In this large sample of patients with severe ExRA, we found Felty's syndrome to be associated with HLA-DRB1*0401. There was no significant difference in the global distribution of HLA-DRB1 or HLA-DQB alleles compared with non-extra-articular RA controls for any other manifestation or for ExRA overall. Patients with severe ExRA were more likely to carry HLA-DRB1*04 SE alleles, and genotypes featuring a double dose of DRB1*04 SE alleles were associated with rheumatoid vasculitis, Felty's syndrome, and all ExRA combined.
A number of studies have indicated a role for HLA-DR4
related genes in ExRA [26
]. In Caucasians of Northern European origin, severe ExRA has been associated with DRB1*0401/0401
homozygosity in particular [21
]. In a recent meta-analysis of HLA-DRB1
genotyping studies of patients with RA-associated vasculitis conducted by Gorman and coworkers [32
], vasculitis was found to be associated with the genotypes 0401/0401
, and 0401/0101
. In other meta-analyses by the same group, double dose of DRB1*04
SE alleles was associated with radiographic signs of progressive joint damage in northern European Caucasians [17
], but there was no significant association between SE and the presence of rheumatoid nodules [33
]. Taken together, these findings indicate that DRB1*04
SE double gene dose is associated with disease severity in RA, and that such genotypes may contribute specifically to risk for severe ExRA manifestations.
On the other hand, there was considerable heterogeneity across individual ExRA manifestations. The association between Felty's syndrome and DRB1*0401
is well established [34
]. In contrast, we did not observe any significant association with single or double DRB1*04
gene dose for patients with pericarditis, neuropathy, or interstitial lung disease. This indicates that the importance of HLA-DRB1
alleles may be variable for different manifestations, although our failure to detect an effect could be due to sample size or selection.
Severe ExRA manifestations tend to cluster in individual patients with RA [36
]. The high prevalence of vasculitis in patients with Felty's syndrome observed in the present study is consistent with the literature [37
], and may in part be due to shared genetic factors such as HLA-DRB1*04
alleles. In a survey of the community-based Olmsted county RA cohort [36
] we found clustering of a number of different ExRA features, including a frequent co-occurrence of vasculitis with neuropathy and rheumatoid lung disease. We made similar observations in the present study. Such clustering may be explained by both genetic and environmental factors.
The association between HLA-DRB1
genotypes and RA disease severity, including ExRA, has been interpreted as reflecting the importance of T cells in the pathogenesis of RA [26
]. HLA-DR and other MHC molecules are involved in presentation of antigens to T cells, and in positive and negative selection of T cells in the thymus. Because there appears to be a stoichiometric relationship between MHC molecules on the cell surface and positive selection mechanisms in thymic maturation of T cells, it has been suggested that the explanation for the gene dose effect seen in ExRA is its effect on T-cell diversity [21
]. The T-cell repertoire in patients with RA is markedly contracted, with less diversity and emergence of dominant T-cell clonotypes [39
]. T-cell abnormalities in patients with ExRA include expansion of CD8+
large granular lymphocytes [40
] and of immunosenescent CD4+
], and extensive CD4+
infiltrates in RA-associated interstitial pneumonitis [43
]. The importance of HLA-DRB1
genes and other genes with a role in T-cell selection and T-cell function for these phenomena require further study.
In accordance with previous studies, we found patients with ExRA to be more likely to be RF positive and ANA positive [22
]. This suggests a role for both B cells and T cells, possibly including dysregulated B cell-T cell interaction, in ExRA.
New genetic associations that were not postulated and have not been reproduced should be interpreted with caution. Given the nonsignificant results of the global distribution tests, the associations between ExRA and some rare DRB1 and DQB1 alleles (i.e. DRB1*12 and DQ4) are probably due to chance. The negative global test for HLA-DRB1 alleles in ExRA overall also suggests that the impact of DRB1*04 SE on the risk for severe ExRA manifestations is not strong, although it is reproducible in separate patient samples.
The lack of association between ExRA and HLA-DQB1 alleles, and the lack of association with HLA-DRB1-DQB1 haplotypes favors a specific role for HLA-DRB1 genes in ExRA, rather than secondary associations due to linked genes. Nevertheless, we cannot exclude the possibility that linkage disequilibrium with other genes in MHC explain our results.
The patients included in this study were recruited from four different centers, and the background RA population from which they were sampled is not fully characterized, at least not for the patients seen at Lund University Hospital and at the Mayo Clinic. On the other hand, these patients were recruited during a period when there was particular interest in patients with severe ExRA at each of the centers, suggesting that they should reflect the majority of patients with ExRA seen and be representative of the ExRA population as a whole.
In multicenter studies of genetic markers, ethnic heterogeneity of the studied patient samples must be considered. However, the majority of the patients included at the Mayo Clinic were Caucasians of northern European origin, similar to the patients from southern Sweden. Thus, our result could be generalized to RA patients with this ethnic background but not to other populations.