There is a growing body of evidence that Chlamydia pneumoniae is the causative agent of vasculitis in cardiovascular disease. Since research in Crohn's disease also indicates its vascular nature (supported further by other analogies and findings below) a hypothesis that  Chlamydia may be the causative agent evolved.
 

CHLAMYDIA PNEUMONIAE AND CARDIOVASCULAR DISEASE.
Campbell LA, Kuo CC, Grayston JT, Department of Pathobiology, University of Washington, Seattle 98195, USA. lacamp@u.washington.edu  Emerg Infect Dis 1998 Oct-Dec;4(4):571-9

Chlamydia pneumoniae is a ubiquitous pathogen that causes acute respiratory disease. The spectrum of C. pneumoniae infection has been extended to atherosclerosis and its clinical manifestations.
Seroepidemiologic studies have associated C. pneumoniae antibody with coronary artery disease,
myocardial infarction, carotid artery disease, and cerebrovascular disease. The association of C.
pneumoniae with atherosclerosis is corroborated by the presence of the organism in atherosclerotic
lesions throughout the arterial tree and the near absence of the organism in healthy arterial tissue. C.
pneumoniae has also been isolated from coronary and carotid atheromatous plaques. To determine
whether chronic infection plays a role in initiation or progression of disease, intervention studies in
humans have been initiated, and animal models of C. pneumoniae infection have been developed.
This review summarizes the evidence for the association and potential role of C. pneumoniae in
cardiovascular disease.
 

CHLAMYDIA TRACHOMATIS AND INFLAMMATORY BOWEL DISEASE--A COINCIDENCE?
Orda R, Samra Z, Levy Y, Shperber Y, Scapa E, Department of Surgery A, Assaf Harofeh Medical Center, Zerifin, Israel. J R Soc Med 1990 Jan;83(1):15-7

Serological tests of 35 patients suffering from inflammatory bowel disease were compared to those
of 35 healthy controls. The tests were performed using the indirect immunoperoxidase assay.
Ninety-three per cent of 15 patients with Crohn's disease had IgG antibodies against Chlamydia,
compared to 26% in the control group. In the 20 patients with ulcerative colitis, 45% had IgG
antibodies against Chlamydia, compared to 10% in the control group. High serum titres of IgG
antibodies were found in most of the patients with inflammatory bowel disease, mainly with Crohn's
disease, while weak reactions appeared in most of the controls in which antibodies were detected.
These results suggest a high incidence of Chlamydia infection in the studied patients with
inflammatory bowel disease, especially in those with Crohn's disease. The possible association
between Chlamydia trachomatis and inflammatory bowel disease is discussed.
 

CROHN’S DISEASE. NEW CONCEPTS OF PATHOGENESIS AND CURRENT APPROACHES TO TREATMENT.
James SP, Strober W, Quinn TC, Danovitch SH, Mucosal Immunity Section, National Institute of Allergy and Infectious Diseases, Bethesda, Maryland 20892. Dig Dis Sci 1987 Nov;32(11):1297-310

One theory of the pathogenesis of Crohn’s disease is that rather than being caused by a unique environmental agent, it is the result of an abnormal immune response in the gastrointestinal tract.  Recent studies indicate that Crohn’s disease in its early stages is frequently associated with the presence of circulating antigen-non-specific suppressor T cells. Such T cells are also found in experimental inflammation caused by Chlamydia organisms in the gastrointestinal tract of nonhuman primates. Taken together, these data suggest that the suppressor T cells are markers of an underlying and persistent, antigen-specific immune response to an as yet unidentified antigen or set of antigens.  We postulate that this underlying antigen-specific response is the result of a primary immunoregulatory abnormality involving an imbalance between the effects of antigen-specific helper and suppressor T cells which recognize a common antigen or antigens present in the mucosal environment.

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