Telomere erosion: a new link between HLA DR4 and rheumatoid arthritis?

Update

TRENDS in Immunology

susceptibility to Listeria monocytogenes infection. J. Immunol. 168, 4667 – 4673 25 Girardin, S.E. et al. (2003) Nod1 detects a unique muropeptide from Gram-negative bacterial peptidoglycan. Science 300, 1584 – 1587 26 Eriksson, S. et al. (2003) Nitric oxide produced by murine dendritic cells is cytotoxic for intracellular Salmonella enterica sv. Typhimurium. Scand. J. Immunol. 58, 493 – 502

Vol.25 No.7 July 2004

339

27 Svensson, M. et al. (2000) Salmonella enterica serovar Typhimuriuminduced maturation of bone marrow-derived dendritic cells. Infect. Immun. 68, 6311 – 6320

1471-4906/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.it.2004.05.004

Telomere erosion: a new link between HLA DR4 and rheumatoid arthritis? Mike Salmon1 and Arne N. Akbar2 1

Department of Rheumatology, MRC Centre for Immune Regulation, Division of Immunity and Infection, Medical School, The University of Birmingham, Birmingham, B15 2TT, UK 2 Windeyer Institute of Medical Sciences, Royal Free and University College Medical School, 46, Cleveland Street, London, W1T 4JF, UK

Previous models used to explain the association between HLA DR4 and rheumatoid arthritis (RA) focused on the presentation of ‘arthritogenic’ peptides. These failed because of the diversity of HLA molecules that are associated with the disease. Patients with RA have accelerated erosion of telomeres in peripheral T cells. Recent research shows that this might be a function of HLA DR4 expression and is common to all blood cells. This offers a new perspective on the mechanism of HLA disease associations and also a link to T-cell homeostasis, which is known to be important in RA. ‘Find the gene, cure the disease’. That marvelously optimistic mantra cited by geneticists throughout the 1980s and early 1990s was the driving force behind the cloning of the human genome and an enormous amount of genetic mapping in a whole range of complex diseases. Unfortunately, the answers that resulted were rarely clear and never simple. In many cases, the genes were either unknown or their functions appeared to have no rational link to the disease process. In recent years, knockout mouse models have frequently been used to try to determine the biology of such genes. Unfortunately, genetic interactions tend to modulate the function of cells and indeed systems in a frequently unpredictable manner. One of the best-established genetic associations with any disease is that of certain subtypes of HLA DR4 with rheumatoid arthritis (RA). Stastny first reported this link in 1978 [1]. He showed that the DR4 subtype HLA Dw4 (now called DRB1 *0401) is considerably overexpressed in patients with RA. In the late 1980s a combination of events led to a rapid clarification of our understanding of this association. RA is principally associated with subtypes of DR4, namely *0401, *0404 and *0405, and also to a lesser extent with DR1 [2 – 4]. By contrast, the DR4 subtypes *0402 and *0403 show no association with RA. Oddly enough, the Native American Yakima population have a Corresponding author: Mike Salmon ([email protected]). Available online 28 May 2004 www.sciencedirect.com

high prevalence of RA, which is associated with a DR6 subtype *1402 [5]. Comparisons between alleles that were associated with RA and those that were not showed that the focus of susceptibility lay in a short stretch of amino acids, between positions 67 and 74 of the a-helix encoded by the DRb chain; usually termed the conserved third allelic hypervariable region (3AHVR) or the ‘shared epitope’ [6]. This was confirmed by the prediction of further associated molecules. The most prominent function of HLA DR molecules is to present short peptide antigens to CD4þ T cells and to select, within the thymus, the receptors that those T cells will carry into the periphery. For precisely these reasons, most workers in the field have tacitly assumed that the HLA association with RA reflects, either, the binding and presentation of an ‘arthritogenic’ peptide or the selection of particular T-cell receptors (TCRs) in the thymus [7– 9]. Unfortunately, the first of these is implausible, because the molecules associated with RA, including subtypes of DR4, DR1, DR6 and DR10, for example, are different outside the 3AHVR motif; they are not capable of presenting a common peptide that would be excluded from other family members. Strenuous efforts to find evidence of an association between particular TCRs and susceptibility to RA have proved largely unsuccessful [10]. To complicate the issue further, it has become apparent that the HLA association with RA is restricted to severe disease, of the sort typically found in hospital clinics. The majority of patients managed in the community with mild disease show no association at all [11,12]. Various attempts have been made to square this particular circle, including suggestions that shared epitope-containing HLA DR molecules form less stable complexes with class II-associated invariant chain [13], and conversely that they form very high affinity complexes with peptides in which arginine residues have been converted to citrulline [14]. This is particularly interesting because antibodies to citrullinated peptides are common in patients with RA and appear to be present in many cases well before the disease starts [15]. A rather clever possibility was suggested some years ago by the sequence

