether B. microti invades reticulocytes, as does the canine pathogen B. gibsoni, we infected the susceptible inbred mouse strains C.B-17.scid and DBA/2 with a clinical isolate of B. microti. Staining of fixed permeabilized red blood cells with 4,6-diamidino-2-phenylindole or YOYO-1, a sensitive nucleic acid stain, revealed parasite nuclei as large bright dots. Flow cytometric analysis indicated that parasite DNA is primarily found in mature erythrocytes that expressed Babesia antigens but not the transferrin receptor CD71. In contrast, CD71-positive reticulocytes rarely contained Babesia nuclei and failed to express Babesia antigens. Accordingly, the frequency of YOYO-1-positive, CD71-negative cells strongly correlated with parasitemia, defined as the frequency of infected red blood cells assessed on Giemsastained blood smears. Importantly, the absolute numbers generated by the two techniques were similar. Parasitemia was modest and transient in DBA/2 mice but intense and sustained in C.B-17.scid mice. In both strains, parasitemia preceded reticulocytosis, but reticulocytes remained refractory to B. microti. In immunocompetent C.B-17 mice, reticulocytosis developed early, despite a marginal and short-lived parasitemia. Likewise, an early reticulocytosis developed in resistant BALB/cBy and B10.D2 mice. These studies establish that B. microti has a tropism for mature erythrocytes. Although reticulocytes are rarely infected, the delayed reticulocytosis in susceptible strains may result from parasite or host activities to limit renewal of the mature erythrocyte pool, thereby preventing an overwhelming parasitemia.

Elimination of Babesia microti Is Dependent on Intraerythrocytic Killing and CD4+ T Cells

“…In contrast, erythrocyte death results in replacement of the mature parasite-permissive erythrocytes with less mature reticulocytes and this repopulation correlates with the appearance of intra-erythrocyte crisis forms and parasite clearance. This is consistent with a previous study that showed that B. microti primarily invades mature erythrocytes in mice and reticulocytes are rarely infected (43). That study concluded that the intensity of reticulocytosis varied with the degree of parasitemia, and that the timing of reticulocytosis appeared to be delayed in susceptible (DBA/2 C.B.10.scid) versus resistant strains of mice (C.B-17, BALB/cBy, B10.D2.).…”

Borrelia burgdorferi Promotes the Establishment of Babesia microti in the Northeastern United States

“…On days 7, 14, 28, and 42 after nymphal infestation of P. leucopus mice, 50 µL of peripheral blood were obtained via submandibular venipuncture. Blood cells were fixed in glutaraldehyde, permeabilized in Triton X-100, and treated with DNase-free RNase, as previously described [50] . Parasites were detected using the nucleic acid dye YOYO-1 iodide.…”

Host Erythrocyte Environment Influences the Localization of Exported Protein 2, an Essential Component of the Plasmodium Translocon

“…NYLS3, an IgG1 monoclonal antibody (MAb) that recognizes PyExp1 (PyHep17) was a kind gift from Stefan Kappe (Seattle Biomedical Research Institute, Seattle, WA) (34). Rat anti-mouse CD71 (BD Biosciences, San Jose, CA) and mouse anti-human CD71 (eBiosciences, San Diego, CA) were used to stain mouse and human reticulocytes, respectively (35,36). Preparations of host cell cytoplasmic and parasite-associated proteins were evaluated by immunoblotting using polyclonal rabbit anti-P. falciparum aldolase (Abcam, Cambridge, MA) and rabbit antiPyMSP8 (37).…”