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  • HERV-K113 is not associated with multiple sclerosis in a large family-based study.

    14 December 2017

    Numerous studies have invoked a role for retroviruses in multiple sclerosis (MS). Most have identified human endogenous retroviruses (HERVs) as possible etiological agents. The majority of HERVs originate from ancestral infection and then become progressively disabled by mutations over millions of years of primate evolution. Their presence in 100% of healthy humans, together with the paucity of functional retroviral genes, argues strongly against a causal role in disease. Recently, a new class of insertionally polymorphic HERVs has been described that is present in only a proportion of the population. One of them, HERV-K113, is notable for open reading frames for all of its genes and is found in 0-28% of humans with widespread geographic and racial variation. Thus HERV-K113 is a credible candidate for causing disease in a manner comparable to infectious retroviruses. Genomic DNA samples from 951 patients with MS were tested for the presence of the HERV-K113 allele by PCR, with their unaffected parents (n = 1902) acting as controls. HERV-K113 provirus was found in 70 out of 951 (7.36%) patients with MS and was not significantly increased compared to the combined parent group (6.52%). The results do not support an association between this endogenous retrovirus and MS. This study also emphasizes the need for large cohorts with controls for race and geographic location when examining possible links between polymorphic HERVs and disease.

  • Evolutionarily conserved antigens in autoimmune disease: implications for an infective aetiology.

    12 December 2017

    The immune system has evolved to eliminate or inactivate infectious organisms. An inappropriate response against self-components (autoantigens) can result in autoimmune disease. Here we examine the hypothesis that some evolutionarily conserved proteins, present in pathogenic and commensal organisms and their hosts, provide the stimulus that initiates autoimmune disease in susceptible individuals. We focus on seven autoantigens, of which at least four, glutamate decarboxylase, pyruvate dehydrogenase, histidyl-tRNA synthetase and alpha enolase, have orthologs in bacteria. Citrullinated alpha-enolase, a target for autoantibodies in 40% of patients with rheumatoid arthritis, is our main example. The major epitope is highly conserved, with over 90% identity to human in some bacteria. We propose that this reactivity of autoantibodies to shared sequences provides a model of autoimmunity in rheumatoid arthritis, which may well extend to other autoimmune disease in humans.

  • Heightened immune response to autocitrullinated Porphyromonas gingivalis peptidylarginine deiminase: a potential mechanism for breaching immunologic tolerance in rheumatoid arthritis.

    12 December 2017

    BACKGROUND: Rheumatoid arthritis (RA) is characterised by autoimmunity to citrullinated proteins, and there is increasing epidemiologic evidence linking Porphyromonas gingivalis to RA. P gingivalis is apparently unique among periodontal pathogens in possessing a citrullinating enzyme, peptidylarginine deiminase (PPAD) with the potential to generate antigens driving the autoimmune response. OBJECTIVES: To examine the immune response to PPAD in patients with RA, individuals with periodontitis (PD) and controls (without arthritis), confirm PPAD autocitrullination and identify the modified arginine residues. METHODS: PPAD and an inactivated mutant (C351A) were cloned and expressed and autocitrullination of both examined by immunoblotting and mass spectrometry. ELISAs using PPAD, C351A and another P gingivalis protein arginine gingipain (RgpB) were developed and antibody reactivities examined in patients with RA (n=80), individuals with PD (n=44) and controls (n=82). RESULTS: Recombinant PPAD was a potent citrullinating enzyme. Antibodies to PPAD, but not to Rgp, were elevated in the RA sera (median 122 U/ml) compared with controls (median 70 U/ml; p<0.05) and PD (median 60 U/ml; p<0.01). Specificity of the anti-peptidyl citrullinated PPAD response was confirmed by the reaction of RA sera with multiple epitopes tested with synthetic citrullinated peptides spanning the PPAD molecule. The elevated antibody response to PPAD was abolished in RA sera if the C351A mutant was used on ELISA. CONCLUSIONS: The peptidyl citrulline-specific immune response to PPAD supports the hypothesis that, as a bacterial protein, it might break tolerance in RA, and could be a target for therapy.

  • Antibodies to several citrullinated antigens are enriched in the joints of rheumatoid arthritis patients.

    12 December 2017

    OBJECTIVE: High titers of specific anti-citrullinated protein antibodies (ACPAs) are frequently present in the serum of rheumatoid arthritis (RA) patients, but their presence in synovial fluid is less well characterized. The purpose of this study was to compare the levels of antibody to 4 well-defined citrullinated candidate RA autoantigens in serum and synovial fluid and to determine whether antibodies to one citrullinated antigen are dominant over another. Furthermore, we studied their relationships with mutated citrullinated vimentin (MCV), a newly identified RA-specific serum assay, and the classic cyclic citrullinated peptide (CCP) in the synovial fluid of well-defined HLA-DR groups. METHODS: Paired serum and synovial fluid samples from 290 RA patients and serum samples from 100 age- and sex-matched healthy controls were analyzed for the presence of anti-MCV and anti-CCP antibodies and for reactivity to citrullinated fibrinogen, alpha-enolase, type II collagen, and vimentin. A total of 219 of the 290 patients were genotyped for the HLA-DR shared epitope alleles. RESULTS: Significantly higher proportions of antibodies against all RA-associated citrullinated antigens were found in synovial fluid as compared with serum. This was also true for the MCV and CCP responses but not for non-RA-associated anti-tetanus toxoid antibodies. As expected, we found a high correlation between citrullinated vimentin and MCV responses. All synovial fluid ACPAs were predominantly associated with HLA-DRB1*04 alleles and were confined to the CCP+/MCV+ subset of patients. CONCLUSION: MCV and CCP positivity represent a similar subset of RA patients, whereas ACPAs with different fine specificities fall into subgroups of anti-CCP+/anti-MCV+ patients. The levels of all specific ACPAs were elevated in synovial fluid, suggesting that there is local antibody production and/or retention of ACPAs at the site of inflammation governed by RA-predisposing genes.