BS is a complex disorder with aberrant T-cell activation in the initiation and perpetuation of the disease [4]. The lack of a specific diagnostic test impedes the early detection of BS. TCR, a distinct immune biomarker of T cells, has been applied to better understand and track the potential disease driven peptide [32]. Since TCR repertoire can change greatly with the onset and progression of diseases, depicting TCR profile could help to illustrate the diverse immune status [18], and predict the outcome of disease [23].
For the first time, we employed high-through sequencing to uncover the TRB repertoire in BS with certain major organ involvements - namely panuveitis, intestinal and cardiovascular involvement [33]. As with other autoimmune diseases [19, 34-36], a diminished diversity and increased clonality of TCR repertoire were demonstrated in our cohort, which might indicate an ongoing autoreactive clonotype expansion in BS. By using V-J pairing index, we identified a diagnostic tool to discriminate BS from HC. To note, the difference of V-J pairing and the inter-group similarity among subgroups of BS reveal distinct T cell signatures among BS patients with varied organ involvements, which partially explains the immune pathogenesis of the clinical heterogeneity of BS [3]. Last but not least, we identified the preference of clones and V-J combinations between active and inactive disease, which could help developing TCR biomarkers for detecting active BS.
Our study suggested that the differences in V-J pairing and specific clones between BS patients and HC could represent the disease-associated T cells’ responses. TCR repertoire is shaped by biases during V(D)J recombination and by the subsequent expansion and deletion of certain clones upon antigen recognition during T cells’ development in the thymus and later in the periphery [37, 38]. The V(D)J recombination mechanism generates many more antigen receptor sequences than others [38]. Before further interpretation, we should better consider the context. First, TCR clonal sharing is determined by the same epitope, which means the antigen is presented by the same or similar MHC-alleles [23]. Although the exact mechanism by which TCR repertoire interacts with peptide/MHC has remained controversial [39], MHC Class II loci play a crucial role in determining thymic selection of the V-gene usage in TCR repertoire [40, 41]. However, due to the constraint of sample size, we could not apply expression quantitative trait locus (eQTL) mapping to test for associations between genetic variation and TCR V-gene usage in the current study. Moreover, the immense level of receptor degeneracy [42] and cross-reactivity of TCRs could underestimate the presence of TCR responses to common autoantigens between patients [43]. Notably, TRBJ2-7 gene usage was more enriched in patients with intestinal involvement and cardiovascular lesions than in patients with uveitis. Previous studies have proved the TRBJ2-7 segment was abnormally expressed in patients with systemic lupus erythematosus [36, 44], and chronic hepatitis B [45]. Collectively, public TCR clones existing in BS subgroups could suggest that distinct antigen-specific T cell cloning and expansion is associated with different phenotypes.
Additionally, our study demonstrated the diverse V-J combinations and distinct clone usage between different disease activity groups. However, the association of distinct TCR and disease activity is not conclusive in other connective tissue diseases. Lu et al.[46] found the Gini index of the V-J gene combinations and SLE-associated clones was correlated with the disease activity, while a longitudinal study of TCR clonotypes revealed that the repertoire diversity remained unchanged in 9/11 patients with either active or quiescent SLE [19]. In RA, TCR diversity and the abundance of differentiated Th17 T cells were significantly correlated with disease activity [35]. Analyses of collagen-stimulated blood cells from early HLA-DR4 RA patients showed that TRBV25-bearing T cells were more frequently expanded in those with moderate to high disease activity [47].
Our study has several limitations. First, due to the rarity of patients with major organ involvement, the sample size was relatively small, which could impair the statistical power. Considering potential factors that may confound the TCR characteristics, future studies of larger cohorts and long-term outcome measurements in BS patients and matched controls are required to better understand the immunological significance of TCR changes [48, 49]. Second, the current study did not include patients with only skin and (or) articular lesions, who make up the majority of BS. An essential biomarker might predict which patients with only skin and (or) articular manifestations are prone to develop major organ involvement. Thus, subsequent research should find the specific TCR profiles of BS patients with major organ involvement. Currently we investigated TCR profiles at the pan-T-cell level, which could mix the distinct features of TRB from different cell subsets.
In summary, our study highlights the essential roles and dynamic complexity of the T cell immune responses in BS pathophysiology. BS patients exhibit a skewed TRB repertoire. The V-J combination, along with unique clones, could distinguish active from inactive BS, which suggests clonal expansions during disease activation. TCR repertoires could provide novel quantitative parameters for disease monitoring in BS.