Chlamydia pneumoniae antigens, rather than viable bacteria, persist in atherosclerotic lesions.

AIMS To evaluate the nature of the presence of Chlamydia pneumoniae or of other members of the order Chlamydiales in atherosclerotic lesions. METHODS Consecutive sections of 13 carotid artery specimens obtained at necropsy and of C pneumoniae infected HEp2 cells were analysed using: (1) immunocytochemistry (ICC) to detect C pneumoniae membrane protein; (2) in situ hybridisation (ISH) using a polymerase chain reaction (PCR) fragment of the omp1 gene to detect C pneumoniae specific DNA; (3) ISH using an oligonucleotide probe to detect Chlamydiales specific 16S rRNA; (4) PCR to detect C pneumoniae 16S rDNA; and (5) in situ DNA nick and labelling (TUNEL) to detect fragmented DNA. RESULTS Staining by ICC and ISH of infected HEp2 cells showed characteristic inclusions. Chlamydia pneumoniae membrane protein was demonstrated in macrophages in advanced atherosclerotic lesions (six of six), but not in fatty streaks (none of two), or normal arteries (none of five). ISH assays using both probes and PCR were all negative, indicating the absence of both specific C pneumoniae DNA and Chlamydiales specific 16S rRNA. Only after treatment with DNAse I were uniformly sized dots demonstrated by the TUNEL assay in inclusions of infected HEp2 cells. The TUNEL assay showed a similar staining pattern in macrophages in five carotid artery specimens, of which four were also positive for C pneumoniae membrane protein. Both macrophage populations were morphologically similar and were similarly distributed. CONCLUSIONS No evidence was obtained for the involvement of other members of the order Chlamydiales in atherosclerosis. The presence of C pneumoniae antigen in the absence of DNA and 16S rRNA suggests that antigens, rather than viable bacteria, persist in atherosclerotic lesions.