340

Update

TRENDS in Immunology

homology of the 3AHVR with sequences present in both a bacterial heat-shock protein DNAJ [8] and an Epstein – Barr virus encoded protein gp110 [9]. The latter is a powerful T-cell epitope, the former a good B-cell epitope. Lymphocytes that recognize these sequences cross-react well with the 3AHVR motif, suggesting perhaps that the HLA-derived peptide itself was the ‘arthritogenic motif’. Unfortunately, this was not sustained by replication and suffered the same problem that such peptides cannot be presented by all of the requisite HLA subtypes. So, 27 years after finding the gene, we still do not know how it contributes to RA, and we certainly cannot cure the disease! Viewed from this perspective, a recent publication from Scho¨nland et al. [16] offers a welcome new insight into the relationship between HLA DR4 and rheumatoid arthritis. They reported previously that patients with RA have an accelerated loss of telomeres in peripheral T lymphocytes [17]. Telomeres are repeating units of DNA code found at the end of chromosomes. They have a key role in preventing aberrant chromosomal translocations by splicing the end of the chromosome into a folded loop. Telomeres are eroded with each cell division, although mechanisms for extension, notably the enzyme telomerase, are present in many cell types, particularly germ cells [18]. Nevertheless, in somatic human cells, telomeric erosion defines a limit to the life of non-malignant cell clones. The telomere-erosion model of ageing was first proposed by Olovnikov in 1973 [19], and it has recently been confirmed as having a key role in immune senescence [20]. Intriguingly, mice have considerably longer telomeres than humans, but live for only a fraction of the time [21]. Many possible explanations could be proposed for the accelerated loss of telomeres in patients with RA. It might, for example, reflect accelerated differentiation associated with the chronic inflammatory state [22]. Such a relationship has been reported previously in CD4þ T cells from patients with atopic dermatitis or psoriasis [23], neither of which are associated with shared epitope HLA alleles. However, in their recent report, Scho¨nland et al. appear to show quite conclusively that the phenomenon is actually associated with the expression of DR4 [11]. Healthy DR4 þ volunteers had significantly shorter telomeres than DR4 2 subjects. The accelerated erosion appeared to occur before the age of 20 years, after which DR4 þ individuals continue to erode their telomeres at the same rate as everyone else. However, DR4 þ neonates, and also sperm produced from DR4 þ adults, showed entirely normal telomere lengths. The shortened telomeres did not appear to reflect reduced telomerase activity. A key point was that the shortened telomeres were not restricted to CD4þ T cells. Granulocytes showed precisely the same effect, suggesting that the focus of activity is in haemopoeitic stem cells. This observation raises several key issues. First, it suggests an alternative function of HLA DRB1 genes that is not associated with antigen presentation Second, as the authors acknowledge, it is possible that the true association is with a linked gene, rather than with DR4 itself. If so, this would suggest that it might not really be associated with RA at all, because the principle focus of susceptibility in RA has clearly been mapped to the 3AHVR of the DRB1 gene [6]. Linkage disequilibrium seems less likely than a www.sciencedirect.com

