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

Abstract

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-␥), a cytokine that is crucial for host defenses against intracellular pathogens, was investigated by using IFN-␥-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-␥ 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-␥ 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/l on day 8. The bacterial DNA burden was also highest on day 8 and then declined after IFN-␥ levels peaked. In contrast, IFN-␥-deficient mice had a markedly elevated HGE bacteria burden with morulae concentration of 282 ؎ 48 cells/l on day 5 (P ‫؍‬ 0.004) and 242 ؎ 63 cells/l on day 8 (P ‫؍‬ 0.005). Rickettsemia resolved in immunocompetent and IFN-␥ deficient mice after 2 weeks, while both the immunocompetent and the IFN-␥-deficient mice had increased serum antibodies against aoHGE antigens at this time point. These data demonstrate that the HGE agent elicits a prominent IFN-␥ response in mice and that IFN-␥ is important in controlling the degree of rickettsemia during the early phase of infection, while IFN-␥ independent mechanisms play a role at later time points.

Kinetics of CD11b/CD18 Up-Regulation During Infection with the Agent of Human Granulocytic Ehrlichiosis in Mice

“…However, it is likely that the extent of CD11b/CD18 activation and its role in aoHGE clearance is partially dependent on IFN-␥. This is consistent with other reports of the dominant role of IFN-␥ in controlling HGE infection (Akkoyunlu and Fikrig, 2000;Dumler et al, 2000;Martin et al, 2001).…”

“…As a T helper 1 cytokine, IFN-␥ is thought to play a primary role in host defenses against intracellular microbes. Indeed, IFN-␥ has been shown to dominate the murine cytokine response to aoHGE infection and limit early bacteremia (Akkoyunlu and Fikrig, 2000;Martin et al, 2001). However the role of IFN-␥ in neutrophil activation during aoHGE infection is unknown.…”

Highly Conserved Regions of the Immunodominant Major Surface Protein 2 of the Genogroup II Ehrlichial Pathogen Anaplasma marginale Are Rich in Naturally Derived CD4+ T Lymphocyte Epitopes that Elicit Strong Recall Responses

“…Although Th lymphocyte responses against E. phagocytophila MSP2 have not been reported, our results target regions in this protein to test as potential epitopes. In light of the high levels of IFN-␥ induced by peptide 12, and its sequence conservation, this MSP2 region may be important for inducing T lymphocyte-dependent protection against both A. marginale and E. phagocytophila (16,18). Experiments are in progress to more precisely define the immunodominant T lymphocyte epitopes on A. marginale MSP2 and to determine which sequences may serve as "universal" epitopes for bovine class II molecules.…”

“…Furthermore, IFN-␥ is associated with protective immunity against A. marginale (20,26) and the related HGE agent (18). NO, produced by phagocytes in response to IFN-␥, appears to be important for inhibition of E. risticii and E. phagocytophila in a murine model (43,44).…”

Experimental Infection of C3H/HeJ Mice with the NY18 Isolate of Anaplasma phagocytophilum

“…While mice do not consistently develop signs of clinical disease as described in humans, horses, or nonhuman primates, they do develop an acute infection and associated pathologic changes. 1,[7][8][9][10]12,13,15,23,24,31,38,39,42,46 In this study we report the results of a pathologic study of acute A phagocytophilum infection in C3H/HeJ mice and discuss the relative importance and advantages of this mouse model in contrast to other A phagocytophilum models and hosts.…”