Lyme Arthrities

The arthritic form of Lyme disease was first reported in the 1970s by Allen Steere, who described the condition in a group of children (and a few adults) residing in and around the town of Lyme, Connecticut. Lyme arthritis can strike when Borrelia burgdorferi introduced into the skin by an Ixodes tick burrows into deeper tissues and ends up in the joints, usually the knee. Swelling results from an inflammatory response to B. burgdorferi residing in the joint. Lyme arthritis is treated with antibiotics, which destroy the bacteria driving inflammation. Unfortunately, arthritic symptoms endure in ~10% of treated patients despite the complete or almost complete eradication of the infection, as determined by negative PCR tests for B. burgdorferi DNA in joint fluid. Such cases are called antibiotic-refractory Lyme arthritis, which can persist for months or sometimes years. In severe cases cartilage and bone erode. Although the pathogenesis of antibiotic-refractory Lyme arthritis could involve persistence of small numbers of B. burgdorferi (or their antigens) in the joints, investigators have been seeking an autoimmune mechanism to explain the prolonged attack on joint tissue by the immune system after the spirochetes have been cleared.

Many autoimmune diseases are linked to variants of HLA (immunity) genes such as those encoding the MHC class II complex. Antibiotic-refractory Lyme arthritis is associated with MHC class II variants that are able to bind to fragments of the B. burgdorferi protein OspA (outer surface protein A) encompassing amino acid residues 165 through 173. Antigen-presenting cells whose MHC class II molecules display OspA165-173 peptides on their surface stimulate T cells that recognize the OspA peptide. How OspA165-173-reactive T cells cause autoimmunity has been an area of intensive research, yet a clear answer has not emerged.

One potential pathway to autoimmunity is molecular mimicry, in which a cross-reactive host protein in the joint continues to stimulate OspA165-173-specific T cells even after the eradication of B. burgdorferi by antibiotics. Although the simplicity of the molecular mimicry model is appealing, exhaustive efforts to find a cross-reactive autoantigen that stimulates OspA165-173-specific T cells have failed. Moreover, levels of OspA165-173-reactive T cells decline soon after initiation of antibiotic therapy despite continuing arthritis following treatment. Thus, chronic arthritis does not seem to involve molecular mimicry driven by a cross reaction between the OspA165-173 epitope and a self-antigen in the joint. It is possible that molecular mimicry involves another B. burgdorferi antigen that is able to bind the MHC class II variants found in genetically susceptible individuals.

Other potential routes to autoimmunity in antibiotic-refractory Lyme arthritis patients emphasize the role of the high levels of key proinflammatory cytokines and chemokines found in their joint fluid, levels even higher than those found in treatment-responsive patients prior to initiation of antibiotic therapy:
In a model known as bystander activation, the immune response to OspA165-173 (or another B. burgdorferi antigen) causes an excessive inflammatory response that activates other T cells that react to autoantigens in the joint.
The immune system is unable to turn off the intense inflammatory response associated with OspA165-173 after the spirochetes are cleared from the joint.

Although much attention has been focused on the role of host genetics, a recent study indicates that the genetics of the pathogen could also influence the course of Lyme arthritis. In the July 2009 issue of Arthritis and Rheumatism, Allen Steere and his collaborators showed that antibiotic-refractory Lyme arthritis is associated with different strains of B. burgdorferi. The strains were typed from joint fluid samples collected before or during antibiotic treatment. Among the methods available to group B. burgdorferi isolates, they used the 16S-23S ribosomal RNA intergenic spacer type (RST), of which there are three. Antibiotic-refractory arthritis was defined as joint swelling lasting for at least 3 months after the start of antibiotic treatment. Antibiotic treatment consisted of 8 weeks of oral antibiotics or up to 4 weeks of antibiotics administered intravenously. Joint fluid from all 17 patients in the study tested positive by PCR for B. burgdorferi DNA prior to or during antibiotic treatment.

The authors found that all 7 Lyme arthritis patients infected with RST1 strains had the antibiotic-refractory form. Joint fluid was obtained after antibiotic treatment from 5 of the 7 patients; all 5 samples tested negative for B. burgdorferi DNA by PCR. In contrast, 2 of 6 and 3 of 4 infected with RST2 and RST3 strains, respectively, were successfully treated with antibiotics (see the table below from the Jones et al. 2009 article). A larger number of samples is needed to demonstrate that the difference observed between RST1 and RST2 strains is statistically significant, but there is a clear trend towards RST1 infections having the greatest association with antibiotic treatment failure and RST3 having the least, with RST2 having an intermediate effect. The duration of arthritis also depended on the infecting RST strain.
How do RST1 strains cause arthritis to persist even after the apparent eradication of the spirochetes by the recommended course of antibiotics? The investigators proposed that RST1 strains provoke a stronger inflammatory response in the joint than RST2 or RST3 strains. Coupled with an immune response to OspA165-173 in genetically susceptible patients, this could cause inflammation to continue at high levels even after elimination of the spirochetes from the joints. RST1 strains may be more likely than the other genotypes to spark intense joint inflammation even in patients who are not genetically prone to antibiotic-refractory arthritis.

In future studies, it would be interesting to see if proinflammatory cytokine levels are related to the RST type that infects the joint. Ultimately, researchers need to identify the B. burgdorferi gene or genes whose variation among the RSTs causes the different treatment outcomes of Lyme arthritis.
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