Gamma Interferon Dominates the Murine Cytokine Response to the Agent of Human Granulocytic Ehrlichiosis and Helps To Control the Degree of Early Rickettsemia

The cytokine response to the agent of human granulocytic ehrlichiosis (HGE) was assessed in a murine infection model and the role of gamma interferon (IFN-γ), a cytokine that is crucial for host defenses against intracellular pathogens, was investigated by using IFN-γ-deficient mice. The agent of HGE (aoHGE) is an obligate intracellular bacterium that survives within neutrophils: morulae (vacuoles containing HGE organisms) are evident in polymorphonuclear leukocytes of experimentally infected immunocompetent mice for 1 to 2 weeks. We now show that IFN-γ levels increase during early infection of C3H/HeN or C57BL/6 mice with HGE bacteria. Moreover, in response to aoHGE extracts or concanavalin A, splenocytes from ehrlichia-infected mice produced more IFN-γ and less interleukin-4 than controls, suggesting that aoHGE partially skewed the immune response towards a Th1 phenotype. Absolute concentration of morulae containing neutrophils in blood was 122 ± 22 cells/μl on day 8. The bacterial DNA burden was also highest on day 8 and then declined after IFN-γ levels peaked. In contrast, IFN-γ-deficient mice had a markedly elevated HGE bacteria burden with morulae concentration of 282 ± 48 cells/μl on day 5 (P = 0.004) and 242 ± 63 cells/μl on day 8 (P = 0.005). Rickettsemia resolved in immunocompetent and IFN-γ deficient mice after 2 weeks, while both the immunocompetent and the IFN-γ-deficient mice had increased serum antibodies against aoHGE antigens at this time point. These data demonstrate that the HGE agent elicits a prominent IFN-γ response in mice and that IFN-γ is important in controlling the degree of rickettsemia during the early phase of infection, while IFN-γ independent mechanisms play a role at later time points.

本研究在小鼠感染模型中评估了人类粒细胞无形体病（human granulocytic ehrlichiosis, HGE）病原体诱导的细胞因子应答，并通过使用γ干扰素（gamma interferon, IFN-γ）缺陷小鼠，探究了IFN-γ——这种对宿主抵御胞内病原体至关重要的细胞因子——所发挥的作用。HGE病原体（aoHGE）是一种专性寄生于中性粒细胞内的胞内菌：在实验感染后的免疫健全小鼠的多形核白细胞中，可观察到桑葚体（即包裹HGE病原体的空泡），该现象可持续1至2周。本研究证实，C3H/HeN与C57BL/6小鼠在早期感染HGE菌后，体内IFN-γ水平会显著升高。此外，在受到aoHGE提取物或刀豆球蛋白A刺激时，感染无形体病小鼠的脾细胞分泌的IFN-γ水平较对照组更高，而白细胞介素-4（interleukin-4）的分泌量则更低，这提示aoHGE可将宿主免疫应答部分偏向Th1表型。感染后第8天，血液中携带桑葚体的中性粒细胞绝对浓度为(122±22)个/μL；细菌DNA载量同样在第8天达到峰值，随后在IFN-γ水平达到峰值后逐渐下降。与之形成鲜明对比的是，IFN-γ缺陷小鼠的HGE细菌载量显著升高：在感染后第5天，其血液中携带桑葚体的中性粒细胞浓度为(282±48)个/μL（P=0.004）；第8天时该浓度为(242±63)个/μL（P=0.005）。免疫健全小鼠与IFN-γ缺陷小鼠均在感染2周后清除了立克次体血症；且在此时间点，两类小鼠体内针对aoHGE抗原的血清抗体水平均有所上升。上述实验数据表明，HGE病原体可在小鼠体内诱导显著的IFN-γ应答；IFN-γ在感染早期对控制立克次体血症的严重程度发挥关键作用，而感染后期的病原体清除则依赖于IFN-γ非依赖型免疫机制。