Chlamydia pneumoniae (CP) has been linked with atherosclerosis. While several studies have shown that CP contributes to the acceleration of atherosclerotic lesions, any studies on the initiation of atherosclerosis are sparse. The present study investigated whether CP infection could initiate atherosclerotic lesions in rats that are known to be resistant to atherosclerosis; further, we investigated if these lesions do form, then how does the CP participate in this and develop of atherosclerosis in these rats.

Thirty 11-week-old Otsuka Long-Evans Tokushima Fatty (OLETF) rats, thirty type 2 diabetic rats and thirty age-matched Long-Evans Tokushima Fatty (LETO) rats that were maintained on a high-cholesterol diet were either mock-inoculated or inoculated intranasally 3 times at 11, 13 and 15 weeks of age. The serum levels of the lipid profiles, plasminogen activator inhibitor-1 (PAI-1), monocyte chemoattractant protein-1 (MCP-1) and C-reactive protein (CRP) were measured by performing ELISA at 24 weeks and 40 weeks of age. The atherosclerotic lesion areas were analyzed, and immunohistochemical staining using chlamydia genus-specific monoclonal antibody and PDGF-B was performed in the ascending aorta at 40 weeks of age.

Immunohistochemical staining with using specific monoclonal antibody demonstrated CP infection in the vessel walls. The serum PAI-1 level of the OLETF rats was higher than that of the LETO rats (p<0.05) regardless of the state of the CP infection, but there were no differences in the serum MCP-1 and CRP levels between the OLETF rats and the LETO rats. While no atherosclerotic lesion was observed in the mock-infected LETO rats, early-to-advanced atherosclerotic lesions were found in the other rat groups. CP-infected OLETF rats showed more advanced atherosclerotic lesions and greater mean lesion areas than the other rat groups (LT-N, 0 mm²; LETO-CP, 3.29±1.23 mm²; OT-N, 4.91±2.11 mm²; OT-CP, 9.20±4.62 mm²)(p<0.05). The characteristics of the atherosclerotic lesions in the rats were intimal thickening that was mainly composed of smooth muscle cells. The atherosclerotic lesion area positively correlated with the presence and the extent of PDGF-B staining in the aortic wall (p<0.01).

Chronic infection of CP in the vessel walls initiated the development of atherosclerosis in the LETO rats and it accelerated the atherosclerosis in the OLETF rats. CP-induced smooth muscle proliferation and the resultant intimal thickening may be mediated by PDGF-B in these atherosclerotic lesions.