34 CHAPTER 2 paired with. Because our studies are based on next-generation sequencing analysis of TCR mRNA, our results might be influenced quantitatively by differences in expression among different types of T cells [9]. However, in vitro studies showed that TCR surface expression and TCR mRNA levels are equal for naive and memory CD4 and CD8 T cells, whereas effector T cells were shown to have at most 2-fold higher levels of TCR mRNA and TCR surface expression, indicating that potential bias in RNA-based TCR studies is probably minimal [9,38,39]. Theoretically, there is a risk of amplification bias due to the fact that some TCR primers might be more efficient than others. However, our system uses an initial linear amplification procedure to prevent this. Furthermore, recent analysis using unique molecular identifiers in our protocol showed excellent correspondence with the results of our linear amplification protocol (data not shown). The reported findings may potentially be extended to other forms of arthritis. In a pilot study in psoriatic arthritis (PsA) patients, we earlier showed substantial overlap in TCR repertoire between ST in both knees in one PsA patient, whereas overlap between synovial repertoires and PB was limited in two PsA patients [40]. These preliminary results are comparable to the results shown in this study and support the notion that TCRβ clones in ST are different compared with PB in both diseases. Clearly, the findings in PsA need confirmation in larger patient groups. Follow-up studies on B cell receptor clonality in both diseases and in other immune-mediated inflammatory diseases may shed more light on the composition of the B cell repertoire in different compartments and different phases of the disease. In RA, this would follow up on an earlier study that showed dominant B cell clones shared among multiple joints of the same patient but not in PB [30]. In conclusion, we show substantial overlap of the TCR repertoires between inflamed ST biopsy specimens from different joints or from different sites within one joint in the same patient, whereas SF does not fully reflect the T cell repertoire in the inflamed ST. This implies that for T cell (receptor) analysis, tissue biopsies are required but that tissue collection might be simplified using less invasive procedures [e.g., through high-quality, US-guided, or blind needle biopsies [27,37]. More importantly, it shows that underlying T cell responses in the ST share a uniform specificity, which suggests that Ag- and/or receptor-specific therapies targeting a limited set of TCRβ clones in individual patients may be feasible in RA and possibly other inflammatory joint diseases, including PsA or even the immune-mediated inflammatory disease group at large.
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