Murat V. Kalayoglu

Induction of Macrophage Foam Cell Formation by Chlamydia pneumoniae

Foam cell formation is the hallmark of early atherosclerosis. It was found that the intracellular bacterium Chlamydia pneumoniae induces foam cell formation by human monocyte-derived macrophages. Exposure of macrophages to C. pneumoniae followed by low-density lipoprotein (LDL) caused a marked increase in the number of foam cells and accumulation of cholesteryl esters. Foam cell formation was not inhibited by the antioxidant butylated hydroxytoluene nor fucoidan, suggesting that lipid accumulation did not involve scavenger receptors. In contrast, addition of heparin, which blocks binding of LDL to the LDL receptor, inhibited C. pneumoniae-induced foam cell formation, suggesting that the pathogen induced lipid accumulation by dysregulating native LDL uptake or metabolism (or both). These data demonstrate that an infectious agent can induce macrophage foam cell formation and implicate C. pneumoniae as a causative factor in atherosclerosis.

Cellular Oxidation of Low-Density Lipoprotein by Chlamydia pneumoniae

A spectrum of clinical and epidemiologic studies implicate infectious agents, including Chlamydia pneumoniae, in the pathogenesis of atherosclerosis. The complexity of atherosclerotic disease necessitates examining the role of infection in the context of defined risk factors, such as high levels of native low-density lipoprotein (LDL). Although native LDL does not have atherogenic properties, cellular oxidation of LDL alters the lipoprotein into a highly atherogenic form. In this report, C. pneumoniae and chlamydial hsp60, an inflammatory antigen that was recently localized to atheromas, were found to induce cellular oxidation of LDL. These data provide initial evidence that an infectious agent can render LDL atherogenic and suggest a mechanism whereby C. pneumoniae may promote atheroma development.

Chlamydial virulence determinants in atherogenesis : The role of chlamydial lipopolysaccharide and heat shock protein 60 in macrophage-lipoprotein interactions

Data from a spectrum of epidemiologic, pathologic, and animal model studies show that Chlamydia pneumoniae infection is associated with coronary artery disease, but it is not clear how the organism may initiate or promote atherosclerosis. It is postulated that C. pneumoniae triggers key atherogenic events through specific virulence determinants. C. pneumoniae induces mononuclear phagocyte foam cell formation by chlamydial lipopolysaccharide (cLPS) and low-density lipoprotein oxidation by chlamydial hsp60 (chsp60). Thus, different chlamydial components may promote distinct events implicated in the development of atherosclerosis. Data implicating cLPS and chsp60 in the pathogenesis of atherosclerosis are discussed and novel approaches are presented for attempting to elucidate how these putative virulence determinants signal mononuclear phagocytes to modulate lipoprotein influx and modification.

Chlamydia pneumoniae and atherosclerosis: Links to the disease process ☆ ☆☆

Chlamydia pneumoniae is an obligate intracellular prokaryotic human pathogen responsible for a significant portion of atypical pneumonia and associated with a variety of chronic sequelae, the most significant of which is atherosclerosis. The organism is endowed with several attributes that may contribute to the development of atherosclerotic lesions or promote tissue damage at the site of an existing lesion. Two key events that are directly involved in the atherogenic process include the development of foam cells from macrophages and the oxidation of lipoproteins at the site of lesion development. The former process allows for deposition of cholesterol-containing low-density lipoprotein (LDL) and the latter can contribute directly to tissue damage locally. We have hypothesized that C pneumoniae may interact with mononuclear phagocytes in ways that are consistent with the view that this organism contributes to atherosclerotic lesion development. We have demonstrated that the presence of C pneumoniae causes macrophage foam cell formation and lipid oxidation with murine and human cells cocultured in the presence of LDL. In addition, we have provided evidence that implicates 2 putative chlamydial virulence factors in the development of these pathologic processes. Chlamydial lipopolysaccharide has been shown to cause macrophages to develop into foam cells in the presence of LDL, and the 60-kDa chlamydial heat shock protein (cHsp60), a known pathogenesis-inducing protein, has been found to contribute to oxidation of LDL in the presence of macrophages. Work is currently underway to define mechanisms involved in these processes and to further refine the putative role of C pneumoniae in atherogenesis and atherosclerotic lesion development.

