Lymphocytes and macrophages are infected by Theileria equi, but T cells and B cells are not required to establish infection in vivo.
Authors: Ramsay Joshua D, Ueti Massaro W, Johnson Wendell C, Scoles Glen A, Knowles Donald P, Mealey Robert H
Journal: PloS one
Summary
# Editorial Summary: Theileria equi Infection and Lymphocyte Requirements Theileria equi establishes a characteristic biphasic infection in horses, beginning with intracellular development within leukocytes before progressing to parasitaemia within red blood cells and causing potentially fatal haemolytic disease. Using flow cytometry and immunofluorescence microscopy, Ramsay and colleagues identified that whilst T lymphocytes, B lymphocytes, and monocyte/macrophages can all be infected in vitro, the intraleukocytic stage shows broader cellular tropism than previously recognised. The critical finding emerged from experimental infection of horses with severe combined immunodeficiency (SCID)—animals entirely lacking functional B and T lymphocytes—which nevertheless developed patent erythrocytic parasitaemia, demonstrating that adaptive immunity is dispensable for completing the parasite's life cycle. This contrasts sharply with *Theileria parva* and *Theileria annulata* infections in cattle, suggesting *T. equi* has evolved less restrictive mechanisms for leukocyte invasion and intracytoplasmic development. For practitioners, these findings indicate that whilst lymphocyte-directed immune responses may influence disease severity or parasitaemia control, alternative cellular targets within the innate immune compartment warrant investigation as potential therapeutic or vaccine intervention points.
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Practical Takeaways
- •Theileria equi establishes infection through innate immune mechanisms independent of adaptive immunity, meaning vaccination strategies must focus on non-lymphocyte-dependent pathways or early parasite stages
- •The intraleukocyte schizont stage can persist in multiple white blood cell types, complicating diagnostic and therapeutic targeting strategies
- •Understanding T. equi tropism to macrophages may inform treatment approaches targeting the intracellular parasitic stage
Key Findings
- •Theileria equi infects B lymphocytes, T lymphocytes, and monocyte/macrophages in vitro, with a broader host cell range than previously reported
- •SCID horses lacking functional B and T lymphocytes developed patent erythrocytic parasitemia when inoculated with T. equi sporozoites
- •B and T lymphocytes are not required to establish or complete the T. equi life cycle in vivo
- •T. equi leukocyte invasion mechanisms are less restricted and common to multiple leukocyte subsets compared to T. parva and T. annulata