Vol.25 No.7 July 2004

direct association with the DRB1 alleles. Unfortunately, the design of this new study is rather limited; the authors only included individuals with DRB1 alleles known to be associated with RA, these were: HLA-DRB1*0401, *0404, 0405 and *0408. Surprisingly, they did not study people with any of the DR4 subtypes, such as *0402 or *0403, which are not associated with susceptibility to RA, or indeed any of the non-DR4 shared eptitope-containing susceptibility alleles. It is perhaps significant, however, that patients who express a combination of *0401 and *0404 tend to do particularly badly, developing severe disease at an early age. Nevertheless, a clear understanding of the role of accelerated telomere erosion in the DR4 association with RA will require confirmation that the phenomenon does not occur in those DR4 subtypes that are not associated with susceptibility to RA but does occur in other conserved 3AHVR alleles. In the study by Scho¨nland et al., the loss of telomeres in CD4þ T cells was associated with the development of markers of cell senescence, including loss of CD28 expression [16]. This is important, regardless of the true association between DR4 subtypes and telomere erosion, because it ties-in closely with long-standing observations that peripheral T cells in the blood and synovium of patients with RA show an advanced state of differentiation [22]. It is possible that telomeric erosion, rather than disease-driven T-cell proliferation, might account for this differentiation state, however, there is evidence suggesting the reverse. Some subsets of CD4þCD25þ regulatory Tcells are generated in the periphery from highly differentiated T cells with short telomeres [24,25]. There are a lot of these cells in both the blood and synovium of patients with RA and they have proved to be highly suppressive in vitro [26]. This would support a model of disease driven proliferation rather than telomeric erosion per se for the generation of highly differentiated CD4þ T cells in RA, although the inflammation clearly persists despite the presence of these cells. Convincing mouse models of RA have proved elusive. Scho¨nland et al. suggest why this might be the case because telomere erosion appears to have little, if any, role in mouse immune homeostasis [16]. This would clearly suggest that mouse models of RA are not at all practical. It is perhaps a little ironic that the most convincing such model, with symmetrical erosive disease, has recently been developed by modification of the TCR-associated ZAP-70 (z-chain-associated protein of 70 kDa) gene [27]. It is quite possible that the work of Scho¨nland et al. might be viewed in the future as a landmark paper, showing a fundamentally new mechanism for an HLA DR disease association and also for linking the primary disease association of RA with T-cell homeostasis, which is known to be important. For the moment, however, we are left with some tantalising questions. Not least of which is exactly how does expression of HLA-DR4 lead to the erosion of telomeres in haemopoeitic stem cells? References 1 Stastny, P. (1978) Association of the B cell alloantigen DRw4 with rheumatoid arthritis. N. Engl. J. Med. 298, 869– 871

Update

TRENDS in Immunology

2 Nepom, G.T. et al. (1987) Identification of HLA Dw14 genes in DR4þ rheumatoid arthritis. Lancet 2, 1002 – 1004 3 Wordsworth, B.P. et al. (1989) HLA-DR4 subtype frequency in rheumatoid arthritis indicate that DRB1 is the major susceptibility locus within the human leucocyte antigen class II region. Proc. Natl. Acad. Sci. U. S. A. 86, 10049 – 10053 4 Gregersen, P.K. et al. (1986) Molecular diversity of HLA-DR4 haplotypes. Proc. Natl. Acad. Sci. U. S. A. 83, 2642 – 2646 5 Willkens, R.F. et al. (1991) Association of HLA-DW16 with rheumatoidarthritis in Yakima indians – further evidence for the shared epitope hypothesis. Arthritis Rheum. 34, 43 – 47 6 Salmon, M. (1992) The immunogenetic componant of susceptibility to rheumatoid arthritis. Curr. Opin. Rheumatol. 4, 342 – 347 7 Winchester, R.J. and Gregersen, P.K. (1988) The molecular basis of susceptibility to rheumatoid arthritis: the conformational equivalence hypothesis. Springer Semin. Immunopathol. 10, 119– 139 8 Albani, S. et al. (1992) The susceptibility sequence to rheumatoid arthritis is a cross reactive B cell epitope shared by the Escherichia coli heat shock protein dnaJ and the histocompatibility leukocyte antigen DRB10401 molecule. J. Clin. Invest. 89, 327 – 331 9 Roudier, J. et al. (1989) Susceptibility to rheumatoid arthritis maps to a T cell epitope shared by the HLA DW4 DR b1 chain and the Epstein – Barr virus glycoprotein gp110. Proc. Natl. Acad. Sci. U. S. A. 86, 5104 – 5108 10 Salmon, M. and Gaston, J.S.H. (1995) The role of T lymphocytes in rheumatoid arthritis. Br. Med. Bull. 51, 332 – 345 11 Gough, A. et al. (1994) Genetic typing of patients with inflammatory arthritis at presentation is predictive of outcome. Arthritis Rheum. 37, 1166 – 1170 12 van Zeben, D. et al. (1991) Association of HLA-DR4 with a more progressive disease course in patients with rheumatoid arthritis. Arthritis Rheum. 34, 822 – 830 13 Patil, N.S. et al. (2001) Rheumatoid arthritis (RA)-associated HLA-DR alleles form less stable complexes with class II-associated invariant chain peptide than non-RA-associated HLA-DR alleles. J. Immunol. 167, 7157 – 7168 14 Hill, J.A. et al. (2003) The conversion of arginine to citrulline allows for a high affinity peptide interaction with the rheumatoid arthritisassociated HLA-DRB1*0401 MHC class II molecule. J. Immunol. 171, 538 – 541

