Preclinical models for Chlamydia pneumoniae and cardiovascular disease: hypercholesterolemic mice

Seroepidemiologic studies have shown an association of Chlamydia pneumoniae antibody with atherosclerosis. Compelling additional evidence has accumulated, in that the organism has been found within atherosclerotic lesions throughout the arterial tree by multiple methods. C. pneumoniae has also been isolated from coronary and carotid atheromatous plaques. Although these studies support a potential role for C. pneumoniae in atherogenesis, confirmation of a causal relationship requires the use of animal models and intervention studies. We have focused on the evaluation of mouse models to address the hypothesis that, following upper respiratory tract infection, lung macrophages are infected, disseminate to the aorta, and alter the onset or progression of atherogenesis. ApoE-deficient knock-out and C57BL/6J mice were used. The apoE-deficient mouse develops atherosclerotic lesions spontaneously on a regular diet in a time- and age-dependent manner. This knock-out strain was developed on the background of the C57BL/6J mouse, which only develops atherosclerosis on a high-fat/high-cholesterol diet. To investigate whether infected macrophages constitute a vehicle for dissemination of C. pneumoniae in vivo, mice were inoculated intranasally or intraperitoneally. The organism was detected in harvested alveolar and peritoneal macrophages at all time points following intranasal and intraperitoneal inoculations, respectively, and in peripheral blood mononuclear cells following inoculation by both routes. In another experiment, alveolar and peritoneal macrophages from intranasally and intraperitoneally inoculated mice were adoptively transferred by intraperitoneal injection to uninfected mice. Subsequently, C. pneumoniae was detected in lung, spleen, abdominal lymph nodes and/or thymus of recipient mice. In control experiments, UV-inactivated C. pneumoniae DNA was not detected in alveolar or peritoneal macrophages beyond 5 min after inoculation in vivo. These cumulative results demonstrate that C. pneumoniae infects macrophages in vivo and that macrophages can serve as a vehicle for dissemination to other sites. To answer the question of whether the organism disseminates to and persists in the aorta, 8-week-old mice were infected intranasally. Following single or multiple inoculations in apoE-deficient mice, C. pneumoniae was detected in the lung and aorta for 20 weeks postinfection. In contrast, in C57BL/6J mice, the organism did not persist in the aorta following a single intranasal inoculation, but could be detected up to 7 weeks postinfection in multiply inoculated mice. Significantly, in apoE-deficient mice with developed atherosclerotic lesions, the organism was found in foam cells within the lesions by immunocytochemical staining. These studies show that persistent C. pneumoniae infection occurs in atherosclerotic lesions in the aorta in the apoE-deficient knock-out mouse model. Infection of the aorta also occurred in C57BL/6J mice but was more transient. Both models should be useful in studying the pathogenic role of C. pneumoniae in atherogenesis and for determining if therapy prevents dissemination of infection. To this end, we have evaluated the susceptibility of C. pneumoniae to roxithromycin, a new macrolide, in vitro in cell culture and in vivo in the apoE-deficient mouse model. In vitro results compare favorably with other new macrolides, quinolines, and tetracyline. Preliminary studies in the apoE model of persistent infection suggest that the organism is detected less frequently in the lung and aorta by PCR following treatment. These promising preliminary studies warrant further investigation.