Chlamydia pneumoniae-infected monocytes exhibit increased adherence to human aortic endothelial cells

Interactions between monocytes and endothelial cells play an important role in the pathogenesis of atherosclerosis, and monocyte adhesion to arterial endothelium is one of the earliest events in atherogenesis. Work presented in this study examined human monocyte adherence to primary human aortic endothelial cells following monocyte infection with Chlamydia pneumoniae, an intracellular pathogen associated with atherosclerosis by a variety of sero-epidemiological, pathological and functional studies. Infected monocytes exhibited enhanced adhesion to aortic endothelial cells in a time- and dose-dependent manner. Pre-treatment of C. pneumoniae with heat did not effect the organisms capacity to enhance monocyte adhesion, suggesting that heat-stable chlamydial antigens such as chlamydial lipopolysaccharide (cLPS) mediated monocyte adherence. Indeed, treatment of monocytes with cLPS was sufficient to increase monocyte adherence to endothelial cells, and increased adherence of infected or cLPS-treated monocytes could be inhibited by the LPS antagonist lipid X. Moreover, C. pneumoniae-induced adherence could be inhibited by incubating monocytes with a mAb specific to the human beta 2-integrin chain, suggesting that enhanced adherence resulted from increased expression of these adhesion molecules. These data show that C. pneumoniae can enhance the capacity of monocytes to adhere to primary human aortic endothelial cells. The enhanced adherence exhibited by infected monocytes may increase monocyte residence time in vascular sites with reduced wall shear stress and promote entry of infected cells into lesion-prone locations.

Characterization of low-density lipoprotein uptake by murine macrophages exposed to Chlamydia pneumoniae

Exposure to Chlamydia pneumoniae is correlated with atherosclerosis in a variety of clinical and epidemiological studies, but how the organism may initiate and promote the disease is poorly understood. One pathogenic mechanism could involve modulation of macrophage function by C. pneumoniae. We recently demonstrated that C. pneumoniae induces macrophages to accumulate excess cholesterol and develop into foam cells, the hallmark of early atherosclerotic lesions. To determine if C. pneumoniae-induced foam cell formation involved increased uptake of low-density lipoprotein (LDL), the current study examined macrophage association of a fluorescent carbocyanine (DiI)-labeled LDL following infection. C. pneumoniae enhanced the association of DiI-LDL with macrophages in a dose-dependent manner with respect to both C. pneumoniae and DiI-LDL. Interestingly, increased association was inhibited by native LDL and occurred in the absence of oxidation byproducts and in the presence of antioxidants. However, enhanced DiI-LDL association occurred without the participation of the classical Apo B/E native LDL receptor, since C. pneumoniae increased DiI-LDL association and induced foam cell formation in macrophages isolated from LDL-receptor-deficient mice. Surprisingly, DiI-LDL association was inhibited not only by unlabeled native LDL but also by high-density lipoprotein, very low density lipoprotein, and oxidized LDL. These data indicate that exposure of macrophages to C. pneumoniae increases the uptake of LDL and foam cell formation by an LDL-receptor-independent mechanism.

Emerging strategies in the diagnosis, prevention and treatment of chlamydial infections

Chlamydial infections cause a spectrum of diseases affecting millions of individuals worldwide. Chlamydia trachomatis is the most common sexually transmitted bacterium and the causative agent of trachoma, the leading cause of preventable infectious blindness in the world. The unique intracellular life-cycle and the chronic, persistent nature of chlamydial infections have hindered efforts to develop optimal diagnostic, preventive and treatment strategies for these pathogens. The reported association of C. pneumoniae with atherosclerosis and adult onset asthma suggests that these organisms may play an important role in the aetiology of chronic diseases. Treatment options involving new classes of antibiotics or new versions of existing antibiotics that may provide greater specificity and utility for long-term use against chlamydiae are much needed. In addition, new combinations of therapies are needed to manage existing chronic conditions and at the same time to alleviate any role that chlamydiae could be playing in these conditions. Novel strategies that address these shortcomings are apparent in recent research efforts. This review summarises patent claims of emerging strategies to diagnose, prevent and treat chlamydial infections. In addition, current efforts to develop suitable vaccine candidates are highlighted.

