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Infection and Immunity, April 2000, p. 1827-1833, Vol. 68, No. 4
0019-9567/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

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

Mustafa Akkoyunlu and Erol Fikrig*

Section of Rheumatology, Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut 06520-8031

Received 8 October 1999/Returned for modification 9 December 1999/Accepted 22 December 1999

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-gamma ), a cytokine that is crucial for host defenses against intracellular pathogens, was investigated by using IFN-gamma -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-gamma 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-gamma 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-gamma levels peaked. In contrast, IFN-gamma -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-gamma deficient mice after 2 weeks, while both the immunocompetent and the IFN-gamma -deficient mice had increased serum antibodies against aoHGE antigens at this time point. These data demonstrate that the HGE agent elicits a prominent IFN-gamma response in mice and that IFN-gamma is important in controlling the degree of rickettsemia during the early phase of infection, while IFN-gamma independent mechanisms play a role at later time points.

* Corresponding author. Mailing address: 608 Laboratory of Clinical Investigation, Section of Rheumatology, Department of Internal Medicine, Yale University School of Medicine, 333 Cedar St., New Haven, CT 06520-8031. Phone: (203) 785-2453. Fax: (203) 785-7053. E-mail: ef6{at}emailmed.yale.edu.

Infection and Immunity, April 2000, p. 1827-1833, Vol. 68, No. 4
0019-9567/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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