Discussion
We found that local joint inflammation in SLE is dominated by IL-6 and IL-17. IL-6, a pleiotropic cytokine, can influence the function of several immune cells such as B and T cells, orientating their fate towards Th17 and T follicular helper phenotypes 13. Its local expression in SLE inflamed large joints might also suggest a more acute joint pathology and an effect on recruiting neutrophils. Indeed, most of the SLE SF samples contained only neutrophils and became acellular following the cell purification step. Previous studies investigating IL-6 plasma concentration in relation to SLE arthritis have shown that elevated IL-6 was associated with active ongoing arthritis but not with CRP levels 14, 15 and also a correlation between IL-6 plasma level and joint involvement15. In our study, IL-6 SF levels also correlated with IL-6 serum levels, albeit IL-6 concentration was much lower in serum, suggesting a more relevant local role for this cytokine.
IL-17A, known to trigger the recruitment of neutrophils and monocytes to the site of inflammation 16 was abundant in SLE SF samples, similarly to what has been observed in ankylosing spondylitis17. In recent years, several studies have highlighted the possible implications of the IL-17 axis in SLE pathogenesis18. IL-17A has been found to be increased in sera of SLE patients compared to healthy controls, expressed in kidney tissue of patients with lupus nephritis and to predict an unfavorable histopathological response to immunosuppressive treatment19. Current clinical trials are evaluating IL-17A inhibition in refractory lupus nephritis (https://www.clinicaltrials.gov/ct2/show/NCT04181762).
This is the first study that, to our knowledge, investigates the cellular composition of SF in SLE. In most cases, the numbers of SF mononuclear cells were low, thereby only the fluid could be preserved. Still, we were able to analyze mononuclear cells from two synovial fluids from SLE patients. Similar to other inflammatory arthritides20, 21 an increase of Tregs could be observed in SLE SF. We could also identify a PD-1highHLA-DRhigh TPH population in SF of SLE patients. This T-cell subset has been described in ACPA+ RA patients and functions by promoting B-cell responses and antibody production22. Similarly, PD1highCXCR5- TPH have been described in SLE where they appear to drive the dynamics of pathologic B-cell responses, correlate with disease activity and are expressed in kidney tissue in lupus nephritis23.
In our study, CCR6+ CD4+ T cell subsets were abundant in SLE-SF. CCR6+ is a chemokine receptor expressed on Th17 cells and Th17 differentiation is dependent on the cytokines IL-6, IL-1β and TGFβ24. Th17 cells have previously been reported to increase during SLE flare and to be reduced after successful treatment25. Leipe J et al.26 reported an increase of Th17 cells in psoriatic arthritis and RA patients in peripheral blood and synovium, with an enrichment of Th17 in SF in the affected joint. Similarly, CCR6+ cells were increased in SF of the two analyzed SLE patients compared to peripheral blood.
Due to the increase of IL-17A and CCR6+ T cells in SF of SLE patients, we stimulated SFMC with anti-CD3/CD28 beads to study their capacity to produce cytokines. Thereby, we identified IL-17A producing T cells, primarily represented by CCR6+CD4+ T cells. It is however possible that other cells also could contribute to IL-17A production as both CD4 and CD8, as well as γδ T cells, innate lymphoid cells and mast cells have been identified as IL-17A producers in psoriasis and psoriatic arthritis27.
While CCR6+ was one of the most prominent CD4+ T-cell subsets in SLE-SF, the vast majority of SF CD8+ T cell expressed the transcription factor Eomes and GZMA similar to observations in RA28. GZMA induces pro-inflammatory cytokines such as IL-6, IL-8, IL-1β. Altogether, our analysis suggests an involvement of T cells in the pathogenesis of lupus arthritis.
The main limitation of our study is that it was conducted retrospectively on historical material. Systematic assessment of joint involvement and SLE disease activity were not performed at the time of sampling, thus clinical associations could not be further explored. Moreover, the sampling was performed only on large joints, which are technically more accessible but imply some limitations, since SLE arthritis more typically involves small joints. The long disease duration in the SLE patients with ongoing and previous treatments could also modify the SF composition. Being most often acellular or paucicellular, a strength of our study is the inclusion of two cellular samples of SLE SF.
From a clinical perspective, lupus arthritis is common already from the time of diagnosis but is a manifestation often responding to treatment. This may be a reason why arthritis has been given relatively less attention from a research perspective. Our study cohort is admittedly small but still represents one of the most comprehensive collections of SF material in SLE. It is tempting to speculate that lupus arthritis is less autoimmune in character as compared to rheumatoid arthritis and our data lend support to such a theory by suggesting that it is part of a systemic immune deviation towards Th17 and not joint specific. Indeed, Th17 responses implicate the involvement of innate immunity through neutrophils, cells that historically have been linked to SLE pathophysiology as LE cells8.
In conclusion, our study provided new exploratory insights into the immunology of lupus arthritis. IL-17A and IL-6 levels were high in synovial fluid of SLE patients. The presence of CCR6+ CD4+ T cells, GZMA+ EOMES+ as well as PD1+ HLA-DR+ CD4+ and CD8+ T cells suggest a pathogenic role of T cells in lupus arthritis. Our findings suggest that SLE patients with arthritis may improve if treated with IL-17A blockade for other clinical manifestations.