Collaborative Multidisciplinary Workshop Report: Interface of Lipid Metabolism, Atherosclerosis, and Chlamydia Infection

Gerald I. Byrne, Sonia I. Skarlotos, Carl Grunfeld, Murat V. Kalayoglu, Peter Libby, Pekka Saikku, James T. Summersgill, and Priscilla Wyrick Department of Medical Microbiology and Immunology, University of Wisconsin Medical School, Madison, Wisconsin; National Institutes of Health, Bethesda, Maryland; San Francisco Veterans Administration Center, San Francisco, California; Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts; National Public Health Institute, Oulu, Finland; Division of Infectious Diseases, University of Louisville, Louisville, Kentucky; and Department of Microbiology and Immunology, University of North Carolina School of Medicine, Chapel Hill, North Carolina

Infection, inflammation and age‐related macular degeneration

to effect a number of extracellular immunomodulatory mechanisms. It is highly unlikely that the relationship between infection and AMD is causal. AMD is much too complex a disease to be explained by a single trigger, be it genetic, infectious or environmental. However, the mounting body of evidence linking infection and AMD makes it entirely possible that pathogens can accelerate the disease process in susceptible individuals. An obvious question to address is whether individuals with predisposing polymorphisms (e.g. CFH variant Y402H) who develop AMD are more likely to be infected with C. pneumoniae, compared with those with similar polymorphisms but who remain healthy. Another set of experiments might attempt to colocalize C. pneumoniae or its virulence determinants with complement pathway variants within the choroid/RPE junction in patients with AMD. The difficulty in detecting C. pneumoniae in vivo and scarcity of AMD tissue specimens prevent rapid progress towards elucidating the exact role of infection in AMD. In addition, even if C. pneumoniae does contribute to AMD, it may not be possible to eradicate the infection with conventional antibiotics. However, the notion that an infectious agent can promote AMD is worthy of closer scrutiny, especially now that inflammation is accepted to be an integral part of AMD pathogenesis.

Treatment of chlamydial eye infections

Infections caused by Chlamydiae are among the most common in the world. Three species—C. trachomatis, C. pneumoniae, and C. psittaci—cause infections in humans. C. psittaci infections are acquired from birds and can lead to psittacosis, a febrile illness characterized by pneumonitis and systemic manifestations. In contrast, C. trachomatis and C. pneumoniae are transmitted from human to human. Chalmydia trachomatis is a common sexually transmitted organism, leading each year to 4 million new infections and 25,000 cases of infertility in the United States. The pathogen is also the etiologic agent of trachoma, the leading cause of preventable blindness, afflicting 150 million people worldwide and blinding 6 million individuals. Chlamydia pneumoniae infections, transmitted via aerosol droplets, lead to a myriad of upper respiratory tract diseases including pharyngitis, sinusitis and bronchitis. C. pneumoniae is also a primary cause of lower respiratory tract disease, contributing to nearly one-fourth of community-acquired pneumonias. More recently, C. pneumoniae has been detected in atheromatous plaques isolated from patients with coronary artery disease, and available data suggest that it is an emerging risk factor in atherosclerosis. The high prevalence of chlamydial infections reflect the pathogen’s successful adaptation to their human hosts. Several characteristics of chlamydiae contribute to their success as human pathogens. Chlamydiae are small, intracellular organisms dependent on their host cell for replication and prolonged survival. Their unique multiphasic life cycle permits the pathogen to establish persistent infections, allowing the organism to escape immune clearance yet express virulence determinants that cause chronic inflammation. In addition, long-lasting protective immunity does not develop following chlamydial infection, and reinfections are common. In the eye, these attributes have