Coxiella burnetii, a Gram-negative intracellular bacterium, can give rise to
Q fever in humans and is transmitted mainly by inhalation of infected
aerosols from animal reservoirs. Serology is commonly used to diagnose
Q fever, but the early cellular immune response-i.e., C. burnetii-specific
interferon γ (IFN-γ) production in response to
antigen challenge-might be an additional diagnostic. Detection of IFN-γ responses has been used to identify past and
chronic Q fever infections, but the IFN-γ response in
acute Q fever has not been described. By challenging immunocompetent BALB/c mice with
aerosols containing phase I C. burnetii, the timing and extent of IFN-γ recall responses were evaluated in an acute C. burnetii
infection. Other
cytokines were also measured in an effort to identify other potential diagnostic markers. The data show that after initial expansion of bacteria first in lungs and then in other tissues, the
infection was cleared from day 10 onwards as reflected by the decreasing number of bacteria. The
antigen-induced IFN-γ production by splenocytes coincided with emergence of
IgM phase II
antibodies at day 10 postinfection and preceded appearance of
IgG antibodies. This was accompanied by the production of proinflammatory
cytokines including
interleukin (IL) 6, keratinocyte-derived
cytokine, and IFN-γ-induced
protein 10, followed by
monocyte chemotactic protein 1, but not by IL-1β and
tumor necrosis factor α, and only very low production of the anti-inflammatory
cytokine IL-10. These data suggest that analysis of
antigen-specific IFN-γ responses could be a useful tool for diagnosis of
acute Q fever. Moreover, the current model of C. burnetii
infection could be used to give new insights into
immunological factors that predispose to development of
persistent infection.