Vol.25 No.7 July 2004

15 Goldbach-Mansky, R. et al. (2000) Rheumatoid arthritis associated autoantibodies in patients with synovitis of recent onset. Arthritis Res. 2, 236 – 243 16 Scho¨nland, S.O. et al. (2003) Premature telomeric loss in rheumatoid arthritis is genetically determined and involves both myeloid and lymphoid cell lineages. Proc. Natl. Acad. Sci. U. S. A. 100, 13471 – 13476 17 Koetz, K. et al. (2000) T cell homeostasis in patients with rheumatoid arthritis. Proc. Natl. Acad. Sci. U. S. A. 97, 9203 – 9208 18 Allsopp, R.C. et al. (1992) Telomere length predicts replicative capacity of human fibroblasts. Proc. Natl. Acad. Sci. U. S. A. 89, 10114 – 10118 19 Olovnikov, A.M. (1973) A theory of marginotomy: the incomplete copying of template margin in enzymatic synthesis of polynucleotides and biological significance of the phenomenon. J. Theor. Biol. 41, 181– 190 20 Plunkett, F.J. et al. (2001) The flow cytometric analysis of telomere length in antigen- specific CD8þ T cells during acute Epstein – Barr virus infection. Blood 97, 700 – 707 21 Akbar, A.N. et al. (2000) Differential regulation of CD8þ T cell senescence in mice and men. Mech. Ageing Dev. 121, 69 – 76 22 Matthews, N. et al. (1993) CD45RB exon expression by T lymphocytes from patients with rheumatoid arthritis. Arthritis Rheum. 36, 603– 607 23 Wu, K. et al. (2000) Telomerase activity is increased and telomere length shortened in T cells from blood of patients with atopic dermatitis and psoriasis. J. Immunol. 165, 4742 – 4747 24 Akbar, A.N. et al. (2003) The peripheral generation of CD4þCD25þ regulatory T cells. Immunology 109, 319 – 325 25 Taams, L.S. et al. (2002) Antigen-specific T cell suppression by human CD4þCD25þ regulatory T cells. Eur. J. Immunol. 32, 1621– 1630 26 van Amelsfort, J.M.R. et al. CD4þCD25þ regulatory T cells in rheumatoid arthritis: an activated, highly suppressive population in the synovial fluid. Arthritis Rheum. (in press) 27 Sakaguchi, N. et al. (2003) Altered thymic T-cell selection due to a mutation of the ZAP-70 gene causes autoimmune arthritis in mice. Nature 426, 454 – 460 1471-4906/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.it.2004.05.002

An ongoing series Interactions between haemostasis and inflammation Already published: Coagulation in arthropods: defence, wound closure and healing U. Theopold, O. Schmidt, K. So¨derha¨ll and M.S. Dushay Trends Immunol. 25, 289 – 294

Coming soon: Platelets: defense and signaling in innate and adaptive immunity G. Zimmermann and A. Weyrich UPA and uPAR fibrinolysis, immunity and pathology F. Blasi and A. Mondino Emerging roles of tissue factor in hemorrhagic fever W. Ruf www.sciencedirect.com

341