Microbe hunting in the joints.

Abstract

Rheumatoid arthritis (RA) is considered an autoimmune disease, but the notion that the process may be triggered by an unknown infectious agent has been a longstanding concept in the pathogenesis of RA. After several decades searching for such causative agents in the joints, the direct evidence is still missing. In the June issue of Arthritis Care & Research, Chen et al provided new insights in this area searching with gas chromatography-mass spectrometry (GC-MS) for muramic acid, a component derived from bacterial cell walls, in the joint tissues of RA patients (1). In the case of reactive arthritis (ReA), infectious agents triggering joint inflammation have been well established (2,3). Gastrointestinal infections by enteric bacterial strains, such as Yersinia, Salmonella, Campylobacter, and Shigella species, and urogential infections by Chlamydia species are known to be associated with ReA. One of the strongest links is the demonstration of Chlamydial DNA in the joints of patients with post-Chlamydia ReA by polymerase chain reaction (PCR) (4,5). However, after a prolonged search for bacterial DNA in RA joints, it remains unresolved whether or not an organism or its antigenic fragments persist locally in these joints. It may still be argued that the causative agent is present at such a low concentration that it cannot be detected with current microbiology techniques. To date, many methods have been used in hunting for microbes in joints with RA, as well as those with ReA, ankylosing spondylitis, and osteoarthritis (OA) (Table 1). In their study, Chen et al screened for bacteria in joint tissues of RA patients using a sensitive pan-bacteria PCR, which can detect DNA of almost any bacterial species. However, no bacterial DNA was found in their study. Despite the absence of bacterial DNA, they probed for evidence of the bacterial cell wall, which represents the major constituent of bacteria. The skeleton of bacterial cell walls mainly consists of polysaccharide, peptidoglycan, and cell wall-associated proteins. Peptidoglycan is the major constituent of the gram-positive bacterial cell wall, whereas lipopolysaccharide (LPS) is the dominant component of the gram-negative bacterial cell wall. The schematic structure of the gram-positive bacterial cell wall is shown in Figure 1. In the current study, Chen et al selected muramic acid, a component of the peptidoglycan moiety of the bacterial cell wall. Muramic acid is unique to bacteria and is not present in the mammalian proteome, making it an ideal chemical marker for detecting bacteria (6). Chen et al detected muramic acid by using a GC-MS with a chemical ionization method sensitive enough to detect even picogram amounts (2 pg/injected amount). GC-MS has previously been used successfully to detect muramic acid in joints of septic arthritis patients) (7,8). Chen et al were able to demonstrate the presence of muramic acid in the joints of some RA patients, suggesting persistence of bacterial cell wall components, rather than bacterial DNA, in the joints of chronic RA (duration more than 5 years). Their findings are in line with earlier reports that bacterial cell wall components were present in RA joints by immunohistochemistry, using an antibody against bacterial cell wall peptidoglycan (9). However, this current study makes the important observation that the presence of bacterial cell wall components is not specific for RA, since they were also found in joint tissues of OA patients. The question has remained open how these bacterial cell wall components might play a biologic role if they persist in the joints for prolonged periods. Are they pathogenic? Several studies have implicated bacterial cell wall components as having multiple immunologic activities that may contribute to joint inflammation. 1) The recently discovered receptors of the first line of host defense—the Tolllike receptors (TLR)—recognize a range of microbes and their products. For example, TLR2 recognizes bacterial peptidoglycan (10). 2) Bacterial peptidoglycan is capable of stimulating synovial macrophages to express the costimulatory molecules CD80 and CD86, and to produce proinflammatory cytokines such as tumor necrosis factor and interleukin 6 (11). 3) Certain bacterial cell walls are able to induce experimental arthritis in rats with histopathologic features resembling RA (12,13). 4) Small fragments of the arthritogenic bacterial peptidoglycan, such as muramyl peptides, possess potent proinflammatory capacity (14). All these lines of evidence suggest that the bacterial cell wall components are potential arthritogenic Xiang Zhang, MD, PhD, Robert D. Inman, MD: University Health Network and University of Toronto, Toronto, Ontario, Canada; Cesar Pacheco-Tena, MD, MSc: Hospital Central Universitario, Universidad Autonoma de Chihuahua, and Hospital Clinica del Parque, Mexico. Address correspondence to R. D. Inman, MD, Arthritis Center of Excellence, ECW 8-005, Toronto Western Hospital, 399 Bathurst Street,Toronto, ON, M5T 2S8 Canada. Submitted for publication December 11, 2002; accepted December 22, 2002. Arthritis & Rheumatism (Arthritis Care & Research) Vol. 49, No. 4, August 15, 2003, pp 479–482 DOI 10.1002/art.11186 © 2003, American College of Rheumatology